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© International Telecommunication Union INTERNATIONAL RADIO CONSULTATIVE COMMITTEE
C.C.I.R
DOCUMENTS OF THE Xlth PLENARY ASSEMBLY
OSLO, 1966
VOLUME IV
RADIO-RELAY SYSTEMS SPACE SYSTEMS RADIOASTRONOMY
PART 2 : SPACE SYSTEMS, RADIOASTRONOMY
Published by the . INTERNATIONAL TELECOMMUNICATION UNION GENEVA, 1967 March, 1969
ADDENDUM No. 1
to VOLUME IV OF THE DOCUMENTS OF THE Xlth PLENARY ASSEMBLY OF THE C.C.I.R. Oslo, 1966
Note by the Director, C.C.I.R.
During the Interim Meetings of the Study Groups IV (Space systems and radioastronomy) and IX (Radio-relay systems), Geneva, 1968, it was decided that, owing to the urgency of commencing study of the problems contained therein, certain new Questions and Study Programmes should be submitted for adoption by correspondence, in accordance with Article 14, § 2(1) of the International Telecommunication Convention, Montreux, 1965. Each of these texts has received more than the twenty approvals necessary for their adoption from the Members and Associate Members of the I.T.U. and they have, in conse quence, now become official Questions and Study Programmes of the C.C.I.R. (see Adminis trative Circulars A.C./128 of 18 December, 1968 (Study Group IV) and A.C./129 of 15 January, 1969 (Study Group IX)). These texts are: — Questions 161IX and 17IIX, which are reproduced on separate sheets numbered 178a and 1786; — Question 19/IV, which is reproduced on separate sheets numbered 5806 and 580c; — Study Programme 3B/IX, which is reproduced on a separate sheet numbered 165a; — Study Programmes 2H/IV, 2J/IV, 18A/IV, 19 A/IV, which are reproduced on separate sheets numbered 5676 to 567d and 5806, 580
Page 280. In line 5 of § 4.2, replace “ 30 dB ” by “ 300 dB ”. Page 295. In legend to Fig. 6, replace “ C ” by “ S ”. Page 410. In line 2 of § 6.2, replace “ space service ” by “ communication-satellite service Page 423. Replace the legend to Fig. 2 by:
E = 32 - .25 log10 Holmdal Goonhilly Raisting Goldstone Mill Village Goonhilly (modified) Andover West Ford ”
Page 425. Fig. 4, intervals on jc-axis are incorrectly spaced. Page 430. In Fig. 1, on abscissae, delete “ Thickness of water layer (in.) ”. Page 431. In Fig. 2, on abscissae, delete “ Thickness of water layer (in.) ”. — 165 a —
STUDY PROGRAMME 3B/IX
RADIO-RELAY SYSTEMS FOR TELEVISION
Residues of signals outside the baseband
(1969) The C.C.I.R.,
CONSIDERING (a) that it is desirable to set limits for the residues of signals outside the baseband in radio-relay systems for monochrome and colour television; (b) that these residues may be associated with the transmission of various types of signal, such as colour and sound sub-carriers and continuity pilots; (c) that, in suppressing these residues, it is important to avoid the introduction of excessive group-delay distortion in the baseband; (d) that this suppression can be expressed either as an absolute level or as an attenuation;
decides that the following studies should be carried out: 1. determination of the appropriate limits for monochrome and colour television, for the residues corresponding to : 1.1 signals arising from non-linear distortion within the video-frequency band, in particular* those corresponding to harmonics of the colour sub-carrier(s); 1.2 continuity and other pilots; 1.3 signals corresponding to the frequency of the sound sub-carrier(s); 1.4 any other spurious signals; 2. desirability of expressing these limits as: — a level relative to the nominal peak-to-peak amplitude of the vision carrier, — an attenuation. — 178 a —
QUESTION 16/IX
RADIO-RELAY SYSTEMS FOR TELEVISION AND TELEPHONY
Use of frequencies above about 12 GHz
(1969) The C.C.I.R.,
CONSIDERING (a) that many demands may soon be made for the use of frequencies above about 12 GHz; (b) that special propagation problems exist at frequencies above 10 GHz; (c) that new technological developments are applicable to radio-relay systems operating at frequencies above about 12 GHz;
decides that the following question should be studied: 1. what are the effects on radio-relay systems of the propagation characteristics of frequencies above about 12 GHz, operating wholly within the troposphere; 2. what are the preferred modulation techniques for use at frequencies above about 12 GHz for radio-relay systems; 3. what are the radio-frequency channel arrangements preferred for the frequency bands above about 12 GHz available for use by radio-relay systems?
QUESTION 17/IX
CRITERIA FOR FREQUENCY SHARING BETWEEN RADIO-RELAY SYSTEMS AND COMMUNICATION-SATELLITE SYSTEMS
(1969) The C.C.I.R.,
considering (a) that radio-relay systems are now widely employed throughout the world and make extensive use of the radio-frequency spectrum; (b) that the use of radio-relay systems is expected to continue to expand and that new systems are expected to operate with improved performance and make more efficient use of the radio- frequency spectrum; 178 b
(c) that the use of communication-satellite systems in the shared bands is expected to expand rapidly; (d) that the continued development of both services is desirable; (e) that control of mutual interference between stations of the two services is necessary; decides that the following question should be studied: 1. what levels of interference are acceptable and under what conditions do they apply to radio relay systems in order to facilitate sharing with communication-satellite systems; 2. what limitations are acceptable to radio-relay systems to facilitate the operation of earth station and space station receivers in a shared environment? — 567 a —
STUDY PROGRAMME 2G/IV
TECHNICAL CHARACTERISTICS OF COMMTJNICATION-SATELLITE SERVICE TO AIRCRAFT AND SHIPS
(1968) The C.C.I.R.,
CONSIDERING
(a) that there is a need for more reliable communication between distant land-based terminals and: — aircraft, — ships; (b) that the use of communication satellites promises to provide a service of sufficient reliability: (c) that communication with aircraft and ships may be required for the transmission of telephony or direct printing telegraphy (including data transmission) or both; (d) that in the interest of conservation of the radio frequency spectrum and to minimize the equipment which aircraft and ships carry, there might be overall merit in using the same frequency bands whether the aircraft or ship is communicating with a terrestrial station: — directly, — via a satellite; (e) that the use of common-spacecraft for mobile service to both aircraft and ships might well be advantageous, especially if the same order of frequency were to be suitable for both; (f) that important technical advantages, including those of frequency economy, might arise from the common use of satellites for both communication and navigation;
decides that the following studies should be carried out:
1. the preferred types of orbit to provide satellite communication between land-based terminals and: 1.1 aircraft, 1.2 ships;
2. the preferred frequencies and technical characteristics for: 2.1 satellite-aircraft links, 2.2 satellite-ship links, 2.3 satellite-ground links;
3. the technical feasibility for communication-satellite services for aircraft and ships to share the use of common frequency bands: 3.1 one with the other, 3.2 with the terrestrial aeronautical mobile service, — 567 b —
3.3 with the terrestrial maritime mobile service, 3.4 with the terrestrial aeronautical and maritime radio-navigation services; 4. the technical feasibility of using a communication satellite also for a navigation service. Note. — In this connection, the Director, C.C.I.R. is requested to draw the attention of the Inter national Civil Aviation Organization and the Intergovernmental Maritime Consultative Organization to this Study Programme and, in particular, to invite their cooperation in the study of the navigational aspects.
STUDY PROGRAMME 2H/IY
USE OF FREQUENCY BANDS ABOVE 10 GHz FOR COMMUNICATION-SATELLITE SYSTEMS
(1968) The C.C.I.R.,
CONSIDERING (a) that wide frequency bands are needed for communication-satellite systems, both regional and global; (b) that the technical feasibility of using frequency bands above 10 GHz for the communication- satellite service should be considered; (c) that the use of frequency bands above 10 GHz for these systems would introduce special technical problems, such as the effects of cloud and precipitation, on system performance and reliability;
decides that the following studies should be carried out: 1. the technical problems associated with the use of frequency bands above 10 GHz for com munication-satellite systems; 2. the special techniques which could be used to overcome these problems, for example the use of space diversity reception to minimize the difficulties arising from attenuation due to heavy rain; 3. the conditions under which the use of these techniques would be appropriate; 4. the conditions under which it may be feasible for communication-satellite systems to share frequency bands above 10 GHz with terrestrial services, and the extent to which such sharing might be possible. — 567 c —
STUDY PROGRAMME 2J/IV
COMMUNICATION-SATELLITE SYSTEMS
Technical factors influencing the efficiency of use of the geo-stationary satellite orbit by communication satellites sharing the same frequency bands *
(1968) The C.C.I.R.,
CONSIDERING
(a) that, for communication satellites, the geo-stationary satellite orbit is particularly valuable;
(b) that this orbit can accommodate only a finite number of communication-satellites sharing ■ the same frequency band and certain arcs of this orbit may be in great demand;
(c) that the service requirements are not uniform and may result in different communication- satellite system characteristics;
(d) that the spacing between such satellites is determined by the need to control interference and that where the characteristics of the satellites are different the number that can be accommo dated in a given arc depends also on how they are arranged;
(e) that interference can arise in both the up- and down-paths and is dependent on a number of technical factors;
(f) that the factors involved are interrelated and it is necessary to define the relationships between them in order to establish appropriate criteria providing for the orderly development and most effective use of the geo-stationary satellite orbit;
(g) that the effectiveness of use of the orbit will be further enhanced to the extent that it may be possible to use the same frequencies more than once within a single communication satellite; (h) that the necessary studies should be carried out as a matter of urgency, since the World Administrative Radio Conference (Space) which is to be held in late 1970 or early in 1971 may need technical guidance on these matters;
decides that the following studies should be carried out:
1. those technical characteristics of communication-satellite systems which affect the utilization of the geo-stationary satellite orbit, and the inter-relationships between them;
2. the technical criteria that should be used to ensure an orderly development aiming at the most efficient and effective use of the geo-stationary satellite orbit; 3. the extent to which it may be feasible and desirable to adopt preferred technical characteristics for different geo-stationary communication satellites and earth stations, to improve the overall effectiveness of use of the orbit.
* Other space services will also be using the geo-stationary satellite orbit, operating in their appropriate frequency bands. Note 1. — The following are some of the factors which should be taken into account in carrying out these studies: — the tolerable levels of interference noise in different communication-satellite systems; — the apportionment of thermal, interference and intermodulation noise; — the radiation patterns of the earth station and satellite antennae; — the differences in the values of e.i.r.p. used on the one hand by different earth stations and on the other by different communication satellites; — the required protection ratios resulting from the various baseband processing and modula tion techniques; — factors affecting the multiple use of the same frequencies within a single communication satellite; — errors in communication-satellite position and attitude: — polarization discrimination. 580 a —
QUESTION 18/IV
RADIO PROPAGATION STUDIES USING SPACECRAFT
(1968) The C.C.I.R.,
CONSIDERING (a) that satellite radio beacons, operating on two or more fixed, harmonically related frequencies, now provide a powerful technique for enhancing our knowledge of the nature of the iono spheric conditions which are relevant to space research; (b) that radio methods of measurement using spacecraft provide for the determination to a high degree of accuracy of: — orbital elements of satellites, — terrestrial distances, particularly intercontinental distances over large oceans; (c) that studies are desirable in the field of radio propagation from spacecraft, with the object of enhancing our knowledge of the transmission of radio Waves by and through the ionosphere;
decides that the following question should be studied: 1. what regions of the spectrum are suitable for: 1.1 radio-wave propagation studies to be carried out with the aid of spacecraft; 1.2 ionospheric measurements to be conducted with the aid of radio beacons aboard spacecraft; 1.3 geodetic and tracking measurements to be made using spacecraft carrying radio beacons; 2. which of the above applications would require long-term observations; 3. what specific relationships should exist between the various frequencies used for these applica tions; 4. what is the maximum interference that can be tolerated in each of these applications; 5. what are the factors affecting the sharing of the required frequencies with space research, telemetry and other radio services; 6. what degree of coordination in the location of ground stations will be required if frequency sharing is deemed to be practicable ? 580 b *—
STUDY PROGRAMME 18A/1V
PROTECTION OF FREQUENCY BANDS FOR SPACECRAFT TRANSMITTERS USED AS BEACONS
(1968) The C.C.I.R.,
CONSIDERING that the protection from interference required for research based on the use of spacecraft transmitters as beacons is not necessarily the same as that required for the reception of space research telemetry and for tracking (see Recommendation 364-1);
decides that the following study should be carried out: the criteria to be adopted for the protection from interference (“ noiselike ” or “ CW-type ”) of observations of beacon transmissions from spacecraft.
QUESTION 19/IV
TECHNICAL CHARACTERISTICS OF COMMUNICATION-SATELLITE SYSTEMS AND RADIODETERMINATION-SATELLITE SYSTEMS FOR AIRCRAFT AND/OR SHIPS
' (1968) The C.C.I.R.,
CONSIDERING (a) that the Conclusions of the 51 st meeting of the Committee on the Peaceful Uses of Outer Space were approved by the General Assembly of the United Nations at its XXIInd Session, in particular as regards: — approval of the report of the “ Working Group on a navigation services satellite system ” ; — continuation by I.C.A.O., I.M.C.O. and other specialized agencies of the studies under taken on space techniques; (b) that I.C.A.O. has already begun the study of space-communications for aeronautical require ments, in particular at the meeting of the COM/OPS Division, Montreal, October 1966, and has instituted a specialized panel called “ ASTRA ” ; - 580 c -
(c) that the appropriate sub-committees of I.M.C.O. have also begun the study of space techniques for communications and navigation with respect to maritime requirements, in particular for the safety of life at sea; (d) that in its Recommendations No. AER.2, the Aeronautical Conference, Geneva, 1966, advocated that study be made of the use of space communication techniques in the aeronautical mobile (R) service; (e) that, in its Recommendation No. MAR. 3, the Maritime Conference, Geneva, 1967, invited the C.C.I.R. to study the technical aspects of space-communication systems which offer the potential of fulfilling the maritime requirements with respect to the navigation of ships at sea; (f) that radiodetermination-satellite systems may be an important means of air traffic control; (g) that a number of Administrations are already studying satellite systems for air traffic control and air navigation by satellites; (h) that radiodetermination-satellite systems may contribute considerably to the fulfilment of various operational needs for the safety of life at sea, such as search and rescue, traffic separa tion guidance, collision warnings, etc.; (j) that the frequencies to be used by radiocommunication-satellite systems and radiodetermina tion-satellite systems should be the subject of international agreement, not only to facilitate the setting up of such systems, but also to avoid interference to and from other satellite systems and stations of other services;
decides that the following question should be studied with respect to aircraft and/or sh ip s: 1. what are the preferred types and technical characteristics of the following systems: 1.1 radiocommunication-satellite systems; 1.2 radionavigation-satellite systems; 1.3 radiolocation-satellite systems; 2. what are the preferred frequency bands for such systems; 3. are integrated radionavigation and radiolocation-satellite systems feasible and advantageous; 4. are integrated radiocommunication- and radiodetermination-satellite systems feasible and advantageous; 5. is the sharing of frequencies with other systems feasible, and if so, with what other systems and under what conditions? STUDY PROGRAMME 19A/IV
TECHNICAL CHARACTERISTICS OF COMMUNICATION-SATELLITE SYSTEMS AND RADIODETERMINATION-SATELLITE SYSTEMS FOR AIRCRAFT AND SHIPS
(1968) The C.C.I.R.,
CONSIDERING (a) that there is a need for more reliable communication between distant land stations and: — aircraft; — ships; (b) that systems of communication satellites could be conceived so as to ensure a service of sufficient reliability; (c) that communication with aircraft and ships may be needed for the transmission of telephony or telegraphy (including data transmission, direct printing, facsimile) or both; (d) that in the interest of conservation of the radio-frequency spectrum and to minimize the equipment which aircraft and ships carry, there might be overall merit in: — using the same maritime frequency bands whether the ship is communicating with a land station directly or via a satellite; — using the same aeronautical frequency bands whether the aircraft is communicating with a land station directly or via a satellite; — establishing a joint aeronautical/maritime communication band; (e) that the use of common-spacecraft for mobile service to both aircraft and ships might well be advantageous, especially if the same order of frequency were to be suitable for both; (f) that important advantages, including those of frequency economy, might arise from the common use of satellites for both communication and radiodetermination;
decides that the following studies should be carried out:
1. the preferred types of orbit to provide satellite communication between land stations and: 1.1 aircraft, 1.2 ships;
2. the preferred frequencies and technical characteristics for: 2.1 satellite-aircraft links, 2.2 satellite-ship links, 2.3 satellite-ground links;
3. the technical feasibility for communication-satellite services for aircraft and ships to share _ the use of common frequency bands: ' 3.1 one with the other (aircraft with other aircraft, ships with other ships and aircraft with ships), — 580 e —
3.2 with land stations in the aeronautical mobile service (directly or via a satellite), 3.3 with land stations in the maritime mobile service (directly or with a satellite), 3.4 with earth stations in the aeronautical and maritime radiodetermination services; 4. the technical feasibility of also using communication satellites for a radiodetermination service.
The Director, C.C.I.R., is requested to draw the attention of the International Civil Aviation Organization, the Inter-Governmental Maritime Consultative Organization and the United Nations Committee on the Peaceful Uses of Outer Space to the existence of this Study Programme and, in particular, to invite I.C.A.O. andl.M.C.O. cooperation in this study. INTERNATIONAL RADIO CONSULTATIVE COMMITTEE
C.C.I.R.
DOCUMENTS OF THE Xlth PLENARY ASSEMBLY
OSLO, 1966
VOLUME IV
RADIO-RELAY SYSTEMS SPACE SYSTEMS RADIOASTRONOMY
PART 2 : SPACE SYSTEMS, RADIOASTRONOMY
Published by the INTERNATIONAL TELECOMMUNICATION UNION GENEVA, 1967 PAGE INTENTIONALLY LEFT BLANK
PAGE LAISSEE EN BLANC INTENTIONNELLEMENT Recommendations of Section F (Radio-relay systems)
Reports of Section F (Radio-relay systems)
PART 1: RADIO-RELAY SYSTEMS
Questions and Study Programmes allocated to Study Group IX (Radio-relay systems) — Opinions and Resolutions of interest to this Study Group
Lists of documents
Recommendations of Section L (Space systems and Radioastronomy)
Reports of Section L (Space systems and Radio- astronomy)
PART 2: SPACE SYSTEMS AND RADIOASTRONOMY Questions and Study Programmes allocated to Study Group IV (Space systems and Radioastronomy) -4 Opinions and Resolutions of interest to this Study Group
Lists of documents DISTRIBUTION OF THE TEXTS OF THE Xlth PLENARY ASSEMBLY OF THE C.C.I.R. AMONG VOLUMES I-VI
— Volumes I to VI of the documents of the Xlth Plenary Assembly contain all the C.C.I.R. texts at present in force. — For Questions and Study Programmes, the final (Roman) numeral indicates the Study Group to which the text has been assigned. The plan on page vi shows the Volume in which the various texts of that Study Group can be found. — Recommendations, Reports, Opinions and Resolutions which have been amended by the Xlth Plenary Assembly, have retained their original number, followed by the indication 1 (e.g .: Recommendation 326-1), which is not shown in the Table below. Further details on the numbering system appear in Volume VI.
1. Recommendations
Number Volume Number Volume Number Volume
45 III 237 I 313 II 48, 49 V 239 I 314 IV 75-77 III 240 III 325-334 I 80 V 246 III 335-349 III 100 III 258 III 352-367 IV 106 III 261, 262 v • 368-373 II 136 V 264-266 V 374-379 III 139, 140 V 268 IV 380-406 IV 162 III 270 IV 407-421 V 166 III 275, 276 IV 422, 423 III 168 II 279 IV 425 III 182 III 281-283 IV 427-429 III 205 V 289, 290 IV 430-433 I 212 V 297-300 IV 434, 435 II 214-216 V 302 IV 436-443 III 218, 219 III 304-306 IV 444-446 IV 224 III 310, 311 II 447-451 V
2. Reports
Number Volume Number Volume Number Volume
19 III 175-194 I 292-295 V 32 V 195-198 III 297-316 V 42 III 200-203 III 318-320 III 79 V 204-219 IV 321 I 93 III 222-224 IV 322 0 106, 107 III 226 IV 323-335 I 109 III 227-239 II 336-339 II 111, 112 III 241-266 II 340 0 122 V 267 III 341-344 II 130 IV 269-273 III 345-373 ' III 134 IV 275-282 III 374-397 IV 137 IV 283-290 IV 398-412 V 413-415 0 (l) Published separately. 3. Opinions
Number Volume Number Volume Number Volume
1, 2 I 12-14 IV 21 III 3 IV 15-17 V 22, 23 II 11 III 19 V 24-30 III 31 V
4. Resolutions
Number Volume Number Volume Number Volume
1 III 12, 13 II 26, 27 VI 2-4 II 14-16 III 30, 31 II 7, 8 II 19, 20 III 32 V 10 II 21-23 I 33-36 VI 24 VI ARRANGEMENT OF VOLUMES I TO VI OF THE DOCUMENTS OF THE Xlth PLENARY ASSEMBLY OF THE C.C.I.R. (Oslo, 1966)
V o lu m e I Emission. Reception. Vocabulary (Sections A, B, K and Study Groups I, II and XIV).
V o lu m e II Propagation (Section G and Study Groups V and VI),
V o lu m e III Fixed and mobile services. Standard-frequencies and time-signals. International monitoring (Sections C, D, H and J and Study Groups III, XIII, VII and VIII).
V o l u m e IV Radio-relay systems. Space systems and Radioastronomy (Sections F and L and Study Groups IX and IV).
Volume V Sound broadcasting and Television (Section E, Study Groups X, XI and XII and the CMTT).
V o l u m e V I List o f participants. Minutes of the Plenary Meetings. Resolutions of a general nature. Reports to the Plenary Assembly. List of documents in numerical order.
Note 1. — To facilitate references, the pagination in the English and French texts is the same.
Note 2. — At the beginning of Volume VI will be found information concerning the Xlth Plenary Assembly of the C.C.I.R. and the participation at this meeting, on the presentation of texts (definitions, origins, numbering, complete lists, etc.), together with general infor mation on the organization of the C.C.I.R. — vii —
TABLE OF CONTENTS OF VOLUME IV
Page Distribution of the texts of the Xlth Plenary Assembly of the C.C.I.R. among Volumes I-VI . . iv Arrangement of Volumes I to VI of the Xlth Plenary Assembly of the C.C.I.R...... vi Table of contents of Volume IV ...... vii
PART 1
RECOMMENDATIONS OF SECTION F (RADIO-RELAY SYSTEMS)
F. 1—Interconnection
Rec. 268 Radio-relay systems for telephony using frequency-division multiplex. Inter connection at audio frequencies...... 19 Rec. 270 Radio-relay systems for television. Interconnection at video signal frequencies 19 Rec. 297 Radio-relay systems for telephony using time-division multiplex. Inter connection at audio frequencies...... 20 Rec. 299 Radio-relay systems for telephony using time-division multiplex. Agreement on major characteristics...... 21 Rec. 304 Radio-relay systems for telephony. Interconnection of different systems of multiplexing ...... 21 Rec. 306 Radio-relay systems for television and telephony. Procedure for the inter national connection o f systems with different characteristics...... 22 Rec. 380-1 Radio-relay systems for telephony using frequency-division multiplex. Inter connection at baseband frequ en cies...... 23 Rec. 381-1 Interconnection of radio-relay and line systems. Line regulating and other pilots. Limits for the residues of signals outside the baseband...... 27
F. 2—Radio-frequency channel arrangements
Rec. 279-1 Radio-relay systems for telephony using frequency-division multiplex. Radio frequency channel arrangements for 300-channel systems operating in the 2 and 4 GHz bands...... 30 Rec. 281 Radio-relay systems for television and telephony. Preferred radio-frequency channel arrangements for television...... 31 Rec. 282 Radio-relay systems for television and telephony. Use of special radio-frequency channel arrangem ents . 31 Rec. 283-1 Radio-relay systems for telephony using frequency-division multiplex. Radio frequency channel arrangements for 60r and, 120-channel telephony systems operating in the 2 GHz b a n d ...... 32 Rec. 382-1 Radio-relay systems for television and telephony. Radio-frequency channel arrangements for systems for 600 to 1800 telephone channels, or the equivalent,' operating in the 2 and 4 GHz bands ...... ' 34 — viii —
Page Rec. 383-1 Radio-relay systems for television and telephony. Radio-frequency channel arrangements for systems having a capacity of 1800 telephone channels, or the equivalent, operating in the 6 GHz band...... 38
Rec. 384-1 Radio-relay systems for television and telephony. Radio-frequency channel arrangements for systems with a capacity o f either 2700 telephone channels, or 960 telephone channels, or the equivalent, operating in the 6 GHz band. . . 40
Rec. 385 Radio-relay systems for telephony using frequency-division multiplex. Radio frequency channel arrangements for 60-, 120-and 300-channel telephony systems operating in the 7 GHz b a n d ...... 43
Rec. 386-1 Radio-relay systems for television and telephony. Radio-frequency channel arrangements for systems with a capacity of 960 telephone channels, or the equivalent, operating in the 8 GHz band...... 45
Rec. 387 Radio-relay systems for television and telephony. Radio-frequency channel arrangements for systems with a capacity of 960 telephone channels, or the equivalent, operating in the 11 GHz band...... 48
Rec. 388 Trans-horizon radio-relay systems. Radio-frequency channel arrangements . . 50
Rec. 389 Radio-relay systems for television and telephony. Preferred characteristics of auxiliary radio-relay systems operating in the 2, 4, 6 or 11 GHz bands . . . 51
F. 3—Hypothetical reference circuits and noise
Rec. 289 Radio-relay systems for monochrome television. Permissible noise in the ,hypothetical reference circu it...... 54
Rec. 300 Radio-relay systems for telephony using time-division multiplex. Hypothetical reference circuit for radio-relay systems with a capacity of 60 telephone channels or less...... 56
Rec. 302 Trans-horizon radio-relay systems. Limitation of interference...... 57
Rec. 390 Definitions of hypothetical reference c ir c u its ...... 57
Rec. 391 Radio-relay systems for telephony using frequency-division multiplex. Hypo thetical reference circuit for radio-relay systems with a capacity o f 12 to 60 tele phone channels...... 59
Rec. 392 Radio-relay systems for telephony using frequency-division multiplex. Hypo thetical reference circuit for radio-relay systems with a capacity o f more than 60 telephone channels...... 60
Rec. 393-1 Radio-relay systems for telephony using frequency-division multiplex. Allow able noise power in the hypothetical reference c irc u it...... 62
Rec. 394 Radio-relay systems for telephony using time-division multiplex. Allowable noise power in the hypothetical reference c ir c u it...... 65
Rec. 395-1 Radio-relay systems for telephony using frequency-division multiplex. Noise in the radio portion of circuits to be established over real lin k s ...... 66 — ix —
Page Rec. 396-1 Trans-horizon radio-relay systems. Hypothetical reference circuit for radio relay systems for telephony using frequency-division m u ltip le x ...... 69
Rec. 397-1 Trans-horizon radio-relay systems. Allowable noise power in the hypothetical reference circuit for telephony transmission using frequency-division multiplex . 70
F. 4—Maintenance
Rec. 290 Maintenance procedure for radio-relay systems for telephony using frequency- division multiplex. Measurements to be m a d e...... 72
Rec. 305 Radio-relay systems for television and telephony. Stand-by arrangements . . 72
Rec. 398-1 Radio-relay systems for telephony using frequency-division multiplex. Main tenance measurements in actual traffic...... 73
Rec. 399-1 Radio-relay systems for telephony using frequency-division multiplex. Measure ment of performance with the help o f a signal consisting of a uniform spectrum 75
Rec. 400-1 Service channels for radio-relay systems. Types of service channel to be pro vided ...... 79
Rec. 401-1 Radio-relay systems for television and telephony. Frequencies and deviations o f continuity p i l o t s ...... 80
Rec. 444 Radio-relay systems for television and telephony. Preferred characteristics for multi-line switching arrangem ents...... 82
F. 5—Characteristics
Rec. 275-1 Radio-relay systems for telephony using frequency-division multiplex. Pre emphasis characteristic for frequency-modulation s y s te m s ...... 83
Rec. 276 Radio-relay systems for television. Frequency deviation and the sense of modulation...... 87
Rec. 298 Radio-relay systems for telephony using time-division multiplex. Preferred characteristics...... 87
Rec. 402 Radio-relay systems for television. Simultaneous transmission o f a monochrome television signal and a single sound channel. Preferred characteristics o f the sound c h a n n e l...... 88
Rec. 403-1 Radio-relay systems for television and telephony. Intermediate-frequency characteristics...... 90
Rec. 404-1 Radio-relay systems for telephony using frequency-division multiplex. Fre quency d e v ia tio n 91
Rec. 405 Radio-relay systems for television. Pre-emphasis characteristics for frequency modulation s y s te m s ...... 92
Rec. 406-1 Line-of-sight radio-relay systems sharing the same frequency bands as the space-station receivers of active communication-satellite systems. Maximum equivalent isotropically radiated power of line-of-sight radio-relay system trans mitters ...... 96 REPORTS OF SECTION F (RADIO-RELAY SYSTEMS)
F. 1—Interconnection
Report 134 Radio-relay systems for telephony using time-division multiplex. Technical characteristics to be specified to enable interconnection between any two systems 99 Report 283 Radio-relay systems for telephony using frequency-division multiplex. Tech nical characteristics to be specified to enable interconnection between any two s y s te m s ...... 102 Report 284 Interconnection of auxiliary radio-relay systems at radio frequencies .... 105 Report 285-1 Trans-horizon radio-relay systems. Transmission, interconnection and inter ference . . 106
F. 2—Radio-frequency channel arrangements
Report 286 Trans-horizon radio-relay systems. Radio-frequency channel arrangements for systems using frequency modulation ...... 114 Report 287 Radio-relay systems for television and telephony. Systems of capacity greater than 1800 telephone channels, or the eq u iv a le n t...... 116 Report 374 Interconnection of auxiliary radio-relay systems operating in the same frequency band as the main radio-relay system ...... 120
F. 3—Hypothetical reference circuits and noise
Report 130 Radio-relay systems for telephony. Noise tolerable during very short periods o f time on line-of-sight system s...... 122 Report 288-1 Radio-relay systems for telephony using frequency-division multiplex. Noise in circuits forming part o f very long telephone connections...... 125 Report 375 Radio-relay systems for television and telephony. Noise objectives for pro gramme circuits 2500 km long provided by means of radio-relay systems . . . 127
F. 4—Maintenance
Report 137-1 Duration of interruptions on radio links when switching from normal to stand by eq u ip m en t...... 130
F. 5—Characteristics
Report 289 Radio-relay systems for television and telephony. Preferred characteristics for the simultaneous transmission of television and a maximum o f four sound channels...... 133 Report 290 Radio-relay systems for television and telephony. Preferred characteristics for the transmission of up to six sound channels...... 135 Report 376 Diversity techniques for radio-relay systems...... 139 Page Report 377 Trans-horizon radio-relay systems. Preferred characteristics, permissible noise and signal distortion for the transmission of monochrome television signals . . 148
Report 378 Radio-relay systems for the transmission of pulse-code modulation and other types of digital s ig n a l'...... 149
Report 379 Characteristics of simple VHF or UHF radio equipment for use on trunk con nections in the new and developing countries ...... , . . 150
Report 380 Simple single-channel radiotelephony equipm ent...... 152
Report 381 Two-channel time-diversity telegraph systems for use over radio-relay links 154
QUESTIONS AND STUDY PROGRAMMES ASSIGNED TO STUDY GROUP IX (RADIO-RELAY SYSTEMS): OPINIONS AND RESOLUTIONS OF INTEREST TO THIS STUDY GROUP
Introduction by the Chairman, Study Group I X ...... 155
Opinion 12 Radio-relay systems for television. Maintenance procedures...... 157
Opinion 13 Radio-relay systems for telephony. C.C.I.T.T./C.C.I.R. Joint Working Party on circuit noise...... 157
Opinion 14 Radio-relay systems for television and telephony. Preferred frequency bands and centre frequencies for radio-relay links for international connections. . . 158
Question 1/IX Radio-relay systems for telephony using frequency-division multiplex . . . 159 Study Programme 1A/IX Radio-relay systems for television and telephony. Systems of a capacity greater than 1800 telephone channels, or the equivalent...... 160
Question 2/IX Radio-relay systems for television and telephony. Hypothetical reference cir cuits and circuit noise 161 Study Programme 2A/IX Radio-relay systems for television and telephony. Noise tolerable during very short periods of tim e ...... 161 Study Programme 2B/IX Radio-relay systems for telephony. Noise in circuits forming part of very long telephone connections...... 162 Study Programme 2C/IX Radio-relay systems for television and telephony. Noise objectives for programme circuits 2500 km long provided by means of radio-relay systems 163
Question 3/IX Radio-relay systems for television. Preferred characteristics for the transmis sion of monochrome televisio n ...... 164 Study Programme 3A/IX Radio-relay systems for television and telephony. Preferred char acteristics for the transmission o f more than one sound ch a n n el...... 164
Question 4/IX Radio-relay systems for television and telephony. Service channels .... 165 Study Programme 4A/IX Radio-relay systems for television and telephony. Preferred char acteristics for auxiliary radio-relay systems for the provision o f service channels 166
Question 5/IX Radio-relay systems for television and telephony. Transmission interruptions 167 Study Programme 5A/IX Radio-relay systems for television and telephony. Preferred char acteristics for multi-line switching arran gem en ts...... 167 — xii —
Page Question 6/IX Protection ratios for the operation of communication services within the channels of a broadcasting service...... 168
Question 7/IX Trans-horizon radio-relay s y s te m s ...... 169 Study Programme 7A/IX Trans-horizon radio-relay systems. Radio-frequency channel ar rangements ...... 169 Study Programme 7B/IX Trans-horizon radio-relay systems. Loss in path antenna gain . . 171
Question 8/IX Radio-relay systems for television. Preferred characteristics for the transmis sion of colour television and the simultaneous transmission of colour television and other sig n a ls...... 171
Question 9/IX Characteristics of simple YHF or UHF radio equipment for use on trunk connections in the new and developing c o u n tr ie s...... 172
Question 10/IX Simple single-channel radiotelephony equipm ent...... 173
Question 11/IX Transmission planning for radio-relay systems in the new and developing c o u n tr ie s ...... 174
Question 12/IX Radio-relay systems for the transmission of pulse-code modulation and other types of digital signal ...... 174 Study Programme 12A/IX Radio-relay systems for the transmission of pulse-code modula tion (PCM) and other types of digital signal. Calculation and measurement of the effects of propagation and in terferen ce...... 175
Question 13/IX Radio-relay systems. Diversity techniques...... 176
Question 14/IX Trans-horizon radio-relay systems. Preferred characteristics, permissible noise and signal distortion for the transmission o f monochrome television signals . . 177
Question 15/IX Characteristics of simple UHF radio equipment for use on trunk connections in the new and developing countries...... 178
List of documents concerning Study Group IX (Period 1963-1966)...... 179
List of documents of the Xlth Plenary Assembly established by Study Group IX . . . 199
The following texts,'which are not contained in this Volume, also concern radio-relay systems :
Text Title Volume Reports 241-1, 242 Propagation data required for radio-relay system s...... II
Report 243 Tropospheric-wave propagation curves for application to interference pro- II blems in the range from 1 to 10 G H z ......
Report 244-1 Estimation of tropospheric-wave transmission lo s s ...... II
Rec. 335-1 International radiotelephone sy stem s...... Ill
Rec. 421-1 Requirements for the transmission of monochrome television signals over long distances...... V
Report 316-1 Requirements for the transmission of colour television signals over long dis tances ...... V — xiii —
Page PART 2
RECOMMENDATIONS OF SECTION L (SPACE-SYSTEMS AND RADIO- ASTRONOMY)
L. 1—General Rec. 445 Definitions concerning radiated p o w e r ...... 203
L. 2—Communication satellites
Rec. 352 Active communication-satellite systems for multiplex telephony and/or mono chrome television. Hypothetical reference circuit for intercontinental systems 204 Rec. 353-1 Active communication-satellite systems for frequency-division multiplex telephony. Allowable noise power in the basic hypothetical reference circuit . . 205 Rec. 354 Active communication-satellite systems for monochrome television. Video bandwidth and permissible noise in the hypothetical reference circuit .... 206 Rec. 355-1 Frequency sharing between active communication-satellite systems and ter restrial radio services in the same frequency b a n d s ...... 207 Rec. 356-1 Communication-satellite systems and line-of-sight radio-relay systems sharing the same frequency bands. Maximum allowable values of interference in a tele phone channel of a communication-satellite s y s te m ...... 209 Rec. 357-1 Communication-satellite systems and line-of-sight radio-relay systems sharing the same frequency bands. Maximum allowable values o f interference in a telephone channel of a radio-relay sy s te m ...... 210 Rec. 358-1 Communication-satellite systems and line-of-sight radio-relay systems sharing the same frequency bands. Maximum allowable values o f power flux density at the surface of the earth produced by communication satellites...... 212 Rec. 359-1 Communication-satellite systems and terrestrial radio systems sharing the same frequency bands. Determination o f the coordination d is ta n c e ...... 213 Rec. 360-1 Communication-satellite systems and line-of-sight radio-relay systems sharing the same frequency bands. Criteria for selection of preferred reference frequen cies for communication-satellite sy ste m s...... 214 Rec. 446 Frequency selection and carrier energy dispersal for communication-satellite sy ste m s...... 215
L. 3—Direct broadcasting from satellites No Recommendations in this sub-section
L. 4—Radionavigation by satellite Rec. 361-1 Frequency requirements of radionavigation-satellite system s...... 217 ♦ L. 5—Meteorological satellites Rec. 362 Frequencies technically suitable for meteorological satellites...... 219
L. 6—Maintenance telemetering, tracking and telecommand Rec. 363 Preferred frequency bands for use in maintenance telemetering, tracking and telecommand of developmental and operational satellites...... 220 — xiv —
Page L. 7—Space research Rec. 364-1 Telecommunication links for near-earth research satellites. Frequencies, band- widths and interference criteria...... 221 Rec. 365-1 Telecommunication links for deep-space research. Frequencies, bandwidths and interference criteria ...... 223 Rec. 366-1 Telecommunication links for manned research spacecraft...... 225 Rec. 367 Frequency bands for re-entry communications...... 227
L. 8—Radioastronomy Rec. 314-1 Protection of frequencies used for radioastronomical measurements .... 228
L. 9—Radar astronomy No Recommendations in this sub-section
REPORTS OF SECTION L (SPACE SYSTEMS AND RADIOASTRONOMY) L. 1—General Report 204-1 Terms and definitions relating to space radiocommunication...... 231 Report 205-1 Factors affecting the selection of frequencies for telecommunications with and between spacecraft...... 234
L. 2—Communication satellites Report 206-1 Technical characteristics of communication-satellite systems. General considera tions relating to the choice of orbit, satellite and type of s y s te m ...... 256 Report 207- 1 Active communication-satellite experiments. Results of tests and demonstra tions ...... 268 Report 208- 1 Active communication-satellite systems for frequency-division multiplex tele phony and monochrome television. Form o f the basic hypothetical reference circuit and allowable noise standards ; video bandwidth and sound channel for television...... 275 Report 209- 1 Frequency sharing between communication-satellite systems and terrestrial se r v ic e s...... 277 Report 210- 1 Frequency sharing within and between communication-satellite systems . . 282 Report 211-1 Active communication-satellite systems. A comparative study of possible methods o f modulation...... 303 Report 212-1 Active communication-satellite systems for frequency-division multiplex tele phony and monochrome television. Use of pre-emphasis on frequency-modu- lation s y s te m s ...... 323 Report 213- 1 Factors affecting multiple access in communication-satellite systems .... 328 Report 214-1 Communication-satellite systems. The effects o f Doppler frequency-shifts and switching discontinuities...... 348 Report 382 Determination of coordination distance ...... 356 Report 383 Communication-satellite systems. The effects o f transmission delay .... 369 Report 384 Frequency-sharing between communication-satellite systems and terrestrial radio-relay systems. Energy dispersal in communication-satellite systems with frequency-modulation of the radio-frequency ca rrier...... 378 — XV —
Page Report 385 Feasibility of frequency sharing between communication-satellite systems and terrestrial radio services. Site selection criteria for earth stations in the commu nication-satellite s e r v ic e ...... 389 Report 386 Feasibility of frequency sharing between communication-satellite systems and terrestrial radio services. Maximum power in a 4 kHz band which may need to be radiated in the horizontal plane by active communication-satellite earth stations 391 Report 387 Power flux-density at the surface of the earth from communication satellites 394 Report 388 Techniques for calculating interference noise in communication-satellite receivers and in terrestrial receivers of radio-relay system s...... 398 Report 389 Estimating interference probabilities between space systems and terrestrial radio-relay systems. Propagation considerations...... 404 Report 390 Earth-station antennae for the communication-satellite service...... 405 Report 391 Radiation diagrams of antennae at communication-satellite earth stations, for use in interference studies...... 420 Report 392 Performance of earth-station receiving antennae. Effects of rain on radomes and of solar and cosmic n o is e ...... • . . 426 Report 393 ‘ Exposures of the antennae of radio-relay systems to emissions from communi cation satellites...... 435
L. 3—Direct broadcasting from satellites
Report 215-1 Feasibility of direct sound and television broadcasting from satellites . . . 444
L. 4—Radionavigation by satellites
Report 216-1 Use of satellites for terrestrial radionavigation...... 449 Report 394 Feasibility of frequency sharing between the radionavigation-satellite service and terrestrial services...... 457
L. 5—Meteorological satellites
Report 395 Radiocommunications for meteorological satellite s y s t e m s ...... 461
L. 6—Maintenance telemetering, tracking and telecommand
Report 396 Maintenance telemetering, tracking and telecommand for developmental and operational satellites. Frequency sharing between earth-satellite telemetering or telecommand links and terrestrial services...... ; ...... 479
L. 7—Space research
Report 218 Technical characteristics of telecommunication links between earth stations and spacecraft for research purposes...... 495 Report 219-1 Interference and other special considerations for telecommunication links for manned arid unmanned spacecraft in the space research service...... 511 Report 222-1 Factors affecting the selection of frequencies for telecommunications with spacecraft re-entering the earth’s atmosphere . . . . ;...... 531 — xvi —
Page L. 8—Radioastronomy
Report 223-1 Line frequencies or bands, of interest to radioastronomy and related sciences, in the 30 to 300 GHz range arising from natural p h en o m en a...... 533
Report 224-1 Radioastronomy. Characteristics and factors affecting frequency sharing with other services ...... 537
Report 397 The OH lines in rad ioastronom y...... 548
L. 9—Radar astronomy
Report 226-1 Factors affecting the possibility of frequency-sharing between radar astronomy and other se r v ic e s...... 551
QUESTIONS AND STUDY PROGRAMMES ASSIGNED TO STUDY GROUP IV (SPACE SYSTEMS AND RADIO ASTRONOMY): OPINIONS AND RESOLU TIONS OF INTEREST TO THIS STUDY GROUP
Introduction by the Chairman, Study Group I V ...... 557
Opinion 3 Data on traffic loading and routing for use in developing communication- satellite system facilities...... 560
Question 1/IV Antennae for space system s...... 560
Question 2/IV Technical characteristics of communication-satellite system s...... 561 Study Programme 2A/IV Feasibility of frequency sharing between communication-satellite systems and terrestrial radio services ...... 562 Study Programme 2B/IV Frequency sharing between communication-satellite systems and terrestrial radio services. Wanted-to-unwanted signal r a tio s ...... 563 Study Programme 2C/IV Communication-satellite systems. Feasibility of frequency sharing among communication-satellite systems...... 564 Study Programme 2D/IV Study of preferred modulation characteristics for communication- satellite s y s te m s ...... 565 Study Programme 2E/IV Factors affecting freedom of access in communication-satellite sy ste m s...... 565 Study Programme 2F/IV Energy dispersal in communication-satellite system s...... 566
Question 3/IV Sharing of radio-frequency bands by links between earth stations and space craft ...... 567 Study Programme 3A/IV Space research, maintenance teleinetering, tracking and telecom mand systems. Possibilities of sharing and protection criteria ...... 568
Question 4/IV Technical characteristics of links between earth stations and spacecraft . . 568
Question 5/IV Active communication-satellite systems for frequency-division multiplex telephony. Transmission characteristics o f audio-frequency channels .... 569
Question 6/IV Effects of plasma on communications with sp acecraft...... 570 Study Programme 6A/IV Frequency bands for re-entry com m unications...... 571 — xvii —
Page Question 7/IV Transmission delay, echoes and switching discontinuities in communication- satellite systems...... 571 Question 8/IV Technical characteristics of radionavigation-satellite systems...... 572 Question 9/IV Radiocommunication for meteorological-satellite sy stem s...... 573 Study Programme 9A/IV Radiocommunication aspects of meteorological-satellite systems 573 Question 10/IV Radioastronom y...... 574 Question 11/IV Radar astronomy...... 575 Question 12/IV Feasibility of direct sound and television broadcasting from satellites . . . 576 .Question 13/IV Contributions to the noise temperature of an earth-station receiving antenna 577 Question 14/IV Propagation factors affecting sharing of the radio-frequency spectrum and coordination between space and terrestrial radio-relay s y s te m s ...... 578 Question 15/IV Frequency utilization above the ionosphere and on the far side of the Moon 579 Question 16/IV Shielding effects due to the ionosphere ...... 579 Question 17/IV Shielding effects due to the M o o n ...... 580 List of documents concerning Study Group IV (Period 1963-1966)...... 581 List of documents of the Xlth Plenary .Assembly established by Study Group IV . . . 602
The following texts, which are not contained in this Volume, also concern space systems and radioastronomy :
Text Title Volume Study Programme 5D/V Tropospheric propagation factors affecting the sharing of the radio- frequency spectrum between radio-relay systems, including space and terrestrial telecommunications sy stem s...... II Resolution 2 Tropospheric propagation data for broadcasting, space and point-to-point communications...... II Study Programme 5C/V Tropospheric absorption and refraction in relation to space tele communication system s...... II Report 263-1 Factors affecting propagation in communications with spacecraft...... II Study Programme 20A/VI Characteristics of the ionosphere affecting space telecommunica tion systems . . II Question 7/VI Effects of radio noise in space on communications with spacecraft...... II Study Programme 2A/VII Standard-frequency and time signal emissions from artificial earth satellites...... Ill Report 276-1 Monitoring at fixed monitoring stations of radio emissions from spacecraft . Ill Question 6/VIII Monitoring at fixed monitoring stations of radio emissions from spacecraft . Ill
2 PAGE INTENTIONALLY LEFT BLANK
PAGE LAISSEE EN BLANC INTENTIONNELLEMENT — 203 — Rec. 445
RECOMMENDATIONS OF SECTION L (SPACE SYSTEMS AND RADIO ASTRONOMY)
L. 1 : General
RECOMMENDATION 445
DEFINITIONS CONCERNING RADIATED POWER
The C.C.I.R., (1966)
CONSIDERING
(a ) the continuing utility of the term effective radiated power and its abbreviation e.r.p., as defined in Nos. 98, 99 and 101 of the Radio Regulations, Geneva, 1959, to mean the product of the power supplied to an antenna and the antenna gain relative to a half-wave dipole, particularly in those frequency bands where a dipole or an array of dipoles is a useful antenna;
(b ) the common usage of the term effective radiated power in the higher frequency bands, to mean the product of the power supplied to an antenna and the antenna gain relative to an isotropic radiator;
(c ) that these concepts lead to values differing by 2-15 dB :
UNANIMOUSLY RECOMMENDS
1. that the term effective radiated power and its abbreviation e.r.p., be used only as defined by Nos. 98, 99 and 101 of the Radio Regulations, Geneva, 1959;
2. that the term equivalent isotropically radiated power be adopted with the following definition : Equivalent isotropically radiated power (abbreviation e.i.r.p.) The equivalent isotropically radiated power of an emission is the product of the power supplied to the antenna for this emission and the antenna gain relative to an isotropic antenna. Rec. 352 — 204 —
L. 2 : Communication satellites
RECOMMENDATION 352
ACTIVE COMMUNICATION-SATELLITE SYSTEMS FOR MULTIPLEX TELEPHONY AND/OR MONOCHROME TELEVISION Hypothetical reference circuit for intercontinental systems (Question 2/1V)
The C.C.I.R., (1963) CONSIDERING (a) that it is desirable to establish a hypothetical reference circuit for active communication- satellite systems to afford guidance to designers of equipment and systems for use in inter continental telephone and television networks ; (b ) that with various types of active satellite system, it is possible to arrange that the majority of intercontinental connections may be made with no more than one satellite link, if it is capable of spanning a great circle distance of at least 7500 km ; (c) that, to span the great circle distances of up to 25 000 km, required for a global system, it will be necessary to connect two, and occasionally three links in tandem ; (d) that the overall performance of each satellite link depends only to a small extent on the great circle distance between the earth stations ; (e) that provision for television standards conversion in a hypothetical reference circuit is un desirable ; UNANIMOUSLY RECOMMENDS 1. that a basic hypothetical reference circuit for active communication-satellite systems should consist of one earth-satellite-earth link (see Fig. 1); 2. that the performance for at least a portion of intercontinental connections should take account of the need to connect two, and sometimes, three such links in tandem ; 3. that this circuit should include one pair of modulation and demodulation equipments for translation from the baseband to the radio-frequency carrier, and from the radio-frequency carrier to the baseband respectively.
Intercontinental connection
Communication-satellite space station Earth station Earth station F ig u r e 1 Basic hypothetical reference circuit — 205 — Rec. 353-1
RECOMMENDATION 353-1
ACTIVE COMMUNICATION-SATELLITE SYSTEMS FOR FREQUENCY-DIVISION MULTIPLEX TELEPHONY Allowable noise power in the basic hypothetical reference circuit (Question 2/1V)
The C.C.I.R., (1963 — 1966)
CONSIDERING
(a) that the basic hypothetical reference circuit is intended as a guide to the design and construc tion of actual systems;
(b) that the costs of establishing and maintaining communication-satellite systems are critically dependent on the overall signal-to-noise performance requirements ;
(c) that the total noise power in the basic hypothetical reference circuit should not be such, as would affect appreciably conversation in most telephone calls or the transmission of telephone signalling;
(d) that the extent of fading cannot be determined fully until more experimental data are available, but is not expected to be appreciable in active communication-satellite systems;
(e) that there may be other sources of noise of short duration ;
UNANIMOUSLY RECOMMENDS
1. that the noise power, at a point of zero relative level in any telephone channel in the basic hypothetical reference circuit as defined in Recommendation 352, should not exceed the pro visional values given below: 1.1 10 000 pW psophometrically-weighted mean power in any hour ; 1.2 10 000 pW psophometrically-weighted one-minute mean power for more than 20% of any m onth; 1.3 50 000 pW psophometrically-weighted one-minute mean power for more than 0-3% of any m onth; 1.4 1 000 000 pW unweighted (with an integrating time of 5 ms), for more than 0-03% of any m onth; r
2. that the following Notes should be regarded as part of the Recommendation :
Note 1. — Noise in the multiplex equipment is excluded from the above.
Note 2. — It is assumed, that noise surges and clicks from power supply systems and from switching apparatus (including switching from satellite to satellite), are reduced to negligible proportions and therefore will not be taken into account when calculating the noise power.
Note 3. — In applying the basic hypothetical reference circuit and the allowable circuit noise to the design of satellite and earth station equipment for a given overall signal-to-noise per formance, the system characteristics preferred by the C.C.I.R., as found in its Recommenda tions, should be used where appropriate; where more than one value is recommended, the designer should indicate the value chosen; in the absence of preferred values, the designer should indicate the assumptions used. Rec. 353-1, 354 — 206 —
Note 4. — For frequency-division multiplex telephony, it will be assumed that, during the busy hour, the baseband signal can be represented by a uniform-spectrum signal, the mean absolute power-level of which, at a point of zero relative level, is equal to (—15 + 10 log10iV) dBm for 240 channels or more, and (—1+4 log107V) dBm * for numbers of channels between 12 and 240, N being the number of channels. These formulae apply only to baseband signals without pre-emphasis and using independent amplifiers or repeaters for the two directions of transmission. Further information on the conventional load, in particular in the case of a repeater which is common to both directions of transmission, is given in C.C.I.T.T. Recom mendation G.223 (Blue Book, Vol. Ill, Geneva, 1964).
Note 5. — It is not yet possible to make firm recommendations regarding requirements to be met, if VF telegraphy and data transmission are required over telephone channels in a communica tion-satellite system.
Note 6. — The system should be designed to operate under the noise conditions specified, including noise due to interference within the limits defined in Recommendation 356-1 for line-of-sight radio-relay systems sharing the same frequency bands and noise during periods of adverse propagation conditions such as those resulting from atmospheric absorption and increased noise temperature due to rain. In certain cases, however, additional noise may cause the limits fixed in the general objectives to be slightly exceeded. This should not cause serious concern, provided that the provisions of C.C.I.T.T. Recommendation G.222, § 6, are met.
RECOMMENDATION 354
ACTIVE COMMUNICATION-SATELLITE SYSTEMS FOR MONOCHROME TELEVISION Video bandwidth and permissible noise in the hypothetical reference circuit (Question 2/IV)
The C.C.I.R., (1963)
CONSIDERING
(a) that the hypothetical reference circuit is intended as a guide to designers and constructors of actual systems;
(b) that the costs of establishing and maintaining active communication-satellite systems are critically dependent on the video bandwidth and the overall signal-to-noise ratio to be pro vided and these should, therefore, not be greater than is strictly necessary for acceptable transmission;
(c) that the total noise power in the hypothetical reference circuit should not reach a value that would prevent acceptable transmission of monochrome television signals ;
(d) that the signal-to-noise ratio for a satellite intercontinental circuit should be comparable to, or greater than, the signal-to-noise ratio objectives specified for the continental networks to be interconnected (see Recommendation 421-1);
* It is considered that these formulae give a good approximation in calculating intermodulation noise when N > 60. For small numbers of channels, however, tests with uniform-spectrum random noise are less realistic, due to the wide difference in the nature of actual and test signals. — 207 — Rec. 354, 355-1
(e) that the extent of fading cannot be fully determined until more experimental data are avail able, but is not expected to be appreciable in an active communication-satellite system;
(f) that there may be other sources of short-duration noise ; (g) that the same satellite system may be required to accommodate either television or large numbers of telephone channels ;
(h) that the video bandwidth and the permissible noise levels in the hypothetical reference circuit should accommodate television signals up to and including signals of the 625-line standard ;
UNANIMOUSLY RECOMMENDS
1. that, in the hypothetical reference circuit for active communication-satellite systems, as defined in Recommendation 352, the nominal upper limit of the video bandwidth should be 5 MHz (equivalent noise bandwidth);
2. that the ratio of signal to weighted-noise for continuous random noise at the end of the hypothetical reference circuit, defined in Recommendation 352, evaluated in accordance with Notes 1 and 2 below, should be at least 55 dB for 99% of the time ;
3. that the question of the higher noise levels, that may occur for less than 1 % of the time, should be the subject of further study.
Note 1. — The signal-to-noise ratio for continuous random noise is defined as the ratio (dB) of the peak-to-peak amplitude of the picture signal, excluding synchronizing pulses, to the r.m.s. amplitude of the noise, within the range between 10 kHz and the nominal upper limit of the video-frequency band f c. The purpose of the lower frequency limits is to enable power- supply hum and microphonic noise to be excluded from practical measurements.
Note 2. — The stated value of signal-to-noise ratio is that which shall apply when measured with the appropriate low-pass filter described in Annex II to Recommendation 421-1, with the appropriate weighting network described in Annex III to the same Recommendation, and with an instrument having an “effective time constant” or “integrating time” , in terms of power, of 1 s. (It is assumed that the weighting used would be appropriate to a television sys tem with a video bandwidth of 5 MHz, as specified in Recommendation 421-1.)
RECOMMENDATION 355-1
FREQUENCY SHARING BETWEEN ACTIVE COMMUNICATION-SATELLITE SYSTEMS AND TERRESTRIAL RADIO SERVICES IN THE SAME FREQUENCY BANDS (Question 2/IV)
The C.C.I.R., (1963 — 1966)
CONSIDERING
(a) that active communication-satellite systems and terrestrial radio services share certain bands between 1 and 10 GHz ;
(b) that control of mutual interference between stations of the two services is necessary; (c) that the continued development of both services is desirable ; Rec. .355-1 — 208 —
(d) that it is necessary to restrict the noise contribution, in a telephone channel of either service, caused by interference from stations of the other, to permissibly small amounts ;
(e) that among the means for reducing, to permissible levels, interference between communication- satellite systems and terrestrial radio systems sharing the same frequency bands are :
— on the part of satellite space stations, limitation of the power flux per unit area in unit bandwidth produced at the surface of the earth ;
— on the part of communication-satellite earth stations, limitation of the minimum distance to terrestrial transmitters, appropriate to the technical characteristics concerned and to propagation factors, together with limitation of the maximum power radiated at low angles of elevation;
— on the part of stations in the terrestrial services, limitation of the distance to earth stations, appropriate to the technical characteristics concerned and to propagation factors, together with limitation of the total emitted power and the equivalent isotropically radiated power;
(f) that the application of reasonable constraints on the design of both line-of-sight radio-relay systems and communication-satellite systems can permit the sharing of frequency bands, but that considerable difficulties may arise in sharing frequency bands with other terrestrial ser vices which involve high power transmitters, highly sensitive receivers, and changing areas of coverage;
UNANIMOUSLY RECOMMENDS
1. that, in sharing between line-of-sight radio-relay systems and communication-satellite systems, the noise in a telephone channel arising from mutual interference should be limited to a permissibly small amount, compared to the total allowable noise in the appropriate hypothetical reference circuit, as set out at present in Recommendations 356-1 and 357-1 ;
2. that the control of mutual interference between communication-satellite space stations and line-of-sight radio-relay systems should be through constraints applicable to the use of both, so as to avoid the need for specific coordination procedures between the Administrations operating radio-relay stations and those operating space stations ; these constraints are set out at present in Recommendations 358-1 and 406-1 ;
3. that questions of sharing between communication-satellite systems and terrestrial radio sys tems, other than line-of-sight radio-relay systems, and the bases for such sharing, should receive further study;
4. that, the control of mutual interference between each earth-station of a communication- satellite system and terrestrial radio stations sharing the same frequency bands should be by the application of specific coordination procedures between the Administrations concerned. Recommended procedures are set out at present in Recommendation 359-1. — 209 — Rec. 356-1
RECOMMENDATION 356-1
COMMUNICATION-SATELLITE SYSTEMS AND LINE-OF-SIGHT RADIO-RELAY SYSTEMS SHARING THE SAME FREQUENCY BANDS
Maximum allowable values of interference in a telephone channel of a communication-satellite system
(Question 2/IV, Study Programme 2A/IY)
The C.C.I.R., (1963 — 1966)
CONSIDERING
(a) that communication-satellite systems and line-of-sight radio-relay systems share frequency bands in the range 1 to 10 GHz ;
(b) that mutual interference would increase the noise in both types of system beyond that which would exist in the absence of frequency sharing ;
(c) that it is desirable that the noise due to interference in the telephone channels of communica tion-satellite systems due to the transmitters of radio-relay systems should be a small fraction of the total noise in those systems, as set out in Recommendation 353-1 ;
(d) that it is necessary to specify the maximum allowable interference power in a telephone channel, to determine the maximum transmitter power and effective radiated power of line- of-sight radio-relay stations, and to determine whether specific locations for satellite-earth stations and terrestrial radio-relay stations would be satisfactory;
(e ) that a distribution of one minute mean power, as exemplified in the Annex, would allot to interference a reasonable fraction of the total noise power permitted in the hypothetical reference circuit;
(f) that communication-satellite systems may receive interference both through the satellite receiver and through the earth-station receiver ;
UNANIMOUSLY RECOMMENDS
1. that communication-satellite systems and radio-relay systems sharing the same frequency bands, be designed in such a manner that the interference noise power at a point of zero relative level in any telephone channel of a basic hypothetical reference circuit of a communication- satellite system caused by the aggregate of the transmitters of radio-relay stations, conforming to Recommendation 406-1, should not exceed : 1.1 1000 pW psophometrically-weighted mean power in any hour ; 1.2 1000 pW psophometrically-weighted one-minute mean power for more than 20% of any m onth; 1.3 50 000 pW psophometrically-weighted one-minute mean power for more than. 0-03% of any m onth; i
2. that the following Note should be regarded as part of the Recommendation :
N ote. — The way in which the above values are to be taken into account in the general noise objec tive for communication-satellite systems is defined in Recommendation 353-1. Rec. 356-1, 357-1 — 210 —
ANNEX
10'5
10* Pw
10*3 0,01 0,1 1 10 20
Percentage of any month
Example of possible interpolation
RECOMMENDATION 357-1
COMMUNICATION-SATELLITE SYSTEMS AND LINE-OF-SIGHT RADIO-RELAY SYSTEMS SHARING THE SAME FREQUENCY BANDS
Maximum allowable values of interference in a telephone channel of a radio-relay system
(Study Programme 2A/IV)
The C.C.I.R., (1963 — 1966)
CONSIDERING
(a) that communication-satellite systems and line-of-sight radio-relay systems share frequency bands in the range 1 to 10 GHz ;
(b) that mutual interference would increase the noise in both types of system beyond that which would exist in the absence of frequency sharing;
(c) that it is desirable that the noise due to interference in the telephone channels of existing radio-relay systems from transmitters of satellites and earth stations should be a fraction of the total noise in those systems such that it would not be necessary to change the design objectives for radio-relay systems, as set out in Recommendation 393-1 ; — 211 — Rec. 357-1
(d) that it is necessary to specify the maximum allowable interference power in a telephone channel, to determine the maximum power flux, which can be allowed at the surface of the earth from communication-satellites and to determine whether specific locations for satellite earth stations and terrestrial radio-relay stations would be satisfactory; (e) that a distribution of one-minute mean power, as exemplified in the Annex, would allot to interference a reasonable fraction of the total noise power permitted in the hypothetical reference circuit;
(f) that it is unlikely that any given radio-frequency channel used by line-of-sight radio-relay systems will be used for transmissions both from active communication-satellites and from earth stations;
UNANIMOUSLY RECOMMENDS
1. that communication-satellite systems and line-of-sight radio-relay systems which share the same frequency bands, should be designed in such a manner that, in any telephone channel of a 2500 km hypothetical reference circuit for radio-relay systems, the interference noise power at a point of zero relative level, caused by the aggregate of the emissions of earth sta tions and space stations of the communication-satellite systems, including associated tele metering, telecommand and tracking transmitters, should not exceed : 1.1 1000 pW psophometrically-weighted mean value in any hour; 1.2 1000 pW psophometrically-weighted one-minute mean power for more than 20% of any m onth; 1.3 50 000 pW psophometrically-weighted one-minute mean power for more than 0-01% of any m onth;
2. that the following Note should be regarded as part of the Recommendation.
Note. — The way in which the above values are to be taken into account in the general noise objective for radio-relay systems is defined in Recommendation 393-1.
ANNEX
IQ'4 pW
Percentage of any month
Example of possible interpolation Rec. 358-1 — 212 —
RECOMMENDATION 358-1
COMMUNICATION-SATELLITE SYSTEMS AND LINE-OF-SIGHT RADIO-RELAY SYSTEMS SHARING THE SAME FREQUENCY BANDS Maximum allowable values of power flux-density at the surface of the earth produced by communication satellites
(Question 2/IV)
The C.C.I.R., (1963 — 1966)
CONSIDERING
(a) that communication-satellite systems and line-of-sight radio-relay systems share frequency bands in the range 1 to 10 GHz ;
(b) that, because of such sharing, it is necessary to ensure that the power flux-density, set up at the surface of the earth by emissions from the space stations, does not reach a value that would cause significant interference to line-of-sight radio-relay systems;
(c) that, nevertheless, any limitations of the power flux-density set up at the surface of the earth should not be such as to place undue restrictions on the design of communication-satellite systems;
(d) that for communication-satellite systems using frequency modulation, a method of carrier energy dispersal could be employed to reduce the radio-frequency power flux-density in any 4 kHz band, under conditions of light loading from multi-channel telephony, or from tele vision signals;
(e) that line-of-sight radio-relay systems can be satisfactorily protected from the emissions of communication satellites by placing suitable limits on the power flux density in any 4 kHz ban d ;
UNANIMOUSLY RECOMMENDS
1. that, in frequency bands in the range 1 to 10 GHz shared between communication-satellite systems and line-of-sight radio-relay systems, the maximum power flux-density produced at the surface of the earth by emissions from a space station, for all conditions and methods of modulation, should not exceed
^ —152 + dB rel. 1 W/m2 in any 4 kHz band,
where 0 is the angle of arrival of the wave (degrees above the horizontal);
2. that the aforementioned limit should be assumed to relate to the power flux-density under free-space propagation conditions. — 213 — Rec. 359-1
RECOMMENDATION 359-1
COMMUNICATION-SATELLITE SYSTEMS AND TERRESTRIAL RADIO SYSTEMS SHARING THE SAME FREQUENCY BANDS
Determination of the coordination distance
(Study Programme 2A/IV)
The C.C.I.R., (1963 — 1966)
CONSIDERING
(a) that, where earth stations and terrestrial stations share the same frequency bands, there is a possibility of interference, both as regards the earth station transmissions interfering with reception at terrestrial stations, and the terrestrial station transmissions interfering with reception at earth stations ;
(b) that, to avoid such interference, it will be desirable for the transmitting and receiving fre quencies used by earth stations to be coordinated with the frequency used by terrestrial services, which might be in a position either to receive interference from earth station trans missions or to cause interference to reception at earth stations ;
(c) that this coordination will need to be established within an area surrounding the earth station and extending to the limits beyond which the possibility of mutual interference may be considered negligible;
(d) that this area may sometimes involve more than one Administration;
(e) that such mutual interference will depend upon several factors, including the transmitter powers, antennae gains along the interference paths, the nature of the intervening terrain, and the tolerable interference levels at the receivers ;
(f) that the coordination of the preferred reference frequencies for the communication-satellite service with the frequencies used by terrestrial services may sometimes, especially for radio relay systems, affect the whole area where such a system is used :
(g) that the possibility of interference will need to be examined in detail in each case, taking all factors into account;
(h) that, as a preliminary to this detailed examination, it is desirable to establish a method of determining, on the basis of broad assumptions, a coordination distance from the earth station, i.e. that distance within which mutual consultation between Administrations will be required and beyond which the possibility of interference may be regarded as negligible;
(i) that the Extraordinary Administrative Radio Conference, Geneva, 1963, invited the C.C.I.R. to study the question of coordination distance and to make suitable recommendations to replace the procedure set out in the Annex to E.A.R.C. Recommendation No. 1A;
UNANIMOUSLY RECOMMENDS
1. that account be taken of the international coordination and planning which will be involved, if communication-satellite earth stations are to share frequency bands with terrestrial stations in nearby countries without undue mutual interference; Rec. 359-1, 360 — 214 —
2. that the coordination distance for possible interference to terrestrial radio stations be deter mined by Administrations planning to establish earth stations on the basis o f: — the e.i.r.p. likely to be used in various directions from the earth station; — the maximum antenna gains and receiver sensitivities likely to be used at the terrestrial stations concerned;
3. that the coordination distances for possible interference to earth station reception be deter mined by Administrations planning to establish earth stations on the basis o f: — the expected range of e.i.r.p. likely from terrestrial stations in the pertinent directions ; — the antenna and receiver characteristics likely to be used at the earth station concerned ;
4. that the coordination distances in the above cases be determined on the basis o f: — the appropriate propagation data for overland and oversea paths given in Reports 243 and 244-1 ; — interference powers in telephone channels in the baseband of receiving systems, not exceeding the provisional values given in Recommendations 356-1 and 357-1 ; — the procedure outlined in Report 382, which is intended to replace the procedure set out in the Annex to Recommendation No. 1A of the Final Acts of the Extraordinary Administrative Radio Conference, Geneva, 1963.
RECOMMENDATION 360-1
COMMUNICATION-SATELLITE SYSTEMS AND LINE-OF-SIGHT RADIO-RELAY SYSTEMS SHARING THE SAME FREQUENCY BANDS
Criteria for the selection of preferred reference frequencies for communication-satellite systems
(Question 2/IV, Study Programme 2A/IY)
The C.C.I.R., (1963 — 1966)
CONSIDERING
(a) that communication-satellite systems are expected to make an important contribution to the provision of world-wide and area communication;
(b) that spectrum economy would be enhanced by sharing frequency bands with terrestrial sys tems in the fixed services ;
(c) that it can be demonstrated that, under suitably controlled conditions, sharing with terrestrial line-of-sight radio-relay systems is practicable; — 215 — Rec. 360, 446
(d) that a number of independent communication-satellite systems may be established, including low-capacity as well as high-capacity systems ;
(e) that the selection of frequencies for use by communication-satellite systems requires inter national agreement, so that interference between such systems and with other radio services may be minimized and the most effective use be made of the available frequency spectrum ;
(f) that the preferred reference frequencies initially adopted will have a major influence on sub sequent frequency usage;
(g) that the selection of preferred reference frequencies depends on the, as yet, undetermined technical characteristics of communication-satellite systems, as well as on the known technical characteristics of radio-relay systems ;
UNANIMOUSLY RECOMMENDS
1. that the designation of preferred reference frequencies should : 1.1 provide scope for the development and use, as necessary, of various types of communication- satellite system, e.g., using different orbits, types of satellite, methods of modulation and other technical characteristics ; 1.2 allow for multi-station access, thereby enabling several earth stations to use a given satellite ; 1.3 minimize interference between communication-satellite systems, and between communication- satellite systems and line-of-sight radio-relay systems, to facilitate the planning of both types of system on a coordinated basis ; 1.4 accommodate the long-term traffic requirements forecast for communication-satellite systems ;
2. that the preferred reference frequencies should interleave, as effectively as possible, with the channel arrangements for line-of-sight radio-relay systems recognized in C.C.I.R. Recom mendations ;
3. that there should be provision for high-capacity systems, low-capacity systems and systems using both large and small earth stations.
Note. — The reference frequency is the frequency relative to which the specific frequency or fre quencies assigned to a given communication-satellite system may be specified (No. 87 of the Radio Regulations, Geneva, 1959).
RECOMMENDATION 446
FREQUENCY SELECTION AND CARRIER ENERGY-DISPERSAL FOR COMMUNICATION-SATELLITE SYSTEMS
(Question 2/IV, Study Programme 2A/IV)
The C.C.I.R., (1966)
CONSIDERING
(a) that the shared use of the frequency bands 3400-4200 MHz and 5725-6425 MHz involves coordination of the frequencies to be used at communication-satellite earth stations and at terrestrial radio-relay stations within coordination distance; Rec. 446 — 216 —
(b) that, in certain areas of the world, where terrestrial radio-relay links in these bands are already well-developed or in the process of development, such coordination can lead to technical difficulties in the choice of frequencies for transmission and reception at communication- satellite earth stations;
(c) that these difficulties are least when the techniques employed by communication-satellite sys tems involve the more or less uniform spreading of the signal energy at all times over most of the occupied bandwidth, but that these techniques still require further study ;
(d) that these technical difficulties can arise in those frequency-modulation systems in which periods of relatively light loading occur ;
(e) that the future development of communication-satellite systems is dependent upon the efficient use of the space-station transmitter power and bandwidth ;
UNANIMOUSLY RECOMMENDS
1. that, for communication-satellite systems operating in the bands 3400-4200 MHz and 5725- 6425 MHz, the choice of carrier frequencies be guided by the following principles, unless adherence to these principles places an unacceptable constraint upon the satisfactory design of the communication-satellite system : 1.1 in systems for which the energy cdn be uniformly spread at all times over-most of the band occupied, the relative position of the carrier frequencies of the terrestrial radio-relay and communication-satellite systems is of little or no importance and such techniques are advan tageous ; 1.2 in those frequency-modulation systems in which periods of relatively light loading occur, interference is reduced if communication-satellite system carrier frequencies are placed: — either, as close as possible to the terrestrial radio-relay system carrier frequencies custom arily used in the areas where the earth stations are to be located; — or, outside the first order sidebands corresponding to the maximum baseband frequencies used by such radio-relay systems.
Note. — In frequency-modulation systems carrying television signals, carrier energy-dispersal can provide a useful reduction of interference. While this may not result in a uniform spreading of energy over the band occupied, the relative position of the carrier frequencies of the ter restrial radio-relay and communication-satellite systems will generally be of little importance if adequate dispersal is used.
L. 3 : Direct broadcasting from satellites
No Recommendations in this sub-section — 217 — Rec. 361-1
L. 4 : Radionavigation by satellites
RECOMMENDATION 361-1
FREQUENCY REQUIREMENTS OF RADIONAVIGATION-SATELLITE SYSTEMS
(Question 8/IV)
The C.C.I.R., (1963 — 1966)
CONSIDERING
(a) that in view of the relatively low information rate likely to be required and in the interest of frequency-spectrum conservation, the use of narrow-band techniques appears to be suit able for radionavigation-satellite systems;
(b) that, for systems using satellites which are in motion relative to the surface of the earth, the bandwidth requirements will be largely determined by the Doppler frequency-shift and the accu racies of transmitter and receiver frequencies ;
(c) that, for systems using satellites which are essentially stationary relative to the surface of the earth, the bandwidth requirements will be largely determined by the modulation techniques which may be employed and the accuracies of transmitter and receiver frequencies ;
(d) that antenna systems of individual radionavigation satellites should be designed to provide the maximum practicable coverage of the surface of the earth ;
(e) that there is a need to minimize the weight of the equipment in the satellite;
(f) that equipment and techniques are at present available, or may be available in the near future, at frequencies in the VHF and UHF ranges (bands 8 and 9) up to about 1000 MHz, the lower frequencies being more suitable for low altitude systems;
(g) that it is possible, in Doppler type systems, to reduce errors caused by ionospheric refraction by the use of two frequencies well separated from each other, and that in this case, system design may be simplified by the use of harmonically related frequencies ;
(h) that for systems based on the principle of directivity, the use of narrow beam width receiving antennae is required; that it is desirable to minimize the physical size of such antennae; and that for a given antenna size, the directivity and resulting accuracy increase with frequency;
(i) that for systems based on the principle of directivity, a study of desirable antenna size and the required accuracy indicates that frequencies above 10 GHz should be used; studies to date indicate that the atmospheric absorption due to water vapour and oxygen is significantly less in the region below 20 GHz and in a narrow region near 35 GHz;
3 Rec. 361-1 — 218 —
(j) that this service would require a high degree of radio-frequency protection ;
(k) that, in the interests of spectrum economy, it is desirable, and may prove possible, to accom modate maintenance telemetering signals required for radionavigation-satellite systems on the radio-frequency channels provided for radionavigation satellites;
UNANIMOUSLY RECOMMENDS
1. that for radionavigation satellite-systems in general: 1.1 frequencies in the VHF, UHF and SHF ranges (bands 8, 9 and 10) are technically suitable ; 1.2 use of narrow-band systems is preferable where techniques permit; 1.3 where frequency combination is feasible, the maintenance telemetering signals for radionavi gation satellite-systems should be accommodated on frequency channels provided for the radionavigation functions; 1.4 that experimentation to investigate the feasibility of several possible systems be conducted in appropriate radionavigation frequency bands ;
2. that for Doppler, time measuring and path difference radionavigation satellite-systems : 2.1 frequencies in the VHF and UHF ranges (bands 8 and 9) are technically suitable;
3. that for radio-sextant radionavigation systems : 3.1 frequencies of the order of 35 GHz are technically suitable. Where lower accuracy is accept able, frequencies between 10 and 20 GHz may be used. — 219 — Rec. 362
L. 5 : Meteorological satellites
RECOMMENDATION 362
FREQUENCIES TECHNICALLY SUITABLE FOR METEOROLOGICAL SATELLITES (Question 9/IV, Study Programme 9A/IV)
The C.C.I.R., (1963)
CONSIDERING
(a) that meteorological satellites have demonstrated their value to mankind;
(b) that meteorological satellites will soon be operating in a routine manner as indicated in Report 395;
(c) that certain bands are now allocated internationally to the meteorological aids service;
(d) that certain of the frequency needs of meteorological satellites may be satisfied through the use of the meteorological aids allocations established at present;
UNANIMOUSLY RECOMMENDS
1. that bands 8, 9 and 10 are technically suitable for narrow-band and wide-band meteorological data transmission;
2. that as meteorological satellites become more fully developed, and taking due account of reliability requirements and economical use of the frequency spectrum, frequencies within the bands used for meteorological satellites will be technically suitable for tracking, mainten ance telemetry and telecommand;
3. that frequencies allocated to the radiolocation service in bands 10 and 11 are technically suitable for use by the precipitation detection radar and cloud detection radar on board meteorological satellites. Rec. 363 — 220 —
L. 6 : Maintenance telemetering, tracking and telecommand
RECOMMENDATION 363
PREFERRED FREQUENCY BANDS FOR USE IN MAINTENANCE TELEMETERING, TRACKING AND TELECOMMAND OF DEVELOPMENTAL AND OPERATIONAL SATELLITES (Question 4/IV)
The C.C.I.R., (1963)
CONSIDERING
(a) that the frequencies technically suitable for maintenance telemetering, tracking and tele command of developmental and operational radionavigation, meteorological and communi cation satellites lie in the range from 100 MHz to 10 GHz ; the lower part of this range being generally more suitable for lower altitude earth-satellites and the higher part of this range being generally more suitable for higher altitude earth-satellites ; (b) that the integration of maintenance telemetering, tracking and telecommand links with data transmission and communication systems has many potential advantages such as system simplicity, efficient use of redundant modules, precision in tracking and efficient use of the spectrum;
UNANIMOUSLY RECOMMENDS
1. that bands of frequencies below 1 GHz, preferably below 600 MHz, are the most technically suitable for maintenance telemetering, tracking and telecommand of developmental and operational satellites;
2. that bands of frequencies between 1 and 10 GHz are also technically suitable for maintenance telemetering, precision tracking systems, tracking and telecommand for high-altitude satel lites ; further, that because the frequency range is so broad, appropriate bands in the lower portion, centre portion and upper portion of this region be considered ;
3. that bands of frequencies above about 10 GHz are technically suitable for maintenance tele metering, tracking and telecommand during the re-entry of satellites into the earth’s atmo sphere (see Report 222-1);
4. that, as systems such as meteorological, radionavigation and communication satellites become more fully developed, and taking due account of reliability requirements and economical use of the frequency spectrum, frequencies within the bands used for data transmission or communications be also used for maintenance telemetering, tracking and telecommand, where feasible. — 221 — Rec. 364-1
L. 7 : Space research
RECOMMENDATION 364-1
TELECOMMUNICATION LINKS FOR NEAR-EARTH RESEARCH SATELLITES Frequencies, bandwidths and interference criteria
(Questions 3/IV, 4/IV, 7/VI)
The C.C.I.R., (1963 — 1966)
CONSIDERING
(a) that suitable operating frequencies, required radio-frequency bandwidths, and limiting inter ference criteria for near-earth satellite telecommunication links are determined by the technical considerations set forth in Report 219-1 ;
(b) that, based on past experience, it might be expected that from 20 to 30 active research near- earth satellites will be in orbit simultaneously ;
(c) that radio frequencies from about 100 MHz to 10 GHz are suitable for near-earth satellite telecommunication links, and that the variety of functions and the conditions under which they must be performed divides this region into the following suitable regions : 100 to 300 MHz, 300 MHz to 1 GHz, and 1 to 10 GHz;
(d) that telemetering data links typically require radio-frequency bandwidths ranging from 10 kHz to 3 MHz per link in the frequency range from 100 MHz to 10 GHz, with a preference for 10 kHz in the lower portion, and 3 MHz in the upper portion ; that wideband data and television links typically require radio-frequency bandwidths ranging from 50 kHz to 20 MHz in the frequency range from 100 MHz to 10 GHz, with preference for 50 kHz in the lower portion and 20 MHz in the upper portion of this range ;
(e) that tracking links typically require radio-frequency bandwidths ranging from 10 kHz to 3 MHz per link in the frequency range from 100 MHz to 10 GHz with a preference for 10 kHz in the lower portion and 3 MHz in the upper portion of this range ;
(f) that the operating noise temperatures of earth-station telemetering and tracking receiving systems, usually range from 30°K above 1 GHz to 3000°K at 0-1 GHz (equivalent to —214 dBW per hertz and —194 dBW per hertz);
(g) that earth-station antennae for satellite communication do not normally use angles of elevation below 3°;
(h) that typical operating noise temperatures for receivers in spacecraft will be approximately 600°K (—171 dBW per kHz) for near-earth research satellites, but measures can be taken to protect the spacecraft receiving system against interference approximately 10 dB greater than this noise level; Rec. 364-1 — 222 —
(i) that telecommand can normally be satisfied by one data link channel per craft with radio frequency bandwidths ranging from 10 kHz at 0-1 GHz to 500 kHz at 10 GHz, largely determined by the Doppler shift which may be of the order of 1 x 10~5 of the carrier frequency ;
(j) that coding techniques and use of narrow-beam antennae will make possible the control of several craft using the same radio-frequency telecommand channel;
(k) that, for some sharing situations between near-earth research satellites and certain represen tative terrestrial services, separations of several hundreds of kilometres between the earth ter minals may be required and that, in many parts of the world, separations of this magnitude are not readily attainable ;
(I) that sharing among near-earth research satellites is desirable and feasible;
(m) that difficulties can be expected when sharing frequencies between near-earth research satel lites and other services, due to the technical problems of furnishing the required protection against terrestrial services;
UNANIMOUSLY RECOMMENDS
1. that the range between 100 and 300 MHz is technically suitable for near-earth research satellites, with each satellite using radio-frequency bandwidths varying from 10 to 300 kHz per link, for accomplishing telemetering, wideband data, tracking and telecommand functions;
2. that the range between 300 and 1000 MHz is technically suitable for near-earth research satellites, with each satellite using radio-frequency bandwidths varying from 30 to 500 kHz per link, for accomplishing telemetering, wideband data, tracking and telecommand functions;
3. that the range between 1 and 10 GHz is technically suitable for near-earth research satellites with the following requirements : — for tracking, 3 MHz per link; — for telemetering, from 500 kHz per link near 1 GHz to 3 MHz per link near 10 GHz ; — for wideband data links and television 500 kHz per link near 1 GHz to 20 MHz per link near 10 GHz; and — for telecommand, 100 to 500 kHz per link to be shared with tracking when feasible;
4. that consideration be given to the availability of coherently related frequencies spaced 6% to 10% of the higher frequency for precision tracking systems ;
5. that sharing be accomplished to the maximum extent feasible among near-earth research satellites;
6. that the protection criterion for earth receiving sites be established as follows : for frequencies greater than 1 GHz, the total time for which the power spectral density of noise-like interference or the total power of CW-type interference in any single band and all sets of bands 1 Hz wide is greater than —220 dBW per hertz at the input terminals of the receiver shall not exceed 0-1 % of the time; for frequencies less than 1 GHz, the permissible interference may be increased at the rate of 20 dB per decreasing frequency decade; — 223 — Rec. 364-1, 365-1
7. that the protection criterion for receivers in spacecraft be established as follows : for frequen cies greater than 300 MHz, the total time during which the power spectral-density of noise-like interference or the total power of CW-type interference in any single band or all sets of bands 1 kHz wide is greater than —161 dBW per kHz at the input terminals of the receiver shall not exceed 0-1% of the time; for frequencies less than 300 MHz, the permissible interference may be increased at the rate of 20 dB per decreasing frequency decade;
8. that note be taken of the difficulties to be expected in sharing between near-earth research satel lites and stations in other services.
RECOMMENDATION 365-1
TELECOMMUNICATION LINKS FOR DEEP-SPACE RESEARCH Frequencies, bandwidths and interference criteria
(Questions 3/IV, 4/IV and 7/VI)
The C.C.I.R., (1963 — 1966)
CONSIDERING
(a) that suitable operating frequencies, required radio-frequency bandwidths, and limiting inter ference criteria for deep-space telecommunication links are determined by the technical considerations set forth in Report 219-1 ;
(b) that cosmic noise and man-made noise militate against the use of frequencies lower than 100 MHz and that atmospheric noise and absorption preclude the use of frequencies much higher than 10 GHz;
(c) that the precision tracking required for acceptable guidance of spacecraft to the Moon and the planets and for manoeuvres in their vicinity result in a preference for frequencies between 1 GHz and 10 GHz;
(d) that the typical operating noise temperatures of earth station receivers, operating at frequencies greater than 1 GHz, will be 25°K (—215 dBW per hertz) for most missions with design margins .of less than 6 dB ; and that, for reception at frequencies less than 1 GHz, the system noise temperature is increased by cosmic noise approximately as the inverse of the square of the frequency;
(e) that the typical operating noise temperature of a receiver in a spacecraft, operating at fre quencies greater than 300 MHz, will be 600°K (—201 dBW per hertz) with design margins of 6 to 10 dB; and that for reception at frequencies less than 300 MHz, the system noise tem perature is increased by cosmic noise approximately as the inverse of the square of the fre quency ;
(f) that water vapour in the atmosphere can produce significant degradation in the quality of deep-space communications, at frequencies higher than 4 GHz ; Rec. 365-1 — 224 —
(g) that typical deep-space systems will use directional antennae both on the earth and on the spacecraft to transmit sufficient information. These antennae will have a surface precision of between 1 mm and 5 mm, regardless of diameter, so that frequencies between 1 and 10 GHz will generally be more suitable for efficient transmission;
(h) that Doppler shifts can be of the order of 1 x 10-4 of the carrier frequency ;
(i) that interruptions in communications of more than five minutes per day could seriously alfect the success of the mission ;
(j) that real-time television from the lunar surface is practical with frame rateis, resolutions, and qualities comparable to commercial television on earth; that photographic facsimile from the nearer planets is similarly practical;
(k) that it is practical and desirable to effect telemetering, data transmission, and tracking func tions on the same space-earth link, and telecommand and tracking functions on the same earth- space link;
(I) that to effect precision tracking, a pair of coherently related frequencies with a separation ranging from 6% to 10% of the higher frequency is required ;
(m) that the angular spacings of the Moon and the planets are such that the same frequency may be used for probes at different celestial coordinates, but that different spacecraft in the vicinity of the same coordinates or in the antenna beamwidth may require different frequencies;
(n) that geographical separations which permit sharing between terrestrial services and deep-space research operations are typically several hundreds of kilometers, and may be greater than 500 km in the absence of terrain shielding; that separations of this magnitude are not readily obtainable in many parts of the world ; and that the spacecraft are visible over large areas of the globe;
(o) that considerable difficulties can be expected, when sharing frequencies between deep-space research operations and other services, due to the technical problems of furnishing the required protection against both terrestrial and near-earth satellite transmissions ;
(p) that, although the exploration of deep space has just begun, certain recommendations are possible and necessary at this time ;
UNANIMOUSLY RECOMMENDS
1. that frequencies for deep-space telecommunication links be located in the frequency band between 100 MHz and 10 GHz :
1.1 the band between 100 MHz and 1 GHz is generally more suitable for narrow-band telemetering, tracking, and telecommand from launch to intermediate distances;
1.2 the band between 1 GHz and 6 GHz is generally more suitable for wideband telemetering, precision tracking, and telecommand from launch to extreme distances ; 1.3 the band between 6 GHz and 10 GHz is generally more suitable for very wideband telemetering and very precise tracking at various distances ; 1.4 special consideration should be given to the availability of coherently related frequencies, spaced at 6% to 10% of the higher frequency, for precision tracking systems ; — 225 — Rec. 365-1, 366-1
2. that spectrum space of the order of 2 to 4 MHz per link (due account being taken of the dependency of the radio-frequency bandwidth on the type of modulation used), is technically suitable for the transmission of wideband information for lunar flights ;
3. that spectrum space of the order of 2 to 4 MHz per link is technically suitable for the trans mission of wide bandwidths for precision tracking ; 3.1 that tracking be time-multiplexed with lunar television or planetary facsimile where practic able ;
4. that frequencies be shared among deep-space probes with different celestial coordinates but generally not with spacecraft with the same celestial coordinates;
5. that the protection criterion for earth receiving stations be established as follows : for frequencies greater than 1 GHz, the total time during which the power spectral density of noise-like interference or the total power of CW-type interference in any single band and all sets of bands 1 Hz wide, is greater than —220 dBW per hertz at the input terminals of the receiver shall not exceed five minutes per day; for frequencies below 1 GHz, the permissible interference may be increased at the rate of 20 dB per decreasing frequency decade ;
6. that the protection criterion for spacecraft receivers be established as follows : for frequencies greater than 300 MHz, the total time during which the power spectral density of noise-like interference or the total power of CW-type interference in any single band and all sets of bands 1 kHz wide, is greater than —171 dBW per kHz at the input terminals of the receiver shall not exceed five minutes per day; for frequencies less than 300 MHz, the per missible interference may be increased at the rate of 20 dB per decreasing frequency decade.
7. that note be taken of the difficulties to be expected in sharing frequencies between deep-space research and other services. Note. — The above criteria are, at the present time, identical with those for manned research spacecraft.
RECOMMENDATION 366-1
TELECOMMUNICATION LINKS FOR MANNED RESEARCH SPACECRAFT (Questions 3/IV, 4/IV)
The C.C.I.R., (1963 — 1966)
CONSIDERING
(a) that, the frequencies, bandwidths, and interference protection criteria for manned research space-flight telecommunication and tracking functions, are generally similar to those of unmanned near-earth and deep-space flight (see Reports 205-1, 219-1 and 222-1 and Recom mendations 364-1, 365-1 and 367), with the following special considerations;
(b) that manned space-flight has periods of launch, earth-space, mid-course manoeuvres, terminal manoeuvres, rendezvous, abort (premature termination), re-entry and recovery, which are in many respects similar to unmanned space-flight, but due to safety-of-life considerations, are much more critical; Rec. 366-1 — 226 —
(c) that, in contrast with unmanned space-flight, two-way voice communication is a vital part of manned space-flight and is required to aid command and control, guidance, navigation and other telecommunication functions;
(d) that manned space-flight will continue to be in the research phase for several years and, as such, can be expected to require the highest telecommunication and tracking reliability, high data-rate instrumentation such as television, and special telecommunication links for emer gency purposes;
(e) that the presence of man aboard a spacecraft generally increases the information transmission per spacecraft by approximately 25% and that manned space-flight operations often involve the use of compound (multi-stage) spacecraft, each part requiring separate telecommunica tion and tracking;
(f) that typical telecommunication links used for voice, telecommand, and telemetering require approximately 100 kHz per link with links required for both directions and for two spacecraft engaged in such near-proximity operations as rendezvous, and that a need therefore can exist lor approximately 500 kHz of bandwidth per mission ;
UNANIMOUSLY RECOMMENDS
1. that for most communication and tracking functions, manned research space-flight be consi dered the same as unmanned near-earth and deep-space research flight with respect to fre quency regions and interference protection criteria ;
2. that frequencies in the HF range below 25 MHz are technically suitable for voice communica tion during the near-earth and recovery phases of manned space-flight, to extend the range beyond the normal line-of-sight propagation associated with higher frequencies;
3. that, to accommodate special requirements resulting from safety-of-life considerations, for reliable, line-of-sight, two-way telecommunication links having a bandwidth of as much as 500 kHz per mission, the frequency range from about 20 MHz through the lower portion of band 9 (the frequency range from 100 to 600 MHz being generally most suitable, but lower frequencies being usable), is technically suitable;
4. that the protection criterion for the earth receiving stations be established as follows : for fre quencies greater than 1 GHz, the total time for which the power spectral density of noise-like interference or the total power of CW-type interference in any single band and all sets of bands 1 Hz wide, is greater than —220 dBW per hertz at the input terminals of the receiver shall not exceed five minutes per day; for frequencies below 1 GHz, the permissible interference may be increased at the rate of 20 dB per decreasing frequency decade ;
5. that the protection criterion for receivers on manned spacecraft be established as follows : for frequencies greater than 300 MHz, the total time during which the power spectral density of noise-like interference or the total power of CW-type interference in any single band, and all sets of bands 1 kHz wide, is greater than —171 dBW per kHz at the input terminals of the receiver shall not exceed five minutes per day ; for frequencies less than 300 MHz, the per missible interference may be increased at the rate of 20 dB per decreasing frequency decade;
6. that, when necessary for emergency purposes, manned space-flight use the recognized emer gency frequencies in accordance with the Radio Regulations, Geneva, 1959 ; — 227 — Rec. 366-1, 367
7. that the above considerations and recommendations apply only to the initial phases of manned space-flight research, and that a study be undertaken to determine the possible impact of advanced manned space-flight research missions such as space laboratories, cargo ferries, etc., it being quite probable that such missions will require world-wide frequency allocations. Note. — The above criteria are, at the present time, identical with those for deep-space research.
RECO M M EN D A TIO N 367
FREQUENCY BANDS FOR RE-ENTRY COMMUNICATIONS
(Question 6/IV)
The C.C.I.R., (1963)
CONSIDERING
(a) that spacecraft re-entering the earth’s atmosphere are enveloped in a self-induced plasma;
(b) that electromagnetic radiations to and from the vehicle may suffer severe attenuation and other detrimental effects due to the existence of the plasma ;
(c) that communications with, and tracking of, the vehicle may be imperative during the re-entry phase to ensure a successful mission ;
(d) that the selection of frequency bands for re-entry communications and tracking is dictated partly by the parameters of the induced plasma ; (e) that the use of such bands requires international agreement, since the phases of re-entry flight may extend over one or more orbits of the earth ; (f) that the only proved solution to the re-entry communication problem to date involves the use of frequencies greater than the critical frequency of the plasma sheath ; (g) that critical frequencies of the plasma sheath can approach or exceed 10 GHz ; (h) that frequencies of 10 GHz and higher are affected appreciably by the earth’s atmosphere; (i) that the bands available at present for space research purposes above 15 GHz are technically suitable for some re-entry communications ;
UNANIMOUSLY RECOMMENDS that both the critical frequency of the plasma sheath and the atmospheric effects be con sidered in the selection of frequencies for re-entry communications (see Reports 205-1 and 222-1). Rec. 314-1 — 228 —
L. 8: Radioastronomy
RECOMMENDATION 314-1
PROTECTION OF FREQUENCIES USED FOR RADIO ASTRONOMICAL MEASUREMENTS (Question 10/IV)
The C.C.I.R., (1953 — 1956 — 1959 — 1966)
CONSIDERING (a) that the development of radioastronomy has already led to major technological advances, particularly in receiving techniques, and to improved knowledge of fundamental radio-noise limitations of great importance to radiocommunication, and promises further important results; (b) that protection from interference on certain frequencies is essential to the advancement of radioastronomy and the associated measurements ; (c) that, for the observation of known spectral lines, certain bands at specific frequencies are of particular importance; (d) that account should be taken of the Doppler shifts of the lines, resulting from the motion of the sources, which are in general receding from the observer; (e) that, for other types of radioastronomical observation, a certain number of frequency bands are in use, the exact positions of which in the spectrum are not of critical importance, but of which the centre frequencies should be approximately in the ratio of two to one ; (f) that the movement of the Moon produces occultations of radio sources, permitting unique radioastronomical observations of high resolution which are particularly important at metre wavelengths; (g) that the sensitivity of radioastronomical receiving equipment, which is still steadily improving, greatly exceeds the sensitivity of communications and radar equipment; (h) that harmful interference to radioastronomy can be caused by terrestrial transmissions reflected by the Moon, by aircraft, and possibly by artificial satellites; (i) that certain types of radioastronomical observation require long periods of uninterrupted recording, sometimes up to several days ; (j) that some degree of protection can be achieved by appropriate frequency assignments on a national rather than an international basis ; (k) that, nevertheless, it is impracticable to afford adequate protection without some international agreement, and the Final Acts of the Extraordinary Administrative Radio Conference, Geneva, 1963, have made provision for sufficient protection for a few of the frequencies allocated to radioastronomy, but that there is still a need for adequate protection in other parts of the spectrum, particularly at frequencies below 1 GHz, and for an increased band width at some higher frequencies ; (I) that, in its Recommendation No. 11 A, the Extraordinary Administrative Radio Conference, Geneva, 1963, recommended that the next Ordinary Administrative Radio Conference should give further consideration to the provision of improved frequency allocations for radio- astronomy ; 229 — Rec. 314-1
UNANIMOUSLY RECOMMENDS
1. that radioastronomers should be encouraged to choose sites as free as possible from inter ference ;
2. that Administrations should afford all practicable protection to the frequencies used by radio astronomers in their own and neighbouring countries;
3. that, in addition to the protection afforded to the band 1400-1427 MHz, which is already allocated to radioastronomy on an exclusive world-wide basis, particular attention should be given to securing improved international protection to bands of the order of 5 to 10 MHz in width, which will permit observations of the following natural line emissions :
Line Line frequency (MHz) Deuterium 327-4 OH
4. that the bands allocated for standard-frequency and time-signal emissions at 2-5, 5, 10, 15, 20 and 25 MHz should not include anything other than the standard-frequency and time signal emissions, thus permitting their use for reception in radioastronomy. (Observations in a single band 50 to 100 kHz in width and in the vicinity of 15 MHz would be technically preferable for radioastronomy) ;
5. that consideration be given to securing improvement in the international protection of the series of frequency bands above 30 MHz, now available to the radioastronomy service, in accordance with the Final Acts of the Extraordinary Administrative Radio Conference, Geneva, 1963 ;
6. that Administrations, in seeking to afford protection to particular radioastronomical observa tions, should take all practical steps to reduce, to the absolute minimum amplitude, harmonic radiations and other spurious emissions falling within the band of the frequencies to be pro tected for radioastronomy.
L. 9 : Radar astronomy
No Recommendations in this sub-section PAGE INTENTIONALLY LEFT BLANK
PAGE LAISSEE EN BLANC INTENTIONNELLEMENT — 231 — Rep. 204-1
REPORTS OF SECTION L (SPACE SYSTEMS AND RADIOASTRONOMY)
L. 1 : General
REPORT 204-1 *
TERMS AND DEFINITIONS RELATING TO SPACE RADIOCOMMUNICATIONS
(1963 — 1966)
Terms and definitions concerning space systems, services and stations are included in the Final Acts of the Extraordinary Administrative Radio Conference, Geneva, 1963 (Annex 1, Section II A) and are not reproduced in this Report.
Spacecraft ** The term “spacecraft” should be used to refer to any type of space vehicle, including an earth satellite or deep-space probe, whether manned or unmanned, and also those rockets and high-altitude balloons which penetrate the outer atmosphere and into space.
Deep space Space at distances from the Earth approximately equal to or greater than the distance between the Earth and the Moon.
Space probe A spacecraft designed to escape the major portion of the gravitational field of the Earth and follow a trajectory which will enable it to perform a specific function in space.
Satellite A body rotating about another body (such as a planet) and having a motion primarily and permanently determined by this body’s force of attraction; by extension, a natural satellite of a planet may itself have a satellite. Note. — A body so defined which rotates about the Sun is called a planet or planetoid.
Primary body The body which primarily determines the motion of a satellite.
Orbit ** The path in space described by the centre of mass of a satellite or other object in space, sub jected to natural forces only, such as the gravitational attraction of the primary body, the action of other bodies, radiation pressure and atmospheric drag.
* This Report was adopted unanimously. ** The definitions of the terms marked with two asterisks differ from those existing in the Final Acts of the Space Extraordinary Administrative Radio Conference, Geneva, 1963 (Annex 1, Section II B). Rep. 204-1 — 232 —
Unperturbed orbit (of a satellite) The orbit of a satellite in the idealized condition in which it is subjected only to the attraction of the primary body, effectively concentrated at its centre of mass ; it is a conic section with the centre of mass of the primary body at a focus.
Orbital elements (of a satellite or other object in space) The parameters by which the shape, dimensions and position of the orbit of a body in space, and the position in time of the body in the orbit with respect to a reference point, can be defined in relation to a specified system of reference.
Orbital plane The plane containing the radius vector and the velocity vector of a satellite, the system of reference being that specified for defining the orbital elements. Note. — In the idealized case of the unperturbed orbit, the orbital plane is fixed relative to the primary body.
Direct orbit (of a satellite) The orbit of a satellite in which the projection of the centre of mass of the satellite on the primary plane of the system of reference turns in a direct sense. Note. — By convention, the direct sense is counter-clockwise when viewed from the north pole of the primary body, which has also been defined in a conventional way. For an earth satellite, the direct sense is that of the rotation of the Earth.
Retrograde orbit (of a satellite) The orbit of a satellite in which the projection of the centre of mass of the satellite on the primary plane of the system of reference turns in a retrograde sense.
(Angle of) Inclination of an orbit (of a satellite) ** The angle between the plane of an orbit and the primary plane of the system of reference ; for a satellite near to the primary body, the equatorial plane of that body is usually taken as the primary plane ; by convention, the angle of inclination of a direct orbit is an acute angle, and the angle of inclination of a retrograde orbit is an obtuse angle.
Ascending (descending) node The point where the orbit of a satellite or planet intersects the primary plane,‘which contains two of the coordinates in the system of reference, the third coordinate of the satellite or planet increasing (decreasing) when passing through that point; for an earth satellite, the primary plane of the system of reference is the equatorial plane, and the third coordinate is increasing from south to north.
Circular orbit (of a satellite) An orbit of a satellite in which the distance between the centres of mass of the satellite and of the primary body is constant.
Elliptical orbit (of a satellite) An orbit of a satellite in which the distance between the centres of mass of the satellite and of the primary body is not constant.
Equatorial orbit (of a satellite) An orbit of a satellite, the plane of which coincides with that of the equator of the primary body.
** See page 231. — 233 — Rep. 204-1
Inclined orbit (of a satellite) An orbit of a satellite which is not equatorial.
Polar orbit (of a satellite) An inclined orbit with an inclination of 90°. The plane of a polar orbit contains the polar axis of the primary body.
Apoastron The point in the orbit of a satellite or planet which is at a maximum distance from the centre of mass of the primary body.
Periastron The point in the orbit of a satellite or planet which is at a minimum distance from the centre of mass of the primary body.
Apogee ** The point on the orbit of an earth satellite which is at a maximum distance from the centre of the Earth ; the apogee is the apoastron of an earth satellite.
Perigee ** The point on the orbit of an earth satellite which is at a minimum distance from the centre of the Earth ; the perigee is the periastron of an earth satellite.
Altitude of the apogee (perigee) ** The altitude of the apogee (perigee) above a specified reference surface serving to represent the surface of the Earth.
Period (anomalistic) ** The time elapsing between two successive passages of a satellite or planet to its periastron.
Nodal period The period of time elapsing between two consecutive passages of a satellite or planet through the ascending node.
Phased satellite
Station keeping satellite (deprecated) A satellite, the centre of mass of which is maintained in a desired relation relative to other satellites, to a point on the Earth or to some other point of reference such as the sub-solar point. Note. — If it is necessary to identify those satellites that are not phased satellites, the term “un phased satellites” may be used. The term “random satellite” is deprecated.
Attitude-stabilized satellite A satellite with at least one axis maintained in a specified direction, e.g. toward the centre o the Earth, the Sun or a specified point in space.
** See page 231. Rep. 204-1, 205-1 — 234 —
Synchronous satellite A satellite, for which the mean sidereal period of revolution about the primary body is equal to the sidereal period of rotation of the primary body about its own axis. Note. — A synchronous earth satellite must be synchronized to the sidereal period of the Earth or length of the mean sidereal day, which is about 23 hours 56 minutes.
Sub-synchronous satellite A satellite, for which the sidereal period of rotation of the primary body about its own axis is an integral multiple of the mean sidereal period of revolution of the satellite about the primary body.
Stationary satellite ** A synchronous satellite with an equatorial, circular and direct orbit. A stationary satellite remains fixed in relation to the surface of the primary body.
REPORT 205-1 *
FACTORS AFFECTING THE SELECTION OF FREQUENCIES FOR TELECOMMUNICATIONS WITH AND BETWEEN SPACECRAFT
(Questions 2/IV, 4/IV, 12/IV, 8/IV, 9/IV, 10/IV and 7/VI, Study Programmes 5C/V and 19A/VI)
(1959 — 1963 — 1966) 1. Introduction The purpose of this Report is to summarize the relationships between frequency and radio propagation and other technical factors which influence radiocommunication in space, to determine a basis for the selection of frequencies for communication between the Earth and a spacecraft, or for communication between spacecraft. Section 2 considers, in a general manner, the propagation factors affecting communi cation with spacecraft: §§ 3 and 4 apply this information to typical systems of communication with and between spacecraft respectively, and to examine the dependence of the signal and noise levels on frequency.
2. Factors affecting communication with spacecraft Rapid advances in the use of spacecraft and expanded demands for communication has intensified the requirements for space communications. Usually, only modest transmitter powers are available in the spacecraft and, therefore, careful engineering of circuits for space craft is necessary, paying particular attention to the selection of radio frequencies. The basis of selection of optimum frequencies may b e: the available signal-to-noise ratio for a given transmitter power ; the minimum probability of interference ; or other considerations. The discussion in this section considers, as its first objective, the way in which propa gation effects control the signal strength and the noise level. However, it is recognized that the signal-to-noise ratio may not be the sole criterion of frequency selection, and some com ments are made on refraction effects and other factors which may disturb the character of the signal and cause problems in location and tracking.
* This Report was adopted unanimously. ** See page 231. — 235 — Rep. 205-1
2.1 Radio “windows’’
All communication between the Earth and spacecraft must pass through the earth’s atmosphere, including the troposphere and ionosphere. Communication between space craft will usually involve radio paths outside the influence of the lower atmosphere of the earth.
The atmosphere is frequency selective, allowing some frequencies to pass through readily while severely attenuating others. A range of frequencies, in which waves penetrate the atmosphere readily, is often called a “window”.
Two principal ranges of frequencies pass readily through the atmosphere [1], They are : — the range between ionospheric critical frequencies and frequencies absorbed by rain fall and atmospheric gases (about 10 MHz to 20 G H z); — the combined visual and infra-red ranges.
The atmosphere is known to be partially transparent in a third range of frequencies below the gyrofrequency [2]. Such radio waves propagate through the ionosphere by the extraor dinary mode of propagation. Since the bandwidths required for most space applications are not available in this frequency range, it is not considered in this Report.
The range 10 MHz to 20 GHz is of more immediate interest, particularly to the C.C.I.R., for the purposes of communication with spacecraft. The upper practical limit of this range may be below 10 GHz during heavy rainfall, and the lower limit may be above 70 MHz de pending upon the degree of solar activity, the geographical position of the earth station and the geometry of the signal path. On the other hand, the “window” may extend from as low as 2 MHz for polar locations during the night [3], to as high as 50 GHz at locations at high altitude free from rain. Fig. 1 illustrates these general frequency limits.
Although the lower frequency-limit of the window is essentially limited by ionospheric influences, the use of frequencies near the lower limit of the window may be restricted by noise from extra-terrestrial or atmospheric sources. Interference problems may also be more difficult at these frequencies.
At higher frequencies, the main factors influencing the signal-to-noise ratio are: — the signal power available under free-space propagation conditions, — the absorption in the atmosphere, — radiation from natural extra-terrestrial sources, — thermal noise radiation arising from atmospheric absorption, and also from the Earth via si de-lobes in the directivity pattern of the antenna.
2.2 Free-space propagation
Although great distances are involved, the propagation medium in space is essentially transparent to radio waves over a wide range of frequencies and performance estimates for this range may be based on free-space propagation. The dependence on frequency of the input power to a receiver under free-space propagation conditions depends upon the type of antenna at the transmitter and the receiver. This dependence is shown by the following free-space propagation formula :
Pr = (A/4tt)2 (PtGtG jd 2) = (c/4tt)2 (.PtGtGrlf*d2) Rep. 205-1 — 236 —
where Pr — available power from the receiving antenna, P, = radiated power, A = wavelength in free-space, / = frequency, d — distance between transmitter and receiver, G, = power gain of the transmitting antenna, relative to an isotropic antenna, G> = power gain of the receiving antenna, relative to an isotropic antenna, c = velocity of light in free-space.
If both the transmitting and receiving terminals of a free-space communication link use omnidirectional antennae or if the beamwidths at both terminals are fixed:
Pr is proportional to Pt/f2d2
so that the received power increases as the frequency is decreased.
If one terminal of a free-space communication link uses a directional antenna of fixed physical size and can operate with narrower and narrower beamwidths as the frequency is increased, while the other terminal uses an omnidirectional antenna or an antenna of fixed beamwidth, e.g. a directional antenna on the earth’s surface (Gt = Const. f 2)* and an omni directional antenna on a spacecraft:
Pr is proportional to Pt/d2
so that the received power is independent of frequency.
If both terminals of a free-space communication link use directional antennae of fixed physical size and can operate with narrower and narrower beamwidths as the frequency is increased, e.g. a directional antenna on the earth’s surface and a directional antenna on a more elaborate spacecraft (Gt = Const. / 2 and Gr = Const, f 2) * :
Pr is proportional to Ptf 2(d2
so that the received power increases as the frequency is increased.
Fig. 2 illustrates the power available for free-space propagation, when both the antenna at the earth station and the antenna at the space station have maximum physical size and minimum limitations on beamwidth. Within the “radio window”, maximum available power depends on the operational requirements and antenna design which usually establish the physical size of the antenna and the limitations on beamwidth.
2.3 Signal attenuation Actual propagation conditions may vary substantially from free-space conditions, par ticularly at frequencies near the edge of the radio “window”, and it is necessary to correct for tropospheric and ionospheric effects to obtain a true estimate of the dependence on fre quency. For higher frequencies, this correction is primarily to take account of attenuation due to atmospheric gases, clouds and rainfall.
* In the direction of maximum antenna-gain. — 237 — Rep. 205-1
Fig. 3 indicates the attenuation of the signal for a vertical path in a clear atmosphere as a function of frequency. The curves are based on a theoretical estimate for an atmos phere typical of Washington, D.C. in August [4], and illustrates the frequency-dependence expected in a moderately humid area. Additional theoretical and experimental work is necessary to determine atmospheric absorption completely. The attenuation, when the path is not vertical, will be increased by a factor independent of frequency [5]; approximate values of this factor are given in Table I for various angles of elevation.
T a b l e I
Effect o f the angle of elevation on atmospheric absorption
Angle from the horizontal (degrees) 0. l 2 3 4 5 7-5 10 30 90
Factor, by which the attenuation (in dB) for a vertical path must be multiplied 80 40 25 18 14 11 7-5 5-5 2 1
Fig. 4 shows the attenuation of the signal per 1 km of path due to rainfall, and to cloud and fog [6], [7]. Estimation of absorption due to rainfall is complicated by variation of drop- size distributions for the same rainfall rate and by turbulence, which may produce a different water content in the air than indicated by surface measurement. Fig. 4 applies to a typical drop-size distribution in steady rainfall. It must, however, be remembered that the effective antenna noise temperature increases during rainfall; also, changes of antenna impedance during heavy rainfall may further degrade the receiving conditions. Estimation of the percentage of time for which a given attenuation occurs requires statistical information on the horizontal and vertical extent of rainstorms, and on cloud thickness and coverage. For example, an estimate based on statistics for Southern England for cloud [8] and rain, making assumptions concerning the dimensions of rainstorms, shows that the attenuation exceeded for 1% of the time is about 0-25 dB due to rain and about 0-7 dB due to cloud, for 5° elevation and a frequency of 4 GHz. Near the low-frequency edge of the window the absorption in the ionospheric D-, E- and F-regions increases with decreasing frequency. For most terrestrial locations, even at 20 MHz, this influence will rarely produce an attenuation greater than 1 dB at vertical incidence or 6 dB at low angles of elevation. However, much larger values have been observed at high latitudes in the presence of aurorae, in particular during short-lived drop-outs [9]. The low-frequency edge of the window is dependent on time and varies according to the local and regional ionospheric characteristics. Complicated ionospheric refraction and reflection phenomena can produce focusing and defocusing phenomena, in particular under conditions of scintillation (see § 2.5) and near the frequency limit of reflection [10].
2.4 Refraction phenomena A radio ray passing through the lower atmosphere is bent by an amount depending on the angle of incidence and the thickness of the atmosphere traversed. The effect is largely independent of frequency, and its magnitude greatest at small angles of elevation. The Rep. 205-1 — 238 —
magnitude of the bending is significant only for very small angles of elevation. Fig. 5 shows the average amount of bending versus apparent angle of arrival for two extreme types of atmosphere [11]. It is clear that the error in angle of arrival is very small for angles of elevation greater than 5°, although it can be of the same order as the antenna beamwidths which are likely to be used for communication-satellite systems. Ionospheric refraction also increases the apparent elevation, but it is important only near the low frequency edge of the window and decreases considerably with increasing fre quency. It is essentially given by the Sellmeier dispersion formula [10]. The effect varies greatly according to the time-variations of the electron density and also with regional varia tions, the highest values being found in tropical regions. Anomalous effects occur frequently near the lower frequency limit of the window [10]. Ionospheric ducting may occur at alti tudes below roughly 500 km and may be important at frequencies up to several hundred MHz [12],
2.5 Scintillation and scatter
Scintillation, i.e. .fluctuation with time in the amplitude and direction of arrival of the signal, occurs when inhomogeneities in the refractive index are present which vary in time. The time variation results from motions of these inhomogeneities in the atmosphere or from motion of the spacecraft. Under these conditions there may be a lack of phase coherence, so that typical phase effects (see § 2.6) cease to be well defined. Large aperture antennae will not realise their full plane wave gain under these circumstances, so that an apparent attenuation is observed. Tropospheric scintillation occurs mainly at very low angles of elevation. Although measurements show that the effect may not be negligible with high directivity antennae [13] even at higher angles of elevation, experience has shown that, with a typical earth-station antenna, fluctuations in signal strength are rarely important for angles of elevation greater than 3°. Theoretical studies referred to in Doc. IV/56 (Federal Republic of Germany), 1963- 1966, indicate that tropospheric scintillation would set an upper limit to the possible gain of an earth-station antenna which is of the order of 80 dB at an angle of elevation of 5°. Ionospheric scintillations have been observed quite frequently related to auroral phe nomena [10], [14]. Such observations have been reported on frequencies up to 1 GHz [15]. Scattering of energy over a wide range of angles may also occur. Although the atte nuation and distortion of a signal resulting from this is generally quite negligible, the frac tion of energy scattered may sometimes be a cause of interference.
2.6 Phase effects
A Doppler frequency-shift is produced if the path length of a radio link varies with time, giving a continuous change of phase. The received frequency/' is increased relative to the transmitted frequency / for a decreasing path length, and is decreased for an increasing path length. In the simplest case of free-space propagation the formula is
A /// = ( / ' —/ ) / / = — vr/c
where vr is the rate of change of path length and c is the velocity of light. The frequency shift A /is a constant percentage of the transmitted frequency, and therefore increases with increasing frequency. For a low-altitude satellite, the fractional change has a maximum value of up to 2xl0~5. The change is, however, greater for rockets and for satellites on take off and re-entry and, at times, also for space probes. — 239 — Rep. 205-1
The Doppler effect gives frequency variations during the passage of a satellite which must be allowed for in considering frequency allocations for earth-space systems. Second- order difficulties may arise from the fact that the absolute frequency shift is not exactly cons tant throughout the frequency band occupied by the transmission.
Tropospheric refraction has only a negligible influence on Doppler shift. However, the ionospheric influence cannot be neglected near the lower frequency limit of the window, up to a few hundred MHz. This ionospheric influence is, again, variable with time and location. Considerable effects have been observed at frequencies below 100 MHz [10].
The Faraday effect appears as a consequence of double refraction in the ionosphere in the presence of the magnetic field of the earth. This gives rise to a rotation of the plane of polarization of a linearly polarized plane wave. The effect depends on the electron density and on the strength and direction of the magnetic field. The total number of rotations is often several hundred in the vicinity of the lower frequency edge of the window. The Faraday rotation may occasionally reach a value as high as 150° at 1 GHz [16].
2.7 Noise
To determine optimum frequencies, the variation of background radio noise within the radio window must also be considered. This noise arises by radiation from natural terrestrial and extra-terrestrial sources, man-made noise, and by radiation from the absorb ing atmosphere around the earth, and may be received in both the main and subsidiary lobes of the antenna. Radio noise is frequently described in terms of an effective antenna tem perature.
Of the natural sources cosmic noise predominates at the lower edge of the radio window. Toward the upper edge, noise due to water vapour and oxygen absorption in the atmosphere becomes increasingly important.
There are three types of cosmic noise, namely a background radiation (mainly from the galaxy), radiation from point sources, and solar noise. The magnitude of the background cosmic noise decreases as the frequency is increased and becomes negligible in comparison with the receiver noise at frequencies above about 1 GHz [17]. The point sources are of very small angular width and are rarely intercepted by an earth-station antenna.
The Sun is also of small angular width and is rarely intercepted by an antenna beam. However, its apparent noise temperature may be very high and the noise received via side- lobes may be significant in some cases. Solar noise is dependent on the frequency and on the degree of solar activity. The apparent noise temperature of the quiet sun varies from almost 1 000 000° in the VHF band to about 10 000° at 10 GHz. It also varies with the sunspot cycle ; for example, at 4 GHz it varies from 23 000° at sunspot minimum to 90 000° at sunspot maxi mum [25], Occasional short bursts associated with particular events on the Sun may, at the lower frequencies, give a peak noise power many orders of magnitude greater than that from the quiet sun but, as the frequency rises, the relative power of the bursts becomes much less. At sunspot maximum, when bursts are most frequent, the noise temperature exceeded for 1% of the time is only about 50% above the quiet-sun value for a frequency of 4 GHz. The amount of solar noise received by an antenna clearly depends on the apparent temperature of the Sun, and on the fraction of the antenna beam intercepted by the Sun.
In addition to noise from extra-terrestrial sources, noise also originates in the earth’s atmosphere. When viewed through an absorbing region at a temperature Tlt the effective sky temperature due to a source at a temperature T0, which fills the antenna beam, is given by
(1 - a) Tx + a T0 Rep. 205-1 — 240 —
where cc, the power transmission factor of the absorbing region, is related to the attenuation L (dB), by the formula L= 101og10(l/cc)
Fig. 6 shows the apparent sky noise temperature due to background cosmic noise and to atmospheric absorption as a function of the angle of elevation for the frequency range from 100 MHz to 40 GHz. This graph is for typical summer conditions but does not include the effects of cloud and rain. The two curves for cosmic noise correspond to the limiting cases with the antenna directed toward the maximum noise radiation, near the galactic centre, and with the antenna directed toward the minimum noise radiation. In estimating antenna temperatures, the antenna pattern and radiation from the earth’s surface must also be con sidered.
The noise due to absorption in cloud and rain may be calculated from the formula given above, if the temperature and rate of attenuation and their distribution along the path are known. During heavy rain or dense fog, antenna temperatures at the highest frequencies considered here will tend to approach the temperature of the atmosphere.
3. Communication with spacecraft
Fig. 7 combines some of the more important technical factors (on the basis of [4, 18 to 24]) to illustrate the general frequency dependence of available signal and noise powers in a simple satellite system, namely an unstabilized satellite with an isotropic antenna 1000 km from an earth station which is located at sea level. The data refer to the link from space to earth which is thought to be generally the weaker one. It is assumed that the dia meter of the antenna at the earth station is limited to 20 m and the minimum beamwidth is 0-2°. Under these conditions, the available signal power remains constant under clear atmospheric conditions up to the frequency at which the beamwidth limitation is reached (about 5 GHz). At higher frequencies the limiting beamwidth is used and there is a decrease in available signal power. There is also increased atmospheric absorption at the higher frequencies.
The noise power is estimated for a parabolic antenna in a moderately humid sea-level location (Washington D.C., August) and includes earth and solar-noise contributions from typical side lobes of a simple parabolic antenna. Careful design for side-lobe suppression will result in somewhat lower noise values. The same general frequency dependence holds for any fixed beamwidth antenna on the spacecraft.
Fig. 8 illustrates the relationship between the available signal and noise powers and the frequency in a more elaborate communication satellite system, namely a stabilized stationary satellite using an earth oriented directional antennae. For the circuit parameters assumed, the available carrier power increases with frequency until the beamwidth limitation of the satellite antenna is reached (20° beamwidth at about 2 GHz for a paraboloid 0-5 m in dia meter) ; the available signal power is then essentially constant until the beamwidth limitation of the earth station is reached (0-2° beamwidth at about 6 GHz for a paraboloid 18 m in dia meter). Beyond this frequency there is a decrease in available power as frequency is increased. Fig. 9 illustrates the relationship between available signal and noise and the frequency in an elaborate stationary satellite system using very directive antennae with an elevated earth station in a moderately humid area (Washington D.C., August) and also in this area during moderate rainfall. Fig. 10 illustrates the typical service area of a stationary satellite as a function of the angle of elevation for reception. Fig. 11 shows the variation in service area at various altitudes of the satellite. — 241 — Rep. 205-1
4. Communication between spacecraft
The choice of frequencies for radio links between spacecraft which is considered in this section, is determined largely be limitations on the spacecraft antennae as determined by the spacecraft mission, and by cosmic noise. When using simple omnidirectional antenna, solar noise is an important limiting factor. Three configurations are considered. The simplest uses omnidirectional antennae on both spacecraft. The second has directional antennae on one of the spacecraft and the most advanced has directional antennae on both.
For spacecraft to spacecraft communications, the controlling factors on system noise are cosmic and receiver noise. Cosmic noise decreases somewhat more rapidly than the square of the frequency as shown in Fig. 6. If the spacecraft are in the vicinity of the Moon, the Earth, or the other planets, the temperature and masking effects of these bodies must be included.
In calculating received power-levels for these systems, use can be made of the formulae in § 2.2 of this Report.
It is obvious that spacecraft having directional antennae will require high angular sta bility, so angular motions are small compared with the antenna beamwidth. Also, depending on the difficulty of search, acquisition and tracking of the signal being received, the beam width of the antenna may have to be larger than that determined only by spacecraft stability and maximum permissible antenna area. Thus, both the gain and antenna area may be limited. Under these conditions, a particular transition frequency f t, above which the system gain is limited by the minimum acceptable beamwidth and below which it is limited by the maximum area of the antenna, may be defined by the following relation :
A2 = C2jf2 = 4lrAmlGm
where Am = maximum permissible antenna area; Gm — maximum permissible antenna gain, relative to an isotropic antenna; A, = free-space wavelength corresponding to the transition frequency.
The transition frequency, f t, is the one frequency at which the maximum permissible values of both antenna area and antenna gain may be used. For the particular case of omnidirectional antennae on both spacecraft, the frequency variation of signal-to-noise ratio for receiver noise temperatures of 300°K and 1500°K is shown in Fig. 12. It is seen that for this configuration, there is a broad optimum operating region extending from about 10 MHz to 150 MHz or more, the low-frequency limit being determined by cosmic noise [18]. Much lower spacecraft receiver noise temperatures than those considered may eventually be achieved.
Similar curves are shown in Fig. 13 for a directional antenna on one of the spacecraft and an omnidirectional antenna on the other. The optimum frequency region depends on the transition frequency of the directional antenna. Above this frequency, the received signal decreases as the square of the frequency because of the beamwidth limitation. Below this frequency, the curves are controlled as before by the receiver noise temperature and the frequency variation of cosmic noise. It is seen from the diagram that when the transition frequency is very high, the optimum frequency region tends to broaden considerably.
For similar directional antennae on both spacecraft, the received power will decrease on each side of the transition frequency as the square of the frequency, if one disregards noise of cosmic origin. For systems presently envisaged, the optimum frequencies may lie any where in a range between about 1 and 30 GHz depending on maximum permissible antenna areas and antenna gains. Rep. 205-1 — 242 —
5. Summary
Communication between earth and space is possible within two broad frequency bands ; about 10 MHz to 20 GHz and in the infra-red and optical regions. With the current state of development, the lower frequency band is of more immediate interest, particularly to the C.C.I.R. and has been considered in this Report. The upper limit of the band considered is dependent upon tropospheric conditions and the lower limit depends upon ionospheric conditions and cosmic noise. The band, therefore, is not sharply defined, but is dependent upon geographical location and time of operation. Within the band, the optimum frequency will depend upon the specific communication system. For communication with spacecraft, when the spacecraft has an omnidirectional antenna, the available signal and noise powers are essentially constant over a broad frequency range. The practical upper frequency limit depends upon the minimum beamwidth which will permit acquisition and tracking at the earth station, as well as upon atmospheric conditions. For communication systems using unstabilized spacecraft, the available signal-to-noise ratio is constant over a broad frequency range, with the practical upper frequency limit depend ent upon maximum antenna size and minimum beamwidth limitations at the earth station. As systems become more refined through stabilized spacecraft and the ability to use narrow-beam antennae, the upper frequency limit increases, and may extend to above 15 GHz for more elaborate systems if reception is not required at very low angles. Optimum frequencies for communication between spacecraft depend upon whether omnidirectional or highly directional antennae are necessary to provide the service required. For omnidirectional antennae on both spacecraft, there is a broad optimum frequency region between about 10 MHz and 150 MHz [1]. For a directional antenna in one spacecraft, the desirable region lies between about 300 MHz and 3 GHz, and for directional antennae on both spacecraft between about 1 GHz and 30 GHz. These values may, however, be different if the effect of solar noise is important.
B ibliography
1. H a y d o n , G. W. Optimum frequencies for outer space communication. J. Research NBS, 64D, 105-109 (March-April, 1960).
2. H e l l iw e l l, R. A. and M o r g a n , M. G. Atmospheric whistlers. Proc. IRE, 47, 200 (1959).
3. Z a c h a r isen , D. H. World maps of F2 critical frequencies and maximum usable frequency factors. NBS Technical Note No. 2 (April, 1959).
4. B ea n , B. R. and A bbo tt, R. Oxygen and water vapour absorption of radio waves in the atmosphere. Geofisica pura e applicata, Milano, 37, 127-144 (1957).
5. P er l m a n , S., K elley , L. C., R u ssell, W. T. Jr. and St u a r t , W. D. Concerning optimum fre quencies for space communication. IRE, CS-7, 167 (September, 1959).
6. K e r r , D. E. Propagation o f short radio waves. McGraw-Hill Book Co., Inc. New York, N .Y. (1952).
7. Sa x to n , J. and H o p k in s . H. S. Proc. I.E.E., 98, Pt. Ill, 26 (1951).
8. J ones. Journal Royal Meteorological Society, 65 (January, 1961).
9. R a w e r , K . Effect of propagation in space communications. U.R.S.I. Symposium on Space Com munication Research, Paris (1961). — 243 — Rep. 205-1
10. R a w e r , K. Propagation problems with space radio communications. Journal o f Research, NBS Section D (1962).
11. Sz u l k in . Average radio-ray refraction in the lower atmosphere. Proc. IRE, 40 (5), 554 (1952).
12. W oyk (C hvojkova), E. The refraction of radio waves by a spherical ionized layer. Journal Atmo. Terr. Phys., 16, 124-135 (1959).
13. N o r t o n , K. A. et al. An experimental study of phase variations of line-of-sight microwave trans missions. National Bureau o f Standards (U.S.A.), Monograph No. 33 (1961).
14. A a ron s, J. et al. Atmospheric phenomena noted in simultaneous observations of 1958. Planetary and space science, 5, 169-184 (1961). 15. Ko, H. G. Amplitude scintillation of a radio star at ultra high frequency. Proc. IRE, 48, 1871- 1880 (1960).
16. In g a lls, R. P. A study o f UHF space communication through an aurora using the Moon as a reflector, Planetary and Space Science, 1, 272-280 (1961).
17. P ierce and K o m pfn e r . Transoceanic communication by means of satellites. Proc. IRE, 47, 372 (1959). 18. Radioastronomy issue. Proc. IRE, 46, No. 1 (January, 1958).
19. L a it in en , P. O. and H a y d o n , G. W. Analysis and predictions of sky-wave field intensity. Radio Propagation Agency Technical Report No. 9 (August, 1950), U.S. Army Signal Radio Propagation Agency, Fort Monmouth, N.J.
20. N o r t o n , K. A. Maximum range of a radar set. Proc. IRE (January, 1947).
21. Sm it h , G. O. Radio communications across space, ship to ship, and ship to planet. Journal o f the British Interplanetary Society, Vol. 12, 1, 13 (1953).
22. C a st ru c c io , P. A. Communications and navigation techniques of interplanetary travel. IRE Transac tions on aeronautical and navigational electronics (December, 1957).
23. W h eelo n , A. D. Mid-course guidance techniques for space vehicles. National conference of aero nautical electronics (1958).
24. M e n g e l, J. T. Tracking the earth satellite and data transmission by radio. Proc. IRE, 44, 755-760 (June, 1956). 25. Solar-Geophysical Data (Monthly), C.R.P.L., Boulder (Col.), U.S.A. B ' L J H F E : A M K : D G C e.251 24 — 244 — Rep.205-1 : Noise due to absorption in a clear atmosphere, assuming an elevation angle of 5°. of angle elevation an assuming atmosphere, clear a in absorption to due Noise : : Typical signal level in heavy rain (16 mm/h), vertical depth 1 km, assuming an elevation angle of 5°. of angle elevation an km, 1 assuming depth vertical mm/h), (16 rain heavy in level signal Typical : fet finshrc absorption. ionospheric of : Effect 5°. of angle elevation an of assuming conditions day-time during level signal : Typical emit o aaood ewe hl-oe points. half-power between paraboloid of : Beamwidth Mnmmcsi os. aiu aus ilb on o ehge b aot 5 dB. 15 about by higher be to found be will values Maximum noise. cosmic Minimum : : Minimum frequency to assure penetration of earth’s ionosphere : polar region-oblique path ;tropical path region-oblique :polar ionosphere earth’s of penetration assure to frequency Minimum : : Typical signal level for a vertical path in a clear atmosphere. clear a in path vertical a for level signal Typical : : Noise level corresponding to a temperature of 70°K. of temperature a to corresponding level Noise : Efc o ayn amshrc odtos n eeain angles. elevation and conditions atmospheric varying of Effect : Available power (dB below 1W) Saerf: stoi atna tasitr oe, W bnwdh 1 H, itne 00 km) 1000 distance kHz, 1 bandwidth, 1W, power, transmitter antenna, isotropic (Spacecraft: hr ilsrtn te eea rqec lmt i a ipe at t saerf cmuiain system communication spacecraft to earth simple a in limits frequency general the illustrating Chart region-vertical path. region-vertical Sga ee drn da ih-ie odtos n absorption). (no conditions night-time ideal during level Signal Mnmm rqec t asr pntain ferhs oopee:toia einolqe path. : tropical ionosphere region-oblique earth’s of penetration assure to frequency Minimum aaood daee, 0 , fiiny 55% efficiency, m, 20 diameter, Paraboloid, 15 dB gain above isotropic antenna antenna isotropic above gain dB 15 H Feuny GHz Frequency MHz at tto nen : antenna station Earth F gure r u ig 1 ------
— 245 — Rep. 205-1
Frequency (GHz)
F ig u r e 2 Variation of the available power, dictated by the physical size of antennae which have a fixed minimum beamwidth requirement, as a function o f frequency (1W - Free space — 1000 km) Minimum beamwidth: earth-station antenna 0-1° spacecraft antenna 1 ° A B Earth-station antenna diameter limited to (m) 100 20 Spacecraft antenna diameter limited to (m) 3 1
Frequency (GHz)
F ig u r e 3 Theoretical one-way attenuation for a vertical path in a moderately humid area (Washington DC in August) A : at sea-level B : at 2 km above sea-level Rep. 205-1 — 246 —
F ig u r e 4
Attenuation due to precipitation
Attenuation in rainfall of intensity : A : 0-25 mm/hr (drizzle) B : 1 mm/hr (light rain) C : 4 mm/hr (moderate rain) D: 16 mm/hr (heavy rain) E : 100 mm/hr (very heavy rain)
Attenuation in fog or cloud : F : 0-032 gm/m3 (visibility > 600 m) G : 0-32 gm/m3 (visibility about 120 m) H: 2-3 gm/m3 (visibility about 30 m) Error (degrees) Error in angle of elevation due to tropospheric refraction tropospheric to due elevation of angle in Error A B : tropical maritime air maritime tropical : : polar continental air continental polar : 27 — 247 — F gure r u ig 5
Rep. 205-1 Rep. Rep. 205-1 — 248 —
Frequency (GHz)
Figure 6 Apparent sky temperature due to background cosmic noise and atmospheric absorption for the angles o f elevation indicated on the curves
A : maximum cosmic noise B : minimum cosmic noise 5 Available power (I) or noise (II) (dB below 1W) Available signal, “heavy” rainfall, 1 km in depth in km 1 rainfall, “heavy” signal, - Available — Available signal, atmosphere typical of Washington D.C. in August in D.C. Washington of typical atmosphere signal, Available m 20 diameter, maximum : antenna station Earth — adit: 0 MHz 10 antenna earth-station the of elevation of angle :A A Bandwidth: pccatatna: isotropic : antenna Spacecraft : typical maser noise maser typical : Theoretical signals and noise in a simple space radiocommunication system radiocommunication space simple a in noise and signals Theoretical Unstabilized satellite, 1000 km, 1W, earth terminal at sea-level at terminal earth 1W, km, 1000 satellite, Unstabilized minimum beamwidth 0-2° beamwidth minimum Frequency (GHz) Frequency 29 — 249 — F gure r u ig 7
Rep. 205-1Rep. Rep. 205-1Rep. Available power (I) or noise (II) (dB below 1W) ------A m 9 and 18m diameters maximum : antennae Earth-station adit 1 MHz 10 : Bandwidth m 5 diameter maximum : antenna Spacecraft ageo lvto fteerhsain antenna earth-station the of elevation of angle :A : typical maser noise maser typical : Stabilized stationary, earth oriented, satellite, 2 kW, earth station station earth kW, 2 satellite, oriented, earth stationary, Stabilized Theoretical signals and noise in a in noise and signals Theoretical Available signal atmosphere typical of Washington D.C. in August in D.C. Washington of typical atmosphere signal Available vial inl “ev”rifl, k n depth in km 1 rainfall, “heavy” signal, Available
iiu emit 22° beamwidth minimum minimum beamwidth 0-2° beamwidth minimum Frequency (GHz) Frequency 20 — 250 — “ F typical igure 8 ” space radiocommunication system radiocommunication space
at sea-level
Available power (I) or noise (II) (dB below 1W) Stationary highly stabilized, earth oriented, satellite, 1W, earth station 2 km above sea-level above km 2 station earth 1W, satellite, oriented, earth stabilized, highly Stationary A ------adit 10 MHz 100 : Bandwidth ageo lvto o h at-tto nen 4 antenna earth-station the of elevation of angle :A pccatatna: aiu daee, m 3 diameter, maximum : antenna Spacecraft m 20 diameter, maximum : antenna Earth-station : typical maser noise maser typical : Theoretical signals and noise in a more elaborate space radio radio system space communication elaborate more a in noise and signals Theoretical Available signal, atmosphere typical of Washington D.C. in August in D.C. Washington of typical atmosphere signal, Available Available, “moderate” rainfall, 1 km in depth in km 1 rainfall, “moderate” Available,
minimum beamwidth 0-2° beamwidth minimum 0-05° beamwidth, minimum Frequency (GHz) Frequency 21 — 251 — F gure r u ig 9
Rep. 205-1Rep. F ig u r e 10 Chart showing a typical service area of a stationary satellite system 5 — e. 205-1 Rep. —253
F ig u r e 11 Chart showing the size of the typical service area as a function o f the height o f the satellite Rep. 205-1 Rep.
System signal-to-noise ratio for communication between spacecraft, both with omnidirectional antennae omnidirectional with both spacecraft, between communication for ratio signal-to-noise System Signal-to-noise ratio (dB—arbitrary reference) 0 0 0 50 00 00 10000 5000 1000 500 100 50 10 ------eevrnietmeaue 300°K temperature noise Receiver B A : maximum cosmic noise cosmic maximum : : minimum cosmic noise cosmic minimum : Frequency (MHz) Frequency eevrnietmeaue 1500°K temperature noise Receiver 24 — 254 — F gure r u ig 12
System signal-to-noise ratio for communication between spacecraft, one o f which has a directional antenna directional a has which f o one spacecraft, between communication for ratio signal-to-noise System
Signal-to-noise ratio (dB—arbitrary reference) ------A Gm)(Am is, maximum antenna surface (m2) surface antenna maximum : A B : : “ : — - : maximum cosmic noise cosmic maximum : eevrnietmeaue 300°K temperature noise Receiver eevrnietmeaue 1500°K temperature noise Receiver minimum cosmic noise cosmic minimum Frequency (MHz) Frequency F aiu nen gain antenna maximum 25 — 255 — gure r u ig 13 Rep. 205-1 Rep. Rep. 206-1 — 256 —
L. 2 : Communication satellites
REPORT 206-1
TECHNICAL CHARACTERISTICS OF COMMUNICATION-SATELLITE SYSTEMS
General considerations relating to the choice of orbit, satellite and type of system
(Question 2/IY)
(1963 — 1966) 1. Introduction
Various orbits and types of satellite have been proposed for communication-satellite systems; it is the aim of this Report to consider the characteristics, advantages and dis advantages of the various types of satellite system from the technical point of view. It should, however, be recognized that the choice of system or systems will depend not only on the technical factors involved, but also on operational, administrative and economic factors. It is noted that, since the development of communication-satellite systems is necessarily an evolutionary process, and because all operational and transmission requirements may not be met by a single type of system, more than one system or type of system may eventually be incorporated into the world telecommunication network. The problems arising from the combination of communication-satellite systems on a technically compatible basis, so that they may be interconnected with one another and with other transmission systems, are out lined. The use of communication-satellite systems in the world telecommunication network requires consideration of the type and volume of traffic (telephone, telegraph, data, television, etc.) to be accommodated, the routing of the traffic, the probable locations and capacities of earth stations and the overall performance required. Traffic aspects are a matter for the C.C.I.T.T./C.C.I.R. Plan Committee to consider; performance aspects of telegraphy, telephony and data transmission are matters for the C.C.I.T.T. to decide; the CMTT deals with television. These factors, nevertheless, have a marked influence on the design of communication-satellite systems; they determine, for example, the extent of the need for multi-station access, the capacities to be provided in satellites and the frequency requirements. The design of earth stations is also affected. Finally, the present state of developments is reviewed, and an indication is given of matters requiring further study and investigation, with a broad indication of the possible time-scale of future developments.
2. Possible types of orbit
The possible orbits include circular and elliptical orbits in the equatorial and polar planes of the Earth, and orbits inclined at an angle to the equatorial plane.
* This Report was adopted unanimously. — 257 — Rep. 206-1
The heights and periods of interest for communication-satellite systems may be class ified, for convenience, as follows : — low altitude orbits : heights from 1000 to 5000 km ; periods from 2 to 4 hours (approx.); — intermediate-altitude orbits : heights from 5000 to 20 000 km ; periods from 4 to 12 hours (approx.); — synchronous orbit: approximate height 36 000 km ; period 24 hours.
The periods given are for circular orbits, and the heights are relative to the surface of the Earth.
It is assumed, with the synchronous orbit, that the satellites move from West to East, the period of 24 hours being the same as that of the rotation of the Earth on its axis. When such an orbit is in the equatorial plane of the Earth, the satellites are stationary since they appear to be motionless relative to observers on Earth. Synchronous inclined orbits are possible, but the satellites are then not stationary. Sub-synchronous satellites have periods corresponding to an integral fraction of the period of rotation of the Earth; examples are as follows :
Approximate height (km) 20 000 14 000 10 500 Period (hr) 12 8 6
Unphased satellites are non-synchronous satellites, the height, orbital period, and orbital plane of which are established within a reasonable degree of accuracy, but which are not precisely controlled. Hence, there is relative motion, of a predominantly random value, between satellites. Although orbits above the synchronous height of 36 000 km are possible, they have no significant advantages for communication-satellite systems.
Circular orbits in or near the equatorial plane of the Earth would have the advantage that perturbations of the orbits due to the oblateness of the Earth, and perturbations due to the Moon and Sun, are minimized, thus facilitating the use of station-keeping and attitude- stabilized satellites (see §§ 3.2 and 3.3). Satellites in elliptical orbits move relatively slowly near apogee and are thus visible for longer times between certain pairs of suitably placed locations on earth, as compared with circular orbits of the same period; on the other hand, the mutual visibility between other pairs of locations may be less. The elliptical orbit inclined at about 64° to the equator is of interest since the apogee remains at approximately the same latitude. These characteristics of elliptical orbits are of greater advantage for systems of regional, as opposed to world-wide, coverage ; in the latter circumstance, elliptical orbits show no special advantages and are at a disadvantage as regards satellite phasing. In addition, it would be difficult to use earthward-directed antennae efficiently on such satellites.
The higher orbits have the advantage of providing greater coverage of the Earth from each satellite, so that fewer satellites would be required for world-wide coverage. On the other hand, a more powerful rocket launcher would be required to put a given payload into the higher orbits. The use of circular equatorial, or near-equatorial, orbits in the West-to- East direction would enable larger payloads to be put into orbit than in the East-to-West direction, by taking advantage of the rotational velocity of the Earth. ;The use of only the equatorial plane with an equatorial launching site, as opposed to the multiple orbital planes required for polar or inclined orbit systems, would simplify the launching problem, provided that a satisfactory equatorial site is available.
To minimize the risk of damage to solar cells and other solid-state devices due to intense proton and electron radiation in the inner region of the van Allen belt, it may be desirable, with a view to achieving long-life satellites, to avoid orbits passing through altitudes of from Rep. 206-1 — 258 —
about 1500 to 5000 km in and near the equatorial plane, unless suitable radiation-resistant devices or screening methods can be devised.
The choice of orbital height is also important from the aspect of transmission delay over the earth-satellite-earth path (see Reports 214-1 and 383).
3. Possible types of satellite Satellites useful for communication may be classified as passive or active ; stabilized or unstabilized ; and phased or unphased.
3.1 Passive and active satellites Passive satellites may be either natural or made by man.
The only known natural satellite of the Earth is the Moon. Tests over a period of years have shown that, when time delay is not a controlling factor, e.g. telegraphy, the Moon may be used successfully for transmission of information occupying a few kilocycles per second of bandwidth. The transmission losses encountered on an earth-moon-earth circuit are very large. Moreover, such a circuit is not continuously available.
Man-made satellites suitable for use in a passive satellite-communications system involve two principal approaches :
— comparatively large thin-walled structures which are inflatable in space; — orbiting dipoles.
Two passive communication-satellites are now in orbit. Echo I and Echo II. Each is a thin-walled spheroid of aluminized mylar. Echo I is about 31 m in diameter, while Echo II is about 46 m in diameter. In addition to other tests, these satellites have been used to confirm propagation predictions and to demonstrate voice communications. Investigation is also continuing on methods of increasing the ratio of the radio cross-section to the mass, with the objective of improving the usefulness of passive satellites. A “belt” of dipoles has been placed in a suitable orbit. Being resonant, each dipole re-radiates signals which excite it. However, multipath transmission from the various dipoles in the common volume illuminated by the transmitting and receiving antennae, and Doppler- shift effects, place limitations on the use of this technique. In general, passive satellites are less effective than active ones, because the overall trans mission loss on the earth-satellite-earth path is appreciably greater than for active satellites. In view of the high-powered earth-station transmitters and limited overall transmission bandwidths characteristics of passive systems, attention is now being concentrated on active satellites for high-capacity systems.
3.2 Attitude-stabilized satellites An attitude-stabilized satellite is designed to maintain one or more of its axes in a specified direction or directions, e.g. towards the centre of the earth or towards a fixed position in space. The simplest form of attitude stabilization is spin stabilization of the rotational axis. Spin stabilization may be passive (as in t e l s t a r ) or active (as in s y n c o m ) . In the latter case, pulsed gas jets can be used to correct both the spin axis and perturbations of the orbit. With active spin stabilisation, a phased antenna array can produce an earth-directed antenna beam. — 259 — Rep. 206-1
Three-axis stabilization of a non-spinning satellite permits directing conventional antennae toward the Earth, and orienting solar cell panels toward the Sun. Earth-directed satellite antennae provide gain which helps overcome the greater path loss associated with higher orbits. Without satellite antenna gain, more powerful transmitters and larger launching vehicles would be needed. The development of techniques and equipment to provide attitude-stabilization systems of long life and high reliability present major problems. These problems may be somewhat reduced by Use of circular equatorial orbits, due to the smaller perturbations experienced in such orbits. Satellites which are not attitude-controlled or which are spin-stabilized are less complex, hence offering the advantages of simplicity in design and operation, and the probability of longer useful life in orbit.
3.3 Phased and unphased satellites Proposals have been made for the use of satellites distributed at random in a variety of orbits. Such “unphased satellites” may be either active or passive, and could be attitude- controlled. A station-keeping satellite is one the position of which, relative either to other satellites in the same system or to a point on earth, is maintained within given limits. Such satellites could be, and in general would be, attitude stabilized. The advantages of unphased satellites as compared with phased satellites would be greater simplicity of design and operation, and the probability of a longer useful life in orbit. On the other hand, fewer phased satellites would be needed to provide a given quality of communica tion service, and would simplify system operation.
4. Possible types of communication-satellite system
4.1 Unphased satellite systems Broadband telecommunication, by means of satellites similar to those which might be used in an unphased satellite system, has now been successfully demonstrated and extensively tested. The extension from this stage to a world-wide operational system would not require the use of any basic new techniques or principles. This type of satellite system offers the means to proceed with an operational system at the earliest possible date. An appreciable number of satellites, each carrying one or more broadband microwave repeaters, are needed to provide high continuity, world-wide service. However, useful service can be started with a small number of satellites, with the service being extended when needed by the addition of more satellites. Because of their simplicity, the individual satellites would be the cheapest of the various types and it is likely that three or more could be launched with a single rocket of current type. For example, with some 12 to 18 such satellites, in orbit at heights of about 11 000 km, a substantial amount of telecommunication service would be possible. In fact, service would be available for 95 to 99% of the time between North America and Europe. With expansion to 48 unphased satellites, e.g. 24 in a polar orbit and 24 in a nearly equatorial orbit, service would be available over 99% of the time over most of the major communication routes of the world. The above-mentioned possibilities could be achieved from existing launching sites. It may be noted that an unphased satellite system which includes polar orbits is the only one of the systems commonly proposed which is capable of complete world-wide area coverage. In an unphased satellite system, the statistical distribution of satellite positions is such that the failure of one, or a few, diminishes the reliability of service only a little. The wide-band repeaters tested in unphased satellites are capable of carrying one broad band telecommunication channel or a number of narrower-band channels. This makes it possible to serve both large and small earth stations, or groups of stations, as desired. Rep. 206-1 — 260 —
The earth-station antennae for a random satellite system must be able to follow the satel lite across the sky, but experience has shown this to be easier than originally expected. Future stations can be simpler than today’s experimental stations. Coordination between earth sta tions is required but, fortunately, the positions of satellites can be predicted in advance for extended periods. Information on satellite positions and their assignment to particular com munication paths can be determined and shared among the earth stations well ahead of time.
The transmission delay introduced into communication links routed via unphased satellite systems at intermediate altitudes will be substantially less than that of a stationary satellite and in some instances would be less than in a sub-synchronous satellite system.
4.2 Medium-altitude phased satellite systems
4.2.1 General As compared with unphased satellite systems, phased satellite systems—particularly those using closed, i.e. recurrent, earth tracks—can be designed to use an appreciably reduced number of satellites. (A closed earth-track orbit is one in which the earth track is the same from one day to the next. The satellites may be in either equatorial or sub-synchronous inclined orbits.) Furthermore, the concept of employing only a few fixed or semi-fixed active arcs along the satellite orbit can be exploited in some of these systems. This has operational and economic advantages in materially reducing the number of earth-station antennae required and also in restricting the range of steering needed for a given earth-station antenna ; it also eases the problems of coordinating the use of frequencies with neighbouring terrestrial radio-relay systems. However, each earth station would need to be equipped with a greater number of antennae than for stationary satellite systems.
Particular examples of such systems are those involving the 12-hour and the 8 -hour sub-synchronous orbits. In these cases, each satellite would be visible to each earth station either once or twice per day and world-wide coverage would be provided using about six overlapping zones each spanning a few hours of local time. All stations in a given zone might use the same satellite at the same time and be in single-hop communi cation with each other.
It is to be noted that the zoning principles would also be useful for television relaying, since the time difference of a few hours in each zone would correspond to a convenient maximum difference in time which could normally be accommodated by “live” television programmes ; on the other hand, world-wide television links could be established when necessary via interconnecting earth stations.
A measure of satellite redundancy which could well have important advantages is also inherent in such systems. On failure of a single satellite, adjacent satellites may be used in a normally non-active part of its orbit, to make good many of the circuits which would otherwise have been interrupted.
4.2.2 Single closed earth-track systems In order that two satellites in non-equatorial equally-inclined circular orbits should follow the same track relative to the Earth, the intersections between the equator and the two orbit planes must be so chosen as to compensate for the rotation of the Earth during the time interval between the equatorial crossings of the satellites. Thus the satellites in single earth-track systems employing non-equatorial orbits are generally all in different orbit planes. Such systems, when they do not have equatorial orbits, use sub-synchronous satellites (i.e. satellites with periods of, for example, 8 or 1 2 hours). — 261 — Rep. 206-1
When a number of earth stations use a single closed earth-track system, there are advantages to be gained from employing only particular sections of the track (active arcs). The length of each active arc should be just sufficient to ensure that it always contains at least one satellite. The area of the Earth, from which the whole of an active arc can be seen, is a “zone of coverage” and all stations in a zone may be interconnected via the same satellite . The use of only one active arc per zone permits large areas of coverage to be established. A zone of coverage is enclosed by an outer boundary which, in the case of equatorial satellites, is exactly defined by the coverage overlap of the two satellites at the instant of hand-over. In the case of inclined orbits, this coverage overlap remains a very good approximation to the zone of coverage, but care should be taken that any curvature of the active arc does not lead to the exclusion of stations at or near the edges of the coverage overlap. Individual pairs of stations which are suitably positioned could, by agreement, use the satellites outside the active arcs ; however, since these stations would be working to an additional satellite they would require extra antennae. It can be shown that not more than about six coverage zones would be needed to provide the great majority of circuits likely to be required on a single-hop basis. A small proportion of circuits would need two satellite links in tandem, or extension of single-hop satellite circuits by long terrestrial circuits. The earth track of all equatorial satellites is unique, i.e. the Earth’s equator, and this offers certain operational advantages. Satellites stationed equally around a circular equatorial orbit need not be sub-synchronous. Moreover, such configurations lend themselves more readily to the multiple launching of satellites. However, systems using inclined orbits can be designed to optimize particular coverage requirements. The relative merits of the two types of system will also be affected by space technology and by launching considerations.
4.2.3 Multiple closed earth-track systems If two or more sub-synchronous satellites are placed in the same inclined circular orbit, but with different phases, their tracks will not as a rule coincide due to the Earth’s rotation but will stay closed and thus form a group of closed tracks. In such systems, satellites placed in different orbital planes, but equally inclined, may form a single track group. Some configurations of these systems may provide reasonably large coverage zones. To set up a multiple closed-track system, a number of satellites can be placed in the same orbital plane from each launching vehicle. It is easier to solve the problem of spare satellites, since only one spare satellite need be provided for all the satellites of any one orbital plane.
4.2.4 Non-closed earth-track systems If a satellite on an inclined orbit is not sub-synchronous, i.e. if its orbital period is not an integral fraction of the sidereal day, its track in relation to the Earth will differ from one day to the next. The altitude of such a satellite is not a critical factor. Scheduling the use of such satellites sets a problem similar to that met within an unphased satellite system.
4.3 Stationary satellite systems From a stationary satellite, the earth subtends an angle of approximately 17° 30', which corresponds to a maximum great-circle earth distance of approximately 17 0 0 0 km, assuming a minimum elevation of 5° for the earth-station antenna. The corresponding area of the Earth, Rep. 206-1 — 262 —
from which a stationary satellite would be visible at an antenna elevation of 5° or more, is approximately a third of the total area of the Earth. Alle arth stations within this area could use this satellite continuously, without periodic hand-over from a setting satellite to a rising one, as will be required for satellites in any non-synchronous orbit.’ Moreover, earth stations could use fixed directional antennae, having very limited beam-steerability. Such antennae can be simpler and less expensive than the multiple installations of steerable antennae which are needed with all other satellite systems.
The distance from a stationary satellite to its farthest earth stations will be about 42 000 km, requiring about 140 ms for propagation between such a station and the satellite. Thus, allowing for additional delay in associated terrestrial communication circuits, telephone conversations via a stationary satellite may have round-trip delays of about 0 -6 s.
To the extent that the satellite is stationary, this delay time is constant and Doppler frequency shifts would be negligible. The small amount of Doppler shift could facilitate the use of single-sideband modulation systems which, in turn, can facilitate flexibility of inter connection between earth stations.
The stationary satellite is also unique, in that the first such satellite can provide essentially uninterrupted service to earth stations located within a third of the area of the Earth. The first such satellite might be placed at longitude 50° W, over the delta of the Amazon River, from where it could be used from all of South America, most of North America and Western Europe, a large part of Africa and even from parts of Greenland and Antarctica. A second such satellite might later be placed over the Indian Ocean, thus covering all of Europe and Africa, plus much of Asia. Additional satellites would probably be added to the stationary orbit, as needed, until they girdled the Earth. Thus, in time, most earth stations could have access to two or more satellites, for long easterly or westerly routes. For example, with only two satellites at the locations mentioned above, stations in Europe and Western Africa would have access to both and would have one-hop coverage of about two-thirds of the earth.
To maintain a stationary satellite in a fairly precise position and to maintain its attitude so that the antennae are always pointing towards the earth, requires additional complexity over an uphased unstabilized satellite which could reduce reliability. Also, launch vehicles having greater thrust and more accurate guidance systems are required for orbital injection than are necessary for low or intermediate altitude random orbit systems. The extent of the problems, and the effort required to solve them, can only be determined as further experiment ation is carried out.
4.4 Integrated satellite systems
Since the development of satellite systems will be an evolutionary process, it is likely that more than one system, or type of system, will be used operationally in the world communica tion network, the earlier systems being of relatively simple design, and the later ones of greater complexity, providing increased satellite capacity and larger numbers of earth stations to accommodate expanding traffic requirements. Furthermore, transmission considerations, and in particular the problem of transmission delay, may indicate a need for both intermediate- altitude and stationary satellite systems.
It is convenient to use the term “sub-systems” to describe the individual elements forming part of an integrated communication-satellite system. For example, a stationary satellite sys tem might form one such sub-system, and an intermediate-altitude system a second sub system.
It is considered desirable, for the following reasons, that the various sub-systems be developed and established on a planned and integrated basis : — 263 — Rep. 206-1
— to avoid mutual interference between the sub-systems; — to achieve the maximum practicable degree of technical compatibility between the sub systems ; — to ensure the possibility of interconnection, both between the sub-systems and with ter restrial transmission systems ; — to enable the capacities of satellites and earth stations to be increased as the traffic expands, with a minimum of additional equipment at earth stations ; — to avoid, as far as possible, obsolescence of earth-station equipment and satellites.
The avoidance of mutual interference between sub-systems will require a suitable choice of radio-frequency channel assignments on an internationally agreed basis. It may also neces sitate a selection of orbit configurations to minimize difficulties due to satellite eclipsing.
A degree of technical compatibility between sub-systems would help to minimize the amount of equipment of different designs required at earth stations and avoid equipment obsolescence. For example, steerable antennae provided to accommodate unphased satellites, or the limited range of azimuthal and elevation angles needed for an intermediate-altitude equatorial-orbit satellite-system, could also be used for a stationary satellite-system. In another example, satellites in different sub-systems might use similar modulation characteristics, so that the earth-station transmitting and receiving equipment would be interchangeable.
The interconnection of communication-satellite sub-systems with one another, and with terrestrial transmission systems, will be facilitated by compatible standards of transmission performance and of baseband characteristics, e.g. of the video bandwidth for television and the multiplexing arrangements for multi-channel telephony and telegraphy.
Attention should also be given to the problem of accommodating traffic growth and provid ing for increased numbers of earth stations, without involving obsolescence either of earth station equipment or of satellites. In this connection, there would be advantages in replacing satellites which have failed by satellites of increased capacity, but of compatible design, in the same orbits.
5. Communication-satellite systems in the international telecommunication network
5.1 General considerations The expansion of the international telecommunication network on a world-wide basis is proceeding at an accelerating pace. It is expected that satellite communications will play an important part in meeting this expansion. Communication satellites will not only be capable of providing large groups of trunk-lines between major traffic centres, but can also provide access to remote points in the world, having relatively low traffic density. Such facilities, it is hoped, will thus supplement the present HF radio network facilities and give a significant improvement in the quality of service. The C.C.I.T.T., at its Illrd Plenary Assembly in 1964, considered plans which have been developed for a world-wide telephone network employing semi-automatic and automatic type operation. This involves a numbering plan, a routing plan and a transmission plan for such world-wide service. The international circuits required for the plan will join together the national networks throughout the world in a global telecommunications network. Com munication-satellite systems could be an important contributor to these facilities, particularly for the very long international circuits between continents. At the same time, the C.C.I.T.T. plan has set up requirements and limits which should be met by the communication-satellite facilities for the world-wide service.
It is possible that the unique features of communication-satellite systems may suggest alternative arrangements of the world plan. As presently envisaged by the C.C.I.T.T., this Rep. 206-1 — 264 —
is a hierarchy of world switching centres through which traffic is routed to its destination. Communication-satellite systems could provide direct access to widely separated points throughout the world, possibly by-passing many of these switching points, thus resulting in certain improvements in noise performance. However, it is too early to estimate the signi ficance of such factors in the world routing plan, but they will clearly require study.
The estimates of the international traffic and its routing, requested by the C.C.I.R. from the C.C.I.T.T./C.C.I.R. Plan Committee, will provide a basis for communication-satellite system planning.
5.2 Performance o f satellite systems
A large background of experience in telecommunications has been built up throughout the world, based on terrestrial systems. This experience has been brought together and made available through the constituent organs of the I.T.U. Circuits derived from communication- satellite systems should conform to these recommendations, so that they can be used in inter national connections on a world-wide scale.
While various types of circuit derived from terrestrial systems differ considerably in their characteristics, a fact which is recognized and accepted, the circuits derived from commu nication-satellite systems will have some unique characteristics. These arise primarily from the great distances traversed by the signals in going from earth to satellite and back to earth. The propagation time involved in transmission via/atellites can be very large, compared to terrestrial systems and can introduce new and significant problems (see Report 383). It is important to note, that the problem does not arise to any significant extent with one-way transmission systems such as are involved in point-to-point television, telegraph, and facsimile services. Performance characteristics of satellite circuits with respect to noise, transmission loss and stability are similar to terrestrial circuits and do not present unusual problems.
5.3 Integration o f communication-satellite circuits into the C.C.I.T.T. routing plan
The routing principles for telephony of the C.C.I.T.T. propose a hierarchy of interna tional switching points through which traffic is routed to its destination. These international traffic centres, CT, are of three grades designated CT1, CT2 and CT3, respectively. The highest ranking centres, CT1, are completely interconnected by direct international circuits. Since the number of CT1 centres is small (the present plan includes seven), the circuits inter connecting them will be very long intercontinental facilities. Each CT1 will serve a region and will be connected to lower ranking CT2 centres. Since each CT1 region will be of continental proportions, the circuits from CT2 to CT1 may also be very long. In addition to these basic circuits, the routing principles include and encourage the provision of direct circuits between any two points having sufficient traffic volume. These latter circuits would by-pass some switching points and thus provide better service.
Integrating high-altitude satellite telephone circuits into the world-wide routing and transmission plan introduces problems arising from the long propagation time of such circuits. The hierarchical routing plan recommended by the C.C.I.T.T. may need to be reviewed to take account of these problems. In the opinion of the C.C.I.T.T., the routing plan should ensure that international connections do not infringe the recommended limits of the mean one-way propagation times given in C.C.I.T.T. Recommendation P.14, §§ A (a) and A (b) (G.114, §§ A (a) and A (b)). Connections involving the use of two high-altitude satellite cir cuits in tandem must be avoided for the telephone service except in exceptional circumstances, e.g. where the only alternative is an HF radio circuit connection of the traditional type. Furthermore, there will need to be some restrictions on the total length of terrestrial circuits, which can be accepted for connections including one high-altitude satellite circuit. — 265 — Rep. 206-1
The consequences of the above limitations are not completely evident at this time. How ever, it seems clear to the C.C.I.T.T. that high-altitude satellite circuits should hot be used for CT3 to its parent CT2 or CT1, or for CT2-CT1 within the same CT1 region. It also seems clear that high-altitude satellite circuits may be used in CT1-CT1 links only when some means is found to comply with the requirements of Recommendation P.14, §§ A(a) or A(b), for the overall connection. Transversal routes of the type CT2-CT2 and CT3-CT3 could be permitted, since there is less chance of getting links in tandem on such connections which would result in very long propagation times. The pertinent section of C.C.I.T.T. Recommendation E.15 reads as follows : “From a transit exchange, the various groups for routing a call are used in the following order: (a) direct high-usage (transversal) route (if it exists); (b) high-usage (transversal) routes which can by-pass a part of the final route, beginning with those that end up at the transit centres nearest to the terminal incoming centre (“Far-to- near sequences”) ; (c) as a last choice, the final route passing through the parent transit centres of decreasing category of the outgoing zone and then of increasing category of the incoming zone, therefore of the form : CT3—CT2—CT1 —CTX—CT1—CT2—CT3
However, at the outgoing end, a route which is not the theoretically final route can be set up with a low loss-probability, such that no overflow has to be provided to another route and, in particular, to the theoretical final route (this latter being by-passed, as it were). As a general rule, a high-usage route is used for traffic to the zone of the transit centre where this route ends. Nevertheless, the same route can be used for traffic to another zone of the same order, on condition that the route between the second and third transit centres is of low loss-probability.”
5.4 The influence o f multiple-access communication-satellites on the routing plan The proposals which have been made for communication-satellite systems, having mul tiple access and facilities for interconnection, offer a very effective means of obtaining circuits of the types indicated above. Such systems make it easier to justify direct circuits between points of low traffic density with consequent improvement in service. Traditional telecommunication systems are able to provide an adequately low probability of lost calls, on an economical basis, only when they have high usage. However, some of the proposed high-connectivity communication-satellite systems may provide a similar lost-call probability by establishing single or multiple circuits, on demand, between any stations in the system on a call-by-call basis without economic penalties. As a consequence, such systems may be able to satisfy the low lost-call probability criterion of C.C.I.T.T. Study Group XIII without requiring high-usage routes. Routing by satellite is influenced by other factors such as the availability of connections between an earth station and its potential users and by the relative economics of terrestrial and satellite systems.
6. Effect of “outages” occurring in communication-satellite systems 6.1 Possible types o f outage Interruptions to service may occur in communication-satellite systems under certain con ditions of operation. In particular, breaks will occur on those occasions when no satellite is mutually visible to the intercommunicating earth stations. Breaks may also occur in systems for other causes such as individual satellite failure or solar eclipse (for satellites not provided with batteries designed to cover eclipse periods). In some systems, interruptions may also
6 Rep. 206-1 — 266 —
occur due to the transit of the Sun across the earth-station antenna beam (see Report 392) or of another satellite of an unphased system. When a predictable break occurs, the interruption time will generally be greater than the duration of the break itself. For most services it would be necessary to refuse new traffic for a significant period prior to the beginning of each satellite outage. In all systems the effects of breaks occurring due to satellite failure will, of course, depend upon the frequency of such failures and the time needed to launch and position a replacement satellite in orbit. Apart from breaks due to such satellite failures, the outages resulting in systems of unphased satellites may be of relatively short duration individually, e.g. from a few seconds up to one or two hours. Calculations have shown that, for systems using unphased satellites, the effective total outage time per month in the telephone service is very approxi mately twice that arising from the non-mutual visibility of satellites, due to the need to provide a suitable guard interval. Non-predictable outages, and predictable but brief outages, which would not require diversion of telephony, may cause particular difficulty in telegraph facsimile data and television transmissions because of the normal one-way nature of such transmissions, and the strict message continuity required. The problems include the difficulty of informing the sender of the part of the transmission which has been lost.
T a b l e I
Distribution of duration of international calls
Duration Number of calls (2) Percentage of total calls (min) O
Under 1 10 1-03 1-2 44 4-50 2-3 145 14-90 3-4 77 7-90 4-5 90 9-25 5-6 90 9-25 6-7 68 6-96 7-8 71 7-27 8-9 83 8-52 9-10 38 3-90 10-11 35 3-58 11-12 38 3-90 12-13 32 3-28 13-14 20 2-05 14-15 9 0-93 15-16 10 1-03 16-17 13 1-33 17-18 16 1-65 18-19 4 0-41 19-20 5 0-51 20-30 52 5-35 30-40 12 1-23 40-60 11 1-13 Over 60 2 0-21
Total calls 975
(9 The duration shown excludes operating time. (*) All calls are telephone conversations ; no data or facsimile transmissions are included. — 267 — Rep. 206-1
6.2 Distribution o f duration o f calls in the telephone service The amount of lead time (guard interval), necessary to divert telephone traffic in anticipa tion of an outage, is related to the probability of not interrupting a call in progress. In this connection, a sample of about 1 0 0 0 long-distance international calls has been classified according to duration (see Table I). Outages will have two direct results on a telephony service, viz. :
— because of the need to refuse traffic at some time before the satellite outage occurs, the overall quality of service will be reduced ;
— nevertheless, a certain proportion of calls will have to be interrupted at the time of the satellite outage, this percentage being less the greater the guard interval.
In practice, these effects will have to be weighed one against the other, in determining what would be the optimum value of guard interval to adopt. It may also be noted, that the additional outage time (due to the necessary guard interval), could be reduced if it were practicable to provide stand-by circuits in another telecommunica tion system. For example, if it were practicable to provide 20% stand-by circuits via another system, and to refuse traffic for only 1 2 minutes before a satellite outage, then virtually all calls still in progress at the commencement of the outage could be transferred to the stand-by circuits. However, it is by no means certain that it will be practicable to operate in this manner, except perhaps in the case of a few routes for which both alternative routing and easy access to traffic transfer-points exist. All of these factors which affect the overall quality of service should be taken into account when considering the suitability of satellite circuits for inclusion in a world-wide automatic network as defined by the C.C.I.T.T.
7. Future developments
7.1 Additional information required While past work has advanced the technology to the point where the feasibility of com munication-satellite systems has been demonstrated, considerable work remains to be done to assure the development of an economic operational system, e.g.:
— to obtain information on the location, type and strength of damaging radiation and its variation with tim e;
— to increase further the life expectancy of satellites through circuit design techniques, mechanical design improvements and component improvements; — to improve reliable, long-lived control and stabilization of attitude ;
— to develop accurate and reliable methods of launching multiple satellites with a single launch vehicle;
— to develop reliable methods of placing heavy pay-loads into precise, high equatorial orbits; — to obtain further information on the significance of propagation time due to long trans mission paths, and appropriate corrective measures for echo ;
— to develop methods of controlling the flow of information and of exploiting facilities, particularly in systems with facilities for multi-station access and interconnection. Rep. 206-1, 207-1 — 268 —
Other developments which would contribute to the improvement of operational com munication-satellite systems include: —- ion engines for better attitude-stabilization and phasing of satellites, and possibly, for injection into orbit; — nuclear power supplies ; — specially shaped beam antennae ; — steerable antennae; — improvements in radio-frequency power amplifiers; — inter-satellite relaying.
7.2 General time scale , Much information on the above problems will be obtained during the course of experi ments planned or being carried out at the present time and also during the course of com mercial operations. Additionally, a number of scientific satellites will be launched during the next two years to investigate the radiation problem.
Bibliography
1. D a lg leish , D . I. and J effer is, A. K. Some orbits for communication-satellite systems affording multiple access. Proc. I.E.E., 112, 21 (January, 1965).
2. L u t z , S. G. and D orosheski, G. Coverage and overlap of satellites in circular equatorial orbits, with applications to multi-access systems. Accepted for publication in Proc. IEEE.
REPORT 207-1 *
ACTIVE COMMUNICATION-SATELLITE EXPERIMENTS Results of tests and demonstrations (Question 2/IV) (1963 — 1966) 1. Introduction
The first phase of experimentation with communication satellites—score, courier, telstar, relay, syncom, intelsat-i and m o l n i y a - i — is virtually complete : commercial trans mission through communication satellites, commenced in 1965. This Report contains summaries of the principal characteristics of the experimental com munication-satellite programmes listed above.
2. Project SCORE [1] 2.1 The satellite s c o r e , the world’s first communications satellite, was successfully launched on 18 Decem ber 1958 into an elliptical orbit initially inclined 30°, with a perigee of 180 km and apogee of 910 km. The space vehicle was the Atlas missile used as the platform for two identical
* This Report was adopted unanimously. — 269 — Rep. 207-1
communications packages installed in pods along the sides. Satellite components included : communications receiver and transmitter, tracking beacon, control unit, recorder, d.c.-d.c. converter, zinc-silver oxide batteries, and separate slot antennae for transmission and recep tion. VHF frequencies were used for both communications and tracking; for communica tions, 132 MHz down link and 150 MHz up link, with 40 kHz bandwidth, and 108 MHz for the tracking beacon. Power output was 8 W. The storage battery pack, which was designed to have a nominal life of 2 weeks, ran down on 30 December 1958 and ended the life of the satellite.
2.2 Earth stations Four mobile earth-stations were located across continental United States, in California, Arizona, Texas and Georgia, plus a control station in New Jersey. Each station consisted of an operations van, antenna and power trailer. The operations van contained standard com munications equipment including both 250 W and 1000 W transmitters, and multiplex tele graph equipment capable of either 7-channel frequency-division multiplex operation or single channel 850 Hz frequency-shift operation. Standard telegraphy sets were used for receiving, transmitting and preparing messages on perforated tape. The antenna system was an array of four circularly polarized helices with a screen reflector mounted on a modified searchlight pedestal; gain varied from 10 dB at 108 MHz to 16 dB at 150 MHz.
2.3 Principal results Communications traffic, both voice and single and multi-channel telegraphy, was carried. Normal operations employed the delayed repeater technique but some messages were exchanged without storage. The system demonstrated the feasibility of world-wide communications over continental distances by means of an active communications satellite.
3. Project COURIER [2]
3.1 The satellite
The active satellite c o u r i e r was launched on 4 October 1960 into an elliptical orbit initially inclined 28°, with an apogee of 1350 km and a perigee of 960 km. A sphere 1-3 m in diameter and weighing 228 kg, the satellite was fabricated from honeycomb moulded fibre- glass and was completely covered by nearly 20 000 silicon solar cells. Functionnally, the electronics were divided into two groups : VHF (108-150 MHz) command, telemetering and beacon equipment and the microwave (nominal 2 GHz) communications system. The micro wave equipment consisted of four transmitters, four receivers, one receiver baseband combiner and two antennae, completely transistorized except for the transmitters. The VHF- equipment was composed of two acquisition and two telemetry transmitters, two command receivers and one antenna. Useful satellite life ended 21 October 1960, the assumed cause being failure in the command circuitry.
3.2 Earth stations Two earth-stations were provided, one in New Jersey and one in Puerto Rico. Each of these contained two functional systems : a ground complex for data handling and transmission, installed in mobile vans, and a tracking antenna and associated equipment. This antenna was an 8-5 m diameter dish (steel mesh parabola), with a multi-frequency feed mounted on a 12 m steel tower. The operations equipment in the vans corresponded to that in the satellite. The VHF-part of it consisted of one 1 kW transmitter, two receivers and the antenna ; the micro wave part consisted of one transmitter, four receivers, one receiver baseband combiner and the antenna. The microwave equipment also provided speed buffering, storage and high speed readout equipment for data transmission. Rep. 207-1 — 270 —
3 .3 Principal results The c o u r i e r satellite was in view of each station for 1 0 to 15 minute periods during which time, for 4 minutes, it accepted messages from one ground station, stored them and, on com mand, delivered them to the other station during a pass. While normal operations employed this delayed repeater technique, some messages were exchanged without storage. Speed of opera tion was this system’s most notable achievement, with data transmitted at the rate of 55 000 bits per second or 13 million bits for each 4 minutes.
4. Project TELSTAR [3]
4.1 The satellite t e l s t a r i, launched 10 July 1962, was the first active communications satellite to be employed for intercontinental communications without storage. It was employed successfully for hundreds of experimental transmissions before it failed, in February 1963, due to mal functioning of the command decoder circuitry, t e l s t a r ii was launched on 7 May 1963, and at the time of writting (May, 1966) is still fully operational, although it is now seldom used, due to unavailability of earth stations for experimental operations.
Spacecraft and orbital data
T e l s t a r i TELSTAR II Weight (nominal) ( k g ) ...... 79-5 79-5 Diameter (cm ) ...... 87 87 Altitude of perigee (k m ) ...... 953 969 Altitude of apogee (km) ...... 5685 10 793 Inclination...... 44-8° 42-7° Stabilization...... Inertial spin Attitude control ...... Magnetic torque
4.1.1 Communication sub-system characteristics The t e l s t a r communications subsystem operated at 6390 MHz (up-link) and 4170 MHz (down-link) with a repeater bandwidth of approximately 50 MHz, and an e.i.r.p. of approximately 2 W. Antennae were circularly polarized, with nearly circular patterns in the plane perpendicular to the spin axis of the spacecraft.
4.2 Earth stations The stations primarily involved in the t e s l t a r programme were : A.T. and T. station at Andover, Maine, having a 335 m2 aperture horn reflector antenna, radome enclosed, a 2 kW TWT transmitter, and a system noise temperature of less than 50°K. The C.N.E.T. station at Pleumeur Bodou, having the same principal characteristics as the Andover station. The G.P.O. station at Goonhilly Downs, with an unenclosed 26 m diameter parabolic reflector antenna, a 5 kW TWT transmitter and a system noise temperature of about 120°K (later improved to 50°K). The D.B.P. station at Raisting, with a 9 m diameter Cassegrain antenna, unenclosed, a 10 kW klystron transmitter and a system noise temperature of about 350°K.
4.3 Principal experimental results More than 250 technical tests and 400 demonstrations were performed via t e l s t a r proving the ability of a space-borne broadband microwave repeater to handle multi-channel telephony, television, data, and phototelegraphy communication services. The satellite provided a great deal of scientific information on the performance and degradation of electronic equipment in — 271 — Rep. 207-1
the space environment in addition to studying the environment itself. Experience was gained in the construction and operation of large tracking earth stations designed for eventual com mercial use. Improved resistance to radiation and a higher orbit have resulted in appreciably extended life for telstar ii.
5. Project RELAY [4, 5]
5.1 The satellite Two r e l a y satellites were put in orbit, the first on 13 December 1962 and the second on 21 January 1964. Initial orbit characteristics were as follows :
RELAY I RELAY II Period (m in)...... 185-1 194-7 Altitude of perigee (km) 1312 2088 Altitude of apogee (km) 7423 7413 Inclination...... 47-5° 46-0°
Both satellites weighed about 78 kg. They were 8 -sided polygons about 75 cm across and 1-3 m high. Spin stabilization was employed, with magnetic coils for attitude control. Each carried two wideband frequency-translating communication repeaters which received signals at 1725 MHz and retransmitted them at 4080 MHz with an e.i.r.p. of approximately 10 W (single carrier). r e l a y i failed in December 1964, after more than two years of successful operation despite a malfunctioning regulator in one of the TWT power supplies, and anomalous performance in the command receivers, r e l a y ii was still fully operational on 18 April 1966.
5.2 Earth stations The r e l a y satellite was controlled from either of two test stations, located at Nutley, New Jersey and Mojave, California. Other earth stations which participated in r e l a y experi ments were the A.T. and T. station at Andover, Maine; the C.N.E.T. station at Pleumeur Bodou, France ; the G.P.O. station at Goonhilly Downs ; the station at Rio de Janeiro, Brazil; the Telespazio station at Fucino, Italy; the I.T. and T. station at Nutley, New Jersey; the D.B.P. station at Raisting, Germany; the R.R.L. station at Kashima, Japan; the K.D.D. station at Juo-machi, Japan ; the C.T.N.E. station at Grinon, Spain ; and the Scandinavian (F.T.S.K.) station at Rao, Sweden.
5.3 Principal results All of the planned r e l a y transoceanic experiments were conducted successfully including relay of multi-channel telephony, telegraphy, phototelegraphy and television in colour and in black and white. The quality of signal transmission in all categories met or exceeded minimum operational standards which are based on C.C.I.R. Recommendations. Cosmic radiation has caused the spacecraft power system to degrade gradually. Radiation degradation measure ments on solar cells have demonstrated that N on P type solar cells are more resistant to radiation damage than P on N type cells.
6. Project SYNCOM [6]
6.1 The satellites The s y n c o m programme provided engineering and scientific experience in the operation of a 24-hour orbit communication satellite, s y n c o m i was launched from Cape Kennedy on 14 February 1963 ; radio contact was lost just as the spacecraft achieved near-synchronous orbit, s y n c o m n was launched on 26 July 1963, and s y n c o m h i was launched on 19 August 1964. Rep. 207-1 — 272 —
Data on spacecraft and initial orbit
SYNCOM I : SYNCOM II SYNCOM III Weight (k g ) ...... 39 36 33 Diameter (cm ) ...... 71 71 71 Height (cm ) ...... 39 39 39 Period (m in)...... 1426-6 1454 - 1423 Altitude of perigee (km) .... 34 124 35 520 34 726 Altitude of apogee (km) .... 36 954 36 627 36 354 Inclination...... 33-5° 33-1° 0-31 Reaction control system ...... N 2 and N 2 and H2o2 h 2o 2 h 2o 2 Stabilization...... Inertial spin
6.1.1 Characteristics o f communication sub-system The s y n c o m communications system consists of two independent frequency trans lation repeaters employing redundant travelling wave tube transmitters. The up-link operated at approximately 7360 MHz. The satellite transmitter had an e.i.r.p. of 2 to 4 W at about 1800 MHz.
6.2 Earth stations
The earth stations participating in the s y n c o m programme were: A transportable station at Lakehurst, N.J., having an unenclosed 9-2 m antenna and a system noise temperature of 200°K. Stations at Fort Dix, N.J. and Camp Roberts, California, each having an 18-4 m antenna and a system noise temperature of 230°K. A shipboard station with a 9-2 m antenna (under radome), with a system noise-temperature of 200°K. The R.R.L. station at Kashima, Japan, with a 7 kW transmitter and a 10 m antenna (Olympics television transmission only). A station at Point Mugu, California, having a 26 m antenna and a system noise temperature of 28°K (Olympics television reception only).
6.3 Summary o f communication experiments The communication experiments aboard s y n c o m ii consisted of three major groupings :
— demonstration tests, performed primarily with live inputs, including single-channel telephony, single-and multi-channel telegraphy and facsimile; — technical performance tests, primarily pre-recorded, including single- and multi-channel telephony and telegraphy, simultaneous telephony and telegraphy, and phototelegraphy tests; — technical characteristics tests, both live and taped, including statistical measurements of envelop delay, signal-plus noise-to-noise ratio and voice-frequency performance.
The Fort Dix, Camp Roberts and shipboard stations confirmed that transmission below an angle of elevation of 7-5° yields degraded results and that operation above 7-5° is free from the selective fading effects present in microwave radio-relay systems. syncom ii, unlike syncom iii, was not designed for video transmission, but narrowband television was successfully transmitted from Fort Dix to the Bell Laboratories’ station at Andover, Maine. Vocoder operation was successfully conducted through the satellite, with no problems caused by transmission delay. Multiple-access tests, using the spread-spectrum technique, were also demonstrated. — 273 — Rep. 207-1
s y n c o m h i provided daily transmission of video material of the 1964 Olympic Games from Kashima, Japan, to Point Mugu, California, with daily programmes of 3 to 4 hours. Several two-way communication tests with commercial aircraft were conducted through syncom hi. Initial tests consisted of telemetry and telecommand transmissions between the satellite and an aeroplane in flight from San Francisco to Honolulu. syncom ii power output had degraded about 30% in the first 900 days of operation, while s y n c o m m has degraded only 5% in 500 days, due to the more radiation-resistant N-P ty p e solar cells used. It appears that sufficient power is available to operate the s y n c o m ii repeater at full power until late 1968 and at reduced power until 1973. syncom iii can operate at full power even when the solar array is degraded 80%, thus the repeater can be expected to remain operational for 15-30 years (assuming no other failures), although the station-keeping fuel on-board is expected to be exhausted by about 1970.
7. Project INTELSAT-I (Early Bird) 7.1 The satellite intelsat-i was launched on 6 April 1965. It is in a quasi-stationary orbit centred on approximately 30° West longitude.
Data on spacecraft and initial orbit Weight (k g ) ...... 39 Size (cm)...... 73 (diam.), 59 (high) Altitude of apogee (km) ...... 35 841 Altitude of perigee (km) ...... 35 809 Inclination ...... 0-085° Period (min) ...... 1436-5 Drift...... 0-ll°/day, West
7.1.1 Characteristics o f communications sub-system The intelsat-i repeater consists of two receivers, each with a 25 MHz passband, centred on 6301 and 6390 MHz. The incoming signals are frequency translated to 4081 and 4160 MHz, and combined to drive one of two travelling-wave tubes. The nominal e.i.r.p. per carrier is 10 W. The antenna pattern is toroidal, and is “squinted” approximately 7° off normal to the spin axis, to provide optimum coverage of the North Atlantic basin.
7.2 Earth stations The previously mentioned stations at Andover, Goonhilly Downs, Pleumeur Bodou, Raisting, and Fucino, have been modified and are presently operating commercially with INTELSAT-I.
7.3 Summary o f experiments intelsat-i was launched as part of an experimental programme for commercial operations. These operations commenced on 28 June 1965. The satellite has been used for extensive tests, during operations in the latter half of 1965, to ascertain user reactions to the delay associated with circuits using synchronous satellites. The results of these tests are still being evaluated to determine their significance to commercial operations.
8 . Project MOLNIYA-I [7, 8 , 9] 8.1 The satellite The first satellite m o ln i y a - i was launched in an elliptical orbit on 23 April 1965. Rep. 207-1 — 274 —
Spacecraft and initial orbit data Altitude of apogee (km) 40 000 (above Northern hemisphere) Altitude of perigee (km) 500 (above Southern hemisphere) Inclination ...... 65° Approximate period . . 12 h Transmitter power output 40 W Satellite antenna gain (the antenna is directed automatically towards the earth) 18 dB
8.2 Earth stations There are stations in Moscow and Vladivostok separated by a distance of about 7000 km. These stations have the following technical characteristics : Transmitter power output 5 kW Noise temperature of the receiving system (with antenna noise) . . . 230°K Type of operation : television, multi-channel telephony, telegraphy and phototelegraphy services.
8.3 Experimental results 8.3.1 A large number of experimental and commercial telephone calls were made from Moscow to Vladivostok and vice versa. The quality of the communication service was good. Transmission delay gave rise to no significant degradation. 8.3.2 Black-and-white television transmission quality was adequate. 8.3.3 Colour television transmissions were made using the SECAM-III system from Moscow to Paris, and similarly from Paris to Moscow, through earth-stations near Moscow and Pleumeur-Bodou (appropriate modifications were made to the equipment at Pleumeur-Bodou). The transmissions showed quite satisfactory results [9].
Bibliography
1. Project score. Proc. IRE, Vol. 48, 4 (April, 1960).
2. R a t c l iffe , J. A. The experimental investigation of space. Journal I.E.E., Vol. 7, 84 (December, 1961).
3. The t e ls t a r system. B.S.T.J., Vol. XLII, 4 (July, 1963).
4. A bramson, B. N . and Brady, M. A. The relay communications satellite system. Programme and Conference digest o f the International Conference on satellite communications o f the I.E.E., 22-28 (November, 1962).
5. Schreiner, W. A. The communications satellite relay. 14th International Astronautical Congress, Paris. I AC Paper No. 81 (25 September-1 October, 1963).
6. Gatland, K. W. Telecommunication satellites. Iliffe Books Ltd., Prentice-Hall Inc., Englewood Cliffs N.J., 130-155 (1964).
7. Fortuchenko, A. D. The U.S.S.R. communication satellite molniya-i. Telecommunication Journal, 10 (October, 1965).
8. 70 years of Radio. Moscou publishing house Sviazizdat (1965).
9. C.C.I.R. Doc. IV/251 (France and U.S.S.R.), 1963-1966. — 275 — Rep. 208-1
REPORT 208-1 *
ACTIVE COMMUNICATION-SATELLITE SYSTEMS FOR FREQUENCY-DIVISION MULTIPLEX TELEPHONY AND MONOCHROME TELEVISION
Form of the basic hypothetical reference circuit and allowable noise standards; video bandwidth and sound channel for television
(Question 2/1V)
(1963 — 1966)
1. Form of the basic hypothetical reference circuit As an aid to the designers of active communication-satellite systems, it is desirable to establish basic hypothetical reference circuits and the allowable noise powers for frequency- division multiplex telephony and monochrome television. It has been found possible to recommend a common basic hypothetical reference circuit for both frequency-division multiplex telephony and monochrome television, based on a single satellite link (earth station-satellite-earth station) (see Recommendation 352). In arriving at this Recommendation, the following factors have been taken into account.
1.1 Active communication-satellite systems for intercontinental connections are likely to use orbital heights in the range from about 8000 to 36 000 km, including the stationary-satellite system at an orbital height of 36 000 km. In these circumstances, the majority of intercon tinental connections, both for telephony and television, could be made by a single satellite link, since it would be capable of spanning great-circle distances of up to at least 7500 km. However, a global satellite system will need to span great-circle distances of up to at least 25 000 km for a proportion of intercontinental telephone connections, and for some tele vision connections. Since the stationary satellite system is limited to a maximum great- circle distance of about 17 0 0 0 km, it may be desirable for telephony to extend such a satel lite link by terrestrial links to span the longer distances. With lower orbital heights, e.g. in the range 8000 to 15 000 km, two, or sometimes three, satellite links in tandem may be needed to span great-circle distances up to 25 000 km. It is to be noted that considerations of transmission delay in telephony may require the selection of orbital heights and number of satellite links, to reduce the delay to values acceptable to the users of long-distance con nections.
1.2 Transmission delay is not significant for television, transmission via two satellite-links in tandem, at stationary or intermediate orbital height, would be possible and necessary for the longer distances up to 25 000 km.
* This Report was adopted unanimously. Rep. 208-1 — 276 —
2. Allowable noise standards
2.1 General considerations It is clearly essential that the allowable noise standards for active communication-satellite systems should be commensurate with those adopted for other long-distance transmission systems ; in this connection the general principles established by the Joint C.C.I.T.T./C.C.I.R. Special Study Group C on circuit noise for telephony, and the CMTT for long-distance television transmission are particularly relevant. However, there are certain characteristics of communication-satellite systems which should be mentioned. The radio path-length (earth-to-satellite-to-earth), between any two earth stations, will depend on the satellite orbit employed, and may vary with time. However, in any specific communication-satellite system, the variation of the path loss between satellite and earth station will not exceed a few decibels, irrespective of the great-circle distances between co operating earth stations or the position of the satellite in its orbit within the range of mutual visibility to the earth stations. It may therefore be concluded, that the noise performance will not vary by more than a few decibels with the distance between the earth stations. The limited experience available so far suggests that there is comparatively little signal fading on communication-satellite links for wave directions exceeding a few degrees in eleva tion and for radio-frequencies up to at least 6 GHz . However, rain, cloud, snow and sleet may, at times, cause the noise level to increase by several decibels, the increase being greater for frequencies above about 6 GHz. Thus, the noise level at the output of a satellite link will be stable and low most of the time, with increases of a few decibels for a small percentage of time, subject to a seasonal variation. Detailed and comprehensive experimental data of noise performance are not yet available; for this reason, the allowable noise for small percen tages of time must be regarded as provisional until such data have been obtained.
2.2 Allowable noise in the basic hypothetical reference circuit: frequency-division multiplex telephony In addition to the general considerations mentioned above, Recommendation 353-1 was prepared giving objectives in the light of the advice of Joint C.C.I.T.T./C.C.I.R. Special Study Group C. The maximum value of 10000 pW (psophometrically weighted), for the mean noise in any hour in any telephone circuit of the basic hypothetical reference circuit, would correspond to 1-3 pW/km on a 7500 km great-circle distance or 0-65 pW/km on a 15 000 km great-circle distance . This objective is commensurate with those for other long distance transmission media, as are also the objectives for the other integrating periods, i.e. 1 min and 5 ms.
2.3 Allowable noise in the basic hypothetical reference circuit: monochrome television Recommendation 421-1 indicates that, for a 2500 km hypothetical reference circuit corres ponding to an international television circuit, used for the transmission of 625-line television signals in a 5 MHz video bandwidth, the signal-to-weighted noise ratio should exceed 52 dB for all except 1 % of the time, the signal being the peak-to-peak value (excluding synchronizing pulses), and the noise being the weighted r.m.s. value measured on an instrument with an effective time constant in terms of power of one second. For the basic hypothetical reference circuit for active communication-satellite systems (see Recommendation 352), comprising a single satellite link, it is recommended that, for 625-line signals with a 5 MHz video bandwidth, an overall signal-to-weighted noise ratio of 55 dB be adopted (see Recommendation 354). The weighting factors for uniform and tri angular spectrum random noise are given in Recommendation 421-1 for various television systems. With two satellite links in tandem, the signal-to-weighted noise ratio would then be 52 dB. — 277 — Rep. 208-1, 209-1
3. Video bandwidth in the basic hypothetical reference circuit The following points have been taken into account in preparing a Recommendation on the nominal upper limit of the video frequency band in an active communication-satellite system for television:
— the video bandwidth should be adequate for acceptable transmission of television signals up to and including 625-line standards ; — the need, for economic reasons, to provide a video bandwidth, no wider than is strictly necessary; — the desirability that the width of the baseband for television should be compatible with that for high-capacity frequency-division multiplex telephony.
Taking these factors into account, it is recommended that the nominal upper limit of the video band in the basic hypothetical reference circuit for monochrome television be 5 MHz. However, in view of the desirability of providing for the future transmission of colour televi sion signals with a chrominance sub-carrier of about 4-43 MHz, it is suggested that the desig ners of communication-satellite systems bear in mind the possible need for a video bandwidth slightly wider than 5 MHz, e.g. 5-5 or even 6 MHz should this be economically practicable.
4. Simultaneous transmission of a sound channel and a television picture To avoid excessive differences in transmission delay between a television picture signal and the corresponding sound signal, there will often be advantages in transmitting both over the same satellite link. In this event, a wider baseband may be needed, to accommodate the sound signal (e.g. on a separate sub-carrier in the baseband). Alternatively, the sound signal might be transmitted by time-division multiplex with the video signal, e.g. using the synchronizing pulses or the blanking intervals, without the need for a wider baseband. How ever, occasion may arise where the sound may be transmitted by some other means, e.g. by submarine cable, a land-line circuit or by another satellite or radio channel, and undesirable differences between sound and picture could arise. The appropriate tolerable values for these differences in propagation time are indicated in Report 412.
REPORT 209-1 *
FREQUENCY SHARING BETWEEN COMMUNICATION-SATELLITE SYSTEMS AND TERRESTRIAL SERVICES (Question 2/1V)
(1963 — 1966) 1. Introduction In considering frequency sharing between communication-satellite systems and terrestrial radio services, there are four interference criteria which must be satisfied: — the signals from the satellite must not cause harmful interference to the receivers of the terrestrial service, as in A in Fig. 1 ;
* This Report was adopted unanimously. Rep. 209-1 — 278 —
— the signals from satellite earth-stations must not cause harmful interference to the receivers of the terrestrial service, as in B in Fig. 1 ; — the signals from terrestrial stations must not cause harmful interference to the receivers of satellite-system earth stations, as in C in Fig. 1 ; — the signals from terrestrial stations must not cause harmful interference, in the satellite receivers, as in D in Fig. 1.
2. Sharing parameters A determination of whether sharing between two systems is possible depends on the following factors : — the maximum allowable value of interference either in a telephone or in a television channel, at the output of the system subject to this interference ; — the number of specific interference paths between which the total allowable interference must be divided; — the ratio of the powers, or the ratio of the power spectral-densities, of the wanted signal and the unwanted signal, at the input to the receiver, which would just result in the allow able value of interference at the output of the receiver, taking account of the types of modulation involved; — the power, or the power spectral-density, of the interfering transmitter; — the transmission loss along the unwanted signal propagation path, including effective antenna gain, basic transmission loss, and the effect of the polarizations concerned ; — the power, or the power spectral-density, of the wanted transmitter ; — the transmission loss along the wanted signal propagation path, including the effective antenna gains, and basic transmission loss.
The specific methods for achieving practical sharing between satellite systems and ter restrial systems include the following : — a limitation on the power, or power spectral-density, radiated by the interfering transmitter or, in the case of satellite transmitters, a limitation of the power flux-density at the surface of the Earth; — a specified method of computing the distance within which earth station transmitters and terrestrial transmitters may produce harmful interference to receivers of systems sharing the same bands.
These matters are summarized in Recommendation 355-1 and, in respect to sharing with line-of-sight radio-relay systems, in the documents to which that Recommendation refers; a further relevant document is Report 387. In respect to sharing with trans-horizon radio- relay systems, the Annex considers some of the factors which must be taken into account. In respect to sharing with other terrestrial systems, further study is required. The relevant num bers in the Radio Regulations are : R470 B, C, D, G, H, I, O, P and Q.
Bibliography
1. H ayashi, S. On the interference characteristic of the phase-modulation receiver for multiplex trans mission. Journal of the Institute o f Electrical Communication (Japan), 35, 522-528 (November, 1952).
2. Curtis, H. E. Interference between satellite communication systems and common carrier surface systems. B.S.T.J., 41, 921 (1962).
3. M edhurst, R. G., Hicks, E. M. and Grossett, W. Distortion in frequency-division-multiplex FM systems due to an interfering carrier. Proc. I.E.E., 105, B, 282 (May, 1958). — 279 — Rep. 209-1
4. H a m er, R. Radio interference in multi-channel telephony FM radio systems. Proc. I.E.E., 108, B, 75 (January, 1961).
5. M e d h u r st , R. G. RF spectra and interfering carrier distortion in FM trunk radio systems with low modulation ratios. IRE Trans., CS-9, 107 (June, 1961).
6. B o r o d it c h , S. V. The calculation of admissible value of radio-frequency interference in multi-channel radio-relay systems, Electrosviaz, 1 (1962).
7. M e d h u r st, R. G. FM interfering carrier distortion. General formula. Proc. I.E.E., 109, B, 149 (March, 1962).
8. C ha m berlain, J. K. and M e d h u r st, R. G. Mutual interference between communication satellite and terrestrial line-of-sight radio-relay systems. I.E.E. International Conference on Satellite Commu nications (November, 1962). Proc. I.E.E., 111, 519 (March, 1964).
9. M ed h u r st , R. G. and R oberts, J. H. Expected interference levels due to interactions between line- of-sight radio-relay systems and broad-band satellite systems. I.E.E. International Conference on Satellite Communications (November, 1962). Proc. I.E.E., I l l , 524 (March, 1964).
ANNEX
SHARING OF FREQUENCY BANDS BETWEEN COMMUNICATION-SATELLITE SYSTEMS AND TRANS-HORIZON TERRESTRIAL RADIO-RELAY SYSTEMS
1. Introduction In view of the allocation by the Extraordinary Administrative Radio Conference, Geneva, 1963, of a number of frequency bands to the communication-satellite service and to the fixed service (including trans-horizon systems) on a shared basis, and in view of the possibility of other bands being similarly allocated at a future Administrative Radio Conference, this Annex examines the conditions under which the communication-satellite systems and trans horizon systems can share the same frequency band, without causing undue mutual inter ference.
2. Trans-horizon radio-relay systems Trans-horizon systems have wide differences in system parameters—for example, trans mitter powers from a few hundred watts to 50 kW, antenna diameters from 3 m to 35 m, base band capacities from 1 telephone channel to 1 television channel, receiver noise figures from 1 dB to 12 dB. It is usually necessary, economically, to choose the system parameters that best suit each specific system and sometimes each specific link. The operating margins that would permit standardization tend to be either not available technically or not feasible eco nomically. It seems unlikely that trans-horizon radio-relay systems will make any extensive use of parallel radio-frequency channels as in line-of-sight systems.
3. Geometric considerations The geometric relations of exposure of satellites to the antenna beams of terrestrial radio relay stations are outlined in Report 393. Although the narrower beamwidths of trans horizon antennae tend to reduce the exposure probabilities to various satellite orbit sys tems, the greater transmitter power, receiver sensitivity and antenna gain all increase the probability of significant interference from such beam exposures and even from exposures to major sidelobes. Rep. 209-1 — 280 —
Additionally, trans-horizon links are frequently used between small and greatly separated islands, and in other similar circumstances which limit the choice of possible path directions and which thus preclude this means of avoiding orbit exposures.
4. Interference considerations
4.1 Interference to and from satellites The equivalent isotropically radiated power of a trans-horizon terminal may be of the order of 85 to 90 dBW, i.e. not greatly dissimilar from that of typical earth stations. A satellite in the main lobe of a trans-horizon antenna would therefore'receive unwanted and wanted signals of the same order of power, if a frequency were shared in the up-path. If a frequency were shared in the down-path, the unwanted signal in the trans-horizon receiver would be about —110 dBW, which is of the same order as the median value of the wanted signal, and would therefore cause a virtual circuit outage.
4.2 Interference to and from earth stations The problem of coordination distance between earth stations and trans-horizon stations is essentially similar to that of coordination distance between earth stations and line-of-sight stations, except for the larger path basic transmission loss. The loss required to make inter ference negligible ranges from about 190 dB, when neither terminal looks at the other, to about 30 dB when both stations look at each other (complementary directions in azimuth but beyond line-of-sight).
It should be noted, that much more is known about downward fading in trans-horizon propagation than about the upward fading that is significant in estimating coordination distance. The usual statistics of trans-horizon loss can be seriously distorted above the median value by ducting due to temperature inversions, which have been known to increase the signals received over trans-horizon paths by as much as 60 to 70 dB above the median values for substantial periods of time. Local topographic features below the scattering region can create ducting on particular paths with a much higher prevalence than the average for the region or type of region.
It is advisable to measure the propagation loss in a path likely to suffer interference during a time when temperature inversions along the path are most likely to occur. Basic transmission losses greater than 250 dB are difficult to measure with transportable equipment.
For stationary satellites, the problem of coordination is eased somewhat by the fact that the antenna of the earth station will always point in one direction, rather than in various directions, as when it is tracking a moving satellite.
5. Conclusions
5.1 It appears likely that the problem of coordination can be solved in most actual situations. It would be eased in a particularly difficult situation, if an unshared frequency band were avail able, to which the frequencies of the offending link could be transferred.
5.2 Sharing with a system of stationary satellites would require a restriction over a small part of the surface of the Earth on the range of permissible azimuth directions for trans-horizon links. This restriction will probably not be considered so limiting as to prevent sharing. — 281 — Rep. 209-1
Satellite
Radio-relay station Earth station Radio-relay station operating in the 6 GHz band operating in the 4 GHz band
E : transmitter R : receiver
F ig u r e 1
Interference paths between communication-satellite systems and terrestrial radio-relay systems ------: wanted signal ------: interfering signal Note. — The frequencies shown are typical of bands allocated to — earth-to-satellite transmission, — satellite-to-earth transmission.
7 Rep. 209-1, 210-1 — 282 —
5.3 Systems of random satellites in inclined orbits appear at present to require such large restric tions on permissible azimuth directions for trans-horizon links over so much of the world that sharing does not appear to be feasible.
REPO RT 210-1 *
FREQUENCY SHARING WITHIN AND BETWEEN COMMUNICATION-SATELLITE SYSTEMS (Question 2/IV, Study Programme 2C/IV) (1963 — 1966) 1. Introduction The extent to which the same frequencies may be used, without causing harmful inter ference, by different satellites within the same communication-satellite system, and by dif ferent systems of communication satellites, is a subject of considerable importance, bearing as it does on the efficient use of the frequency spectrum. The possible use of frequency sharing may be affected by : — the number of satellites sharing a given frequency channel; — the radiation pattern of the earth-station antennae ; — any difference in polarization between wanted and interfering signals ; — the relative operating power flux-densities of the wanted and interfering signals, both at the satellites and at the earth stations ; — the interference reduction factor between the input to the wideband space-station, and/or earth-station receiver, and the demodulated output at the earth station. It appears likely that early global systems will use frequency-modulation, multiple access being achieved by passing multiple carriers through a common repeater. Because of the relatively high levels of intermodulation associated with this method of operation and the limited equivalent isotropically radiated power (e.i.r.p.) of the satellite transmitter, likely to be available, it will be necessary to use high modulation indices. The resulting flux densities associated with these transmissions may be very low. The cases considered in this Report are those in which either the interfering system or the wanted system, or both, are FM systems of this type.
2. Ratio of interfering-to-wanted carrier levels The ratio of interfering-to-wanted carrier powers at an earth station can be expressed as follows : 10 log10 — K — P(cp) — L (dB) (1)
where C = the wanted carrier power at the input to the receiving system (W); X = the interfering carrier power at the same point (W); K = an allowance for the difference in operating flux densities of X and C (dB); P (9 ) = the discrimination of the earth-station antenna as a function of the angle, 9 , relative to the main beam axis (dB). (A typical antenna pattern is given in Report 391 ; L = an allowance for the difference in polarization between X and C (dB).
* This Report was adopted unanimously. — 283 — Rep. 210-1
Typical relative power flux-densities for multi-carrier frequency-modulation systems are shown in Table I. The values given in the Table are based on the following assumptions : — the total psophometrically weighted noise power in a telephone channel under clear weather conditions is 10 000 pW. Of this noise, 1000 pW is allocated to interference from line-of-sight radio-relay systems, 500 pW to group-delay distortion, 1500 pW to thermal noise arising in the earth-to-satellite path and 7000 pW to the satellite-to-earth path (including intermodulation noise due to non-linearity in the satellite); — the carrier-to-intermodulation noise ratio at the satellite output is 10 dB ; — the pre-emphasis improvement in the top channel is 2-5 dB ; — the mean power of the multiplex signal is in accordance with Note 8 of Recommenda tion 393 ; — the ratio of the peak to the r.m.s. power of the multiplex signal is taken as 10 dB when calculating the receiver noise bandwidth, but when calculating the occupied radio-fre quency bandwidth, an equivalent value of 13 dB is taken to allow for the larger bandwidth necessary in the satellite ; — the top baseband frequency in hertz (f m) is given by f m = 4200 N where N is the number of voice channels ; — the radio-frequency bandwidths are taken as twice the appropriate peak frequency deviation plus twice the top baseband frequency; — that at the 1 dB threshold point in the top baseband channel (see Report 212-1), the ratio of the carrier power to the total noise power (i.e. thermal noise power plus inter modulation noise power), in a bandwidth of one hertz, is given by :
10 log10 (C/w) = 1 0 log10 [(2 fd/k) + 2 fm\ + 10 where C = the carrier power (W); n = the total noise power in a bandwidth of one hertz (W); fa = the peak frequency deviation of the multiplex signal (hertz); k = 6x10«/(/* + 4-1 x 105) ; — the operating margin, i.e. the amount by which the received carrier power to down-path thermal noise ratio can decrease before threshold is reached, is 6 dB ; — that a guard band of 26% of the required bandwidth is provided between adjacent radio frequency channels in the satellite.
T a b l e I
Characteristics o f multi-carrier frequency-modulation emissions
Number of channels 200 too 50 20 10 5
Radio-frequency bandwidth, including guard bands (MHz) 30 17-5 10-5 5-3 3-0 1-8
R.m.s. multi-channel deviation (MHz) 2-5 1-47 0-89 0-45 0-26 0-15
Channel test-tone deviation (MHz) 0-98 0-66 0-46 0-28 0.185 0-12
Power flux-density at the Earth relative to that of a 200 channel transmission (dB) 0 - 4 -7 -6 -1 1 -7 -1 4 -6 -17-5 Rep. 210-1 — 284 —
3. Protection ratio required The ratio of interference power to signal power in the worst telephone channel can be expressed as follows:
10 log10 (I/S) = 10 log10 CXfC)-B (dB) (2)
where B — the interference reduction factor (dB) between the input to the wideband space- station and/or earth-station receiver and the demodulated output at the earth station (B is sometimes called the “receiver transfer factor”) ; S = the test-tone signal power and is taken to be 1 mW at a point of zero relative level; I = the demodulated (unweighted) interference power in mW introduced into the worst channel at a point of zero relative level.
Then from (1) and (2):
10 logj0 (I/S) - K-P (9 ) —B—L (dBmO) (3)
For the purpose of this study, a value of 500 pW is taken as the allowable weighted inter ference power from each source of interference; the value of 10 log10 (I/S) unweighted will be -60-5 dB and the required protection ratio will be :
P (9 ) + L = 60-5 + K-B (dB) (4)
The value of the interference-reduction factor, B, depends on the type of modulation used on the wanted and interfering carriers. Some examples are considered in Annex II and the required protection ratios are evaluated in each case using, where appropriate, the relative power flux-densities and polarization protection discussed in § 2 .
4. Relation between protection ratios required and those likely to arise in practice The required protection ratios, for the particular cases discussed in Annex II, can be summarized as follows:
— between or within multiple-carrier systems, the worst case of interference would occur when an emission carrying a relatively large number of channels interfered with an emission carrying a relatively small number of channels. For example, in the case of a carrier of 200 channels interfering with a carrier of ten channels, 27 dB protection would be required; — for a single carrier frequency-modulation emission of, say, 2 0 0 0 channels interfering with a small-capacity frequency-modulation carrier of, say, 10 channels, 31 dB protec tion would be required; — for a single-sideband emission interfering with a small-capacity frequency-modulation emission, the required protection could be between about 0 dB and 47 dB depending on frequency separation ; if it is assumed that small separations between the frequency- modulation carrier-frequency and that of the single-sideband signal could be avoided, it is probable that protection ratios no greater than 31 dB need be called for;
— for a single carrier frequency-modulation emission of, say, 2 0 0 0 channels interfering with a digital system the required protection would be of the order of 31 dB.
The calculations leading to these results have so far assumed full loading of frequency- modulation emissions. In practice, it is to be expected that energy dispersal will be used to avoid the occurrence of high spectral densities in narrow bands during light loading conditions. If it is now assumed that the spectral power-density over a band of a few MHz would never exceed the value during full loading by more than about 3 dB, the protection ratios required — 285 — Rep. 210-1
when the interfering emission is from a frequency-modulation system would be increased by 3 dB. The maximum protection ratio likely to be required for any of the systems considered is therefore taken to be 34 dB.
Generally, in the case of two satellites near to one another (whether they form part of a single system or belong to independent systems), the extent of any interference resulting depends upon whether they both receive signals from their corresponding earth stations at the time of proximity. If they do so, then the form of treatment given in previous sections of this Report will apply. If not, i.e., if one satellite is intentionally energised from the ground and the other only unintentionally, then the effect of any interference will be less marked. This may occur, for example, in an unphased satellite system when the separation between adjacent satellites is temporarily small, or in the case where an unused satellite is in the vicinity of a stationary satellite. In these cases, off-beam antenna gain reductions will apply both to the illumination of the unwanted satellite and to the reception of its unwanted emission. If the output spectral power density of the space station repeater is a function of the flux illuminating the satellite, the power spectral density produced by the unwanted satellite at the earth station would be below its normal operating value. Quite small angular separations between satellites might be tolerable in such situations.
In practice, the protection afforded will arise primarily from the radiation pattern of the earth-station antenna. However, it may prove practicable to afford part of the protection by using opposite senses of circular polarization for two systems sharing the same frequencies. The degree of protection will be dependent on the type of antenna used. With paraboloids of short focal length and with horns, the protection afforded by cross-polarization may decrease rapidly off the main beam ; however, for certain types of antenna (e.g. paraboloids of long focal length), appreciable additional protection may be practicable over a wide angle about the main beam.
If the protection is afforded solely by the angular discrimination of the earth-station antenna, then it can be seen from Fig. 4 of Report 391 that, assuming the use of an antenna of 26 m diameter with a gain of 58 dB, an angle of separation of 2° is required to give 34 dB protection. For the same protection ratio, angles of approximately 3-5° and 5° are required, respectively, for antennae of 14 m diameter with a gain of 53 dB and antennae of 9 m diameter with a gain of 49 dB.
The angles of separation arising between satellites of the same system and of different systems are considered in Annex I. As examples, the cases of sharing between stationary satellites and sub-synchronous satellites in equatorial or 30° inclined orbits are considered here. If the sub-synchronous system uses equatorial orbits of 8 -hour period, the angle of separation will be less than 2° for periods of up to about 5 minutes for stations within about 8 ° of the equator. Where the sub-synchronous system is a 30° inclined single earth-track system, it is possible to avoid angles of separation of 2 ° or less by ensuring that the stationary satellites are at least 16° from the equator crossings of an 8 -hour orbit system or at least 9° from those of a 12-hour orbit system. The earth track of a 12-hour system and the range of positions of the stationary satellites which ensure at least 2° separation are shown in Fig. 1.
5. Summary
This study shows that for the particular systems examined, to keep interference from other satellites and earth stations down to a level of 500 pW from each source, angular separation^ between satellites of about 2° or more will be necessary in most cases. However, smaller separations than these and the resulting higher noise levels may be acceptable, if they occur for only small percentages of the time.
For the combinations of orbits considered, it is concluded that interference between a stationary satellite and an 8 -hour equatorial satellite may limit frequency sharing. Similar Rep. 210-1 — 286 —
Longitude
^ 1 7 \ C 7 7
180' 90' 0" 90' 180' W E
F ig u r e 1
Relative positions o f medium-altitude and stationary satellites
------Typical earth track of medium-altitude satellites i------1 Suggested range of locations of stationary satellites for a minimum angle of separation of 2° — 287 — Rep. 210-1
conclusions may be drawn for the case when the medium altitude system uses 1 2 -hour orbits, although the interference will be less frequent. On the other hand, frequency sharing between stationary satellites and an inclined medium altitude 1 2 -hour single earth-track system should present few problems, though it will impose restrictions on the relative positions of stationary satellites and the inclined medium-altitude orbit. Sharing between stationary satellites should also be possible provided that separations between satellites of about 2 ° or more are maintained at all times. These conclusions will of course be modified if the assumptions underlying them are varied. For example, sharing will be more difficult if earth-station antennae of less than 26 m diameter are used or if the assumed degree of energy dispersal is not attained. On the other hand, the likelihood of interference between communication-satellite systems may be considerably reduced if suitable precautions are taken. Such precautions include the use of frequency planning, diversity of polarization, suitable modulation methods and, where this is practicable, the use of more directional antennae on satellites. These aspects of the problem need further study.
ANNEX I
ANGLES OF SEPARATION OCCURRING BETWEEN SATELLITES
1. Introduction In conducting studies of interference within and between communication-satellite sys tems, it is necessary to investigate the angles of separation, subtended at an earth-station, that may occur, and to estimate the distribution in time of the magnitude of these angles. This Annex confines itself to such an investigation for several possible systems and combina tions of systems of communication-satellites. Systems other than those investigated in this Annex can be envisaged and should be further studied.
2. Angles of separation occurring between satellites of independent systems 2.1 Independent stationary and sub-synchronous equatorial systems Figs. 2-5 show the loci of the angles of separation subtended at earth-stations by a stationary satellite and a sub-synchronous satellite at an altitude of 14000 km, when the ins tantaneous longitudinal separations are 0°, 5°, 10° and 15° respectively. Table I gives the minimum subtended angles from an earth station, anywhere in the area of visibility of the satellite, for a range of longitudinal separations of the satellites.
T a b le II
Minimum angles subtended by stationary and sub-synchronous satellites
Instantaneous longitudinal separation of a stationary satel lite from a sub-synchronous satellite (d eg rees)...... 5 10 15 20 25
Minimum angle subtended by stationary and sub-synchro nous satellites (degrees)...... 0 1 6 11 16 Rep. 210-1 — 288 —
2.2 Independent stationary and medium-altitude inclined orbit systems In this case the loci of constant angles of separation subtended at the surface of the Earth will form the same pattern as when the medium-altitude system uses equatorial orbits. However, the positions of the loci relative to the geography of the Earth will be different and the patterns will also occur in a different sequence. For this case, the approach adopted is to determine the angles of separation subtended at the centre of the Earth necessary to ensure that there is no point on the surface of the Earth at which the subtended angle of separation is zero. It can be shown that, for medium- or high-altitude satellites where small angles are con cerned, the angle subtended at a point near the edge of the visibility area by two satellites is given by the approximate expression :
radians
where 9 = angle subtended at a point on the surface of the earth near the limit of visibility ; 9 ' = angle subtended at the centre of the Earth; r = radius of the Earth ; rx — radius of the lower satellite o rb it; r2 = radius of the higher satellite orbit.
For the present example, the higher satellite is assumed to be stationary and the lower satellite is assumed to be in a circular orbit of 12-hour period (rx & 26 500 km). Then, expressing 9 and 9 ' in degrees instead of radians, the expression becomes :
9 = 9 ' —5-2°
Thus, when 9 ' = 5-2°, the angle of separation subtended at the edge of the area of visibility will be zero. If 9 ' is less than 5-2°, then there will be a point within the area of visibility having zero separation, while for any specified minimum satellite separation from any point within the area of visibility, 9 ' must exceed 5-2° by this amount. Fig. 1 of the Report illustrates the relative locations of stationary satellites and the earth- track of one type of medium-altitude single earth-track system using satellites in orbits with a 12-hour period inclined at 30°. The locations are chosen so that there is a separation of at least 7-2° between the stationary satellite positions and the closest part of the earth-track of the medium-altitude system, ensuring a minimum satellite separation of 2 ° in the area of visibility. The sections of the orbital plane of the stationary satellites within which angles of separation of 2° would arise, extend for 9° of longitude on either side of the ascending and descending nodes of the earth-track of the medium-altitude system. For frequency sharing between stationary satellites and a system using orbits with an 8 -hour period, inclined at 30°, the sections of the orbital plane which should be avoided extend for about 16° longitude on either side of the ascending and descending nodes for 2 ° minimum satellite separation in the area of visibility. The problem of frequency sharing between stationary satellites and multiple earth-track medium-altitude systems is likely to be much worse than in the case of single earth-track systems, because of the more numerous equatorial crossings of multiple track systems. As a result of this, the choice of suitable locations for stationary satellites would be severely limited.
2.3 Independent polar and equatorial systems In this case, the polar system is assumed to use unphased satellites at an altitude of 14 000 km and the equatorial system is assumed to be either stationary or sub-synchronous. Small angles of separation will arise only for small percentages of the time and the calculations are, therefore, made to determine the distribution of the angle of separation in time. — 289 — Rep. 210-1
The effect of interference into random systems is, in general, less serious than that into stationary or sub-synchronous systems because of the increased possibility in the former case of using alternative satellites ; only interference from the random into the stationary or sub- synchronous system is, therefore, considered. Angles of separation have been estimated for an earth-station at 45° latitude, on the same longitude as the equatorial satellite. However, they apply approximately to earth-stations in other locations. Referring to Fig. 5, it is seen that there will be an area A where the cone of interference of the antenna intersects the orbital sphere of the polar satellites. For an antenna directed at an equatorial satellite, either stationary or sub-synchronous, the area of interference will be approximately circular and will lie close to the equator. If the semi-angle of the cone of inter ference of the antenna is it (902/180 x 360 If there are n satellites in the polar system, then the probability that one or more of them will be in the cone of interference is multiplied by n. Thus, the probability p that a satellite of the polar system and a specific satellite of the equatorial system will be within a cone of semi-angle 9 is /> = /nr (9 0 2/ l 80x360 For the station at 45° latitude and polar satellites at an altitude of 14 000 km, 9' = 0-89. Therefore p = 0-000031 mp2. The values of 9 for a range of values of p are given in Table III for a system of 24 polar satellites. Table III Angles o f separation arising for various percentages of the time (24 polar satellites) p (%) 0-01 0-1 1 10 20 9 (degrees) 0-37 1-1 3-7 11-5 16 For different values of n it may be noted that 9 is inversely proportional|to sjn. If, for example, there were 1 2 polar_satellites, then for 0 -1 % of the time there would be one or more polar satellites within 1-1 -]/2 an 1 -6 ° of the equatorial satellite. Rep. 210-1 — 290 — For a sub-synchronous repetitive earth-track polar system, the distribution of the angles of separation would depend on the particular relationship of the track of the polar satellites on the Earth and the location of the equatorial satellites. 3. Angles of separation occurring between satellites in the same system 3.1 Stationary system For a stationary satellite system providing world coverage, large angles of separation would normally arise. However, it may be considered desirable to locate two stationary satel lites at approximately the same longitude, and the angle of separation subtended at any'earth station will be approximately equal to the angle subtended at the centre of the Earth. 3.2 Sub-synchronous station-keeping equatorial system In this case the angular spacing between adjacent satellites is not likely to be less than 24°. For a system at an altitude of 14 000 km the corresponding minimum angle of separation at earth stations would then be about 25°. 3.3 Random polar system The distribution of angle of separation as a function of the time depends on the orbits of the satellites and the locations of the earth stations. Generally, it will be possible to avoid interference by using a third satellite if one is visible. On occasions, however, no other satel lite will be visible and the interference will be unavoidable. The angles of separation, T able IV Angles of separation arising for various percentages of the time 12 satellites 24 satellites (4 in each of 3 planes) (4 in each of 6 planes) p (%) 0-05 0-1 1 2 0-01 002 0-1 0-2 In any random system there will be occasions when no satellite is visible simultaneously to a particular pair of stations. For the route considered above, using the 12- and 24-satellite ' systems respectively, these occasions would amount to 3-1% and 0-1% of the time. Rep. 210-1 — 291 — F ig u r e 2 Angle of separation subtended at the Earth by a stationary satellite (35 900 km) and a sub-synchronous equatorial satellite (13 850 km) (Longitudinal separation o f satellites 0°) A : Limit of visibility of sub-synchronous satellite B : Stationary satellite C: Sub-synchronous satellite Rep. 210-1 — 292 — F ig u r e 3 Angle of separation subtended at the Earth by a stationary satellite (35 900 km) and a sub-synchronous equatorial satellite (13 850 km) (Longitudinal separation o f satellites 5°) A : Limit of visibility of sub-synchronous satellite B: Stationary satellite C: Sub-synchronous satellite — 293 — Rep. 210-1 F ig u r e 4 Angle of separation subtended at the Earth by a stationary satellite (35 900 km) and a sub-synchronous equatorial satellite (13 850 km) (Longitudinal separation of satellites 10°) A : Limit of visibility of sub-synchronous satellite B : Stationary satellite C: Sub-synchronous satellite Rep. 210-1 — 294 — F ig u r e 5 Angle of separation subtended at the Earth by a stationary satellite (35 900 km) and a sub-synchronous equatorial satellite (14 000 km) (Longitudinal separation of satellites 15°) A : Limit of visibility of sub-synchronous satellite A' : Limit of visibility of stationary satellite B : Stationary satellite C : Sub-synchronous satellite — 295 — Rep. 210-1 Figure 6 Interference between a polar-satellite system and a synchronous equatorial-satellite A : Area of intersection between the cone of interference of the antenna and the orbital sphere of the polar satellite E: Earth station C ; Orbital sphere of the polar satellite X: Synchronous satellite Rep. 210-1 — 296 — ANNEX II PROTECTION RATIO REQUIRED 1. Introduction This Annex considers interference where either the interfering and/or the wanted system is a frequency-modulation system, and evaluates the protection ratio required as given by equation (4) of the Report. 2. Interference between multiple-carrier frequency-modulation systems The most difficult situation from the sharing point of view will arise if the multiple car riers are at non-uniform spacings across the band. This situation could arise if the carriers of either system are designed to transmit unequal numbers of channels, the frequencies and band widths being chosen to obtain the maximum channel capacities. In the extreme case, each system might use carriers bearing any number of channels between, say, 5 and 200, these carriers being randomly intermixed and occupying (with the minimum practicable guard-bands) the whole of the available bandwidth. Clearly it would be almost impossible in this case to maintain large separations between all of the carrier fre quencies of the two systems. The interference reduction factor, B, between two frequency-modulation carriers with telephony basebands and with Gaussian spectra, has been derived from [1] or [2] and is given by: B = - 1 0 log1 0 | / 2i»/[P(S/)3.2/j. V2^]}.{exp [-(/ 0 -/) 2/2//] + exp [-(/„ + /) 2/2/,2]} dB (1) where / = baseband frequency of wanted signal; b = channel bandwidth (3-1 kHz); P — pre-emphasis factor ; 5 / = r.m.s. channel test tone deviation for wanted system; /, = V-V+4'; A ’ A = total r.m.s. deviations of the two signals ; / 0 = frequency separation between the two carriers: Values of B have been calculated for interference between combinations of carriers carr ying various numbers of channels and for a range of carrier spacings. The interfering signals were assumed to consist of a number of carriers each carrying the same number of channels and occupying adjacent frequency allocations spread across the bandwidth of the wanted signal as shown in Fig. 7. The required protection ratio [P( 3. Interference between a multiple carrier frequency-modulation system and a system using single sideband modulation up and single carrier frequency-modulation down 3.1 Down link Since Figs. 8 to 10 indicate that frequency-modulation carriers, with small numbers of channels, are more susceptible to interference than those with large numbers of channels, only interference into the multiple carrier frequency-modulation system need be considered. It is assumed that no significant reduction of receiver threshold is possible for large numbers of channels and the carrier-to-noise ratio in the down link of the single-sideband/ frequency-modulation system is therefore taken as about 16 dB. With a deviation chosen to meet C.C.I.R. noise objectives under clear weather conditions in a system of say 2000 channels, the minimum power per unit bandwidth required at the earth-station receiver input will be about -130-6 dBW per 1 MHz at the centre of the band (assuming 50°K receiving system noise temperature). The corresponding figure for the 200-channel carrier is -135-1 dBW per 1 MHz (i.e. 4-5 dB lower). From Fig. 10 it can be seen, that a protection ratio of about 26-7 dB is required when a 200-channel carrier interferes with a 10-channel carrier. The protection required when the interfering signal is 2000 channels on one carrier is, therefore, about 31 dB (i.e. 4-5 dB greater). The appropriate corrections should be made if the down link employs phase modulation (instead of frequency modulation as assumed). 3.2 Up link Because of the much higher power per unit bandwidth in single-sideband systems com pared with wide deviation frequency-modulation systems, we need only consider interference from the single-sideband system into the frequency-modulation system. Assuming an up-path thermal noise allowance in the single-sideband system of 4000 pW, the single-sideband mean signal power per 3-1 kHz band will be : -1 5 -1 0 log (4000 x 10-9) - 39 dB above the noise power in a 3-1 kHz band or 38 dB above the noise in a 4 kHz band. If the wanted frequency-modulation system uses satellite receivers of similar sensitivity to those of the interfering single-sideband system and the frequency-modulation system up-path thermal noise allowance is say 500 pW (i.e. the frequency-modulation advantage reduces 4000 pW to 500 pW), then, to reduce interference from the single-sideband signal into the frequency- modulation system to 500 pW, a protection ratio of 38 dB would be required from the radiation pattern of the transmitting antenna of the single-sideband system. Allowing for possible differences in the sensitivity of satellite receivers, and departures from the assumed up-path thermal noise allowance of 500 pW for the frequency-modulation system, the protection ratio required might well vary from say 25 dB to 47 dB. The above values assume that the band occupied by the single-sideband transmissions covers the full width of the frequency-modulation signal. As the bandwidth requirements for single-sideband are very modest it may be possible to allocate a portion of the spectrum either for exclusive use of single-sideband or for sharing with wideband frequency-modulation signals only, the single-sideband being well separated from the centre of the frequency-modulation signal. In the latter case the interference, when compared with the above figures, will be reduced by the factor : 2/( /, V 2t t )/ exp —[ ( z + /)2/2//] dz + / e x p - [ ( z -/) 2/2//] dz where f x and f y are the limits of the single-sideband signal relative to the frequency-modulation carrier frequency and f s is the r.m.s. multi-channel deviation of the frequency-modulation signal. s Rep. 210-1 — 298 — As an example of this, consider a 120-channel frequency-modulation carrier in a multiple carrier system sharing with a 1200-channel single-sideband system. Typical r.m.s. deviation for the frequency-modulation signal would be about 1-7 MHz and the bandwidth allowed would be about 20 MHz (including guard bands). The single-sideband signal would occupy 5 MHz and might perhaps be located in the band 5 to 10 MHz from the frequency-modula tion carrier. The interference would be reduced by about 27 dB compared with full overlap of the systems. The probable discrimination required by the earth-station antenna would, therefore, only be about 11 dB. 4. Interference between a frequency-modulation system and a digital system The characteristics of digital modulation systems, which may be used for future commu nication-satellite systems, are not at present known. However, it is likely that the radio- frequency power in any digital system will be spread over a very wide band. If the digital system is designed to provide a high degree of multiple access, then individual channels or small groups of channels could occupy individual bands of radio-frequency spectrum and could, therefore, be fully overlapped by a frequency-modulation carrier. It is this case which would probably prove to be the worst from the interference point of view, and it is likely that a protection ratio of about 31 dB might be required to ensure that interference does not significantly degrade the digital system performance. This figure is based on the assumptions that: — in the digital system, the maximum permissible interference spectral power density is 10 dB below the noise spectral power density ; — the frequency-modulation system is designed to operate at a carrier-to-noise ratio of about 16 dB. Further study of these assumptions may show that a more favourable protection ratio is possible. The determination of interference from a digital transmission into a frequency-modula tion system is not attempted here, because of the uncertainty about the type of digital system likely to be used. However, it seems unlikely that the spectral power density employed will be greater than that for frequency-modulation systems and therefore unlikely that the inter ference resulting from digital systems will be worse than that resulting from frequency- modulation systems. Bibliography 1. H a y a sh i, S. The computation of interference between desired and undesired signals. Journal Institute of Electrical Communication o f Japan (in Japanese) (1952). 2. M e d h u r st , R . G. and R oberts, J. H . Expected interference levels due to interactions between line- of-sight radio-relay systems and broadband satellite systems. Proc. I.E.E., Vol. 111,3 (March, 1964). — 299 — Rep. 210-1 F ig u r e 7 Typical arrangement o f wanted and interfering frequency-modulation signals A : Interfering signal B : Wanted signal C: Carrier-frequency separation Rep. 210-1Rep. K (dB) 2 6 10 8 6 4 2 0 Carrier-frequency separation (MHz) separation Carrier-frequency Wanted system 200 channels 200 system Wanted 300 — F igure 8 5 15 10 20 -K (dB) 2 6 10 8 6 4 2 0 Carrier-frequency(MHz)separation Wanted system 100 channels 100 system Wanted 31 — 301 — F gure r u ig 9 Rep. 210-1 Rep. P ty-L (dB) Rep. 210-1Rep. -K (dB) 1 3 5 4 3 2 1 0 Carrier-frequency separation (MHz) separation Carrier-frequency Wanted system 10 channels 10 system Wanted 32 — 302 — F gure r u ig 10 ( )J -(& 7 (SP) — 303 — Rep. 211-1 REPORT 211-1 * ACTIVE COMMUNICATION-SATELLITE SYSTEMS A comparative study of possible methods of modulation (Question 2/TV, Study Programme 2D/IV) (1963 — 1966) 1. Introduction The purpose of this Report is to give general information concerning some typical modula tion systems which can be used in active communication-satellite systems. The information can also be used to provide an indication of mutual interference with radio-relay systems operating in the same frequency band. However, not all the assumptions have been verified experimentally. Several factors enter into the choice of a method of modulation for active communica tion-satellite systems, some of them being of particular significance in regard to one direction of transmission only. The modulation methods used may not be the same for both directions of transmission. The most important aspects to be considered in this comparison are: — the radio-frequency transmitter power (particularly in the satellite) needed to give accept able signal-to-noise ratio performance; — bandwidth requirements in relation to the traffic capacity to be accommodated, i.e the comparative economy in the use of the radio-frequency spectrum ; — the system flexibility, particularly in regard to the manner by which provision can be made for multi-station access to the system as discussed in Report 213-1 ; — liability to cause interference to, or to receive interference from, systems sharing the same frequency bands; — practicability in the light of current, and probable future, technology. The present study is concerned primarily with multi-channel telephony systems and three distinctly separate methods of modulation appear at this stage to warrant detailed consideration. These are: — frequency-division baseband multiplexing with frequency modulation (FM) of the carrier (FDM-FM); — frequency-division baseband multiplexing with single-sideband modulation (SSB) of the carrier (FDM-SSB); — pulse-code modulation (PCM) of either an FDM baseband or individual telephone chan nels multiplexed in time (TDM), using phase-shift keying of the carrier as the modulation method (FDM-PCM-PSK or TDM-PCM-PSK). These three basic methods are compared, but it should be borne in mind that each method may have variants which are not considered here ; a study of these variants is not expected to greatly affect the conclusions reached. All three methods of modulation can be considered for frequency-division multiple access (FDMA) to the satellite repeater. PCM-PSK can also be considered for time-division multiple access (TDMA) to the satellite repeater. * This Report was adopted unanimously. Rep. 211-1 — 304 — 2. Assumptions made and limitations involved in the comparison 2.1 General assumptions 2.1.1 Detailed calculations refer to multi-channel telephony only. 1200 telephone channels have been assumed as typical of the maximum capacity likely, for the present, to be associated with one wide-band radio-frequency channel. It can be shown that, if the signal-to-noise ratio requirements are met for 960 telephony channels, they can also be met for 625-line monochrome television. 2.1.2 The noise allowance for any telephone channel in the hypothetical reference circuit is given in Recommendation 353-1. As stated in Note 6 of the Recommendation, 1000 pWOp* of the total allowable 10 000 pWOp may be caused by interference from trans mitters of line-of-sight radio-relay systems. This has been taken into consideration, except in the case of PCM, since in normal circumstances no great increase in noise due to outside interference occurs with this method. The noise allocations will be found in the following sections. 2.1.3 It can be assumed, that future communication-satellite systems will work with satellite orbit heights between 10 000 and 36 000 km. Satellite attitude stabilization will permit the use of antennae at the satellite giving a radiation pattern covering only the visible part of the Earth. This type of space-station antenna is assumed throughout this Report. Under these assumptions, transmission loss is practically independent of orbit height and transmission frequency, but is a function of the gain of the earth-station antenna. For a large earth station with a 25 m diameter antenna, the transmission loss is no greater than 122 dB, as shown in the Annex, and this value is used throughout the Report. 2.1.4 The overall system noise temperature is assumed to be 50°K for a large-antenna earth station and 1200°K for the space station. 2.1.5 The effects of transmission delay and its variation (e.g. Doppler shift and pulse width variation), on various methods of modulation, are discussed in Reports 214-1 and 383. 2.2 Basis o f comparison and limitations involved in the study 2.2.1 This study provides a basis for a comparison of modulation methods and indicates the factors which influence optimum solutions for the examples investigated. However, economic as well as technical factors will govern the choice of the modulation method to be adopted in any specific system. Two different situations may be considered : — “bandwidth limitation”, permitting economy in the use of the radio-frequency spectrum; — “power limitation”, permitting economy in the use of space-station power. The calculations for FM and PCM‘methods are based on power limitation, taking advantage of the modulation improvement factor of wide-band transmission or coding systems. Since wide-band systems have characteristic thresholds of operation, an operating margin of 6 dB in the satellite-to-earth link is assumed, a substantial part of which is to allow for the increase in attenuation caused by rain. Threshold for FM is here defined as that value of carrier-to-noise ratio for which the signal-to-noise ratio in the highest telephone channel is 1 dB less than it would be if the linear relation valid above threshold were extrapolated. For simplicity, it is assumed that the carrier-to-noise power ratio at threshold (C/N ) thresh is 10 dB. Furthermore, it is assumed that the threshold value for * pWOp is the psophometrically-weighted mean power at a point of zero relative level. — 305 — Rep. 211-1 PCM is reached when the overall noise in the worst telephone channel is as high for PCM as for FM at threshold. The noise power in a telephone channel originates from various noise sources, which include: — thermal noise arising on the earth-satellite and satellite-earth paths; — intermodulation noise produced by delay distortion; — intermodulation noise produced by the transmission of several carriers in a common space-station repeater (multiple-access); — PCM quantization noise when an FDM-PCM signal is transmitted; — idle channel noise when TDM-PCM signals are transmitted ; — clicks near threshold for both cases of PCM. This Report does not suggest optimum noise allocations. Only simple assumptions are made; the complex problem of noise allocation must be studied further for the methods of modulation considered. 2.2.2 The detailed calculations do not take into account the use of syllabic compandors on telephone circuits for the following reasons : The effect of syllabic compandors on telephone transmission performance is still under study within the C.C.I.T.T. For the time being, the C.C.I.T.T. does not recom mend the systematic use of such compandors (conforming to the present specification) on international circuits. This means that, for the present, compandors should only be used in international circuits on which the noise notably exceeds the design objectives ; international systems should not be designed on the basis of higher noise objectives, with the intention of operating most telephone circuits with compandors. The use of compandors implies demodulation to audio frequencies at some point in the telephone network, where both echo suppressors and compandors might be used. For other services, e.g. telegraphy, compandors offer no benefit, and may even degrade service. When telephone circuits are used for the transmission of telegraphy, a guide to the desired noise performance is given in C.C.I.T.T. Recommendation G.153 (Blue Book, Vol. III). 2.3 Operational modes Two methods of operation are examined : single-access and multiple-access. Two types of space-station repeater are also considered, frequency translating repeaters and the more complicated repeaters which change the method of modulation. The examples given in Section IV cover the cases of 1, 12, 24, 60, 120, 240, 600, 960 and 1 2 0 0 telephone channels per carrier. 2.4 Channel capacity in relation to transmitter powers required at the earth station and the space station The transmission loss on the earth-to-satellite and satellite-to-earth paths is, as has already been stated, a function of the diameter of the earth-station antenna, D. The smaller the antenna, the more the transmitter power needed both at the earth station and at the space station to provide a given number of telephone channels. In addition, so far as the satellite-to-earth path is concerned, the space-station transmitter power required depends also upon the system of the earth-station noise temperature, T. For each of the two paths, a station efficiency factor, r|, may be determined by the follow ing relationships; taking q as unity when D is 25 m and T is 50°K. For the earth-to-satellite path: tw = (D/25)2 Rep. 211-1 — 306 — For the satellite-to-earth path: T)**. = (50/r)(Z)/25)2 The transmitter power required for a given channel capacity is, of course, inversely pro portional to the station efficiency factor, typical values of which are given in Table I. In the Table, the efficiency factors are quoted both numerically and on a decibel scale. T a b l e I Earth-station efficiency factor, iy Type of earth station Large antenna. 25 m Medium antenna, 16 m Small antenna, 9 m 50°K 100°K 150°K Ratio (dB) Ratio (dB) Ratio (dB) Earth-to-satellite...... 1 0 0-41 - 4 013 - 9 Satellite-to-earth ...... 1 0 0-20 - 7 0-04 — 14 3. Transmitter power for single access In this section, the transmitter power for the space station (PSm) and the transmitter power for the earth station are determined for FM, SSB and PCM-PSK. It is assumed in all three cases that the space-station repeater is a frequency-translator. The following formula is easily derived: PSat / PEarth = TEarth / (<7 • Tsat) where Input carrier-to-noise ratio at the space station ^ ~ Input carrier-to-noise ratio at the earth station A value of q — 4 has been assumed here, since this is compatible with present day tech nology. In fact, the particular value, chosen for q, has little bearing on the comparison of methods of modulation, although it is important for planning communication-satellite sys tems. 3.1 Frequency modulation 3.1.1 Required signal-to-noise ratio in a telephone channel due to thermal noise arising in the down path It is assumed, that four-fifths of the allowable noise power of 9000 pWOp (see § 2.1.2) occurs in the down path and that 75% of it is thermal noise. Thus, the thermal noise power in the worst channel is 6750 pWOp (i.e. 1350 pWOp arising in the up path and 5400 pWOp arising in the down path). This corresponds to a signal to psopho- metrically weighted noise ratio (S /N )TD of 52-7 dB. 3.1.2 Required carrier-to-thermal noise ratio on the down path It is assumed that threshold occurs when the ratio of downcoming carrier power- to-total thermal noise power is 10 dB. Thus, assuming an operating margin, m, 1/10 - m (N/C)TD + (N/C)tu where (N jC )TD, (N /C )TU are the ratios of the thermal noise power to the carrier power on the down-path and on the up-path respectively. — 307 — Rep. 211-1 If m is taken as 4 (i.e. 6 dB) and the ratio of the carrier-to-noise ratio on the up path to that on the down path is also taken as 4 (see § 3) then substitution in the equa tion given in the first paragraph of this section leads to (C/N)td = 16-3 dB 3.1.3 General analysis One form of the basic relationship between the carrier-to-noise ratio and the signal-to-noise ratio in a frequency-modulation system is S/N = (C/N) (.Fch/fmf (.Brf/b).P. W where S/N is the ratio of test-tone power (i .e. 1 mW at a point of zero relative level) to the psophometrically-weighted noise power in the highest telephone channel C/N is the carrier-to-noise ratio ; Brf is the occupied radio-frequency bandwidth (Hz); b is the bandwidth of the telephone channel (Hz); Fch is the r.m.s. test-tone deviation per channel (Hz); f m is the mid-frequency of the highest baseband channel (Hz); P is the pre-emphasis improvement factor; and W is the psophometric weighting factor. In this Report, P and W are assumed to have the values of 4 dB and 2-5 dB res pectively. It is assumed that in the expression given above only Br{ and Fch are unknown. To solve for Fch and hence Brf, it is necessary to find a further relation between these two quantities. To do this it is first assumed that the bandwidth Brf is given by Brf — 2(AF + f m) (i.e. the “Carson’s rule bandwidth”), where APis the multi-channel peak deviation. To restrict the intermodulation noise due to bandwidth limiting (“truncation noise”) to a tolerable level, it is necessary to define a suitable relationship between AF and Fch- The following relationship for frequency-division multiplex basebands of more than 1 2 0 channels has been suggested : A F = P ca-IO ^ -d /20 where Ln is a factor which is numerically equal to the equivalent peak power level (in dB) as defined in C.C.I.T.T. Recommendation G.223. It has been found experimentally that the use of this relationship in association with the formula for Brf given previously will limit truncation noise to about 100 pW in a telephone channel. Further restriction of bandwidth results in a sharp increase of noise [3]. Once the test-tone deviation Fch and the radio-frequency bandwidth Brf have been established, the noise power at the receiver input may be calculated from a knowledge of the effective noise temperature of the system. Since the required ratio, C/N, has already been fixed, the necessary carrier power at the receiver input can be derived. The power required at the appropriate transmitter is directly related to the required carrier input power at the receiver by the transmission loss of the system. 3.1.4 Analysis for the case o f threshold extension In § 3.1.2 it was assumed that threshold occurs at a carrier-to-noise ratio of 10 dB. The threshold in the radio-frequency bandwidth may be depressed by the use of known techniques (e.g. frequency feedback or phase-locked demodulators). Rep. 211-1 — 308 — Insufficient information is available on the threshold extension which can be achieved in practice. For the purpose of this study it is assumed that the threshold extension achievable is given by : Threshold extension = 10 log (Brfj Bfb) where Brf is the radio-frequency bandwidth as defined in § 3.1.3 and Bfb — 2[(AF/R) + f m] 6 x 1 0 ® Wherc R = 7m + (0-41 X 106) for /» < 5-« x JO* Hz R = 1 for f m ^ 5-6 x 106 Hz These formulae for R are based on the analysis of [1] and limited experimental evidence. Systems carrying small numbers of channels have yet to achieve the threshold extension implied, but threshold extension demodulators are becoming available which give better results than those indicated by the above expression for the higher values of f m. Reference [2] discusses some practical threshold extension demodulators. It should be emphasized that the use of threshold extension does not modify the basic relationship between signal-to-noise and carrier-to-noise in frequency-modulation systems ; it merely permits the use of lower carrier-to-noise ratios than would otherwise be possible. It is therefore unnecessary to add to the analysis given in §§ 3.1.2 to 3.1 .4. 3.1.5 Using the relationships given in §3.1.2 to §3.1.4 and the assumptions detailed in §§2 .1 and 2 .2 , the following characteristics of a number of frequency-modulation sys tems have been calculated and recorded in Table I I : — Psat: the transmitter power required at the space station ; — Pearth '• the transmitter power required at the earth station; — Brfn : the radio-frequency bandwidth required for a carrier bearing n telephone channels. Fig. 1 shows the required value of average r.m.s. deviation per telephone channel. T a b l e II Bandwidth and transmitter power for frequency-modulation systems Total number of telephone channels 12 24 60 120 240 600 960 1200 Equivalent peak power level Ln (dBmO). . . 19 19-5 20-8 21-2 22-3 - 25 27 27-8 Maximum baseband frequency (MHz) . . 0-06 0-12 0-252 0-552 1-052 2-54 4-028 5-56 Occupied bandwidth Brf (M H z)...... 2-96 4-64 8-1 12-5 19 36-5 51-6 64-7 Transmitter power Psat (d B W )...... -1 7 -9 -1 5 -5 -1 2 -0 -8 -5 -5 -5 -0 -2 2-8 4-8 Transmitter power Pearth (dBW) .... 1-9 4-5 7-5 11-0 14-5 19-5 22-5 24-5 — 309 — Rep. 211-1 3.1.6 To permit a comparison of FM and PCM in accordance with § 2.2.1, it is necessary to determine the noise power in the highest baseband channel of a frequency-modulation system at threshold (Nthres). This comprises up-link thermal noise, down-link thermal noise and intermodulation noise : Nthres = 1350 + (4 x 5400) + 2250 = 25 200 pWOp 3.2 Pulse-code modulation (PCM) The performance characteristics of PCM systems are still under study in C.C.I.T.T. Study Group XV. Little information is available on high quality PCM systems on radio links. For the power limited case considered in this Report, two-phase PSK provides the lowest error rate and the consideration of PCM is therefore restricted to this modulation method. Transmission of a large number of telephone channels is possible, either by encoding individual telephone channels and multiplexing them in time division (TDM-PCM), or by encoding a frequency-division multiplex baseband (FDM-PCM). It is shown in § 3.2.1 that the radio-frequency bandwidth and transmitter powers required both differ by approximately 1 dB in favour of TDM-PCM for a given capacity. Therefore, these cases will not be treated separately. 3.2.1 Replies to C.C.I.T.T. Question XV/33 have indicated that for TDM-PCM, adequate quality may be obtained with seven bits per sample provided non-linear coding is used. Assuming the conventional sampling frequency of 8 kHz per speech channel and adding an eighth digit for signalling and 5% for synchronization, the bit rate for n channels = 1-05 x 64x 103xrc bits/s = 67 200 n bits/s. PCM-transmission of an FDM multi-channel signal with 8 bits per sample results in a quantization noise power well below 10 000 pWOp for FDM-systems with 120 and more channels. If the sampling frequency is chosen as 2-3 f m, f m being the highest base band frequency, and if 5% is added for synchronization, the bit rate is 1 -05 x 2-3 x 8 x f m bit/s. As f m is approximately equal to 4200 n, n being the number of telephone channels, the bite rate for FDM-PCM is 81 x 103 x n bits/s. The bit rate for FDM-PCM is thus higher in the ratio 81/67-2 = 1-2 than the bit rate for TDM-PCM. This means that the bandwidth, and therefore the power for FDM-PCM, are 0-8 dB greater than the cor responding values for TDM-PCM. 3.2.2 For PCM-PSK it is assumed, that the radio-frequency bandwidth required for trans mission (.Brf) is 1-2 x (bit rate). For a given carrier-to-noise ratio, the space-station trans mitter power required is proportional to the radio-frequency bandwidth and, therefore, to the number of channels, n. Thus, analysis for PCM is reduced to the calculation of one example, as all the other cases can be derived by simply modifying the value of n. Since PCM-PSK with synchronous detection does not show a sharp radio frequency-threshold, it is the post-detection signal-to-noise ratio which determines the noise power in a speech channel caused by bit errors. Analysis shows that—irrespective of the number of channels—a bit-error probability of 3x 10-5 corresponds to a noise power of approximately 20 000 pWOp in a telephone channel. This statement holds with negligible error both for PCM-TDM and FDM-PCM. A bit-error rate of 3 x 10-5 obtains when E/N0 = 10 dB, E being the energy per bit in watt seconds and where N0 is the noise power in a bandwidth of one hertz [4]. This corresponds to a radio-frequency carrier-to-noise ratio (C/N), in the occupied band width, of 9-2 dB. Using the same assumptions as in § 3.1.1, but with a threshold carrier-to-noise ratio of 9-2 dB, we find (C /N )TD = 15-5 dB. 3.2.3 Results are presented in Table III. Rep. 211-1 — 310 — T a b l e III Bandwidth and transmitter power required for single access for TDM-PCM-PSK and (in brackets) for FDM-PCM-PSK Total number of telephone channels 12 24 60 120 240 600 960 1200 Occupied bandwidth Brf (M H z)...... 0-97 1-93 4-84 9-68 19-3 48-4 77-4 96-8 (M 7) (2-34) (5-86) (H-7) (23-4) (58-5) (93-8) (117-0) Transmitter power Psat (d B W )...... -1 4 -2 -1 1 -2 -7 -2 -4 -2 -1 -2 2-8 4-8 5-8 (-3 -4 ) (-0 -4 ) (3-6) (5-6) (6-6) Transmitter power Pearth (dBW) .... 5-6 8-6 12-6 15-6 18-6 22-6 24-6 25-6 (16-4) (19-4) (23-4) (25-4) (26-4) There are several methods of improving the performance of digitized telephone channels in a power-limited case. One method is to encode the PCM binary data by processing 4 bits at a time into bi-orthogonal 8 -bit words [5] (PCM-ortho-PSK in Fig. 3). This effectively reduces the calculated threshold at the receiver by about 2-5 dB, but requires twice the bandwidth. It is also possible to encode the telephone channels by a modified delta modulation technique [6 ] using 40 800 bits per channel, including synchronization. Compared with TDM-PCM-PSK this method leads to a reduction of the radio-frequency bandwidth by the factor (40 800/67 200) = 0-61 (-2-1 dB). If, additionally, orthogonal encoding is used, the advantage over TDM-PCM-PSK becomes 4-6 dB. However, telephone channel quality, using modified delta modulation techniques (Delta-ortho-PSK in Fig. 3), is not yet known and such techniques require further study to verify that the appropriate standards can be met. 3.3 Single-sideband transmission Single-sideband transmission is simple in concept and has the merits of greatest economy in bandwidth and efficient use of transmitter power. Unlike frequency-modulation, it is not subject to threshold effect. On the other hand, single-sideband modulation has certain disadvantages relative to the other two methods, e.g. it requires higher transmitter power; it is more sensitive to non- linearity ; it requires highly accurate frequency synchronization, which is particularly difficult to achieve with non-stationary satellites ; it is both more susceptible to interference, and will produce more interference when its signals fall within one base-bandwidth of the carrier of other systems. The same noise allocation is assumed for single-sideband as for frequency-modulation (see § 3.1.1), although for single-sideband it may be more difficult to provide the necessary linearity. In a single-sideband transmitter, it is necessary to consider both mean power and peak power. These may be determined in accordance with C.C.I.T.T. Recommendation G.223 in a manner similar to § 3.1.2. The question of the peak factor to be used for radio-frequency power amplifiers requires further study. — 311 — Rep. 211-1 T a b l e IV Transmitter power required for single-sideband satellite systems for single access Total number 12 of telephone channels 24 60 120 240 600 960 1200 Average transmitter power Psat (dBW) . . -1 -2 0 1-6 2-8 4-3 8-3 10-3 11-5 Peak transmitter power Psat (dB W )...... 14-5 15-0 16-3 16-7 18-0 20-5 22-5 23-5 Average transmitter power Pearth (dBW) . 18-6 19-8 21-4 22-6 24-1 28-1 30-1 31-1 Peak transmitter power Pearth (dBW) .... 34-3 34-8 36-1 36-5 37-8 40-3 42-3 43-3 The use of syllabic compandors would significantly reduce these power requirements (see § 2 .2 .2 ). 3.4 Summary for single access Comparison of Tables II, III and IV shows that, while the required values both of band width and power for frequency-modulation and pulse-code modulation are of the same order, those for single-sideband differ greatly. Where spectrum economy is of great importance, the use of single-sideband may need to be considered. For large numbers of telephone channels per carrier, frequency-modulation is superior to pulse-code modulation for the assumptions made here, since it requires less transmitter power and radio-frequency bandwidth. Furthermore, frequency-modulation is, in contrast to pulse- code modulation, a well-established technique ; the values determined for frequency-modula tion, therefore, may be closer to practice than those given for pulse-code modulation. Some points should be noted in favour of pulse-code modulation : — the signal-to-noise ratio for pulse-code modulation under normal operating conditions is better than for frequency-modulation, as far as idle channel noise is concerned; — under normal operating conditions pulse-code modulation has a very high degree of immunity against interference; — TDM-PCM systems are more readily adaptable to the transmission of digital information. 4. Multiple access Report 213-1 deals with multiple access in a general manner and discusses the problem of mixing stations with different sensitivities. This study supplies supplementary material, mainly in the form of numerical examples. Rep. 211-1 — 312 — T a b l e V Examples o f methods o f modulation for multiple access Earth-station Space station Earth-station References Example transmission receiver FDM-FM Frequency translating repeater; One receiver §4.1 A carrier multiple carriers combined in per carrier Table VI repeater (FDM-FM-FDMA) TDM-PCM- Frequency translating repeater; One receiver §4.2 B PSK or FDM- multiple carriers combined in per carrier Table VII PCM-PSK repeater (PCM-PSK-FDMA) carriers Synchronized Frequency translating repeater; Single wide §§ 3.2.3, 4.3 periodic transmissions multiplexed by time band receiver Table III C transmission division (PCM-PSK-TDMA) of TDM-PCM- PSK SSB-FDM Common SSB receiver; Single wide §§ 3.3, 4.4 D signals modulation conversion ; band receiver Table IV FM transmitter (SSB-FDMA/FM) 4.1 Example A (FDM-FM-FDMA) Transmission of several radio-frequency carriers through a common space-station repeater produces intermodulation noise. The allowable 9000 pWpO noise power in a telephone channel (see § 2 .1 .2 ) must include this source of noise and is therefore reallocated, for the multiple-access case, as follows : 2250 pWOp intermodulation noise arising in the earth station; 3000 pWOp intermodulation noise arising in the common repeater; 750 pWOp thermal noise in the up link; 3000 pWOp thermal noise in the down link. The transmitter power and bandwidth required at each earth station may be calculated in accordance with § 3.1.2. The space-station transmitter power and bandwidth for each carrier may be similarly calculated, and the total transmitter power (Psat) and bandwidth obtained by simple addition. It may be necessary to operate the space-station transmitter somewhat below saturation power (back-off), to maintain the intermodulation noise below the maximum permissible amount. Since present knowledge regarding the necessary amount of back-off is inadequate, Table VI shows only the required transmitter power, and not the saturation power. To avoid possible interference between the several carriers, guard bands may be required. In the calculation, it has been assumed that these amount to 25% of the occupied bandwidth. — 313 — Rep. 211-1 T a b l e VI Example A : Common repeater for multiple FDM-FM carriers : radio-frequency bandwidth and transmitter power required Total Earth-station Number Number Total transmitter power transmitter power of telephone channels bandwith of carriers per carrier (MHz) Psat per carrier (dBW) (dBW) 5 240 135 3-0 15-8 10 120 178 2-7 12-5 20 60 226 2-2 9-0 50 24 332 3-0 5-8 100 12 424 3-3 3-1 A comparison of Psat in Table II with Psat in Table VI, assuming a multi-carrier back-off of 3 dB, shows that multiple access entails a moderate increase in space-station transmitter power which depends on the number of carriers, and an appreciable increase in radio frequency bandwidth. 4.1.1 FM-FDMA with a single telephone channel per carrier Curves B of Figs. 2 and 3 show the requirements for the radio-frequency bandwidth and the transmitter power which have been calculated under the following assumptions : Total noise power (pWOp)...... 10000 Space-station intermodulation noise (pW Op)...... 2000 Earth-station intermodulation noise (pW Op)...... 500 Thermal noise — up link (pWOp)...... 1500 Thermal noise — down link (pWOp)...... 6000 In this case it has been assumed that no pre-emphasis is used, and that the peak-to- r.m.s. ratio of the signal is 10 dB. 4.2 Example B (PCM-PSK-FDMA) The assumptions of § 4.1 are made with regard to noise allocation. On this basis, the required carrier-to-noise ratio (.C/N)TD assuming an operating margin of 6 dB is 16-4 dB, (C/N)thresh is 9-2 dB as in § 3.2.2. To allow for the difference of 0-9 dB between 16-4 dB and the corresponding value of 15-5 dB in § 3.2.2, the transmitter power per carrier must be increased accordingly. The total space-station power (Psat) is found by addition as in § 4.1, again bearing in mind that Psat must remain somewhat below saturation power. In determining the total radio-frequency band width, the same factor of 25% for guard bands is assumed as in § 4.1. 9 Rep. 211-1 — 314 — T a b le V II Example B : Common repeater for multiple PCM-PSK-FDMA carriers : radio-frequency bandwidth and transmitter-power required Number Total Total Earth-station Number transmitter power transmitter power of carriers of telephone channels bandwidth per carrier (MHz) p sat per carrier (dBW) (dBW) 5 240 122 6-7 19-5 10 120 122 6-7 165 20 60 122 6-7 13-5 50 24 122 6-7 9-5 100 12 122 6-7 6-5 A comparison of Tables III and VII shows that, irrespective of the number of carriers, and assuming a multi-carrier back-olf of 3 dB as in § 4.1, multiple access entails a 4 dB increase in space-station transmitter power and a 25% increase in the radio-frequency bandwidth. 4.3 Example C (PCM-PSK-TDMA) In the case of time-division multiple access, each participating earth station is assigned a time slot. Considerations of synchronization and timing determine the minimum duration of a time slot. A detailed review of this modulation method is given in Report 213-1. To permit the proper interleaving of time slots, they must be separated by guard intervals, which are here assumed to require 5% of the frame duration. It is necessary to increase the trans mission rate by the same factor, to compensate for this loss in available transmission time. The bandwidths and powers required can be obtained from the PCM-TDM-PSK examples in Table III by multiplying by 105 and adding 0-2 dB respectively. 4.4 Example D (SSB-FDMA/FM) This example involves modulation conversion at the space station. Since, in this case, the up-path thermal noise does not affect the threshold, it may be ignored in determining the radio-frequency bandwidth and transmitter power required at the space station, and (C/N)TD is then 16 dB, instead of 16*3 dB as given in § 3.1.1. The use of this value gives the following results for a capacity of 1 2 0 0 telephone channels : Brf = 6 6 MHz ; Psat = 4-7 dBW. Since the up path uses single-sideband modulation, the required earth-station transmitter power may be taken directly from Table IV. — 315 — Rep. 211-1 4.5 Summary for multiple access Fig. 4 shows, for all four examples, the total radio-frequency bandwidth (Brf) and space- station transmitter power (Psat) required as a function of the number of space-station carriers, as well as the required earth-station transmitter power (Peanh). System capacity is assumed to be 1200 telephone channels. All carriers are assumed to bear equal numbers of telephone channels. A multi-carrier back-off of 3 dB has been assumed for FDM A. The first three examples would technically permit the inclusion of large and small antenna earth stations in a system on the basis of Table I, but in cases 1 and 2 this is likely to aggravate the space-station intermodulation problem. It should be noted that in PCM-PSK-TDMA, it is alternatively possible to use corres pondingly longer pulses for transmission to less sensitive stations. In SSB-FDMA/FM, all earth stations must demodulate the entire down-path transmis sion, and the system channel capacity is therefore limited by the least sensitive station. A high degree of coordination among earth stations is required for these multiple-access examples. In examples A, B and D, earth-station transmitter power must be controlled to ensure that all carriers have the same power level at the space station, regardless of the variations in transmission loss. It is also necessary to ensure, that frequencies transmitted by the earth stations are received at the space stations in the correct relationship and this requirement is particularly stringent for example D. Similarly stringent requirements of synchronization apply to example C. This Report considers only certain of the modulation methods discussed in Report 213-1. Another method of multiple access, which is equally suitable for different methods of modulation, involves the use of a separate repeater for each carrier. This has not been discussed separately here, since it is a straightforward extension of the single-access cases discussed earlier. 5. Interference between communication-satellite systems and other radio services 5.1 Frequency-modulation Frequency-modulation produces a spectrum with an energy distribution which is a func tion of the modulating signal. With lightly loaded or unmodulated frequency-modulation multi-channel telephony systems, a considerable part of the total energy is concentrated in relatively narrow frequency bands, so that a greater interference potential exists than for a normally loaded frequency-modulation system. This situation may be avoided by carrier energy dispersal. 5.2 Pulse-code modulation Under conditions of light loading, a pulse-code modulation system produces a multiple line spectrum with components spaced at Ijyt, where y is the number of bits per sample and t the bit duration. The effect of loading is to decrease the amplitudes of the line components and to fill in the spaces between the lines. With y = 7, the maximum line amplitude is about 0-3 of the unmodulated carrier power. In the absence of special precautions, the interference potential of lightly loaded pulse-code modulation systems could be significantly greater than that of frequency-modulation systems of similar capacity and performance. However, the interference potential of pulse-code modulation systems under light loading conditions can readily be'reduced by one of several techniques, e.g. the use of modified coding which avoids repetitive patterns. Pulse-code modulation systems are less susceptible to interference than frequency- modulation systems, and much less susceptible than single-sideband systems. Rep. 211-1 — 316 — Attention is drawn to Report 388, which discusses interference to pulse-code modulation systems. 5.3 Single-sideband modulation Single-sideband modulation concentrates the transmitted energy into a bandwidth equal to the width of the baseband, and the power of any telephone channel is confined to a bandwidth equal to that of the channel. As a result, there is a risk of a high level of interference from the single-sideband modulation down-link transmission to terrestrial radio-relay systems which use frequency-modulation, when a single-sideband modulation signal falls within the first set of sidebands of the terrestrial system. Since there is no demodulation improvement associated with single-sideband modulation it is more vulnerable to interference than frequency-modula tion or pulse-code modulation. 6. Power flux Report 387 and Recommendation 358-1 establish the maximum permissible power flux- density produced by a space station at the surface of the Earth. It can be shown that if carrier energy-dispersal is applied, this limit is not exceeded for all examples, except § 3.3. At present, the use of single-sideband modulation in the down path is likely to exceed the permitted value of power flux-density. 7. Summary This Report has considered the advantages and disadvantages of several modulation methods suitable for communication-satellite systems. It has been necessary to make a number of assumptions, some of which require experimental verification. Bibliography 1. E n l o e , L . H. Decreasing the threshold in FM by frequency feedback. Proc. IRE 50 18-32 (1962). 2. L e f r a c k , F., M oo re, H. and N e w t o n , A. An FDM-FM feedback demodulator for tropo and satellite relay communications. U.S. State Department Seminar on Earth Station Technology, Washington D.C. (May 16-27, 1966). 3. K r e il , S., M e t z g e r , E. and H aas, R. Intermodulations-Gerausche bei Satelliten-Systemen mit FM infolge harter Bandbegrenzung des FM Signals (Intermodulation noise arising in satellite links due to reduction of the bandwidth for transmission of FM-signals), NTZ, H. 3 (1966). 4. C a h n , C . R. Performance of digital phase modulation communications. Trans. IRE, Vol. CS-M, 1, 3-6 (May, 1959). 5. V it e r b i, A. J. On coded phase coherent communications. IRE Trans, on space electronics and tele metry (March, 1961). 6. D e J ager F . and G reefkes, J. A . Continuous delta modulation. Globecom VI Symposium, Philadelphia, Pa. (June, 1964). — 317 — Rep. 211-1 ANNEX CALCULATION OF TRANSMISSION LOSS AS A FUNCTION OF SATELLITE ALTITUDE Satellite altitude (km) 10 000 14 000 20 300 36 000 Maximum range to an earth station (for an antenna elevation of 5°) (km) . . 14 900 18 700 25 400 41 300 Transmission frequency (GHz) ...... 4 6 4 6 4 6 4 6 Beamwidth of space-station antenna with at least ± 3 ° allowance for attitude errors (degrees). .... 50-7 42-5 33-7 23-3 Free space loss (dB) . . . -1 8 8 -191-5 -1 9 0 -193-5 -192-6 -196-1 -196-8 -200-3 Space-station antenna gain (dB)...... 8-0 10-0 12-6 16-8 Earth-station antenna gain for a diameter of 25 m ( d B ) ...... 58 61-5 58 61-5 58 61-5 58 61-5 Transmission loss of the system...... 122-0 122-0 122-0 122 Rep. 211-1Rep. vrg rms feuny eito e tlpoe hne rsligfo a ettn o dBmO 0 of test-tone a from resulting channel telephone per deviation frequency r.m.s. Average Average r.m.s. frequency deviation Number of telephone channels, channels, telephone of Number 38 — 318 — F gure r u ig 1 n eesr saesain adit fr utpecrir D-M systems FDM-FM multiple-carrier for bandwidth space-station Necessary Necessary bandwidth of the space-station repeater (MHz) 0 0 0 0 20 0 10 2000 1000 500 200 100 50 20 10 Curve Curve oa nme o oewy eehn channels telephone one-way of number Total B A Oecrir e saesain repeater space-station per One-carrier : Snl canl e carrier per channel Single : 39 — 319 — F gure r u ig 2 Rep. 211-1Rep. 3.1.3 n 3 ig u r e F — 320 — Number of telephone channels, C: PCM-TDM-PSK D : PCM-ortho-PSK E : E : Delta-ortho-PSK @ @ : Values calculated according to § Curve Curve B : Single-channel per FM carrier Curve Curve A : FDM-FM, single carrier (Results for digital systems are independent of the number of channels) (ap) (XU/D) 0TSoi 01 Carrier power (W)-to-noise temperature (in °K) per telephone channel, CjnT, for a margin of 6 dB Rep. Rep. 211-1 u (dBW) Br{ (MHz) 5 0 0 0 100 50 20 10 5 2 b Saesain rnmte pwr Psat power transmitter Space-station (b) a Rdofeuny adit Brf bandwidth Radio-frequency (a) ubr f carriers of Number ubr f carriers of Number 6c 9,- : d BV dE 31 — 321 — !W£_3,5; V^4W 9,3 - 3W )W N B C A D - " Rep.|211-1 Rep. 211-1 — 322 — Number of carriers (c) Earth-station transmitter power per carrier, Pearth * Mean power ** Peak power F ig u re 4 Brf, Psat! Pearth as functions o f the number, m, of carriers — parameters on the curves according to Table V Note : Curves are lettered to correspond to the examples given in Table V. — 323 — Rep. 212-1 REPORT 212-1 * ACTIVE COMMUNICATION-SATELLITE SYSTEMS FOR FREQUENCY-DIVISION MULTIPLEX TELEPHONY AND MONOCHROME TELEVISION Use of pre-emphasis in frequency-modulation systems (Question 2/1V) (1963 — 1966) 1. The use of pre-emphasis in active communication-satellite systems for frequency-division multiplex telephony using frequency-modulation results in a useful improvement in the signal-to-thermal noise ratio in the higher frequency channels of the system and thus enables the space-station transmitter power requirements to be reduced. The use of pre-emphasis for television could modify the energy distribution in the radio- frequency emission of communication-satellite systems, in such a way as to reduce, substan tially in some circumstances, the possibility of interference between communication-satellite systems and line-of-sight radio-relay systems using the same or adjacent channels. The use of pre-emphasis for television may also enable the effective frequency deviation of the communication-satellite system to be increased, thereby improving the signal-to-noise ratio ; however, too large an increase in deviation could offset the reduction of interference potential. The use by different Administrations of the facilities offered by active communication- satellite systems, including the shared use of space-station repeaters, would be facilitated by the use of an agreed pre-emphasis characteristic for such systems employing frequency- modulation. 2. Conclusions In view of the foregoing, and of the potential advantages of pre-emphasis in frequency- modulation systems, it is considered that the preferred pre-emphasis characteristics for frequency-division multiplex telephony and monochrome television should be the subject of further study. In certain experiments with communication-satellite systems using wide-deviation fre quency modulation, the same standard pre-emphasis characteristics have been employed, as are recommended for terrestrial line-of-sight radio-relay systems in Recommendation 275-1 for frequency-division multiplex telephony and in Recommendation 405 for monochrome television. However, the following characteristics, peculiar to communication-satellite systems, may be taken into account: — operation close to the receiver threshold (because of the low level of the received signal and the limited range of its variations), which modifies the thermal-noise spectrum; — the possibility of reducing intermodulation noise, by virtue of the fact that there are only a few circuit elements which are liable to produce such noise. Owing to these special factors, the distribution of the noise sources is different from that obtained in radio-relay systems. In particular, in the case of frequency-division multiplex telephony, the use may be envisaged of a pre-emphasis characteristic different from that recommended for radio-relay links, which would have the same form but a smaller overall * This Report was adopted unanimously. Rep. 212-1 — 324 — range of variation and would be better adapted to the special features of communication- satellite systems. An overall range of variation equal to 6 dB is suggested, and it would be desirable that the Administration concerned make experimental studies of the improvement obtained by the use of the pre-emphasis network described in the Annex. ANNEX PRE-EMPHASIS CHARACTERISTICS OF FREQUENCY-MODULATION SYSTEMS FOR FREQUENCY-DIVISION MULTIPLEX TELEPHONE TRANSMISSION 1. Characteristics of communication-satellite systems Frequency-division multiplex telephony using frequency modulation produces noise of different powers in the different telephone channels. By using pre-emphasis, the relative frequency deviations for these channels can be modified with a view to making the overall noise, thermal noise and intermodulation noise powers equal in these channels. If intermodulation noise is disregarded, the pre-emphasis characteristic should have the same shape as the total thermal-noise spectrum. At operating points near threshold, the total thermal noise consists of two terms of the same order of magnitude, threshold-effect noise and linearly-demodulated thermal noise. The first has a uniform spectrum (white noise), the second a parabolic spectrum. At an operating point far above threshold, as in radio-relay systems, threshold-effect noise is negligible and we again find the well-known parabolic spectrum. Far below threshold, linearly-demodulated thermal noise is negligible and the noise spectrum is uniform. Near threshold, the shape of the total thermal-noise spectrum thus depends on the level of the received signal. Below threshold, the signal-to-noise ratio decreases rapidly as the carrier-to-noise ratio falls. It does not, therefore, seem advisable to operate communication-satellite systems at a carrier-to-noise ratio less than the “ 1 dB threshold point” ; this point is defined as the carrier- to-noise ratio, at which the signal-to-total thermal-noise ratio is 1 dB less than the signal-to- linearly demodulated thermal-noise ratio in the highest baseband channel. At this point, linearly-demodulated thermal-noise is therefore 6 dB more than threshold effect-noise in the highest channel. In the lower channels, threshold-effect noise, which has a uniform spectrum, is relatively greater. The variation in the total thermal-noise power level between the high and the low channels is then about 7 dB. For higher carrier-to-noise ratios at the “ 1 dB threshold point” , this variation in level exceeds 7 dB. In order that the channels be equalized at the 1 dB threshold, the overall range of varia tion of the pre-emphasis characteristic should not be greater than 7 dB. To take account of the effects of intermodulation which are most serious in the bottom channels when pre-emphasis is used, it is proposed that the overall range of variation of the pre-emphasis characteristic should be limited to 6 dB. To facilitate planning, moreover, it is desirable that the r.m.s. frequency deviation caused by the frequency-division multiplex telephone signal should be the same with or without pre-emphasis. 2. Proposed pre-emphasis characteristic For the reasons adduced in § 1, it is proposed that experiments be made with a pre emphasis characteristic such that: — the overall range of variation in the effective frequency band between the outer multi plex telephone channels is 6 dB ; — 325 — Rep. 212-1 — the r.m.s. frequency deviation caused by the frequency-multiplex telephone signal is the same with or without pre-emphasis ; — the pre-emphasis characteristic is derived from, a basic pre-emphasis network, similar to that proposed in Recommendation 275-1. It would appear desirable to try a network, the composite attenuation of which is expressed by the formula: 3-5 2 0 logj0 0 /vj) = 1 0 log10 1 + dB 6-1 where f r = network resonance frequency; = 1-25 fmax (where is the highest baseband frequency) and / is the baseband frequency. The diagram of this network is shown in Fig. la. The diagram of the corresponding de-emphasis network is shown in Fig. lb. The tolerances of the components of the networks a re : — resistors : ± 1 % ; — capacitors : ^ 0-5% ; — f r: i 0-5% (L resonant with C). Note. — The frequency for which the deviation should correspond to that obtained without pre-emphasis is 0-55 f^ x - The variation of the deviation as a function of frequency is shown in Fig. 2. Rep. 212-1 — 326 — 1.12 Rq = 0,455 r ° f r ~ h 2 5 fmax- 2-kJj^C, 0,89 R0 j _ i?p I V c , 0-455 f r = 1-25 f m I— = -2 i r V jL„C 1?! < 0-01 R0 (/ r;) i?2 < 0-01 J?0 (/r) (a) 1,12 R0 I— 2-tt V L1C1 j L2 _ R0 V Co 1-30 / r - l - 2 5 — J R, < 0-01 1?0 (/,) F ig u r e 1 Relative deviation (dB) ,2 ,5 , 02 , 10 ,5 1,5 1,25 1,0 0,5 0,2 0,1 0,05 0,02 Normalized frequency, frequency, Normalized 37 — 327 — F gure r u ig 2 f/fmax max r f x a fm e. 212-1 Rep. Rep. 213-1 — 328 — REPORT 213-1 * FACTORS AFFECTING MULTIPLE ACCESS IN COMMUNICATION-SATELLITE SYSTEMS (Study Programme 2E/IV) (1963 — 1966) 1. Introduction — nature of the problem The satellite in a communication-satellite system is a node in the circuits to the earth stations involved. The problem of multiple access in satellite communications is to achieve, with the satellite, as high a degree of flexibility of interconnection between the earth stations as may be desired. In this respect, the satellite repeater differs from the repeater in a ter restrial radio-relay system. In the terrestrial radio-relay system, the repeaters have not been called upon to provide multiple access of the type and scope which is envisaged with the satellite repeater. In the satellite repeater, multi-channel transmissions from a number of earth stations will be brought together (thus “multiplexed” in a general sense) at the satellite for amplification and retransmission to these same earth stations. In the extreme, one can envisage an enormous and complex satellite having remodulating facilities for each of its earth stations, and having switching equipment to interconnect and reconnect all of these circuits as needed. The objective in the development of multiple-access satellite communication is to approach the communication utility of such an ideal “exchange in orbit”, while at the same time keeping the satellite small and reliable, hence relatively simple, consistent with the state of development. At present, any circuit switching or its equivalent should be accomplished and controlled from the Earth. There still, however, may be a degree of signal processing performed in the satellite. 1.1 Control o f multiple-access at radio-frequencies In a multiple-access communication-satellite system, a portion or the whole of the total number of available channels may be allocated permanently or semi-permanently between pairs of participating earth stations. It is possible, especially in later and more complex systems, that channels will be avail able for use by any of the participating earth stations using one of the following three methods of gaining access. 1.1.1 Controlled access An earth station desiring access to the system must request and obtain access to the system via a network management facility. 1.1.2 Self-ordered access An earth station desiring access to the system would be able (by appropriate means) to ascertain which radio-frequency channels or time slots are available, and conse quently to be able to enter an available channel. (The “channel” can be a radio-fre quency channel, or a channel or group of channels in a frequency-division multiplex or time-division multiplex baseband. In the latter case, an addressing procedure is necessary.) 1.1.3 Un-ordered access In common spectrum or random-access discrete address systems, access would nor mally be gained to a radio-frequency channel without first determining the availability of a channel. * This Report was adopted unanimously. — 329 — Rep. 213-1 If an earth station is to be able to enter a system at any time, sufficient capacity must be provided in the satellite to handle the maximum foreseeable traffic. 1.1.4 General considerations Many proposed methods of multiple access, particularly those in the latter two classes, have contemplated examining the satellite signal to select a channel which appears to be vacant and available. Actually, the examination only reveals that a channel was available T ms before, when T is the maximum earth-satellite-earth pro pagation time, which can be nearly 300 ms with a stationary satellite. 2.] Factors determining the accessibility of a communication-satellite system to a number of earth stations A communication-satellite system should function within the technical and operational framework of the international world network as defined by the C.C.I.T.T. This may either inhibit the application of certain forms of multi-station access, or may lead to adjustments in these world network concepts to accommodate these new techniques of multiple access. The following factors, which will be discussed in subsequent sections, primarily affect the accessibility of communication-satellite systems : — the orbital parameters of the system; — the methods of modulation; — the methods of multiplexing; — the possible diversity of earth-station characteristics, such as transmitting power, antenna gain, and receiving system noise temperature. (Earth-station sensitivity is determined by the ratio of antenna gain to the noise temperature of the receiving system.) Indirectly, the permissible complexity in the satellite, in earth stations, and in terrestrial control networks, will also affect the accessibility. 3. ] The effects of orbital parameters on multi-station access For satellites at a given altitude, there will be an area in space within which a satellite will be visible to two or more earth stations. The radial projection of this area on the Earth is the area of mutual visibility. Alternatively, the sub-satellite point on earth is surrounded by a circle on the Earth enclosing the area from which the satellite is visible above a certain minimum angle of elevation. The diameter of this circle is a function of the altitude ; this circle is the zone of coverage for that one satellite. 3.1 Stationary satellites A stationary satellite has a large and constant zone of coverage, so that all earth stations have continuous direct connectivity with each other using one antenna system per earth station. Although a large propagation delay is associated with this altitude, such a satellite needs minimal compensation for variations in frequency or propagation delay due to changing ranges. With fixed earth-station antennae, a truly stationary satellite would be desirable. How ever, the satellite need not be perfectly stationary, because the antenna beam of a fixed reflector can be moved somewhat by moving the antenna feed, or by other appropriate means. A stationary satellite must carry controllable thrust, adequate for station keeping through out its life. 10 Rep. 213-1 — 330 — 3.2 Non-stationary satellites 3.2.1 Unphased satellites Assuming equal repeater capabilities, satellites for use in unphased satellite systems can be light and simple, because they need not carry controllable thrust equipment. Systems of unphased satellites have a low probability that a particular satellite will be in the area of mutual visibility and, therefore, need a much higher number of satel lites for the same coverage (see Fig. 1). In such systems, two or more satellites may occasionally be sufficiently close to each other to be simultaneously within the beam of an earth station, thus giving rise to interference. Such events are predictable and their effects may be alleviated by using operational procedures or different frequency allocations. Since a large number of satellites are necessary, the probability is large that more than one satellite will be visible to a given earth station. Hence, an earth station requir ing multiple connections would often be able to use more than one satellite, assuming there are sufficient antenna systems at the earth station. Therefore, there is less justifica tion for providing a high degree of multiple access per satellite than for phased or even stationary satellites. However, the increased multiple-access capability must be weighed against the requirement for multiple antennae and handover. 3.2.2 Phased satellites Systems of phased satellites (particularly those following recurrent earth tracks) [8 ] can give the required coverage with a relatively low number of satellites. In general, the greater the altitude of such systems, the greater will be the coverage areas, but the greater also will be the propagation delay. Phased satellite systems, with their defined coverage areas and defined arcs, need less complex systems of earth-station antennae than do systems of unphased satellites. 3.3 General considerations The continuity of the service of a non-stationary satellite system, with respect to the chosen group of earth stations, is defined as the probability of finding at least one satellite in the cor responding area of mutual visibility. As the area over which the earth stations are dispersed increases, the mutual visibility area decreases, necessitating an increase in the number of satellites required to maintain a given probability of finding one satellite in the mutual visibility area. Reducing the area covered by a given group of earth stations increases the area of mutual visibility, but gives rise to other problems. If the coverage zones chosen are small, connection over long distances will require multi-hop routing. This can result in large pro pagation delays and complex operating procedures. Handover is required except with stationary or synchronous near-stationary satellites. Because large steerable earth antennae cannot be moved quickly from a “setting” satellite to a “rising” one, uninterrupted handover requires the use of two such antennae, each with its own transmitter, receiver and tracking system. The choice of a non-stationary satellite raises the problem of the switching discontinuity at handover, due to differences between the lengths of the radio paths. Systems of non- stationary satellites must be capable of compensating for larger Doppler shifts and a greater range of time delay. These considerations are discussed further in Reports 214-1 and 383. — 331 — Rep. 213-1 3.4 Preferred orbital systems There are conflicting factors governing the choice of orbital parameters, but on balance it would appear that the types of system may be placed in the following order of preference so far as multi-station access is concerned: — stationary satellite systems ; — systems using closed earth tracks; — unphased satellite systems. 4. Effects of systems of modulation and methods of multiplexing on multiple access 4.1 Information processing in the satellite, general remarks The following four possibilities have received particular consideration : 4.1.1 A satellite involving a minimum of signal processing, consisting of frequency transla tion of all signals together from the receiving to the transmitting frequency bands, through a single repeater. 4.1.2 A satellite involving frequency translation, accompanied by a change of modulation, as for example from a single-sideband transmission to a phase-modulated transmission. 4.1.3 A satellite employing more than one repeater simultaneously, up to one per earth station or even one per station carrier. 4.1.4 A satellite in which the earth-station signals are demodulated to baseband, then suitably transposed and recombined before modulating the transmitted carrier or carriers. 4.2 Design considerations for satellite repeaters Considerations in the design of present-day satellites affect the choice of modulation systems. One of these is that satellite repeaters have been limited in output power, due to limitations on weight and on primary power, and due to the power demands of sub-systems other than the repeater. Active communication satellites, thus far, have used transmitter output powers of the order of five watts. At present there exist technical possibilities for the increase of this power. For example, the U.S.S.R. m o l n iy a i satellite has a transmitter power output of about 40 W. For simplicity the majority of communication satellites have not yet used nearly as much antenna gain as is possible with earth-subtending, conical antenna beamwidths. The U.S.S.R. satellite m o l n iy a i employs such a higher-gain earth-subtending antenna; this allows the use of less sensitive earth stations. Satellite repeaters used thus far, as well as those contemplated for use in the near future, all employ travelling wave tubes in the final power amplifier. For high efficiency these tubes are operated near saturation. This introduces non-linear effects. The non-linearity of final amplifiers has not been a great problem when using multi channel signals of constant amplitude, such as frequency modulation, or phase-keyed pulse- code modulation. However, when two or more such signals, from different earth stations and at different frequencies, are combined in the same satellite amplifier, their sum loses this con- stant-amplitude property. The combining of these frequency components produces an ampli tude fluctuation with peaks. These will saturate the amplifiers unless there is a power reduc tion (“back-off”) below the single carrier saturation power output of the travelling-wave tube, to prevent excessive intermodulation when several carriers are being amplified. This power back-off usually causes a decrease in total channel capacity with an increase in the number of stations which make simultaneous use of a satellite repeater. The desirability of eliminating or at least reducing such power back-off has motivated or directed much of the study of multiple-access systems. Rep. 213-1 — 332 — 4.3 Specific methods o f modulation and multiplexing 4.3.1 General considerations 4.3.1.1 Multiple access at radio-frequencies The Annex to this Report deals only with methods by which all the radio- frequency signals in the desired multiple-access configuration can share the available transmitter power in a satellite repeater. Each method is classified according to the technique used to associate the received radio-frequency signal with the particular remote earth source. 4.3.1.2 Baseband signal The baseband signal may be in any conventional form such as frequency- division multiplex of message channels, or it may be put into a pulse format. The pulse format may be a time-division multiplex of pulse samples of each message channel, or pulse samples of a frequency-division multiplex of message channels. The pulse samples may use an analogue representation such as pulse amplitude, pulse width or pulse-position modulation or may use a digital representation such as pulse-code modulation (PCM). PCM is attractive for pulse formats, because it is less susceptible to interference and intermodulation. 4.3.1.3 Modulation at radio-frequencies Any conventional form of amplitude-modulation or angle-modulation may be used with any baseband signals. Angle-modulation is preferred for the down link because of the modulation improvement factor. If the baseband signal is in pulse code format, phase-shift keying is preferred, because of its better modula tion improvement factor. If the technique of multiple access at radio-fre quencies dictates intermittent use of the radio-frequency carrier, a pulse format is necessary. 4.3.2 Methods of multiple access at radio-frequencies Four methods, discussed in the Annex, are summarized below. 4.3.2.1 Frequency-division multiplexing at radio-frequencies without change of modula tion in the repeater. Each up-path radio-frequency carrier occupies a particular frequency allocation. All radio-frequency carriers are amplified simultaneously and translated to a new set of frequencies in the down path. The earth receiving station filters out the desired radio-frequency carrier or carriers prior to demo dulation. (It may be necessary in addition to select channels addressed to that station.) 4.3.2.2 Frequency-division multiplexing at radio-frequencies with change of modulation in the repeater. The particular method discussed employs single-sideband sup pressed carrier amplitude modulation for the up path, with all stations employing the same radio-frequency carrier, and each station employing a unique portion of a frequency-division multiplexed baseband. In the satellite repeater, the composite signal from the several earth stations modulates the down-path radio-frequency carrier with angle modulation. The receiving earth station identifies its message channels after it demodulates the down-path radio frequency carrier. Frequency-division multiplexing at radio-frequencies employing multiple repeaters and/or demodulation in the satellite are not discussed in the Annex. Discussions on this type of system may be found in [5] and [19]. — 333 — Rep. 213-1 4.3.2.3 Time-division multiplexing at radio-frequency requires that all participating earth stations transmit periodically in an established sequence such that the communication repeater in the satellite is only amplifying the signals of one earth station at any instant. Thus, the output signal of the satellite repeater is a constant envelope signal and power back-off loss is avoided. A particular earth-station receiver identifies the desired transmission by observing the information in the periodically distributed time slots associated with the corresponding earth station. It is immaterial whether the same or slightly different radio-frequency carriers are used by all earth stations, but minimum system bandwidth is achieved when all use the same radio-frequency carrier. 4.3.2.4 Common-spectrum techniques Common-spectrum systems are those in which signals from all of the participating earth stations make common use of the time-frequency domain, and receiver processing is employed to detect a wanted signal, in the presence of others. By design, two or more signals at the same frequency can exist in stantaneously in such systems. Three typical approaches to providing multiple- access capability a re : — spread spectrum; — frequency-time matrix ; — frequency-hopping. These are discussed in the Annex. 5. The effect of different earth-station sensitivities In principle, it would seem desirable that earth stations handling little traffic be smaller and simpler—and therefore cheaper—than those handling large amounts of traffic. This would imply the use of smaller antennae and perhaps less powerful transmitters and less sensitive (noisier) receivers. Such stations would therefore require more satellite power and/or radio-frequency bandwidth per telephone channel. A small station is most efficient when all of the baseband channels addressed to it are in separate carriers. In contrast, in a time-division multiple-access system the bit-rate could be decreased, in appropriate time-slots, to accommodate less sensitive earth stations. The conclusion seems to be that smaller station access to the system is possible in many systems, but always leads to technically less efficient use of the satellite system as a whole. 6. Conclusion As indicated in this Report, many facets of the design of multiple-access systems are being studied, and there are numerous satellite orbits, modulation and multiplexing methods, methods of gaining access, etc., between which to choose (each with its advantages and limitations). None of these systems has as yet been tried adequately under actual satellite operation. Consequently, a firm order of preference among the alternatives cannot yet be given. However, with so many choices available and so few fundamentally difficult technical problems, the achievement of useful multiple access can be viewed with optimism. The optimum achievement of multiple access is expected to become a major factor in the deter mination of parameters of communication-satellite systems. Rep. 213-1 — 334 — Bibliography 1. A ein, J. M. Multiple access to a hard-limiting communication satellite repeater. IEEE Trans, on Space Electronics and Telemetry, Vol. SET-10, 159-167 (December, 1964). 2. A ltmann, F. The Comsat small station problem. First annual IEEE Annual Communications Con vention Conference Record, 333 (June, 1965). 3. Blasbalg, H. A comparison study of pseudo-noise and conventional modulation for multiple access satellite communications. First Annual IEEE Communications Convention Conference Record, 345-353 (June, 1965). 4. Blasbalg, H., Fremon, D. and K eeler, R. Random access communications using frequency shifted PN signals. IEEE International Convention Record, Part 6, 192-216 (March, 1964). 5. Bray, W. J. Some technical aspects of the design of world-wide satellite communication systems. Pre sented at International Conference on Satellite Communications of the I.E.E., London (November, 1962). 6. Campbell, D. R. MESA. A time division multiple-access system. International Symposium on Global Communications (Globecom VI), Philadelphia (June, 1964). 7. Chapman, Jr., C. R. and Millard, J. B. Intelligible crosstalk between frequency modulated carriers through AM-PM conversion. IEEE Trans, on Communication Systems, Vol. CS-12, 160-166 (June, 1964). 8. D algleish, D. I. and Jefferis, A. K. Some orbits for communication-satellite systems affording multiple access. Proc. I.E.E., Vol. 112, 21-30 (January, 1965). 9. D awson, C. H. An introduction to random access discrete address systems. IEEE International Convention Record, Part 6, 154-158. 10. D ayton, D. S. Multiple access among mobile/ground terminals via future stationary communication satellites. International Symposium on Global Communications (Globecom VI), 79, Philadelphia (June, 1964). 11. Fulton, Jr., F. F. Efficient multiplexing of intermittent transmitters. Western Electronic Show and Convention (WESCON) Paper No. 29/3 (1961). 12. Garner, W. B and Mathwich, H. R. Multiple access technique for commercial communication satellite. National Electronics Conference (NEC) Record, 460-465 (1964) . 13. Glenn, A. B. Code division multiplex. Proceedings 1964 National Winter Conference on Military Electronics, Paper 15-6, also IEEE International Convention Record, Part 6, 53-61, New York, N.Y. (1964). 14. Grossman, H. W. Low-level satellite communications using RACEP. Presented at IEEE 1963 National Space Electronics Symposium, Miami Beach, Fla. (1-4 October, 1963). 15. H amsher, D. H. System concepts for address communication systems. IRE Trans, on Vehicular Communications, Vol. VC-9, 72-76 (December, 1960). 16. Harris, D. P. Unscheduled spectrum sharing in communications. Fifth Annual Global Communi cations Symposium (globecom v), 273-276 (May, 1961). 17. Jacobs, I. The effects of video clipping on the performance of an active satellite PSK communication system. IEEE Trans, on Communication Technology, Vol. COM-13, 195-201 (June, 1965). 18. Jones, J. J. Hard limiting of two signals in random noise. IEEE Trans, on Information Theory, Vol. IT-9, 34-43 (January, 1963). 19. Milne, K ., et al. A channelling arrangement for a nonsynchronous communication-satellite system giving freedom of access for a number of earth stations. Presented at International Conference on Satellite Communications of the I.E.E., London (November, 1962). 20. Prihar, Z. Random access stationary satellite relay system. Philco Corp. Presented at IEEE 1963 National Space Electronics Symposium, Miami Beach, Fla. (1-4 October, 1963). 21. Shaft, P. D. Hard limiting of several signals and its effect on communication system performance. IEEE International Convention Record, Part 2, 103-113, New York, N.Y. (1965). — 335 — Rep. 213-1 22. Sommer, R. C. On the optimization of random access discrete address communications. Proc. IEEE, 52, 1255 (October, 1964). 23. Stewart, J. A. and Huber, E. A. Comparison of modulation methods for multiple-access synchro nous satellite communication systems. Proc. I.E.E., Ill, 535-546 (March, 1964). 24. Sunde, E. D . Intermodulation distortion in multicarrier FM systems. IEEE International Convention Record, Part 2, 130-146, New York, N.Y. (1965). 25. Taylor, J. E. Asynchronous multiplexing. Communication and Electronics (January, 1960). 26. A ltman, F. Power control for Comsat. Multiple access. International Space Electronics Symposium, Las Vegas, Nevada (October, 1964). 27. Gibbons, H. M. A study of multiple access to an active communication satellite employing hard limiting. International Space Electronics Symposium, Las Vegas, Nevada (October, 1964). 28. Lutz, S. G. and D orosheski, G. Coverage and overlap of satellites in circular equatorial orbits, with application to multiple-access communication satellite systems. Proc. I.E.E. London (September, 1966). 29. Lutz, S. G. A survey of satellite communication. Telecommunication Journal, 29, Geneva (April, 1962). 30. Lutz, S. G. Satellite communication. Small station aspects. Telecommunication Journal, 31, 8, Geneva (August, 1964). ANNEX THE EFFECTS OF SYSTEMS OF MODULATION AND METHODS OF MULTIPLEXING ON MULTIPLE ACCESS 1. Introduction The purpose of this Annex is to present a review of some of the techniques of modulation and multiplexing applicable to multiple access in a communication-satellite system. In these discussions the term multiplexing will be used to describe the method of com bining, in the communication-satellite repeater, the radio-frequency signals received from the individual earth stations, as distinct from multiplexing as a means of forming a baseband from several voice-frequency channels. As will be evident from the discussion, the technique employed to combine individual message channels at a particular earth station is not neces sarily the same as the technique employed to combine (multiplex), in the satellite repeater, the signals received from individual earth stations. Three classes of techniques for modulation and/or multiplexing will be discussed : fre quency division, time division and common spectrum. All of these techniques can be employed with essentially the same type of saturated radio- frequency amplifier in the satellite repeater. The design of the receiver unit is influenced by the technique employed. The choice of one of these techniques is not seriously inhibited by the present state of development in satellite repeaters. One important aspect of multi-station access techniques is the extent to which a particular technique permits interconnection of earth stations of different receiving sensitivities (antenna gain/noise temperature). This is referred to as the station mix adaptability of each technique. Using the earth station sensitivities cited in Report 211-1 (25 m antenna, 50°K system noise temperature as compared with 9 m antenna and 150°K), the relative sensitivity differs by a factor of 25 or 14 dB. This can be interpreted theoretically to mean that, to deliver a unit of traffic to the small station with the same quality as to the large station, one must allot about 25 times as much radio-frequency power-bandwidth product. Alternatively, for the same value of satellite power bandwidth product required to deliver one unit of traffic to the small earth station, about 25 units of traffic, with the same quality, can be delivered to a large earth station. Rep. 213-1 — 336 — 2. Particular techniques 2.1 Frequency-division for multiple access (FDMA) 2.1.1 Multiple radio-frequency carriers with no change of modulation For this discussion it is assumed that each carrier is frequency-modulated by a baseband signal composed of single-sideband suppressed-carrier voice channels. How ever, it should be understood that the information on each carrier may also be digitized and encoded before modulating the radio-frequency (see Report 211-1). It is also assumed, that the type of repeater used does not change the type or degree of modulation of the received signal, but merely amplifies the signal, translates its frequency, and re-transmits it. The use of a travelling-wave tube (TWT), or other saturating power amplifier, causes several types of transmission impairment which must be considered in designing a multiple-access system using frequency-division multiplex. The impairments, unique to multiple-carrier operation are intermodulation noise and intelligible crosstalk, both due to the presence of multiple carriers in the TWT. These impairments may be kept within desired bounds by reducing the input power to the TWT in accordance with the number of carriers present; this causes a decrease in output power. This output power back-off usually reduces the number of channels obtainable from the repeater. If limiting is employed in the receiving portion of the repeater, differences in input power between various carriers are accentuated at the repeater output. To ensure that all carriers arrive at the repeater with appropriate power ratios, it is necessary, in multiple-carrier operation, to control the power transmitted to the repeater from each earth station. The actual transmitted power is a function of the number of voice channels modulating the carrier. 2.1.1.1 Orbits The multiple frequency-modulation carrier technique is applicable to medium altitude satellites or synchronous satellites. However, the baseband may have to be corrected for Doppler shift (see Report 214-1). 2.1.1.2 Supervisory control and channel assignment The only supervisory controls needed as a direct result of the modulation technique are for assessing up-link path loss for each station, to permit power control, and for control over channel assignment if necessary. Fixed, continuous assignments could be made for high-density traffic, and time sharing could be used for the more lightly loaded routes. Using these techniques, the system channel capacity can be optimized within the constraints of the traffic requirements, by reducing the number of simultaneous carriers in one repeater. This has the additional advantage of reducing the required bandwidth. Another possible technique of channel assignment is on a per-call basis. In this technique, each voice-channel frequency modulates a separate carrier. Since a large number of carriers are present simultaneously at the satellite, a large back-off may be necessary. However, since only a portion of the talkers are active at any given time, the frequency-modulation carriers may be sup pressed during pauses in speech to reduce the number of simultaneous carriers. Also, the threshold extension for a single channel is large. — 337 — Rep. 213-1 It should be noted that special steps may be necessary to avoid undesirable interaction between certain types of echo suppressor and the voice-actuated, carrier switches. 2.1.1.3 Operation with stations having different sensitivities The amount of satellite power required, when several earth stations receive the same carrier, is determined by the least sensitive station. At the same time, the channel capacity may be determined by the ratio of received carrier power- to-system noise temperature, C/T. It is therefore desirable that C/71 be the same for each earth station receiving on the same carrier. 2.1.1.4 Interference considerations Frequency modulation produces a spectrum with an energy distribution which is a function of the frequencies of the modulating signals and of their modulation indices. With lightly-loaded or unmodulated multi-channel tele phony systems using frequency-modulation, a considerable part of the total energy is concentrated in a relatively narrow frequency band, with the result that a greater interference potential to other systems sharing the same frequency bands exists in this condition than for a normally loaded frequency-modulation system. This interference potential can be reduced substantially by use of carrier energy-dispersion techniques (see Report 384). 2.1.1.5 Status During 1963, theoretical and experimental studies were conducted in the United States concerning multiple access using several FDM /FM carriers. Two effects were given special attention : — intermodulation arising from the non-linearity and undesired AM-PhM conversion of the repeater ; — intelligible crosstalk resulting from modulation transfer by the process of FM-AM-PhM conversion. Intermodulation calculations were performed using an equation represent ing the saturation characteristic of a travelling-wave tube. Results of experi ments using typical satellite travelling-wave tubes agreed with the calculations. Intelligible crosstalk was calculated for the cases considered, based on measured parameters of travelling-wave tubes used. The study supports the feasibility of this multiple-access mode and gives a quantitative assessment of the resulting capacity penalty. This penalty depends on many parameters and system assumptions, so each system configuration must have an independent evaluation. Some general conclusions can still be derived. For example, if identical earth receiving systems are assumed, the relative capacity would give the following: Rep. 213-1 — 338 — Channel cap acity relative to single-carrier operation Number of carriers (°/'o> Normalized in one repeater bandwidth Carrying Carrying 120 channels per carrier 1200 channels per carrier (a) 0>) 1 100 100 1-0 2 70 110 1-45 4 50 90 1-8 8 50 95 2-2 16 40 95 2-6 The difference in channel capacity, relative to single-carrier operation in cases a and b of the Table, is because the improvement realized with threshold- extension receivers depends on the baseband width. For example, if a single carrier had 1200 channels, there is practically no improvement (see § 2, Report 211-1). In this case (column b), the reduced number of channels per carrier in multi-carrier operation permits an increase in the threshold improvement factor. Under this condition, the improvement can, in some cases, exceed the losses due to intermodulation and back-off, so that multi-carrier operation yields a higher number of channels. Column a shows the relative capacity when the single carrier has only 120 channels. Since the threshold-extension receiver is close to optimum feed back improvement at 1 2 0 channels, intermodulation and back-off losses are not recovered, and a loss of capacity is sustained. A loss is sustained in any case if the satellite repeater is bandwidth limited, thus reducing the efficiency of the feedback receiver by not spreading to full capacity. A more complete discussion on this subject will be found in Report 211-1. 2.1.2 Multiple radio-frequency carriers with modulation conversion The technique to be described employs a single-sideband suppressed-carrier amp litude-modulated signal in the up-link. All stations employ the same virtual carrier frequency, with each station employing a unique portion of a frequency-division mul tiplexed baseband. At the space-station repeater, these up-link signals combine to form a continuous baseband. The repeater converts this signal to a single broadband, phase- modulated signal for transmission to all earth stations. This modulation conversion takes place at intermediate frequency, without demodulation to baseband. The specific FDM channels, transmitted at a given time by each earth station, are acquired by, or assigned to, the stations in accordance with the supervisory technique employed. This prevents simultaneous multiple occupancy of a channel. The single-sideband, phase-modulation (SSB/PhM) system can provide either heavy-density trunks or as little as one channel at a time from each to any other earth station. This technique requires accurate control of earth-station transmitter power level, so that all signals arrive at the satellite with substantially equal power density, and it requires a high degree of linearity in the earth-station power amplifier. — 339 — Rep. 213-1 2.1.2.1 Orbits Orbits of synchronous or non-synchronous satellites may be used. How ever, the use of a stationary satellite minimizes the problems of frequency and power control, because its motion relative to the earth stations—and hence the Doppler correction problems—are least. The SSB/PhM technique requires accurate control of frequency, not only to meet the C.C.I.T.T. criterion of no more than 2 Hz shift in the end-to-end connection (see Report 214-1), but also to compensate for any Doppler shift which may occur in the up-path. 2.1.2.2 Operation with stations having different sensitivities To ensure that the necessary channel noise performance is met, all earth- station receiver systems must meet the same minimum requirements, regardless of whether the station carries much traffic or little, since each earth station must receive the entire signal transmitted by the repeater. 2.1.2.3 Channel assignment and control Two methods have been studied : one uses channelling and supervisory centres (controlled access). The other requires that each station should find unused channels for the completion of calls (self-ordered access). Suitable channel assignments and system supervision can be achieved by either method. The channelling and supervisory centre method offers more certain control of the operation of the system, including removal from service of unassignable channels and a single location for recording of system use. On the other hand, it requires two-way supervisory communications between the stations and the channelling and supervisory centre. (However, supervisory channels can be kept to a small fraction of the total capacity of the system.) A standby centre may be desirable in a different location to ensure continuity of system opera tion. An earth station can find an unused channel without requiring supervisory facilities, if a tone (either in-band or out-of-band) is associated with each channel which is in use. If group delay is long, the probability increases of more than one station simultaneously seizing the same channel, although with proper precautions this probability can be made satisfactorily small. 2.1.2.4 Interference In common with all systems employing angle-modulation in the down path, the SSB/PhM system may, when the system is idle or close to idle, contain a sufficient part of its energy concentrated in a narrow frequency band to present a potential source of interference to terrestrial radio-relay systems. Suitable precautions may have to be taken against this eventuality, either by noise- loading of idle channels or by other carrier energy-dispersion techniques. 2.1.2.5 Status Experiments to investigate the feasibility of the single-sideband technique will be conducted by the United States as part of the Applications Technology Satellite (ATS) programme. 2.1.3 Multiple radio-frequency carriers with demodulation in the repeater For discussions of this technique, see [5] and [19]. Rep. 213-1 — 340 — 2.2 Time-division multiple access (TDM A) Time-division multiple access is that system which permits access by the sequential assignment in time of periods of transmission through the satellite repeater. Time-division multiple access is characterized by the durations of the time frame and the time slot. In a communication-satellite system employing time-division for multiple access, each participating earth station is assigned one (or more) time slots, and one complete sequence of earth-station transmissions is the time frame. Considerations of synchronization and timing, associated with the use of time-division in a multiple-access communication-satellite system, set a minimum limit for the duration of the time assignment to an individual earth station. This limitation, plus the necessity for time compression of the baseband signals for discontinuous transmission, leads to a time- division technique in which many pulses are transmitted during each time slot [6 ]. To a large extent the time-frame, time-slot and time-slot content are independent, the time-frame being dependent upon the maximum permissible transmission delay, the time-slot upon the number of stations and the guard time, and the time-slot content being only limited by the modula tion formats which can be transmitted therein. Propagation time is probably the largest delay factor and in the case of the stationary satellite is noticeably long. Thus, systems handling voice traffic cannot allow excess delays from other sources which approach an appreciable fraction of the transmission delay. This determines a maximum value for the time-frame. In the following discussion it will be assumed that the information has been sampled and converted to a pulse format. If the time-frame is made equal to or less than the sampling interval (typically 125 ps for speech), then no storage is required and the system becomes a “real time” system. However, as the time-frame is reduced, synchronization and timing become increasingly critical. The minimum time-slot duration should not be less than twice the uncertainty in propaga tion time, before acquisition, between any earth station and the satellite, so that a trial synchronizing pulse will not overlap adjoining time slots. Guard time between successive time slots must not be less than the minimum time uncertainty associated with ranging after acquisition. The time-frame and time-slot durations will fix the maximum number of simultaneous users in the system. Generally speaking, the time-slot can be as great as several milliseconds if only a few stations are in the system. Practical considerations shorten this duration. Prin cipally this occurs because of required storage capacity. Note.—This pertains to a system employing storage. In a “ real time” system, the time slot is approximately 125 jn (ps), where n is the number of time slots. The time-division multiple-access system, its method of operation and major sub-systems, are best described by examining and defining a generalized timing format. The timing format for other than bit-by-bit interleaved time-division multiple access is illustrated in Fig. 2. It consists of a recurring time-frame of length TF and repetition rate l/7>, containing h time slots. The first time slot is designated the reference time slot. Each of the n time slots contains synchronization, signalling and message information. — 341 — Rep. 213-1 A reference time is provided to permit sequential synchronous interleaving of time slots. This reference time is a coded signal originating in one of the following : — the satellite; — a selected earth-station, and relayed by the satellite; — an earth-station, pre-corrected for up-path effects, so that the signal appears as if it had originated in the satellite. Transmitted time slots are synchronized to the frame reference by continuously correcting for path variations at a rate equal to the derivative of the path delay. The method of modulation and modulation rate within a particular time slot is indepen dent of the others except for frame synchronization and addresser-addressee compatibility. 2.2.1 Orbit considerations Time-division multiple-access techniques are applicable to satellites at medium altitudes and to stationary satellites. However, the interleaving of time slots from various earth stations at the satellite requires accurate knowledge of the range as a function of time. While quadratic-range prediction has been explored and is, in fact, commonplace in the technology of launch guidance, the associated equipment is more complex than that required for linear prediction, which can be implemented using equipment readily available at present. With linear prediction, the guard time necessary between transmissions from various participants to accommodate uncertainties in range measurement, is of the order of the maximum change in range expected over the interval of time corresponding to the round trip from satellite to earth station. Because this technique is sensitive to timing, provisions must be made to ensure synchronization of time slots during hand-over in a medium altitude system (see also Report 214-1). 2.2.2 Channel assignment and supervisory control In a time-division multiple-access system, any time slot in the frame is, at least theoretically, available to any station. The principal problem arising from this inherent flexibility is one of allocating these time slots, or establishing a general network disci pline. A given time slot can be considered either as a route between geographic loca tions or as uniquely assigned to a particular earth station, to be used in whatever manner is required. When a network is organized so that time slots correspond to routes between geographic points, there must be some control as to the order in which these time slots may be used. This function would properly be in the domain of the network control station. If the allocation of time slots to individual stations is considered, then each assigned time slot may be sub-divided according to the needs of that station, remembering the corresponding station and its arrangement. In this network configuration, a network control station is not required. In either network organization, however, there must be order-wire communica tions between the participating stations. This can be handled either in a separate time slot specifically allocated to order-wire communications, or each station may sub- allocate part of its time slots for this function. The fraction of time allocated to order wire need only be quite small in either case (of the order of 0-5%). Rep. 213-1 — 342 — 2.2.3 Operation with stations having different sensitivities In a time-division multiple-access system, the full power of the satellite repeater is available at a particular instant of time for the repeating of signals from any earth station, independent of all the others ; the satellite repeater output is a constant envelope signal. Because of this, a variety of types of station may be mixed in an operating system. Stations with different receiving sensitivities (ratio of antenna gain to system noise temperature) and/or transmitter powers can employ the same repeater, provided they transmit at fates appropriate to the sensitivities of the intended receivers. With the consideration that the frame synchronization signal must be receivable by all sta tions, each station of a given size may communicate with other stations of the same size, independent of the existence of larger or smaller stations. Further, communica tions between stations of differing size can be achieved by adjusting the average power received from the satellite repeater (by adjusting the width and rate of the pulses in the constant envelope signal), to values commensurate with the receiving sensitivity of the smaller station . This adaptability of the system to varying station sensitivities is one of its significant advantages. 2.2.4 Inter modulation In a time-division multiple-access system the signal is, in theory, completely orthogonal and therefore is not characterized by baseband noise resulting from inter modulation products generated in a non-linear repeater amplifier. In most practical implementations, digital modulation will be used, and some form of sampling and quantization will be applied. The resultant quantization distortion (or noise, as it is commonly called) will not be audible in an idle channel and therefore is not considered a contributing factor to the baseband noise. (Delineation of acceptable standards for PCM quantization distortion is a necessary precurser to quantitative comparison of digital systems such as time-division multiple access with others which utilize analogue modulation.) 2.2.5 Interference Presuming that the baseband signal is digitalized, and that the radio-frequency carrier is phase-shift keyed by the baseband signal, the spectral power-density is essen tially independent of the intelligence being carried and can be assumed to be Gaussian in character in any narrow band. In this respect, the potential interference from both earth station and satellite is considerably less than that to be expected from analogue transmission where, in the absence of modulation, discrete lines are evident in the radiated spectrum. Furthermore, the mean power required for either FM or PCM modulation is comparable both at the satellite and the earth station, and is considerably less than that required for single-sideband modulation. In this respect the data in Report 211-1 are applicable. 2.2.6 Status Implementation of a time-division multiple-access system is considered feasible, using present day components and techniques as derived in the fields of communica tions and computers. Direct experiments are only now in the process of being conducted. The areas of importance in these tests are, in general, high-speed logic, wideband power amplification, wideband modulation and demodulation, and time synchronization. — 343 — Rep. 213-1 2.3 Common-spectrum multiple access Common spectrum systems are those in which signals from all of the participating earth stations make common use of the time-frequency domain, and receiver processing is employed to detect a wanted signal in the presence of others. By design, two or more signals at the same frequency can exist at the same time in such systems. Common spectrum systems may employ many different techniques to provide the multiple-access capability, and three typical approaches are : — spread spectrum; — frequency-time matrix; — frequency-hopping. The spread-spectrum system makes use of a deterministic noise-like signal structure to spread the normally narrow-band information signal over a relatively wide band of frequencies. The receiver inverts this operation to reconstitute the original information signal. The spectrum spreading is effected by one of a series of complex codes, and a coded signal entering a receiver using a different code will not be reconstituted. Because the generated codes in a spread-spectrum system are pseudo-noise in character, an interfering code is rejected in proportion to the ratio of the spread-spectrum bandwidth to the information bandwidth. In the spread-spectrum system, synchronization of the receiver code generator to the incoming signal is necessary. The frequency-time matrix system is one which requires the simultaneous presence of energy in more than one time or frequency assignment, to produce an output signal. The requirement for presence in several time and/or frequency slots reduces the probability of mutual interference when a number of users are simultaneously transmitting. In one type of frequency-time matrix system, the information to be transmitted is sampled and digitalized and each information bit is time and/or frequency coded before transmission. For each user in a multiple-access system a different, fixed time and/or frequency code is employed, but all users occupy the same time-frequency domain. This time-frequency matrix system requires minimal synchronization of a receiver to its incoming signal. In a frequency-hopping system, the transmitted frequencies are switched at a rate equal to or lower than the sampling rate of the information transmitted. Selection of the particular frequency to be transmitted is made in a pseudo-random manner from a set of frequencies covering a wide bandwidth. Therefore, the intended receiver must be synchronized to its incoming signal. 2.3.1 Orbit considerations The influence of the orbit upon common spectrum multiple-access systems is most felt in those approaches requiring synchronization between transmitter and receiver. In addition to the expected requirement that all frequency apertures be wide enough to accommodate Doppler frequency shifts (see Report 214-1), the requirement for synchronization means that uncertainties in both time and frequency must be resolved in the process of acquisition (see Reports 214-1 and 383). Synchronization is more difficult for lower-altitude satellites because of larger Doppler frequency errors, more rapidly changing propagation times, and because of the requirements for more frequent handovers. The time required to accomplish synchronization will depend upon the rates at which the necessary search procedures may be performed and these, in turn, are fixed by the information bandwidths in use. It should be noted that the entire problem of synchronization in common spectrum systems can be relieved by the use of timing signals emanating from the satellite, in which case only propagation difference errors from the satellite to the earth stations in question need be resolved. It should also be noted that, in such a common-spectrum multiple-access system once a link is established between two earth stations, these Rep. 213-1 — 344 — stations may employ the link to establish with great accuracy both the propagation delay and the relative clock discrepancies. In this manner the entire system may main tain an accurate accounting of timing errors, and system synchronization problems are greatly reduced. 2.3.2 Flexibility o f channel assignment Common spectrum systems have the capability of establishing communications in a simple, flexible manner, with minimal requirements for supervisory control. A “channel” assignment would consist of the allocation of a “code” or frequency-time pattern for each channel assignment. If it is assumed that the power per voice channel is fixed, then the transmission quality will be determined by the number of voice channels per unit bandwidth. Since common spectrum systems are designed on the basis of tolerating substantial mutual interference, they are inherently less sensitive to system operating anomalies (such as incorrect power control) than are other types of system. Since these errors will be tolerable in common spectrum systems, operational procedures can be directed toward more local control and less centralized control. In this manner, common spectrum systems will be self-adaptive to many unusual conditions and centralized real-time control can be reduced to a minimum. 2.3.3 Operation with stations having different sensitivities The intermix of large and small earth stations in a common spectrum multiple- access system is feasible, but will require some control of the up-path power levels to assure an adequate share of the down-path power to small receiving stations. This exercise of power control must consider the transmitter power, the antenna gain and the receiver noise-temperature as well as non-linearities in the satellite repeater which may suppress weak signals by as much as 6 dB. The requirement for power control may be eased somewhat by operating small receiving stations with a reduced width of baseband; e.g., increased processing gain which will provide a tolerance to low received signal levels at the expense of fewer information channels. Certain common spectrum techniques of the pulse address type have an inherent capability for power control in that the signal power is a function of the information content. 2.3.4 Spectrum utilization, interference To reduce the effect of mutual interference in a common spectrum system j the spectrum bandwidth may be increased without changing the information bandwidth to a point at which the noise power in the earth-station receivers is comparable to the total down-path, wanted signal power. At this point, the ratio of the maximum useful spectrum to the information bandwidth will have been reached and the system will be limited by thermal noise rather than limited by mutual interference. Operation in this manner is efficient but will require very large spectral bandwidths. This large system bandwidth is not necessarily undesirable from the point of view of spectrum utilization and interference, however, because the received power level can be adjusted to be below thermal noise. A concomitant advantage of operating in this fashion is that this spead of power minimizes interference with terrestrial systems. Interference from conventional narrow-band system signals to the up-path trans mission of a common spectrum system will add an interfering signal to the down-path transmission and will, in addition, deprive the system of a portion of the useful down- path power. The interfering down-path signals will be reduced by the processing gain of the earth-station receivers, but the loss of down-path power will reduce the effective signal density on the down-path. If the satellite repeater is a hard limiter, a constant — 345 — Rep. 213-1 envelope interfering signal of substantially more power than the total of wanted signals, may cause a power suppression of the down-path transmission—beyond linear power division—of as much as 6 dB. If the interfering signal is not of constant envelope the weak signal suppression will be necessarily smaller. Interfering signals at the inputs of the earth-station receivers of a common-spectrum multiple-access system are reduced by the processing gain of the receivers. 2.3.5 Intermodulation In addition to thermal noise in common spectrum systems, random interference will occur due to signals generated by other system users and system distortions. The basic considerations for self-interference are different for the several types of common- spectrum system. In a frequency-hopping system, false pulses are received when the set of interfering sources produces certain frequency-time patterns. In addition, can cellation errors will be evident in systems of this type. In synchronous spread spectrum systems all interference tends to become Gaussian as a result of receiver processing and the interfering signal levels are reduced by the processing gain or bandwidth-time product. In pulsed (asynchronous) systems, it is necessary to carry out a statistical analysis of the bandwidth-time processing gain to determine the degree of suppression of interfering signals. 2.3.6 Status While some limited experience does exist in the operation of common spectrum multiple-access systems over satellite links, little has been done with the express purpose of making performance comparisons with more conventional systems. 3. Conclusions The type of modulation and the multiplexing technique employed have an effect on the multiple-access capabilities of a communication-satellite system. Conversely, the choice of modulation and multiplexing techniques will be influenced by the operational configuration of a multiple-access system in terms of the number and characteristics of the participating earth stations, the variability of traffic requirements among the participating stations, and other factors. There are technical problems in all proposed systems and many of the systems described have not yet been tried in operation. Further study and experience are necessary before an order of preference can be given. n Rep. 213-1 — 346 — 100 y— V f t Y v -A & i 50 /- / / y x -+ / x r J A / 3 0 X / cr 20 - i 8 £ D ^ - s'! / / 3 ^ 10 / tfl <£ 1 y A l i 60 2000 4000 6000 8000 10000 12000 14000 Equivalent diameter of the area of coverage (km) F ig u r e 1 Areas of coverage for unphased and equi-spaced equatorial satellite systems (Minimum elevation : 5°; Altitude : 14 000 km) Curve A : Unphased system, q = 99,9% B : Unphased system, q = 99% C : Unphased system, q = 90% D : Unphased equatorial system, q = 99% E : Equi-spaced equatorial system, q = 100% — 347 — Rep. 213-1 A F ig u r e 2 Format o f a time-division multiple-access system A : Recurring time-frame B : Time-slot C, C ': Time-frames D : Guard time Rep. 214-1 — 348 — REPORT 214-1 * COMMUNICATION-SATELLITE SYSTEMS The effects of Doppler frequency-shifts and switching discontinuities (Question 1/TV) (1963 — 1966) 1. Introduction In a communication-satellite system, the received signal will be subject to the following phenomena : — Doppler frequency-shifts due to the relative velocities between satellite and earth stations ; — discontinuities of transmission time-delay and of Doppler shift due respectively to the difference in the lengths of the radio paths and in the different relative velocities, on switching from one satellite to another. This Report considers the probable magnitude of these phenomena and their effect on various types of communication signal. 2. Doppler frequency-shifts (applicable to non-stationary satellites) The magnitude of the total Doppler frequency-shift between the terminals of a communi cation-satellite system depends upon the wavelengths used and the relative velocities of the satellite with respect to the earth stations. The major component of the effect of the Doppler shift, i.e. the shift of the carrier or a reference-frequency of the transmission, can be removed in the receiver; however, it may be necessary also to compensate for the differential shift across the radio-frequency spectrum of the signal that produces a frequency “stretch” or “shrinkage” of the baseband signal. Depending upon the relative locations of the earth stations and the orbit, the Doppler shifts between transmitting earth station and satellite and also between satellite and receiving earth-station can either add or subtract. If 5000 km is taken as a probable minimum orbital height for a communication-satellite, then the “stretch” or “shrinkage” of the baseband signal will not exceed 2 parts in 105. In most practical cases, the orbital height will be greater and the Doppler shift would be considerably less than this, and in the particular case of the stationary satellite, there would be no significant Doppler shift. The maximum value of the Doppler shift, resulting from transmission to or from a space station on a satellite in a circular orbit, can be estimated from the relationship : dz 3-0x KHxF-s where A F = Doppler frequency-shift, F = Operating frequency, s = Number of revolutions per day (24 hours) of the satellite with respect to a fixed point on the Earth. This relationship may also be used for calculating the maximum differential Doppler frequency-shift over a frequency band. A few values of s for various circular equatorial orbit altitudes are provided below (Table I) to facilitate the calculations for individual cases. * This Report was adopted unanimously. It should be read in conjunction with Report 383. — 349 — Rep. 214-1 T a b le I Altitude for circular Period Revolutions per day equatorial orbits relative to the Earth, s (km) (h) 0 35 600 24 1 20 240 12 2 13 940 8 3 10 390 6 4 8 080 4-8 5 6 420 4 6 5 170 3-4 7 4 190 3 For satellites employed to relay signals simultaneously from a number of earth-stations, special consideration of Doppler shift may be necessary. In a frequency-division multiple-access (FDMA) system, each participating station uses a portion of the frequency band of the satellite repeater. Since the transmissions from each station are independent in time, there is no adverse effect from any relative time-shift. There will, however, be a Doppler frequency-shift in the transmission from each station which varies with time. Table II shows the maximum possible Doppler frequency-shifts at the satellite at 6 GHz. The figures are based on equatorial orbits and assume that the satellite moves in the same direction as the surface of the Earth. To prevent interference between adjacent radio-frequency channels caused by Doppler frequency-shifts, guard bands can be used. Depending on the location of the stations, the signal transmitted by one station may be shifted upward, while that from a station on an adjacent channel may be shifted downward. Alternatively, the frequency shifts may be corrected by available techniques. For example, allowing a guard band equal to the maximum possible Doppler frequency- shift shown in Table II for a ten-channel system, the total guard bands would then be 18 times the figures shown ( at 6 GHz). T a b l e II Maximum Doppler frequency-shift Period (h) 6 8 12 24 Approximate altitude (km) 11 000 14 000 20 000 36 000 Minimum elevation of antenna : 5° Maximum Doppler frequency-shift at 6 GHz (kHz) 55-4 37-2 18-5 0-24 C) (0 The Doppler shift given takes account of various perturbations of the 24-hour orbit, assuming that the actual position of the satellite can be held within 60 km of the true stationary position. In this case, the maximum- velocity of the satellite towards, or away from, an earth station will not exceed 5-8 m/s. 2.1 Telephony When frequency-division multiplex telephony is used, it is necessary to limit the band width or the apparent geocentric angular velocity of the satellite to prevent unacceptable differential Doppler frequency-shifts (unless corrections are applied to compensate for the Doppler effects). Rep. 214-1 — 350 — According to C.C.I.T.T. Recommendation G.225, “The difference between an audio frequency applied to one end of the circuit and the frequency received at the other end should not exceed 2 Hz” . The question of error in reconstituted frequency is still under study in C.C.I.T.T. Study Group XV. It may be noted that an error of 2 Hz is not exceeded in a single satellite link, if the product of the baseband (MHz) times the number of revolutions per day of the satellite relative to the earth, s, does not exceed 0-666; however, additional error is likely to be introduced by the multiplex equipment. Doppler effects will also shift the pilot frequencies used in FDM telephony for satellites with such angular velocities. Possible methods which could be used for correction of these shifts are : — a suitable variable time-delay device : — the carrier-frequencies used in the frequency-division multiplex equipment could be automatically controlled to compensate for the effects of Doppler shift and so reduce the overall frequency errors to acceptable small values. The first of these methods has the advantage that it would effectively cancel the errors resulting from the movement of the satellite in a manner similar to that in which they are introduced (i.e. by change in transmission delay during the pass). This method would, therefore, also eliminate all the effects of Doppler shift on the baseband signals and by suitable arrangements, would avoid switching discontinuities when transferring the information-flow from a satellite to the next one in the orbital pattern. Control of the variable delay could be performed, either by using predicted orbit information or on a servo basis employing a pilot signal transmitted from the earth station to the satellite and back to the same earth station (loop method). The loop method has the following advantages : — it would ensure that only the correct frequencies were received at the satellite. This facility could be of particular importance for certain systems, for example, those using closely-spaced channels or blocks of channels with single-sideband modulation in the earth-to-satellite direction; — Doppler frequency “stretch” might to some extent be obviated, e.g. by splitting the receiving bandwidth into appropriately separated portions and providing independent compensations for the blocks of circuits arriving from each of the other earth stations. Alternatively, compensation for the variable delays could be applied only at the receiving end and controlled by pilot signals originating at the distant stations. In this case, the Doppler frequency “stretch” or “contraction” of the baseband would need to be accommodated by adaptations of the frequency-division multiplex equipment at each earth station. Administra tions are requested to submit to the C.C.I.T.T. their recommendations, or findings concerning such adaptations involving control of the earth-station frequency-division multiplex equip ment, either on a loop basis, as is described under the first method above, or on a route-by- route basis. Doppler-shift correction may be necessary in any communication-satellite system using single-sideband amplitude modulation. 2.2 Telegraphy and data transmissions If telephone channels comply with the requirement of C.C.I.T.T. Recommendation G.225 this implies that, for telegraph and data channels derived from such telephone chan nels, the effect of Doppler frequency-shift may be ignored or has been adequately compensated for (see § 2.1). 2.3 Phototelegraphy If phototelegraphy channels are derived from telephone channels complying with the requirement of C.C.I.T.T. Recommendation G.225, the effect of Doppler frequency-shift may be ignored as being adequately compensated for. — 351 — Rep. 214-1 2.4 Wide-band data It should be noted that Doppler correction would need to be provided for carrier-derived phototelegraphy or data channels requiring wider bandwidths than a single telephone channel (e.g. group or supergroup bandwidths). ,2.5 Television The change in field frequency introduced by Doppler frequency-shift is very small. In normal monochrome television practice, the accuracy of the field frequency at the programme source is likely to be the limiting factor as far as disturbance to domestic receivers is concerned and Doppler shift will not be of concern. It may ultimately be desirable to correct for the effects of Doppler shift on colour televi sion signals, but the initial tests done with the satellites have demonstrated that standard colour receivers and in particular those using crystal controlled sub-carrier oscillators, will operate satisfactorily, with the order of Doppler frequency-shift likely to be encountered in a practical communication-satellite system. 3. Switching discontinuities (applicable to non-stationary satellites) Satellites which rise and set can be used by any two or more earth-stations only while mutually visible. These stations must then switch or “hand-over” to another mutually visible satellite, to maintain communication with some orbit systems, or with excessively separated earth stations ; relatively long interruptions may occur when mutual visibility of the first satellite is lost before another satellite has been acquired. Such interruptions can be avoided by the use of controlled, equally separated satellites of sufficient number in orbits having a recurrent earth-track *. Such satellite orbit systems are often referred to as systems of phased satellites. The phased circular equatorial orbit system is the simplest and best known such system. Even though such systems can prevent hand-over interruptions, there will generally be slight discontinuities of overlap of communication between two stations at the instant of hand-over, depending on whether the propagation path via the new satellite is shorter or longer than that via the former satellite. The calculation of these propagation path lengths Or delay times, and their difference, is dependent upon simple geometric relations which are explained in Report 383 on the effects of transmission delays. In the case of multi-hop connections, the switching discontinuities in the different hops will not often be coincident in time, so that the number of discontinuities per 24 hours will be approximately n.m , where n is the number of hops and m the mean number of switching discontinuities per 24 hours per hop. With systems employing phased satellites, the time differences for some pairs of earth stations would not exceed 10 ms ; whilst for other pairs of earth stations, the time differences would be up to 20 ms or even more. In unphased satellite systems, the time differences would have durations between 0 and 30 ms or more. It should be noted that these discontinuities are predictable and that counter-measures are possible. The use of variable delay devices could reduce these switching discontinuities to negligible proportions. Note. — An earth station using any satellite, non-stationary or stationary, may have its circuits interrupted for predictable periods when the satellite in use has approximately the same orientation from an earth station as the Sun or another satellite at the same frequency, or when the satellite uses a solar power supply without batteries and is eclipsed by the Earth. To avoid interruptions of these types, alternate routing via surface circuits or via a different satellite may be used during periods of outage. * D a l g le ish , D . I. and J effe r is, A. K. Some orbits for communication-satellite systems affording multiple access. Proc. I.E.E., Vol. 112, 1, 21-30 (January 1965), Rep. 214-1 — 352 — 3.1 Telephony Time differences, of up to perhaps 20 ms during transfer from one satellite to another, should not cause difficulty with telephone conversations. However, a discontinuity in trans mission of this duration can cause errors in existing telephone MF signalling systems such as the Intercontinental C.C.I.T.T. No. 5 and TASI. Further signalling techniques being contemplated such as C.C.I.T.T. No. 6 that will employ high-speed pulsing rates, may be much more susceptible to errors from this source. 3.2 Telegraphy and data transmissions The effects of present interest are those due solely to differences in transmission time between one satellite path and another, and these are of two types : — lengthening or shortening of telegraph elements when the transmission time differences are relatively large, i.e., exceeding a significant part of an element; — phase discontinuities of voice-frequency tone, sometimes giving rise to telegraph distor tion, whenever the transmission time differences exceed a fraction of the time occupied by one cycle of the highest baseband frequency utilized by a telegraph channel of a broadband system carrying voice-frequency telegraphy [1, 2]. According to preliminary information from one source (see Annex I), it appears that, in an unprotected 50-baud start-stop telegraph channel the average number of character errors caused by discontinuities of up to about 7-5 ms may not exceed about 0-25 per disconti nuity. The average number of character errors increases probably to about 1-0 for discontinuities of duration about 10 to 12 ms, whilst it may approach 2-0 or more for discontinuities of duration up to 20 ms or 30 ms. Time duration of the discontinuities likely to be encountered in non-stationary satellite systems would cause character errors in synchronous telegraph systems and in time-division multiplex telegraph systems. Time discontinuities can falsify selection signals such as used in Telex, causing incorrect routing and, particularly on automatic systems, the possibility of incorrect charging might arise. Automatic error-correcting (e.g. ARQ) equipment is used on some telegraph circuits, for example when the traffic is extended over HF radio links. It may be noted that ARQ would not only protect against errors arising from switching discontinuities, but also against errors arising from other causes. Justification for any special treatment of circuits routed through communication-satellite systems should take into account the relative frequency of error- producing disturbances in the satellite links and in their terrestrial extensions as well as in international circuits using other means. If, after account has been taken of the various sources of error in telegraph channels, it seems necessary to take special measures to deal with errors caused by satellite switching discontinuities, then it appears that consideration might be given to the possibility of using some device such as a buffer store. This might commence to store the telegraph signals on receipt of a “satellite change” signal, and would retransmit at a slightly higher rate after the satellite switching operation. Another method of reducing the number of errors due to satellite switching would be to use a suitable variable delay device. Switching discontinuities of up to perhaps 20 ms would affect data transmission by causing : — errors to occur in one or more blocks, — loss of block phase. Provided the switching from one satellite to another is fairly infrequent, the errors of the first type would not be serious, and would in fact be similar to the effects of occasional switch ing or noise disturbances to be expected on normal line circuits. The loss of block phase results directly from the time discontinuity and has no equivalent in line systems. — 353 — Rep. 214-1 Block phase would thus need to be re-established on data circuits each time a switch from one satellite to another occurs, unless means are adopted to compensate for the delay disconti nuity. However, if the switch-overs are not unduly frequent, and re-phasing of the data transmission system is arranged to take place automatically, the loss of circuit time due to this cause would not be a serious disadvantage. 3.3 Phototelegraphy The effect of these discontinuities would be an immediate displacement (either in an advance or a retard direction), of any succeeding elements of the picture relative to the posi tion before switching. For equipment conforming to C.C.I.T.T. standards and using a drum speed of 60 r.p.m., a delay discontinuity of 20 ms would produce a displacement of about 2% of the picture width, e.g. 0-5 cm displacement in a picture 25 cm wide. This displace ment would be a serious defect in most pictures or in typescript, meteorological charts, etc. With higher scanning rates, the displacement would increase in proportion. The amount of such displacement that could be accepted as tolerable is, of course, a matter to be decided in consultation with the C.C.I.T.T. It seems likely, however, that switching discon tinuities of the order of 20 ms would produce unacceptable distortion in the majority of cases, and would, therefore, need to be avoided, either by suitable delay-compensation techniques or by arranging that the picture transmissions do not occur during switching times. 3.4 Television Switching from one non-stationary satellite to the next is very similar to, and will generally produce the same effects as, switching between “non-synchronous” programme sources, and can result in temporary disturbance to the receiver field time-base. The actual time over which the disturbance exists will vary in practice depending upon the relative phase relationship at the moment of switching, but will normally lie between 0-5 s and 2-0 s. The change in transmission delay on switching may introduce a small discontinuity in the sound signal which, although noticeable, should not be disturbing. As switching in a satellite system will be infrequent, the effects on both vision and sound signals should not prove too serious. 4. Summary The significance of Doppler frequency-shift and switching discontinuities in communica tion-satellite systems varies with the type of service or signal transmitted, and with the charac teristics of the satellite orbit. In general, stationary satellites are not expected to introduce significant Doppler frequency-shifts or switching discontinuities. Non-stationary satellite systems will introduce greater Doppler frequency-shift and switching discontinuities. The major component of the Doppler frequency-shift can be removed in the radio- frequency receiver, but there will remain a “stretch” or “shrinkage” of the baseband spectrum due to differential frequency shift. The effect on monochrome television will be insignificant and the effect on colour television will probably be tolerable. In telephony, with the general use of broadband single-sideband frequency-division multiplexing techniques, the changes in baseband spectrum (differential Doppler) will require compensation in the form of transmission delay equalization of the entire baseband or of automatic control of the carrier frequencies used in the multiplex equipment. It is felt that such compensation is feasible. Telegraph, data and phototelegraphy channels, derived from channels adequately corrected for telephony, should not require any further consideration of Doppler effects. It appears that, unless special steps are taken, time discontinuities due to satellite switch ing may lead to error rates on telegraph channels which, for certain pairs of earth stations with particular orbital configurations, could exceed the desirable limit suggested in C.C.I.T.T. Rep. 214-1 — 354 Recommendation R .54 of 2 errors per 100 000 telegraph characters. Some discussion of this matter, and of possible means of mitigating the effects on telegraph transmission, is given in Annex I. ' The attention of the C.C.I.T.T. and the CMTT is drawn to the problems which may arise in communication-satellite systems due to Doppler frequency-shifts and switching discontinuities ; the C.C.I.T.T. with regard to telephony, telegraphy, phototelegraphy and data transmission and the CMTT for television transmission, including the related sound channel. ANNEX I 1. C.C.I.T.T. Recommendations C.C.I.T.T. Recommendation R.57 calls for a maximum isochronous distortion over a single telegraph link not exceeding 10%. C.C.I.T.T. Recommendation R.54 suggests, in the considerings, an error-rate not exceeding 2 per 100 000 telegraph characters due to distortion as a desirable overall trans mission objective. 2. Telegraph error-rates in 50-baud start-stop telegraph systems In a preliminary series of experiments, the relationship between the duration of switching discontinuities and telegraph error-rate in 50-baud start-stop telegraph systems has been explored. The error-rate is dependent, to a small extent, on the nature of the transmitted text. It appears that the error-rate may not vary greatly when the duration of the switching disconti nuity is varied between 0 and about 7-5 ms ; the average number of errors is then about 0-25 per switching operation. For durations exceeding about 7-5 ms, the error-rate increases; this may be explained by the evident fact that, in these circumstances, the lengthening or shortening of the telegraph elements approaches or exceeds 50% of the duration of the elements. The preliminary experiments suggest that the average number of character errors per discontinuity may be about 1-0 for discontinuities of duration about 10 to 12 ms, whilst it may approach 2-0 or more for discontinuities of duration up to and exceeding 20 ms. These results, as stated above, apply to telegraph signals at a speed of 50 bauds ; the duration of each element is 20 ms and it is not unreasonable to find that, for discontinuities of duration up to about 30 ms, there may not be more than two telegraph character errors. 3. Compensation by means of variable-delay correction devices 3.1 Compensation with moderate accuracy It would be possible to greatly reduce character errors due to satellite switching if suitably controlled variable-delay devices could be connected in tandem with satellite links, so that the overall signal delay could be kept constant. Compensation to an accuracy of the order of 200 ps would deal with character errors due to the lengthening or shortening of telegraph elements. The development of such broadband delay devices would have the additional advantage of substantially eliminating differential Doppler-shift effects in the transmitted baseband ; these would otherwise call for special control of super-group and group translation oscillators to preserve the centring of voice-frequency telegraph signals in their appropriate filter bandwidths. The effects of phase jumps at the instant of satellite switching would remain, and while the character error-rate would be less than that estimated to occur without compensation, a reliable estimate of the probable error-rate would require experimental investigation. — 355 — Rep. 214-1 3.2 Compensation with high accuracy To avoid character errors due to phase jumps at the instant of satellite switching, delay compensation to an accuracy corresponding to ±15° at the highest baseband frequency involved appears to be necessary. For telegraph channels carried in the highest frequency telephone channels of a 1200-channel system with baseband frequencies up to 5 MHz, an accuracy of some 0-01 ps would be required. The probable limit of predicted satellite slant range, and therefore of transmission delay, is of the order of 50 ps. Consequently, direct compensation on a predicted basis in a single step to an accuracy sufficient substantially to remove telegraph errors is impracticable. Consideration might, however, be given to addi tional measures, for example, an electronically controlled variable-delay device, which has its delay changed until the baseband signals over the two satellite paths displayed complete correlation in time, the switch-over then taking place. Another possibility to which attention might be drawn is the employment of a relatively slow “fade-over” instead of an abrupt switch-over. The major effects of sudden phase changes might thereby be avoided and only a small proportion of telegraph channels suffer from amplitude effects. FM voice-frequency telegraph systems can tolerate at least 15 dB reduction of signal level and printed error-rates, of the order of one in 80 000 might be achieved, although this possibility requires theoretical and experimental investigation. The effect of such a “fade- over” on telephone, data and facsimile circuits would need to be assessed. 4. Summary of means of compensation In considering possible methods of mitigating the effects of switching discontinuities on telegraph performance, it must be borne in mind that in any telegraph channel there may be a number of causes of error. Telegraph errors due to satellite-switching discontinuities might be reduced in number b y : 4.1 “buffer store” systems, which would commence to store on receipt of a “satellite change” signal transmitted over the system and re-transmit at a slightly higher rate after completion of the change; 4.2 time discontinuity correction of moderate accuracy used in conjunction with any of the following measures : 4.2.1 placing of the telegraph channels in the lower part of the baseband spectrum ; 4.2.2 inter-satellite switching, which takes place at the point where the telegraph signals are d .c .; 4.2.3 introducing slow “fade-over” devices to mitigate transients caused by rapid switching between satellites ; 4.2.4 recoding of the telegraph information into special codes, such as those developed by Hamming, which give correction facilities without the necessity for retransmission; 4.3 precise compensation of transmission delays to minimize the delay discontinuity at change over. In addition, it would be possible to use ARQ or some equivalent system ; this would be particularly useful in the event that the satellite link is extended by an HF radio link or other type of link liable to introduce a relatively large number of telegraph errors. Rep. 382 — 356 — REPORT 382 * , DETERMINATION OF COORDINATION DISTANCE (Study Programme 2A/IV) (1966) 1. Coordination area and coordination distance When terrestrial stations are located within the coordination area of an earth station sharing the same frequency band, mutual consultation between Administrations is required. Terrestrial stations located outside the coordination area will experience or cause only a negligible amount of interference. The coordination area around an earth station is deter mined by plotting the coordination distance as a function of the azimuth. Since the coordination area is determined before any specific cases of potential inter ference are examined in detail, it must be based perforce on assumed parameters of the terrestrial systems, but the pertinent parameters of the earth station are known. So as not to inhibit the technical development of terrestrial systems, the assumed parameters must lie somewhat beyond those presently employed. The techniques of calculating interference in the general case are discussed in Report 388, but, for the reasons given above, a simplified procedure is desirable for calculating coordina tion distance. This Report gives such simplified procedures to assist Administrations who intend to instal an earth station. Because it is essential to have only simple procedures for calculating coordination distance, detailed calculations regarding precipitation scatter and diffraction (see Report 339) are considered to relate only to calculations of interference probability. 2. Formula for basic transmission loss associated with coordination distance 2.1 General The basic transmission loss is given by the following formula : Lb = (/>, + G,)-Fs-(P -G r) where Pt — power, in dBW, or power density in dBW, in a 4 kHz band, supplied by the inter fering transmitter to the input to the antenna ; Gt = isotropic gain, in dB, of the transmitting antenna of the interfering station in the pertinent direction; Fs = earth station site-shielding factor, in dB ; Pr = maximum permissible interfering carrier power, in dBW, or maximum permis sible interfering power density, in dBW, in a 4 kHz band, in the same units as P„ at the input to the receiver subject to interference ; Gr = isotropic gain, in dB, in the pertinent direction, of the receiving antenna of the station subject to interference, less feeder losses and polarization discrimination, if applicable. In the absence of any definite information, the antenna of the terrestrial system must be assumed to be directed towards the earth station. The operating direction of the earth-station antenna depends on the type of satellite orbit with which the earth station will work. For * This Report was adopted unanimously. — 357 — Rep. 382 stationary satellites, the gain of the earth-station antenna towards the horizon at the azimuth of interest depends on the direction of and the elevation to the satellite positions with which the station will work. The gain does not vary with time. On the other hand, for non-stationary satellites, the antenna gain towards the horizon at the azimuth of interest will vary with time. For systems of phased satellites, the mean antenna power-gain over a typical satellite trajectory forms a reasonable approximation for which it can be assumed the gain will not vary with time. For systems of unphased satellites, it is necessary to consider the number and orbits of the satellites to specify the time variation of the earth antenna gain. Propagation between the earth station and the terrestrial station will occur via one or more modes, of which one may be controlling : — diffraction, for relatively short distances ; — tropospheric scatter, for relatively long distances ; — scatter from precipitation. In the interest of simplifying the procedures, it is reasonable to set a minimum coordina tion distance of 100 km, which removes the necessity of considering the diffraction mode, except under the circumstances of a common horizon. 2.2 Site-shielding factors In cases where earth stations are sited below the level of surrounding or nearby terrain, it is necessary to take account of a site-shielding factor denoting the additional transmission loss of any unwanted signal afforded by this terrain. Simplified values of the site-shielding factor appropriate to the calculation of coordination distance are given in the following Table. These conservative values derive from a more elaborate table taking distance into account (see Report 337), but use of the more elaborate table would involve solution by successive approximation resulting in unwarranted complica tion. In the event of multiple ridges, the site-shielding factor may be considerably higher than given in the Table. In such cases, other values may be agreed upon between the Adminis trations concerned. If, in a given azimuthal direction, an obstacle is seen from the earth station at an angle of elevation, 0, then—for that azimuthal direction—it is necessary, in calculating coordina tion distance, to employ the effective earth-station antenna gain at the angle of elevation, 0, rather than the gain along the horizontal. Elevation of horizon Site-shielding factor (Fs) (degrees) (dB) 0 -1 0 1 -2 10 2-3 17 3-4 23 4 25 These values apply to obstacles 5 to 16 km from the earth station. Nearer obstacles may be within the near-field region of the antenna beam, for which values of Fs are not available. Rep. 382 — 358 — 3. Basic transmission loss from earth station to angle-modulated radio-relay station A formula-is given for Pr, the interfering signal power in a 4 kHz band in § 1.2 of Report 388, which can be inserted into the formula for basic transmission loss as follows : Lb = (Pt + Gt) - F s + 192-6-10 log T - ( I C-N C) + Gr (dB) where T = noise temperature (°K) of radio-relay receiver subject to interference ; Ic = psophometrically weighted interference power (dBmOp) in a telephone channel; Nc = psophometrically weighted thermal noise-power (dBmOp) in a telephone channel, resulting from the noise temperature of the receiver subject to interference ; and Pt is measured in dBW in a 4 kHz band. The following parameters are appropriate to coordination distance : T = 750°K; Nc = —76 dBmOp; Ic = —63 dBmOp for 20% of any month and —43 dBmOp for 0-005% of any month. These figures derive from Recommendation 357-1, under the assumption that there are two entries of interference ; Gr = 42 dB. Inserting these factors into the equation above produces two equations, as follows : Lb (0-005%) = CP, + Gt) —Fs + 173 (dB), Lb (20%) = CP, + Gt) - F s + 193 (dB). I The second equation is relevant only at coordination distances less than the 100 km minimum, so that it need not be used. Coordination distances resulting from the first equa tion are given for various values of P, + Gt—Fs in Fig. 1. These curves enable the coordina tion distance to be directly derived from a knowledge of the earth-station equivalent isotropi cally radiated power in a 4 kHz band, in the given direction, as modified by the site-shielding factor. If the coordination distance lies partly in one zone and partly in another, the procedire given in Annex I should be followed. It is pointed out in Docs. IV/230 (Canada), IV/245 (I.F.R.B.), and IV/246 (I.F.R.B.), 1963-1966, that the coordination distance calculated using the gain of the main beam of an earth station may be greater for angles of elevation below 12° than the coordination dis tance calculated using horizontal gain. If situations arise in which the main beams of the earth antennae will be only a few degrees above the horizon and less than 12° above the horizontal for long periods of time, the following appears to be a suitable procedure for provisional use. Calculate (P, + G ^ n beam—8©), where 0 is the angle of elevation in degrees of the main beam of the earth-station antenna. This value is then compared with that of P, + Gt—Fs and the greater used in determining the coordination distance from Fig. 1. The relation between basic transmission loss and distance has been based on a nominal frequency of 6 GHz and the small variation applicable to other frequencies has been ignored in the interests of simplicity. — 359 — Rep. 382 4. Basic transmission loss from radio-relay station to earth station, angle modulation A formula for Pr, the interfering carrier power, is given in § 1.3 of Report 388, which can be inserted into the formula for basic transmission loss as follows : Lb = (Pt + Gt)—Fs—B —Ic—Pwanted + Gr where B = interference-reduction transfer factor in dB, for which values are given in Report 388, § 1.3 ; Pwanted — power (dBW) of the carrier subject to interference at the receiver input; and Pt is measured in dBW. The following parameters are appropriate to coordination distance: Ic = —66 dBmOp for 20% of any month, and —43 dBmOp for 0-01% of any month. These figures derive from Recommendation 356-1, under the assumption that there are four entries of interference at distances cor responding to 20% of any month, and three entries of interference at the greater distances corresponding to 0-01 % of any month; OP/ + Gt) '= the radiated power of the radio-relay station may safely be taken, for normal coordination purposes, as + 55 dBW, in some areas it may be necessary to produce additional coordination contours for other values of CP, + £,)• Inserting these factors into the equation above produces two equations as follows: Lb (0-01%) = ( - P wanted + Gr) —Fs—B + CP, + Gt) + 40-5 (dB), Lb (20%) = {-Panted + Gr) - F s- B + (P, + Gt) + 63-5 (dB). The second equation is relevant only at coordination distances less than the 100 km minimum, so that it need not be used. The factors B and Pwanted are functions of the number of channels per carrier in the satellite system. Using the equation for B given in § 1.3 of Report 388 and the r.m.s. frequency deviations of typical satellite systems given in Fig. 1 of Report 211-1, the following values of (B+ Pwanted) have been computed for an earth station with a system noise-temperature of 50°K. Number B + P w anted of channels (dBW) 12 -1 0 6 120 - 96 240-1200 - 90 It is now possible to compute the required basic transmission loss as a function of (G>—Fs) - and hence to plot coordination distance as a function of (Gr—Fs). This has been done in Figs. 2, 3 and 4 for the three radio climatic zones defined by the Extraordinary Administra tive Radio Conference, Geneva, 1963, for the specific case of (P, + Gt) = + 55 dBW. Contours for coordination distances for other discrete values of (Pt + Gt) can be pre pared by introducing the appropriate corrections in the Figures. For mixed paths, the procedure given in Annex I should be followed using Figs. 5, 6 and 7, but using the values appropriate to (G>—Fs). II Rep. 382 — 360 — ANNEX I CALCULATION OF THE COORDINATION DISTANCE FOR A MIXED PATH The method to be followed is essentially that adopted in E.A.R.C. Recommendation No. 1A. Figs. 5, 6 and 7 show the value of (e.i.r.p. in 4 kHz— as a function of the path length in each of the two zones separately. The procedure to be followed is illustrated by the following example, in which it is assumed that the (e.i.r.p. in 4 kHz—F^) in the direction of interest is + 10 dBW. In Fig. 8a, an earth station is shown situated 50 km from the coast, and there is an oversea path of 150 km before the coastline of a neighbouring country is reached. It is required to find the coordination distance from the earth station in the given direction using the mixed paths chart of Fig. 8b. The procedure is as follows: 1. Starting from the origin, the distance of 50 km from the earth station to the coastline is set off along the ,4-axis of the chart as indicated by the point A x. 2. The oversea path length of 150 km is then set off parallel to the B-axis of the chart as indicated by the point Bx. 3. The further overland distance required is then measured parallel to the ,4-axis from the point Bx, to the point of intersection with the curve labelled + 10 dBW, as indicated by X. 4. The coordination distance is the sum of the A and B coordinates of the point X. 0 Coordination distance (km) 3 -0 1 0 0 0 0 0 0 60 50 40 30 20 10 0 -10 -20 -30 oriain itne: at sain no ai-ea system radio-relay into station Earth : distance Coordination P, + t s F Gt- C : zone zone : C zone :B zone : A F gure r u ig (dBW/4 kHz) (dBW/4 1 B A C e. 382 Rep. Coordination distance (km) 5 -0 3 -0 1 0 0 0 0 40 30 20 10 0 -10 -20 -30 -40 -50 oriain itne Rdorly ytm no at sain zn A zone : station earth into system Radio-relay : distance Coordination — n nme o canl pr are i stlie system satellite in carrier per channels of number o C, CP, + For t = Gt) Gr—Fs 5 B and dBW 55 + F gure r u ig (dB) 2 T = T 50°K e. 382 Rep. Coordination distance (km) oriain itne Rdorly ytm no at sain zn B zone : station earth into system Radio-relay : distance Coordination = n ubr f hnes e crir n aelt system satellite in carrier per channels of number o ( For Pt + Gt) = + 55 dBW and and dBW 55 + = r s F Gr- F igure (dB) 3 T = 50°K = // 6 — e. 382 Rep. — 363 Coordination distance (km) oriain itne Rdorly ytm no at sain zn C zone : station earth into system Radio-relay : distance Coordination n ubr f hnes e crir n aelt system satellite in carrier per channels of number = o C, CP, + For Gt) 5 B and dBW 55 + = Gr-Fs F gure r u ig (dB) 4 T = 50°K = \\ e. 382 Rep. — 365 — Rep. 382 Path length in zone A (km) F ig u r e 5 Chart for calculation of coordination distance : mixed paths in zones A and B The figures above the curves indicate values of e.i.r.p.— Fs (dBW) Figures in brackets above the curves indicate values of Gr—Fs (dB) Note. — The figures given for G —Fs are for n = 12. For n = 120, add 10 dB, for n ^ 240, add 16 dB. Rep. 382 — 366 — Path length in zone A (km) F ig u r e 6 Chart for calculation o f coordination distance : mixed path in zones A and C The figures above the curves indicate values of e.i.r.n.—Fs (dBW) Figures in brackets above the curves indicate values of Gr—Fs (dB) Note. — The figures given for Gr—Fs are for n = 12. For n = 120, add 10 dB, for n ^ 240, add 16 dB. — 367 — Rep. 382 Path length in zone B (km) F ig u r e 7 Chart for calculation of coordination distance : mixed path in zones B and C The figures above the curves indicate values of e.i.r.p.— Fs (dBW) Figures in brackets above the curves indicate values of Gr—Fs (dB) Note. — The figures given for Gr—Fs are for n = 12. For n = 120, add 10 dB, for n ^ 240, add 16 dB. Rep. 382 — 368 — Land zone A Sea zone B Land zone A Earth-station transmitter (a) Path length in zone A (km) (b) F ig u r e 8 Example of calculation o f coordination distance for mixed paths — 369 — Rep. 383 REPORT 383 * COMMUNICATION-SATELLITE SYSTEMS The effects of transmission delay (Question 7/IV) (1966) 1. Introduction In a communication-satellite system, the transmitted signal will be subject to considerable transmission delay due to the propagation time of the radio path. The magnitude of this effect with various orbits is discussed and it is shown how various types of communication signal may be affected. 2. Transmission delay Overall transmission delay in a communication-satellite system depends largely on the altitude of the satellites and the number of earth-satellite-earth links, or hops, forming the connection. With stationary satellites, the transmission delay is essentially fixed for a given circuit, but if the satellites are in motion relative to the earth, the transmission delay will vary with time. 2.1 Single-hop connection For a single-hop circuit, the minimum possible transmission delay (tmin) occurs when the two earth stations are close together and the satellite is directly overhead. The maximum possible transmission delay (tmax) earth station-to-earth station, is obtained when the satel lite is at the horizon, as seen by both earth stations. Thus, if h is the altitude of the satellite, r is the radius of the Earth, and c the velocity of light, then let R = r + h; further, let 0 be the angle subtended at the centre of the Earth between the directions of the satellite and the earth station. Then the one-way transmission delay for the earth-to-satellite path is: t = — sj R2 + r2—2Rr cos 0 c From this, the minimum and maximum possible transmission delays for one hop between earth stations may be expressed as : tmin = 2 h/c and ______tmax = (2 hjc) / \/ 1 + 2 r/h The minimum transmission delay will in practice be slightly greater than tmin, because the earth stations would naturally be some distance apart. Also, the maximum’transmission delay would be rather less than tmax since the earth stations would, in practice, work to angles of elevation not less than 3° and perhaps generally not less than 5°. However, the above expressions enable the minimum and maximum transmission delays for a single-hop circuit (and the possible range of variation about this mean), to be calculated for different positions of earth stations with sufficient accuracy for present purposes. To allow for the component of transmission delay in the terrestrial extensions from the satellite-system earth stations, an amount of transmission delay should be added, calculated from the formula (taken from C.C.I.T.T. Recommendation G.114): 12 + (0-004 x distance in km) (ms) or 12 + (0-0064 x distance in statute miles) (ms) * This Report was adopted unanimously. It should be read in conjunction with Report 214-1. Rep. 383 — 370 — The 12 ms term is an allowance for the probable existing terminal equipment and some loaded-cable facilities, and the velocity factor of the second term, 0 004 (or 0-0064), is based on the use of high-velocity plant for the major transmission distance. For estimating purposes and since it is improbable that two maximum-delay terrestrial extensions will be used in one connection, it is considered reasonable to take an average value of 30 ms for the total transmission delay of terrestrial extensions, the minimum being of the order of 10 ms, the maximum of the order of 50 ms. Employing these figures plus the transmission delays computed for the tmin and tmax for mulae above, the range of values for total single-hop transmission delay (one way) for communication-satellite systems have been derived and are given in Table I. It will be noted that Table I shows the case of minimum transmission delay between earth stations plus minimum transmission delay due to terrestrial extensions and the opposite case of maximum transmission delay between earth stations plus maximum transmission delay due to terrestrial extensions. In practice, one may well have a maximum of transmission delay between earth stations with terrestrial extensions of minimum transmission delay, or vice versa. T a b le I Overall one-way transmission delay (ms) Non-stationary satellite Non-stationary satellite Stationary satellite (altitude 11 000 km) (altitude 14 000 km) (altitude 36 000 km) Min. Max. Mean Min. Max. Mean Min. Max. Mean Between earth- stations . . . 74 109 92 92 128 110 240 280 260 Terrestrial extensions . . 10 50 30 10 50 30 10 50 30 Total . . . 84 159 122 102 178 140 250 330 290 Fig. 1 gives the maximum one-way propagation time of the radio-path and the great- circle distance of the effective earth-coverage of the satellite, as a function of satellite altitude, assuming that the minimum usable angle of elevation at both transmitters and receivers is 5°. This great-circle distance does not determine the maximum separation distance between earth stations except with a stationary satellite. The hand-over requirement for communication continuity with non-stationary satellites limits the distance between stations to within the coverage overlap of successive satellites. The boundaries for uninterrupted operation of one-hop systems are discussed in Report 206-1. 2.2 Multiple-hop connections Consideration of the geometry of a communication-satellite system shows, that the mean transmission delay will increase in proportion to the number of hops. Assuming that the additional transmission delay for the terrestrial extensions (30 ms mean) will be the same for all cases and, assuming essentially zero transmission delay for the intercommunication between continuous hops, the values given in Table II are obtained. — 371 — Rep. 383 T a b l e II Overall one-way transmission delay (ms) Number of hops Non-stationary satellite Non-stationary satellite Stationary satellite (altitude 11 000 km) (altitude 14 000 km) (altitude 36 000 km) 1 122 140 290 2 214 250 550 3 306 360 The import of these transmission delays will be discussed in the following Sections of this Report. 2.3 Variation in transmission delay as a function o f time In a system using satellites in motion relative to the Earth, the transmission delay in any given circuit will be subject to a gradual change arising from the varying distance between the satellite and the earth stations. In a practical non-stationary satellite system of the type referred to above, the range of variation would not ordinarily exceed 2 0 ms per hop. One method of eliminating, or at least minimizing this change, is to insert a variable delay device in the signal paths at earth stations. The amount of delay thus inserted could be controlled automatically, to maintain the total delay constant at a value slightly greater than the maximum transmission delay of the radio path. The use of such a method could also reduce the effects of switching discontinuities which otherwise could occur at each hand over of circuits to another non-stationary satellite. This is discussed in Report 214-1. The variation in transmission delay is of particular importance in systems employing time division for multiple access (TDMA systems). In such (TDMA) systems, the transmission from each earth station must be inserted into the pulse train at the satellite at the proper time in each frame. Because of variations in the transmission delay, difficulties arise in synchronizing the transmission from the earth stations. Even though synchronism is obtained at one instant, shifts in the relative positions in time of the transmissions from the various stations will occur, unless the time of start of transmission is regulated. As an aid in expressing these changes, the “differential transmission delay” will be defined as the difference in time of arrival at the satellite of signals simultaneously transmitted from Stations A and B. That is, if t a is the transmission delay from Station A to the satellite at any instant, if Tjg is the transmission delay from Station B to the satellite at the same instant, then the differential transmission delay, AT, is A t = t b - t a This quantity is in itself, however, not of great importance. Of more interest is the “variation in differential transmission delay”, VAT, which occurs over a period of time. Table III gives the maximum values of the “variation in differential transmission delay”, VAr max., that can occur for satellites in the four different orbits. (Calculation of the values given in the Tables is explained in Doc. IV/226 (Canada), 1963-1966.) Rep. 383 — 372 — T a b l e III Maximum possible variation in differential transmission delay Orbital period (hr) 6 8 12 24 Approximate altitude (km) 11 000 14 000 20 000 36 000 Minimum elevation o f antenna : 5° VA-r max. (ms) ...... 30-5 32-0 33-7 0-124 (2) FAt max. (frames) C1) ...... 244 256 269 1 (0 In this case it is assumed that each speech channel is sampled 8000 times a second, and that the frame duration- is approximately Vsooo sec., or 125 us. (2) The indicated time shift takes into account various perturbations in the quasi-stationary satellite orbit, assum ing that it is possible to hold the real satellite position within a radius of 60 km from the exact stationary position, in which case the maximum velocity of the satellite towards, or away from, an earth station will not exceed 5-8 m/s. It should be noted that these variations occur slowly (e.g. for a satellite in a 10 000 km orbit and using a minimum antenna elevation of 5°, the variation occurs over a period of about 63 minutes). Nevertheless, there will be some variation in delay during the time it takes a signal to travel the distance from the earth station to the satellite and return. The magnitude of this variation for various orbits is indicated in Table IV, as a fraction of the frame time (125 psec). T a b l e IV Maximum round-trip variation in transmission delay as seen by one earth station Period (hr) 6 8 12 24 Approximate altitude (km) 11 000 14 000 20 000 36 000 Minimum elevation o f antenna : 5° Maximum delay variation (fram es)...... 0-004 0-003 0 -0 0 2 0-00004 0) (0 The indicated time shift takes into account various perturbations in the quasi-stationary orbit, assuming that it is possible to hold the real satellite position within a radius of 60 km from the exact stationary position, in which case, the maximum velocity of the satellite towards, or away from, an earth station will not exceed 5-8 m/s. In the TDMA system, all earth stations transmit on the same carrier frequency. This requires that the transmitter carrier be on only during that interval of the frame assigned to the station. During transmission, the carrier would probably be modulated by phase-shift keying or frequency-shift keying. Because of the Doppler frequency-shift, transmissions will arrive at the satellite and be repeated at frequencies which vary above and below the nominal carrier frequency. To accommodate this shift, the earth-station receivers must be capable of adjusting to the sudden changes in carrier-frequency which will occur. — 373 — Rep. 383 3. Telephony 3.1 Transmission delay and echo Several series of subjective tests have been carried out by members of the C.C.I.T.T., on the tolerance of telephone subscribers to the combined effects of long delays and the echo effects obtained with modern echo suppressors. Based on these results, a revised Recom mendation G.114, on limits for mean one-way propagation time, was adopted at the C.C.I.T.T. Illrd Plenary Assembly, Geneva, 1964. The pertinent Section of C.C.I.T.T. Recommendation G.114 is: “RECOMMENDATION G.114 (P.14) — Mean one-way propagation time A. Limits for connections It is necessary in an international telephone connection to limit the propagation time between two subscribers. Recent tests have shown that international connections probably will not cause adverse subscriber reaction due to the combined effect of delay and echo suppressors if the mean one-way propagation time * is increased from near zero to about 150 ms. As the propagation time is increased beyond 150 ms, subscriber difficulties increase, and the rate of increase of difficulty rises, up to and including the maximum one-way propagation time tested, namely 400 ms. The C.C.I.T.T., therefore, provisionally recommends the following limitations on mean one-way propagation times when echo sources exist and echo suppressors are used: (a) acceptable without reservation, 0 to 150 ms ; (b) provisionally acceptable, 150 to 400 ms. In this range connections may be permitted, in particular, when compensating advantages are obtained; (c) provisionally unacceptable, 400 ms and higher. Connection with these delays should not be used except under the most exceptional circumstances. Until such time as additional significant information permits Administrations to make a firmer determination of acceptable delay limits, they should proceed with caution and full cognizance of the data in Annexes E and F (2nd Part of Volume V bis of the Red Book) in selecting, from alternatives, plans involving delays in range (b) above.” Pending further consideration of these limits, the use of communication-satellite systems for telephony is restricted with respect to the number of links that can be operated in tandem. As was apparent from Table II and its associated discussion, not more than one hop via stationary satellites should be included in one connection. In a system of non-stationary satellites, the permissible number of links in tandem will depend upon the maximum accept able delay and the particular orbit characteristics. It is very desirable that the overall trans mission delay be held within recommended limits by a suitable combination of satellite and terrestrial links for telephone calls which otherwise would be subject to excessive overall effects of delay and echo, even when used with the echo suppressors giving the best known performance on long delay circuits. An approximate assessment has been made of the proportions of telephone traffic which could be accommodated by one-hop, two-hop and three-hop connections, using present knowledge. The results are shown in Table V below. * Mean value of the times in the two directions of transmission. Rep. 383 — 374 — T a b l e V Non-stationary satellite Non-stationary satellite Stationary satellite (altitude 11 000 km) (altitude 14 000 km) (altitude 36 000 km) Number of hops Mean one % Mean one % Mean one % way delay total way delay total way delay total (ms) traffic (ms) traffic (ms) traffic 1 122 80 140 80 290 88 2 214 18 250 18 (550) ( 12) 3 306 2 360 2 With stationary satellites, telephone traffic to points beyond the one-hop coverage area should preferably make use of terrestrial extensions. 3.2 Echo and echo-suppressors New echo-suppressors are available which are designed specifically for the delays associated with stationary satellite systems. In respect to terrestrial extensions of satellite circuits, the C.C.I.T.T. has stated that the delay on each national chain of extension circuits, equipped with echo-suppressors, beyond the last echo-suppressor in a connection, is an important parameter to be taken into account in the design of echo-suppressors. Tests have indicated that mean one-way end-delays of up to 10 ms can be added with no statistically significant increase in degradation using present designs of echo-suppressor intended for satellite-links with long propagation time. As reported in C.C.I.T.T. Study Group XVI Contribution No. 37, tests have been made by the Telephone Association of Canada, the United Kingdom and the A.T. & T. Co., in which two circuits were connected in tandem, each circuit having a separate pair of echo- suppressors. In each test, values of mean one-way delay for one circuit were taken up to that of a stationary satellite and these circuits were equipped with echo-suppressors designed for such conditions. The second circuit had modest values of mean one-way delay (up to 30 ms) and was, in each case, equipped with an echo-suppressor intended for such lower delays. Under these conditions there was no statistically significant increase in degradation over that due to the long-delay circuit alone. Information on three or more interconnected circuits with separate echo-suppressors has not so far been contributed. 3.3 Transmission delay contrast Due to the somewhat longer transmission delay which will be experienced on circuits employing satellite links, a contrast between international telephone calls may be experienced, due to the different facilities employed. Results submitted to the C.C.I.T.T. by the United Kingdom and the A.T. & T. Co. concerning the use of transatlantic cable and satellite circuits, show no statistically significant effects of this sort over the range of exposure obtained in a four-month period. 4. Telegraphy and data transmission When one-way transmission only is involved, delay would not be important. However, the loop delay is sometimes significant, such as when automatic error correction must be applied. Although the circuits provided by communication-satellite systems are unlikely to — 375 — Rep. 383 introduce significant errors in data and telegraph transmission, the earth stations may be linked to more distant terminals by other communication systems, such as HF radio, which are susceptible to errors. It may therefore be advantageous to apply error-correction to the whole circuit, including the satellite link. The loop-delay time is then of importance, since it affects the amount of storage required. The amount of transmission delay, which may be encountered on such composite circuits, is indicated in §§ 2.1 and 2.2. The most common system of telegraph error-correction (ARQ) now in use, should, when using an 8 -character repetition cycle, be able to accommodate loop delays of up to 850 ms. Extended storage would be required in a two-hop stationary satellite system and may be required in a three-hop non-stationary satellite system. For data systems, which use automatic repetition of incorrectly receivedfcode blocks, the storage capacity of the terminal equipment must clearly be made sufficient to accommodate all delays including propagation delay and switching delay. Switched telegraph operation may be confronted with some difficulties as far as long distance automatic selection is concerned : this is a matter which the C.C.I.T.T. should take into consideration. The smooth variation of delay, arising from the variation of path length, is unlikely to present any difficulty on the transmission of either telegraphy or data. 5. Phototelegraphy The absolute value of transmission delay is of no importance, but the smooth variation will produce a “skew” on the received picture, to an extent depending on the amount by which the delay varies during the time taken to transmit the complete picture, which is of the order of 15 min for equipment conforming to C.C.I.T.T. Recommendations and working at 60r.p.m. In an operational communication-satellite system, the delay variation over a 15 min period is unlikely to exceed 1 0 ms, and this would result in a peripheral displacement of the final scanning line from its true position by about 1% of the picture width. This amount of skew distortion would be acceptable in practice. It should be noted, however, that skew distortion can also arise from difference in drum speed between transmitting and receiving terminal equipment. The C.C.I.T.T. recommends that the terminal equipment should be equalized to within a tolerance of 10-5, which is equivalent to a change of delay of 10 ms over a 15 min period. In the most unfavourable circumstances, the delay variation of the satellite system could increase the effect of this speed difference and increase the total skew to about 3% of the picture width. However, such an unfavourable combination of errors would be unlikely to occur frequently. The foregoing refers to equipment conforming to C.C.I.T.T. Recom mendations, but is equally applicable to other types of equipment which provide similar definition standards, despite differences in transmission rate. For colour pictures, transmitted by the colour-separation process, delay variations may prevent accurate registration. 6. Television The transmission delays summarized in § 2 will in no way limit the transmission of television signals, provided that the accompanying sound signals are subject to the same order of delay. Only a small overall time difference between sound and vision signals can normally be tolerated before the sound becomes noticeably out of synchronization with the picture. It is, therefore, desirable that in a communication-satellite system, facilities should be provided to enable the vision and accompanying sound signals to be transmitted together over the system. Rep. 383 — 376 — 7. Summary Overall transmission delay need not be of concern for services utilizing one-way systems, e.g. television, monochrome phototelegraphy, and those telegraph and data systems not requiring error correction. Telegraph and data systems with automatic error-correction may require adaptation to cope with the transmission delays of communication by satellites, which will ordinarily be greater than encountered on other transmission media. For television with an accompanying sound channel, which is the usual case, precautions will be required to see that their relative transmission delays are within acceptable values. Transmission of the vision signal and sound signal over the same satellite system is desirable. Transmission delay takes on added significance in telephony. The results of recent tests, considered by the C.C.I.T.T., Geneva, 1964, and incorporated into Recommendation G.114, indicate that international connections probably will not cause adverse subscriber reaction due to the combined effects of delay and echo-suppressors, if the mean one-way transmission delay is increased from near zero to the order of 150 ms. As the delay is increased beyond 150 ms, subscriber difficulties increase, and the rate of increase of difficulty rises, up to and including the maximum one-way delay tested, namely 400 ms. It should be noted that the satisfactory operation of practical telephony circuits having loop-delays in excess of 300 ms (150 ms one-way), is conditional upon the use of improved design echo-suppressors. 13 Maximum great-circle distance (km) igehp ai ln va stlie mnmm nl o lvto : 5°) : elevation of angle (minimum satellite a via link radio Single-hop uv B: aiu oewy rpgto tm (ms) time propagation one-way Maximum (km) : distance B great-circle Curve Maximum : A Curve liue f h stlie (km) satellite the of Altitude 37 — 377 — F gure r u ig 1 one-way e. 383 Rep. Rep. 384 — 378 — REPORT 384 * FREQUENCY SHARING BETWEEN COMMUNICATION-SATELLITE SYSTEMS AND TERRESTRIAL RADIO-RELAY SYSTEMS Energy dispersal in communication-satellite systems with frequency-modulation of the radio-frequency carrier (Study Programme 2F/IV) (1966) 1. Introduction It is clear from studies of frequency sharing between communication-satellite systems ' and terrestrial radio-relay systems using frequency-modulation that, to ensure that mutual interference between the systems is kept to a tolerable level, it will be essential to use energy dispersal techniques to reduce the spectral energy density of the communication-satellite transmissions during periods of light loading. The reduction of the maximum energy density will also facilitate: — frequency sharing between communication-satellite systems themselves ; and — multiple-carrier operation of broadband transponders. The amount of energy dispersal required obviously depends on the characteristics of the systems in each particular case and this question is appropriate to studies of frequency sharing under Study Programmes 2B/IV and 2C/IV. It is clear, however, that it is desirable that the maximum energy density under light loading conditions should be kept as close as possible to the value corresponding to the conditions of busy-hour loading. In this Report, the results of some theoretical and experimental studies of energy dispersal techniques, applicable to frequency-modulation communication-satellite systems, are reported and suggestions for further experimental work are made. 2. Multi-channel telephony systems Annex I examines a number of methods of maintaining a high degree of carrier energy dispersal in telephony systems, with particular reference to the dependence of the obtainable dispersal on the complexity of the means of dispersal and the attendant increase in occupied radio-frequency bandwidth as a function of distortion. The methods fall into one or other of two general classes; one which adds a dispersal waveform not necessarily of constant magnitude to the input signal and the second which, in addition, effectively controls the deviation sensitivity of the frequency modulator. Various arrangements of these two methods are discussed in Annex I and are illustrated in Fig. 1. Method 1 (a) is the simplest, consisting of the addition of a dispersal waveform of fixed magnitude. The relative effectiveness of this method (i.e. the ratio of the maximum dispersed power per 4 kHz to the maximum power per 4 kHz under full load conditions), using each * This Report was adopted unanimously. — 379 — Rep. 384 of four low-frequency dispersal waveforms is shown in Fig. 2 for an assumed 10% increase in occupied radio-frequency bandwidth. The four waveforms considered are : — a sinusoidal signal; — a sinusoidal signal plus 30% third harmonic added in suitable phase; — a low-frequency triangular waveform ; — a band of low-frequency noise. It is evident from Fig. 2, that the low-frequency triangular waveform (Curve C) is the most effective of these waveforms and that, apart from this, the only one that appears to offer possibilities for general application is that of low-frequency noise (Curve D). While this is seen to be rather less effective (by some 5 dB) it, nevertheless, seems more appropriate to telephony systems than a triangular waveform for the following reasons : — its similarity to a telephony baseband signal; — the frequency band can be readily altered to suit whatever sub-baseband frequency range is available; — it does not depend for its effectiveness on a precisely specified waveform ; — it is easy to generate and apply. It would be useful to study the possibility of using, instead of noise, a known pseudo random signal with uniform spectrum in the low-frequency band. This would make it poss ible to suppress the signal at the receiver, thus avoiding certain disadvantages of this method. However, the use of a band of low-frequency noise, in conjunction with Method 1 (a), would result in an undue increase in the occupied bandwidth if it were desired to approach the busy-hour loading conditions. Hence, Method 1 (6 ), which incorporates automatic means of adjusting the degree of dispersal applied according to the state of the loading of the system, offers a much more attractive arrangement. Method 2 is more complicated than Method 1, but turns to advantage the need to provide energy dispersal by improving system noise performance when the deviation sensitivity is increased under light-loading conditions. The obvious disadvantage of this method is the need to provide overall gain regulation, while the extent of the advantage which would accrue under conditions of light loading is dependent on traffic loading outside the busy periods of the day. Of the variants of Method 2 discussed in Annex I, Method 2 (a), with a band of noise as the dispersal waveform, would seem to be the most suitable one for general applica tion. Uncertainty about the character of baseband spectra under practical conditions, is one of the main problems which militate against an adequate assessment of Method 2 at this stage. In considering the amount of dispersal which can be achieved in practice, it has to be borne in mind that the fully loaded condition will not necessarily provide the degree of dispersal postulated by the Gaussian distribution as in Annex I, § 1. The conventional load, representing mean busy-hour conditions was originally evolved for the purpose of calculating intermodula tion noise and may not be a particularly accurate representation for the purpose of dispersal studies ; this is particularly so for small capacity systems ; i.e. those of 60-channels or less. For this reason, it would be unwise to assume that, in practice, this ideal condition is attained, until there is some convincing support for such an assumption. In the meantime, there appears to be insufficient justification for assuming that it will be found possible, in practice, to maintain the dispersal of carrier energy for all degrees of loading within less than, say, 3 dB of what conventional busy-hour conditions would give rise to ; it may, in fact, prove difficult even to approach within 3 dB, without some increase in radio-frequency bandwidth. 3. Television systems Annex II considers the general problem of energy dispersal techniques for monochrome television systems and gives the results of an experimental study, to determine the subjective effects of the addition of various low-frequency waveforms to 625-line/50 fields-per-second Rep. 384 — 380 — television signals. The results show that a “symmetrical” triangular waveform is preferable to other waveforms considered and that the subjective effect of adding this waveform, of peak-to-peak amplitude up to 50% of the peak-to-peak amplitude of the video signal before pre-emphasis, is negligible provided th a t: — the dispersal waveform is locked in both phase and frequency to the field frequency; — suitable simple means of removing the added waveform are used at the receiving earth station; — the transmission system does not introduce intermodulation between the dispersal wave form and the vision signal. By a “symmetrical” triangular waveform is meant a wave in the form of an isosceles triangle (i.e. with equal rise and fall times). Sawtooth waveforms, with either rise or fall times approaching zero were also considered (see Annex II). If only the television aspects of the problem are taken into consideration, there is little to choose between the 12% and 25 Hz “symmetrical” sawtooth waveforms. There is a slight instrumental advantage to be gained by using the latter and it is considered, therefore, that, for the energy dispersal of 625-line/50 fields-per-second television signals, a synchronized 25 Hz “symmetrical” triangular waveform should be used and it is not expected that signifi cantly different results would be obtained by using a synchronized 30 Hz “symmetrical” triangular waveform with 60 fields-per-second television systems. To determine the dispersal effect produced by this method, it will be assumed, for the purpose of example, that the overall system performance restricts the permissible peak-to-peak amplitude of the dispersal waveform to 30% of the peak-to-peak amplitude of the video signal. Considering a 625-line system employing the normal pre-emphasis network (Recom mendation 405), the peak-to-peak deviation of the dispersal waveform will be 9-4% of the peak-to-peak deviation of the video signal without pre-emphasis. Using the symbol AF(MHz) for the peak-to-peak deviation of the video signal, the dispersal obtained is approximately: /maximum energy per 4 kHz\ / 0-004 \ ^ ^ 10 108 (------total energy------) = 10 l0g (m 4 4 f) = “ (1 4 + 1 0 l0g Ai?) For comparison, the theoretical dispersal obtained in the telephony case, assuming a Gaussian spectral distribution and a peak-to-r.m.s. ratio of 12 dB as in Annex I, is : - ^28 + 10 log = -(1 9 + 10 log AF) (dB) which means that the television case is likely to be only some 5 dB below optimum for some 1 0 % increase in radio-frequency bandwidth. In the interests of bandwidth economy, it would be desirable to be able to control the deviation on the lines of Method 1 (b) of Annex I. It is not obvious that any simple method is possible as it would presumably be necessary to monitor the energy concentration in the radio-frequency spectrum. Finally, it should be noted that the use of the proposed method of energy-dispersal for colour television transmission would demand an even higher degree of system linearity than that required for monochrome transmission. 4. Summary It appears from this work that the most promising methods of energy-dispersal are as follows : — for telephony systems : the addition of a controlled noise signal below the baseband as in Method 1 (b) of Fig. 1 ; — 381 — Rep. 384 — for television systems : the addition of a “symmetrical” triangular waveform synchronized to the picture frequency as in Method 1 (a) of Fig. 1. At this stage, it would seem unwise to assume that the use of these methods would provide energy-dispersal as great as that provided under full-load conditions. The excess of the dispersal power per 4 kHz over that achieved under full-loading conditions (assuming a Gaussian spectrum), is unlikely in practice to be less than 3 dB for telephony and 5 dB for television. If substantial addition to the radio-frequency bandwidth occupied is to be avoided in telephony systems, even this amount of energy dispersal may well present difficulties, especially for low-capacity transmissions. Further experimental work is needed to determine the practicability of these methods. In particular, this work should be directed towards assessing: — whether the amounts of dispersal predicted can in fact be achieved ; — what distortion is likely to result from intermodulation between the signal and dispersal waveforms due to the non-linearities of a practical system; — the relationship between baseband distortion and radio-frequency bandwidth. It is also desirable that a study should be made of the spectra of telephone baseband signals, of the kinds likely to be routed via communication-satellite systems under various conditions of loading and of the resulting radio-frequency spectra. In the light of this information, it should be possible to make a better assessment of the potentiality of Method 2 of Fig. 1 for telephony systems. ANNEX I ENERGY-DISPERSAL TECHNIQUES FOR USE WITH TELEPHONY SIGNALS 1. General In studying ways of achieving high degrees of carrier energy-dispersal, it is useful to know what is the dispersing effect of the fully-loaded baseband signal, to have some reference value with which to compare what can be obtained artificially. It is legitimate, for the general class of wide-deviation frequency-modulation systems under consideration, i.e. those in which the multi-channel r.m.s. deviation (dF) exceeds the highest baseband frequency, which in turn greatly exceeds the lowest baseband frequency, to assume that the mean power spectrum under the conventional busy-hour loading conditions is of Gaussian form. Hence, the dispersing effect obtained under these conditions is : = 10 log10 0-004 I V2tt dF = -(2 8 + 10 log dF) (dB) (dF is expressed in MHz). Possible arrangements for maintaining a high degree of carrier energy under conditions of reduced loading, by the methods discussed in this Annex, are shown diagrammatically in Fig. 1. Rep. 384 — 382 — 2. Dispersal by added waveforms 2.1 Method 1 (a) The simplest way of bringing about some degree of carrier energy-dispersal is to add to the baseband signal, a suitable low-frequency dispersing waveform of fixed magnitude, as in Method 1 (a) of Fig. 1. Of a variety of dispersal waveforms that have been proposed, the following are examined in this Report: — a sinusoidal signal; — a sinusoidal signal plus 30% third harmonic added in suitable phase; — a low-frequency triangular wave ; — a band of white noise. To provide some basis for comparing the efficiencies of these waveforms, the maximum energy spectral density, which they produce when applied to an unmodulated carrier, has been calculated for an assumed 10% increase in occupied radio-frequency bandwidth. The results are plotted in Fig. 2, relative to that which would occur under the conditions of busy-hour loading ; the curves of Fig. 2 have been designated A-D as described above. Some approxi mation occurs here, because difficult questions of the relation between signal distortion and radio-frequency bandwidth limitation have been avoided by assuming: — Carson bandwidth occupancy (with peak-to-r.m.s. ratio of 12 dB) throughout; — that this bandwidth formula may also be applied to the sum of the signal and dispersed r.m.s. deviation when the dispersal is by noise band; — in other cases, that the occupied radio-frequency bandwidth is increased by the peak- to-peak dispersal deviation. The errors so incurred are not thought to be large, and in any case should be in the same sense for all waveforms. As a further approximation, each type of dispersing waveform is represented in Fig. 2 by a single curve. The relation between the typical channel capacities and r.m.s. deviations implied by the two abscissae scales is based on the information given in the Annex to Report 211-1. 2.1.1 Sinusoidal dispersal It is evident from Curve A of Fig. 2 that carrier energy-dispersal by a sinusoidal signal is rather inefficient, while Curve B shows that a sinusoidal signal with 30% of third harmonic is only about 2 dB better. For a typical 20-channel transmission, the maximum power density, in either case, exceeds what occurs under full-loading condi tions by about 10 dB. It is a feature of both these types of dispersal, that the amount by which the dispersed power-density exceeds that at full loading increases with the r.m.s. multi-channel deviation, and hence, with channel capacity. For example, the excess for 1200 channels is about 18 dB. 2.1.2 Triangular dispersal The most effective way, for a given increase in occupied bandwidth, of dispersing the energy present in a single spectral line is, at least theoretically, by the application of a triangular waveform. The dispersed power density is inversely proportional to the permitted percentage increase in radio-frequency bandwidth and Curve D of Fig. 2 shows that, if a 1 0 % increase in occupied bandwidth is permitted, the dispersed power per 4 kHz exceeds that under full-loading conditions by only about 4-5 dB for all numbers of channels. — 383 — Rep. 384 The triangular waveform evidently offers a simple and efficient means of dispersing the energy present in isolated spectral lines of telephony transmissions. It must be remembered, however, that its effectiveness depends upon faithful preservation of the shape of the wave until it appears as frequency modulation, particularly when a high degree of dispersal is required. If 32 dB of dispersal were required for a 1200-channel transmission, for example, flattening of the extremities of the wave by only 0-25% might lead to a local doubling of spectral energy density. 2.1.3 Dispersal by a band o f low-frequency noise A form of carrier energy-dispersal that is fairly easy to generate and to apply, is not critical in its application and shares with triangular dispersal the property of yielding a maximum energy spectral-density, inversely proportional to the amplitude of the waveform, may be accomplished by adding a band of low-frequency noise to the multi-channel baseband. Curve C of Fig. 2 shows that, for a 10% increase in occupied bandwidth, the maximum dispersed power per 4 kHz exceeds that under full-loading conditions by about 9-5 dB for all numbers of channels. 2.2 Method 1(b) An obvious variant of Method 1 (a), would incorporate automatic means for adjusting the degree of artificial energy-dispersal, applied according to the state of loading of the system, as shown in Method 1 (b) of Fig. 1. It might, in fact, be possible in this way, using say, noise- band dispersal, to maintain the maximum energy spectral density of a transmission quite close to its full-loading value without any increase in occupied radio-frequency bandwidth. The performance that could be achieved in practice would depend on the distortion produced by the interaction (via radio-frequency bandwidth limitation and other transmission charac teristics) of the dispersal waveform and isolated tones and active telephone channels under light-loading conditions. It is probable that the matter can only be settled experimentally, since there is as yet no generally accepted way of calculating the distortion that frequency- modulation signals undergo during transmission, even for the simple case of white-noise loading. A particular method that has been proposed for applying the variable degree of dispersal to which the present sub-section relates, relies on filling a suitable proportion of unoccupied telephone channels with simulated speech (i.e. band-limited noise). Although full dispersal could in this way be maintained without increase of bandwidth, the complexity of the apparatus likely to be required for the method is a serious disadvantage, as is the probable necessity for applying it at the audio switchboards from which the baseband originates. 3. Dispersal by automatic deviation control 3.1 General It would clearly be possible to adjust the signal level entering the system frequency modulator so as to maintain the r.m.s. (or peak) frequency deviation at some constant value. ’ The desired level could be obtained merely by subjecting whatever the baseband content happens to be to sufficient amplification, or by so amplifying after the addition of some fixed or variable amount of artificial dispersal. The overall baseband transmission loss of the system would be kept sensibly constant by a compensating adjustment of the post-demodula tion gain through the medium of a system pilot tone. The possibilities are discussed in the following paragraphs. 3.2 Method 2 (a) The most general method of carrier energy-dispersal considered in this Report, of which the others are in a sense degenerate forms, is Method 2(a) in Fig. 1. This consists in adding to the baseband, before the application of automatic deviation control, a source of artificial energy-dispersal whose amplitude is made to depend upon the loading conditions. Rep. 384 — 384 — The use of this method might add little or nothing to the occupied radio-frequency bandwidth. Furthermore, if the application of artificial dispersal were delayed until the approach of light-loading conditions, a valuable decrease in the sensitivity of the system to thermal noise, distortion, and interference might result. The magnitude of this decrease would depend upon what fraction of the fully-loaded baseband power were attributable to speech signals. As for Method 1 (b), some determination of the baseband distortion associated with this method is desirable although, other things being equal, such distortion would be less than in the earlier method, because the increased deviation-per-channel under light-loading conditions would render the system less sensitive to the radio-frequency distortion components produced. With regard to the choice of a means of artificial dispersal to be added to the baseband, this might consist of any of the low-frequency dispersal waveforms considered in § 2. The noise-band waveform resembling one or more perpetually-active telephone channels is perhaps to be preferred, because it is moderately efficient and because it has the same dispersing effect for a given r.m.s. deviation as the baseband signal, permits accurate deviation control by a simple r.m.s. detector and presents no difficulties of application at large amplitudes. 3.3 Method 2(b) As a trivial simplification of the foregoing method, the amplitude of the added dispersing waveform might be set at some fixed value. There would be some increase in occupied radio frequency bandwidth, although not so much as in Method 1 (a) for the same degree of dis persal. 3.4 Method 2(c)* The complete omission of artificial dispersing waveforms from the modulating signal would reduce dispersal by automatic deviation control to its simplest form. The effectiveness of the method would seem to depend on the baseband spectrum retaining some moderate degree of complexity even under light-loading conditions. Unfortunately, it may not be poss ible to count upon this : in the complete absence of telephone channel activity, the system loading would degenerate to a number of pilot tones, carrier leaks and the like. There might be enough of these in a large system to yield some semblance of evenly distributed baseband power, but this is unlikely to be true of low-capacity systems. In such systems, particularly if many of the carrier leaks were at an unusually low level, the lowest levels of loading might derive from a very small number of prominent pilots. It can readily be shown that, if the loading of a system results from only one or two prominent tones in the baseband, the radio-frequency spectral densities may exceed those obtaining under full load conditions by many decibels. It would, therefore, be unwise to rely upon the presence of a few tones to bring about, by application of automatic deviation control alone, a similar degree of energy-dispersal to that which results from full loading. ANNEX II ENERGY-DISPERSAL TECHNIQUES FOR USE WITH TELEVISION SIGNALS 1. Introduction In a television transmission system using frequency modulation, a large proportion of the radiated power may be concentrated on or near the radio carrier frequency under certain modulation conditions, e.g. when a television picture with large areas of the same brightness is being transmitted. Energy-dispersal can be achieved by adding a suitable low-frequency waveform to the video signal before modulation. — 385 — Rep. 384 To obtain information on the degree of degradation which would be introduced when this type of energy dispersal technique is used, an experimental study has been made to determine the subjective effects, on 625-line monochrome television signals, of adding and removing, by several methods, various low-frequency waveforms which are suitable for energy dispersal purposes. 2. Dispersal waveform The amplitude and shape of the dispersal waveform which is added to the video signal before modulation, must produce the required amount of carrier energy dispersal without intro ducing a significant degradation in the transmission performance of the system. This latter requirement also depends upon the efficiency of the method used to remove the added wave form and on the overall linearity of the transmission system. Two forms of triangular wave form (the “symmetrical” triangular and the “sawtooth” waveform), having repetition fre quencies centred around 50, 25 and 12-5 Hz, have been considered in some detail to determine which waveform is to be preferred. It was thought that the most favourable result would be obtained by synchronizing the dispersal waveform to the field-frequency of the television signal and also that the relative phasing of the synchronized signals might, in some cases, give variations in picture impair ment. These effects were examined by using both synchronized and unsynchronized dispersal waveforms and, as far as impairment to the received picture is concerned, the tests showed that there is a considerable advantage to be gained by using a synchronized rather than an unsynchronized waveform. As the generation of waveforms synchronized to the television field-frequency presents no practical problems, the remaining experiments were confined to synchronized waveforms. The process of synchronization should normally ensure correct phasing of the waveform with respect to the television field information. With the 50, 25 and 12-5 Hz “sawtooth” and the 25 and 12-5 Hz “symmetrical” waveforms, all points of inflection will occur during the field-blanking interval and the discontinuities in the slope of the waveform will not appear as an impairment to the picture. With the 50 Hz symmetrical wave-form, only alternate points of inflection can coincide with the field-blanking interval and the remaining points occur at the mid-point in each television field (i.e. across the middle of the picture). It was thought likely that the peak-to-peak level of waveform which would be required for energy dispersal purposes was between 10 and 50% of the peak-to-peak amplitude of the video signal before pre-emphasis and the tests were confined to this range of levels. 3. Removal of dispersal waveform After demodulation at the receiving earth-station, the dispersal waveform must be removed from the baseband signal and two proposals for doing this were examined: 3.1. Waveform cancellation The dispersal waveform can be removed from the baseband signal by “cancelling” it with a locally generated dispersal waveform which is added in antiphase. While this is basically a simple system capable of giving excellent results under poor signal-to-noise conditions, the practical instrumentation is quite complicated and does not appear to have any significant advantages over the following alternative proposal. 3.2 Black-level clamping The effects of the dispersal waveform may be removed from the baseband signal by using a well-established television technique known as “black-level clamping” . The “clamp” is a device which is normally used to remove low-frequency distortion from a television signal by means of a sampling and error correcting process [1 , 2 ]. The amount by which a low-frequency error-signal may be reduced by “clamping” is a function of the frequency of the error signal and of the level of random noise present on the video signal. As initial satellite systems may have to handle video signals having a poor Rep. 384 — 386 — signal-to-noise ratio, the characteristics of the clamps used in these experiments were adjusted to be consistent with the optimum performance which can be obtained with 625-line systems operating under conditions of poor signal-to-noise ratio. A typical characteristic for sinusoidal error-signals is given in Table I. T able I Error-signal frequency (Hz) (sine w a v e )...... 50 25 12-5 p-p error-signal output ------:------—:------(dB) ...... - 1 5 - 2 1 - 2 7 p-p error-signal input (It should be noted that as the frequency of the error-signal decreases, both the efficiency of the clamp and the visibility of flicker on a picture increase ; on a subjective impairment basis, therefore, these two effects tend to cancel out.) The effect of clamping an error signal having a triangular shape produces a result, which is similar to that which would be obtained if the error waveform were differentiated and with the levels which may be necessary in a practical energy dispersal system, a single clamp of the type described does not reduce to an acceptable level the impairments introduced by any of the various waveforms under consideration. At this point a major difference between the “sawtooth” and “symmetrical” waveforms should be mentioned. Because the slope of the “sawtooth” waveform is constant during the “active” part of each television field, the only impairment which may be observed on a picture monitor after the video signal has been clamped, is a slight, and probably insignificant shading across the picture. However, the very high slope of the dispersal waveform during the field blanking interval causes a serious distortion of the waveform during this period, the magnitude of which is dependent upon the level of dispersal waveform being used. In practice, this distortion is most undesirable, as it can interfere with both synchronizing and vertical interval test signals which occur during the field blanking interval. It is also extremely difficult to remove this form of distortion once it has been introduced into the video waveform. With the “symmetrical” waveform, the residual impairment left after a single clamping operation can be observed as a picture impairment. With the 50 Hz waveform, the impair ment appears as a disturbance across the middle of the picture. For the 25 and 12-5 Hz waveforms, a picture “flicker” can be observed. This effect is also dependent on the level of dispersal waveform being used, but the application of a further clamp will reduce the flicker effect to a level where it is imperceptible, even with a dispersal waveform amplitude of 50% of the peak-to-peak amplitude of the video waveform. Although the characteristics of the waveform distortion left after twice clamping the signal are somewhat different in character, the magnitude of the residual distortion when a “sym metrical” dispersal waveform is used is some 10 to 20 dB less than when a “sawtooth” wave form is used. Bibliography 1. Sa vage, D . C. Three types of television signal stabilising amplifier. Report of International Television Conference, I.E.E. London, 251 (1962). 2. D oba, S. and R iek e, J. W. Clampers in video transmission. A.I.E.E. Transactions, 69, A ll (1950). — 387 — Rep. 384 A : Baseband signal input B : R.m.s. detector C : Amplifier 1 D : Amplifier 2 E : R.m.s. detector F : Output to frequency-modulator G : Dispersal waveform Method No control Gain 1 (a) No control Gain 1 (b) Gain Gain 2(a) Gain Gain 2(b) Zero gain Gain 2(c) Full load Full load F ig u r e 1 Simplified block diagram (Possible filters, buffer-amplifiers and gain-regulating pilots omitted) Typical number of channels £ 8 * £ ~ 88 — per 4 kHz when fully loaded (dB) Maximum dispersed power per 4 kHz relative to maximum power Multi-channel r.m.s. deviation (MHz) — 389 — Rep. 385 REPORT 385 * FEASIBILITY OF FREQUENCY SHARING BETWEEN COMMUNICATION- SATELLITE SYSTEMS AND TERRESTRIAL RADIO SERVICES Criteria for the selection of sites for earth stations in the communication-satellite service (Study Programme 2A/IV, § 1) (1966) The following factors should be taken into account when selecting sites for earth stations in a communication-satellite service: 1. Interference factors 1.1 Received power levels at a communication-satellite earth station are very low and the receivers are necessarily sensitive. Consequently, there are possibilities of radio interference : — between terrestrial radio systems which may be sharing the same frequency band (Final Acts of the Extraordinary Administrative Radio Conference, Geneva, 1963 refer); — from harmonic or spurious emissions from feed or mobile transmitters in a different frequency band (e.g. television or radio location devices); — from similar sources within the territory of neighbouring Administrations; — from man-made noise sources (e.g. power lines, ignition systems and other industrial sources). 1.2 When aircraft fly through the antenna beam of the earth station, the resulting re-radiation of energy to or from the earth station may be severe enough to : — render the communication satellite service unusable for a short period ; — interfere with terrestrial radio services in the same frequency band but which are normally protected by the nature of the terrain. 1.3 When an earth station is tracking at small angles of elevation, the effect of antenna sidelobes m ay: — increase the susceptibility of an earth station to radio interference; — increase the possibility of the earth station interfering with terrestrial radio services. 2. Geographical factors 2.1 Whilst the earth station must be favourably located to give maximum visibility of all satellites within the system, it may be necessary to consider the following additional factors when selecting the most suitable site: — plans for nearby terrestrial radio systems which may be projected by the home or neigh bouring Administrations (Recommendation 359-1, Report 209-1 and the Final Acts of the E.A.R.C., Geneva 1963 refer) ; * This Report was adopted unanimously. Rep. 385 — 390 — — the proximity of low-flying aircraft, approach lanes, etc.; — the site should preferably be surrounded by hills for protection against radio interference. However, to ensure maximum satellite availability, angles of elevation of any obstruc tions should not exceed about 3° ; — the ground should not be liable to subsidence ; — the site may be required to accommodate several large antennae without impeding each other’s visibility. 2.2 Atmospheric conditions may significantly affect the residual noise of a communication- satellite service and the ideal climate should be mild and dry, with low wind velocities. 3. Logistic factors 3.1 The site must meet with the approval of the local authorities and it must be possible to enforce building or other restrictions on adjacent land, if required for the interference-free operation of the station. 3.2 Connection to the cable or radio-relay network may be required and the distance to the nearest switching centre should be considered. 3.3 Primary power, adequate water supply and road access should be considered. 3.4 To determine the exact location of an earth station, it may be necessary to choose a site with visible triangulation points. 3.5 A collimation tower may be required. 4. Safety factors 4.1 Consideration should be given to the harmful effects of excessive radiation on human beings and the hazards associated with the handling of inflammable and explosive materials. As a guide, in areas where the radiation density exceeds 10 mW/m2, access should be restricted while the station is in operation. At no time should inflammable liquids be handled or explosive devices be stored in such areas. In areas where people are continuously exposed, a smaller radiation density may be appropriate. 4.2 Where low-flying aircraft may be exposed to these dangers, their operators should be advised accordingly. — 391 — Rep. 386 REPORT 386 * FEASIBILITY OF FREQUENCY SHARING BETWEEN COMMUNICATION- SATELLITE SYSTEMS AND TERRESTRIAL RADIO SERVICES Maximum power in any 4 kHz band which may need to be radiated in the horizontal plane by active communication-satellite earth stations (Study Programme 2A/IV) (1966) 1. Requirements of communication-satellite systems In considering a limit on the permissible horizontally-radiated power of earth stations, it is important to bear in mind the needs of active communication-satellite systems that can reasonably be foreseen. This must include systems for both multi-channel telephony and television. The use of telephony channels to convey signals such as voice-frequency telegraphy, and tones for test or signalling purposes must be taken into account, where this affects the maximum power to be transmitted in any 4 kHz band. Any limit of power so established must be suitable for the various methods of modulation, numbers of telephone channels and earth-station antenna sizes that might be used. It is also necessary to consider the charac teristics of the satellites which may be used, including the apportionment of noise and the satellite antenna gain. Operational requirements for margin and carrier energy dispersal also bear significantly on the final result. In the following, consideration is given to the power requirements for two types of multi-channel telephony system which are illustrative of those likely to require the highest value of transmitted power in any 4 kHz band. The requirements for frequency-modulation television transmission are not thought likely to exceed these values, assuming suitable energy dispersal techniques are employed. The two systems considered are: — a frequency-modulation system using frequency-division multiplex (FDM/FM), in which the earth station transmits a block of 1 2 0 0 channels, carrier energy-dispersal being used to restrict the increase in maximum power in any 4 kHz band in light traffic loading to 3 dB of the value under fully loaded condition ; — single-sideband amplitude-modulation (SSB/AM) systems, in which the transmitted power in any 4 kHz band is independent of the number of channels. In both cases, characteristics have been assumed which would lead to an estimate of the likely maximum earth-station power requirement in any 4 kHz band. The detailed charac teristics and calculations are given in the Annex, and the results show that the e.i.r.p. Ds in any 4 kHz band in the direction of the main beam would be 62 dBW for the frequency- modulation system and 81 dBW for the single-sideband system. 2. Horizontally-radiated power in any 4 kHz band To estimate the maximum e.i.r.p. in the direction of the horizontal plane, the angle of elevation of the antenna is assumed to be 3°, this being the minimum angle for earth-station transmission (see No. 470L of the Radio Regulations). From Report 391, the gain of an earth-station antenna, relative to an isotropic antenna at 3° from the axis, is about 20 dB and * This Report was adopted unanimously. Rep. 386 — 392 — is substantially independent of the size of the antenna. The measured characteristics of various antennae given in Report 391 indicate that gains some 4 dB greater may occur. Assuming a peak value of 24 dB and denoting the in-beam gain of the antenna relative to an isotropic antenna by Gs , the e.i.r.p., EH, in any 4 kHz band in the direction of the horizontal plane for an angle of elevation of 3° is Eh = Ds —Gs + 24 (dBW/4 kHz) The e.i.r.p. required by the two systems are summarized in Table I for two values of earth-station antenna gain, correspondinng at 6 GHz to diameters of about 26 m and 9 m. T a b l e I E.i.r.p. of earth stations (in any 4 kHz band) E.i.r.p. in the direction of the horizontal plane (Ej?) with E.i.r.p. in direction an angle of elevation 3° for in-beam antenna gain, relative System of the main beam to an isotropic radiator of: CDS) G s = 60 dB G s = 50 dB (dBW in any (dBW in any 4 kHz band) 4 kHz band) FDM/FM 62 26 36 SSB/AM 81 45 55 In the cases considered, system characteristics have been assumed which lead to an estimate of the likely maximum value of e.i.r.p. in any 4 kHz band which may need to be radiated by an earth station in the horizontal plane. The highest value (55 dBW in any 4 kHz band) arises with SSB/AM transmission on the up-path when a relatively small antenna is used by the earth station. The e.i.r.p. with FDM /FM is 19 dB lower in both cases of Table I. ANNEX ANTICIPATED RADIATED POWERS OF EARTH STATIONS 1. FM systems The following calculations are made for a system carrying 1200 telephone channels on a single carrier with an r.m.s. channel deviation of 1-1 MHz (5/), due to a 1 mW test tone applied at a point of zero relative level. It is assumed that the noise contribution of the up-path is 1400 pW, so that the unweighted signal-to-noise ratio in a 31 kHz telephone channel is 56 dB, the pre-emphasis advantage is taken as 2-5 dB, and, assuming a satellite receiver noise temperature of 1500°K, the required input radio-frequency carrier power Pr is derived as follows : Pr = 56-2-5-20 log (6 ///J + 10 log kTb (dBW) whence Pr = -95-2 (dBW) — 393 — Rep. 386 where f m is the top baseband channel-frequency, b is the bandwidth of telephone channel, T is the receiver noise-temperature, k is Boltzmann’s constant. For an orbital height of 36 000 km, the propagation loss at 6 GHz and low angle of elevation is 200-4 dB, and the satellite antenna gain is assumed to be 13 dB. The earth station is therefore required to radiate an e.i.r.p. Ds in the direction of the main beam as follows : Ds = -95-2 + 200-4-13 = 92-2 dBW The spectral distribution is approximately Gaussian, so that for a total r.m.s. frequency deviation of 6 -8 MHz, the highest ratio of power per 4 kHz to total power is given by : S = 10 lo g (0-004 / V2tt- 6 -8 ) = -36-3 dB The maximum e.i.r.p. in any 4 kHz band, in the direction of the main beam of the earth station with full traffic loading, is therefore 92-2—36-3 = 55-9 dBW. If then an allowance of 3 dB is made for up-path rain margin, the figure is increased to 58-9 dBW. The power density will be higher with light traffic loading, the increase being limited by the amount of artificial energy dispersal applied. Dispersal techniques are currently under study, but it seems reasonable to assume that it will be found possible to limit the increase of power density in the minimum loading condition to 3 dB. The maximum e.i.r.p. in any 4 kHz band, in the direction of the main beam, would thus be about 62 dBW. 2. SSB/AM systems Assuming the same noise contributions from the up-path and the same receiver noise temperature as in the previous case, the received power per channel Pr for a signal of 0 dBmO is given b y : Pr = 56 + 10 log kTb = -1 0 6 dBW Taking, as in § 1, the orbital height as 36 000 km, the frequency as 6 GHz, and the satellite antenna gain as 13 dB, the earth station e.i.r.p. in any 4 kHz band, in the direction of the main beam, at low angle of elevation, would be : Ds = -1 0 6 -1 3 + 200 = 81 dBW • This is the radiated power for a signal of 0 dBmO. It should be noted that there is a considerable variation of speech signal power among the telephone circuits. The distribution of power of a large number of talkers is Gaussian and the level of a median talker is taken as -13-6 dBmO [1]. Taking a standard deviation of 5-1 dB, the speech power of the highest 1% of the talkers is -1 -7 dBmO. Further, test tone levels in a telephone channel would not nor mally exceed 0 dBmO. It is considered appropriate to take the value of 0 dBmO as the maxi mum power in a telephone channel averaged over an integrating time of a few seconds, and the corresponding value of e.i.r.p. in any 4 kHz band, to be radiated by the earth station, is 81 dBW. B ibliography 1. R ic h a r d s, D. L. Statistical properties of speech signals. Proc. I.E.E., Vol. 111,5,941-949 (May, 1964). 14 Rep. 387 — 394 — REPORT 387 * POWER FLUX-DENSITY AT THE SURFACE OF THE EARTH FROM COMMUNICATION SATELLITES (Question 2/TV) (1966) 1. Introduction Recommendation No. 3A of the Extraordinary Administrative Radio Conference, Geneva, 1963, directs attention to the provisional nature of Recommendation 358 and the need for further study of the problem of maximum allowable limits for power flux-density, leading to a definitive Recommendation at the Xlth Plenary Assembly. In reviewing the information contained in the many contributions to this study, it is appropriate to consider first the manner in which the choice of orbit affects the exposure of radio-relay systems to communication-satellite systems; then to consider the power flux density at the surface of the earth, both from the point of view of the requirements of satellite systems and for the protection of radio-relay systems. 2. Effect of choice of orbit on the exposures of terrestrial systems to satellite systems The liability of a satellite system to cause interference depends on the satellite orbits and the location of the terrestrial systems that are liable to interference from the satellites. Examination has been given [1] to the problem when both systems are chosen at random and curves have been drawn (Fig. 6 of [1]) to show, for a given number of “entries” (i.e. pro duct of number of satellite transmitters and radio-relay receivers whose frequency bands overlap), the variation with time of the effective antenna gain of the radio-relay system in the direction of the interfering satellite system. The case of satellites in equatorial orbits and other orbits with recurrent earth-tracks has also been examined; in this case the radio-relay antenna will receive full in-beam exposure from a satellite, if it is so directed as to intersect the satellite orbit. The extreme case is of course that of a stationary satellite. Both these cases are discussed in Report 393, as are also methods to ascertain whether the beam of a particular radio-relay antenna will intersect the stationary satellite orbit. Information about the extent to which existing antennae of radio-relay systems are directed towards the stationary satellite orbit has been received from U.S.A., United Kingdom, Federal Republic of Germany, Canada, Australia and Japan. The percentage of the total antenna-beam directions which so intersect varies between the networks considered, but out of some 6000 antenna-beam directions reported, about 2 % are so oriented that the stationary satellite orbit is within 1° of the beam axes. More detail is given in the Annex to Report 393. 3. Power flux-density required for communication-satellite systems The required power flux-density for communication-satellite systems has been examined, first, in respect of existing systems requiring large and complex earth-station installations (G/T’exceeding40 dB) and, second, for less sensitive installations (G/Tless than 30 dB), which are representative of those that may result in a more economic overall system when, for example, a very large number of stations operate with a single satellite. In such cases, total system * This Report was adopted unanimously. — 395 — Rep. 387 cost is highly sensitive to the sum of earth station costs. Hence, less expensive stations (i.e. those with smaller antenna aperture and higher system noise temperature) are called for. The former installations represent the best achievable performance but it is important, in recommending limits of power flux-density, to take account of stations of the latter type if the potentialities of these systems are to be realized. The results of this examination show that, while the provisional limits given in Recom mendation 358 just suffice for current power limited satellite systems, they impose restrictions which inhibit the reasonable development of satellite systems for the future. This arises for the following two main reasons : — in wide-deviation frequency-modulation systems, carrier energy-dispersal can be used such that the total power flux-density limit ( —130 dBW/m2) is exceeded without exceeding the limit on power flux-density in any 4 kHz band (—149 dBW/m2). Hence, the former limit is restrictive without providing additional protection to radio-relay systems; — by allowing an increase in the limiting value of power flux-density in any 4 kHz band as the sub-satellite point is approached (see § 4.2), the design of satellite systems with simpler earth stations would be facilitated, which would permit more efficient use of the radio frequency spectrum. Such a relaxation would assist the satellite system designer, yet not increase the potential interference to radio-relay systems. The application of carrier energy-dispersal (see Report 384) to frequency-modulation systems may well enable the maximum power in any 4 kHz band under light loading to be kept to within some 3 dB of full loading. It is therefore appropriate to treat frequency-modu lation systems on the same basis as those using other methods of modulation and the same limits on power flux-density in any 4 kHz band should apply to all systems irrespective of the method of modulation. 4. Limits of interfering power flux-density required for the protection of radio-relay systems 4.1 General considerations In the Annex to this Report it is demonstrated that, in a model radio-relay receiver of high sensitivity, a maximum power flux-density of some —151 dBW/m2 in any 4 kHz band is consistent with the maximum allowable interference in a telephone channel of a 2500 km hypothetical reference circuit permitted by Recommendation 357-1. The in-beam condition assumed could, of course, persist continuously only in the case of a direct exposure from a stationery satellite. No more than a few radio-relay stations of a long system might be exposed to interference. However, some actual short systems may receive the whole of the interference allotted to the 2500 km hypothetical reference circuit and, when the systems are not extended to form part of long built-up connections, this amount could approach the level of noise from all other sources. Safety margins are inherent in any particular case because of the factors listed below (although it is not possible to evaluate these margins in decibels): — the beam of any one of the radio-relay antenna may not intersect the stationary satellite orbit; — there may not be a satellite in the part of the orbit intersected ; — such a satellite may not be using frequencies which could cause interference to the radio- frequency channels which are being received by the radio-relay station; Rep. 387 — 396 — — the radio-relay receiver in question may not be as sensitive to interference as the model system assumed in the Anpex (e.g. not quite in full beam, lower gain antenna, higher noise factor). The continuous full in-beam condition is not typical of interference from systems of non-stationary satellites and, in these cases, the afore-mentioned power flux-density limits would ensure a substantial margin for the protection of radio-relay systems. The values of power flux-density required for the protection of high capacity telephony systems should adequately protect television systems as well. 4.2 Effect o f angle o f arrival on permissible power flux-density Because of the need to achieve high efficiency and to avoid undue complexity in satellite antennae, it must be expected that the power flux-density produced by a satellite will vary over the surface of the earth. This arises from two factors : — for most satellite antennae, the radiation pattern of the antenna is likely to provide some 3 dB less gain towards the horizon than towards the centre of the Earth ; and — the path length (and hence, attenuation) to the horizon is greater than to the point immediately below the satellite. The combination of these factors means that the power flux-density at the sub-satellite point could be some 6-5 dB greater than at the horizon for a satellite at 8000 km, and some 4-2 dB greater for a satellite at 20 000 km. The power flux that is potentially troublesome to radio-relay systems is that which arrives horizontally, so as to enter the beam of a radio-relay antenna, and the value of power flux-density at the sub-satellite point could be many decibels greater than the horizontally arriving value without danger to the radio-relay user. It would therefore be helpful to set a limit to the flux-density as a function of arrival angle. A suitable arrangement would be to vary the allowable value of flux-density as a linear function (in decibels) of arrival angle, allowing a maximum increase of 6 dB for the sub-satellite path. 5. Conclusions 5.1 In determining a maximum level of power flux-density to be recommended, the following criteria should be met as far as possible : — the level should be low enough to avoid exceeding the recommended limits of interference to existing and future terrestrial radio-relay systems using the same frequencies ; — the level should be high enough to allow satisfactory operation of communication- satellite systems, both now and in the future. 5.2 In the light of the preceding sections, it is concluded that: — a total power flux-density of —130 dBW/m2 is unduly restrictive, on the development of frequency-modulation communication-satellite systems and does not provide additional protection to radio-relay systems ; — it seems reasonable that the power flux-density limits should, under any conditions of loading, be —152 dBW/m2 in any 4 kHz band arriving horizontally under free-space conditions and that an increase should be permitted for higher angles of arrival, up to a maximum increase of 6 dB ; — 397 — Rep. 387 — it may be assumed that carrier energy-dispersal will be applied to satellite systems using frequency modulation so that, under conditions of light loading, the maximum power flux-density in any 4 kHz band is only a few decibels higher than for the full load condi tion. Frequency-modulation systems may therefore be considered on the same basis as those using other methods of modulation and the power flux-density limits in any 4 kHz band should be the same for all systems, irrespective of the method of modulation. B ibliography 1. C ham berlain , J. K. and M ed h u r st, R. G. Mutual interference between communication-satellites and terrestrial line-of-sight radio-relay systems. Proc. I.E.E., Vol. I ll (March-April, 1964). ANNEX INTERFERENCE TO A RADIO-RELAY SYSTEM RECEIVER FROM A SATELLITE TRANSMITTING A UNIFORMLY DISTRIBUTED SPECTRUM 1. Introduction As an example of the method of calculation, this Annex evaluates the interference in a telephone channel of a model radio-relay system, resulting from the exposure of one of the radio-relay receivers to a uniformly distributed spectrum transmitted from a stationary satellite. The parameters assumed for the model radio-relay system are: — antenna of 7-5 m2 effective area (about 42 dB gain relative to an isotropic source); — 3 dB feeder attenuation ; — 3 dB polarization discrimination ; — overall system noise temperature, 750°K ; — 25 pW thermal noise (psophometrically weighted) in a telephone channel for a free- space path of 50 km ; — operating frequency, 4 GHz. The thermal noise in a 4 kHz band at the receiver input is : Pn = kTB = -228-6 + 28-8 + 36-0 - -163-8 dBW Taking into account the antenna effective area and the feeder and polarization losses, this thermal noise power corresponds to a power flux-density o f: Fn = -163-8-8-8 + 6 - -166-6 dBW/m2/4 kHz 2. Conditions of fading Fading of both the wanted radio-relay link signal and the unwanted satellite signal will occur at times. On occasions the signals will undergo deep fading, with the satellite signal fading to a greater extent than the radio-relay signal. This condition is relevant to the per missible short-time limits of interference. Rep. 387, 388 — 398 — In considering the limits of mean hourly interference, the worst conditions may arise when the two signals are relatively stable. It is necessary to make this assumption at the present time, but more experimental information on the fading of satellite signals arriving at very low elevation angles is desirable. 3. Hourly mean interference With the above assumption for the hourly mean, it is possible to calculate the interference in terms of a radio-relay link free-space thermal noise power of 25 pW. The permissible value of interference is 1000 pW, which is 16 dB above this thermal noise power. For this case therefore, the permissible power flux spectral-density of interference is: -166-6 + 16 = -150-6 dBW/m2/4 kHz 4. Interference for 0-01% of a month Ignoring the effect of averaging over one minute (which is a conservative assumption), the distribution of the instantaneous carrier-to-interference ratio can be derived from a convolution of the fading characteristics of the two signals. Suitable distributions are given in Fig. 1 of Report 338. It is found that the power flux-density which would result in an interference level of 50 000 pW for 0-01% of a month, is also about -150-6 dBW/m2/4 kHz. - REPORT 388 * TECHNIQUES OF CALCULATING INTERFERENCE NOISE IN COMMUNICATION-SATELLITE RECEIVERS AND TERRESTRIAL RADIO RELAY RECEIVERS (Question 2/IV) (1966) 1. Interference between angle-modulation systems 1.1 General The following methods of calculation assume that the communication-satellite system is either using wide deviation or such energy dispersal that the spectrum can be essentially represented by a Gaussian distribution. For the terrestrial radio-relay system, it is assumed that the deviation is sufficiently small that the spectrum is characterized by a carrier spike and fairly low sidebands. This condition certainly prevails during light loading conditions. Straightforward graphical methods for deriving interference-reduction transfer factors for all relevant cases are given in [2 ]. The interference convolution of the two spectra can be simplified in the following two cases : * This Report was adopted unanimously. — 399 — Rep. 388 1.2 Angle-modulation satellite system interfering with angle-modulation terrestrial radio-relay systems Since the power of the interfering signal is specified in any 4 kHz band, it is convenient to relate the interference in a telephone channel of the radio-relay system to the thermal noise at the input to the receiver subject to interference, as follows : Interference in telephone channel Interfering signal power in any 4 kHz band at the receiver input Thermal noise in telephone channel Thermal noise power in any 4 kHz band at the receiver input 1.3 Angle-modulation terrestrial radio-relay system interfering with angle-modulation satellite systems Since the interfering signal is by no means uniformly distributed over the spectrum, it is not possible to follow the same method. Instead, it is necessary to compute the relationship between the test tone-to-noise ratio in a telephone channel and the wanted-to-interfering carrier ratio. One can define an interference reduction factor B as follows : 1 mW test tone interfering carrier power B = 10 log ------x ------interference power in telephone wanted carrier power channel The factor B can be calculated using the following formula : „ 809 x f f x p x F x exp (fm2/2F2) B = 10 lo g ------Jm where: fm = top baseband frequency (MHz), fd = r.m.s. test tone deviation of the top channel (MHz), F = r.m.s. multi-channel deviation (MHz), p — pre-emphasis factor for the top channel for the satellite system [2 ]. For typical current satellite system designs, B has the following values: Number of channels B in the satellite system (dB) up to 240 33-5 300 32-4 600 29-7 1200 28-0 2. Interference between digital systems and angle-modulation systems Interference from PCM and PCM-FSK systems to FDM angle-modulation systems can be computed by the power-density method of § 1.2. It is necessary to write the spectrum of the interfering signal, which depends on the modulation rate, the frequency deviation, and the shaping of the pulses. These factors have not been standardized for either radio-relay systems or communication-satellite systems. Interference from FDM angle-modulation systems to PCM and PCM-FSK systems is in the early stages of investigation. Rep. 388 — 400 — Details of a study, verified by tests carried out in Japan, are contained in the Annex. The results show that a considerable reduction in interference is possible between angle- modulation systems and PCM systems, as compared to the mutual interference between two angle-modulation systems. 3. Interference between single-sideband systems and angle-modulation systems The power of a single-sideband transmission is expressed in terms of the talker power at a point of zero relative level. The average talker power is usually taken as —15 dBmO, but it may be necessary to take account of the fact that 1 % of the talkers may produce speech levels as high as 0 dBmO. The relationship between peak power and relative level is given by § 6 of C.C.I.T.T. Recommendation G.223. Typical peak powers of earth-station transmitters are given in Table VI of Report 211-1, and of satellite transmitters are given in Table III of Report 211-1. Using these parameters, interference between single-sideband systems and angle-modulated systems can be computed by the methods of § 1 . B ibliography 1. M ed h u r st, R . G. FM interfering carrier distortion : general formula. Proc. I.E.E., 109B, 149 (1962). 2. J o h n s, P. B. Graphical method for the determination of interference transfer factors between interfering FM multi-channel telephony systems. Electronics Letters, Vol. 2, 3 (March, 1966). 3. J o h n s, P. B. Interference between terrestrial radio-relay systems and communication satellite systems. Electronics Letters, Vol. 2, 5 (May, 1966). 4. B o r o d it c h , S. V. The calculation of admissible radio-frequency interference in multichannel radio relay systems. Elektrosvjaz, 1 (1962). 5. B en n e t t , W. R . and R ic e , S. O. Spectral density and autocorrelation functions associated with binary frequency-shift keying. B.S.T.J. 42, 2355-2385 (September, 1963). 6 . P ostl, W. Die Spektrale Leistungsdichte bei Frequenzmodulation eines Tragers mit einem stochasti- chen Telegraphiesignal (Spectral power density with frequency-modulation of a carrier with a stochastic telegraph signal). Frequenz, 17, 107-110 (March, 1963). 7. T jen g T . T jh u n g . Power spectra and power distributions of random and binary F M signals with pre modulation shaping. Electronics Letters, 1, 176-178 (August, 1965). 8 . A n d er so n , R . R . and Sa l z , J. Spectra of digital FM. B.S.T.J., 44, 1165-1189 (July-August, 1965). 9. J a cobs, I r a . The effects of video clipping on the performance of an active satellite PSK communication system. Trans. IEEE on Communication Technology, COM-13, 195-201 (June, 1965). ANNEX INTERFERENCE BETWEEN PULSE-CODE MODULATION SYSTEMS AND ANGLE-MODULATION SYSTEMS 1. In this Annex, the mutual interference between an angle-modulation communication-satellite system and a terrestrial radio-relay system using pulse-code modulation will be examined and a comparison will be made with the interference that would occur if both systems used angle modulation. — 401 — Rep. 388 The radio-relay system using pulse-code modulation considered by way of example, is characterized by: — four-phase differential modulation, — homodyne demodulation, — integral detection, — 240 multiplex channels per radio-frequency carrier. Other system parameters are 4 GHz frequency band, 42 dB antenna gain less 2 dB wave guide loss per feeder, a noise figure of 10 dB, and an equivalent receiver bandwidth of 7-7 MHz. The reduction of the bit error-rate by the aid of a stand-by radio channel or space-diversity technique is not considered here, to simplify the discussion. When integral detection is used, the equivalent bandwidth of thermal noise, B, is equal to the repetition frequency of the PCM pulse train, i.e. B = 7-7 MHz. Therefore, the receiver noise power kTBF is —95 dBm, assuming that the noise figure of the receiver is 10 dB as stated before. The relation between the bit error-rate and the signal-to-noise ratio, when the signal is reproduced by homodyne demodulation from a four-phase modulated wave, is given theoreti cally in Fig. 1. Here the noise is assumed to be Gaussian. In the system using differential modulation as assumed here, two pulse trains obtained from two carrier phases, orthogonal to each other, have bit error-rates twice and four times larger than the value shown in Fig. 1. Therefore, taking the worst case, the signal-to-noise ratio necessary to obtain the bit error-rate of 10~6 in this system is, from the ratio corresponding to the bit error-rate of 0-25 x 10-6 in Fig. 1, 110 dB. The experiments carried out show that an additional allowance of 3 dB must be made for imperfections in the equipment and inter-symbol interference, thus leading to a total signal- to-noise ratio of 14 dB for a bit error-rate of 10-6. Assuming that the bit error-rate of a 2500 km PCM radio-relay system will not exceed 10~6 (corresponding to a signal-to-noise ratio of 14 dB) for more than 0-01% of the time of any month, it is reasonable to assume that the bit error-rate due to interference from commu nication-satellite systems should not exceed 10~6 for more than 0 -0 0 1 % of the time. Due to inter-phase, inter-channel and inter-polarization interference within the PCM radio-relay system, there is already a signal-to-noise ratio of 18-8 dB in the absence of fading. Therefore, the signal-to-noise ratio due to interference from the communication-satellite system cannot be allowed to be lower than 15-8 dB to keep the overall signal-to-noise ratio better than 14 dB for 0-001% of the time. The probability of occurrence of Rayleigh fading in Japan is given in the following empirical formula for a month when the fading is most severe : P = KxQx(f/4y-2xd ^ (1) where: K = 5-1 x 10~9 ; Q — a coefficient depending on the condition of propagation : Q — 0-4 over mountainous terrain, Q — 1-0 over plains, Q = 72/ V h where h = — ^ (m) over sea ; / = frequency (GHz) ; d = span length (km). Rep. 388 — 402 — Taking the probability of occurrence of Rayleigh fading as that given by expression (1) and assuming regeneration once every 280 km of the 2500 km hypothetical reference circuit, it can be shown that the fading margin requirements are 24 dB for a 20 km hop length and 34 dB for a 50 km hop length, which must be added to the 18-8 dB signal-to-noise ratio in the absence of fading. In the four-phase modulation system, the carrier-to-thermal noise ratio is 3 dB higher than the signal-to-noise ratio at the demodulator input. Accordingly, the relation between the signal-to-noise from thermal noise and the receiving microwave power Pr, is as follows : Signal-to-noise ratio (dB) = Pr (dBm) + 95—3 (2) The non-fading microwave carrier power at the receiver input is then given by: Pr = Fd-13-2 (dBm) (3) where Fd is the fading allowance. Hence for the two hop lengths of 20 and 50 km, the carrier power at the receiver input is —49 and —39 dBm, respectively. For comparison, the normal carrier power at the receiver input for a 960-channel FDM-FM system would be about -33-5 dBm. 2. Conclusions It can be seen from the foregoing that PCM systems can operate at lower carrier powers than FDM-FM when using an equivalent band of the RF spectrum. The interference from terrestrial PCM transmitters into angle modulated satellite communication systems would therefore be correspondingly lower. The curve in Fig. 1 (experimentally verified) shows the bit error-rate for four-phase differential modulation as a function of the carrier-to-noise ratio. It will be noted that this is a very steep function and that unlike FM systems it does not have a proportional relationship with the signal-to-noise ratio in the telephone channel. This means that in practice no signifi cant deterioration of the PCM system occurs until the interfering carrier reaches a level 15-8 dB below the wanted carrier. For comparison purposes in typical FM systems, a 50 000 pWOp noise level is reached at a carrier-to-interference ratio of about 25 dB, a difference of about 10 dB from PCM systems. This advantage will only be realized if the PCM system operates at the same carrier level as an equivalent FM system. Accordingly for coordination purposes, the use of PCM systems is expected to offer some 6 dB reduction in isotropically radiated power of the radio-relay transmitter and some 4 dB improvement in interference rejection by the radio-relay receiver. — 403 — Rep. 388 Signal-to-noise ratio (dB) F ig u r e 1 Bit error-rate as a function of signal-to-noise ratio Rep. 389 — 404 — REPORT 389 * ESTIMATING INTERFERENCE PROBABILITIES BETWEEN SPACE SYSTEMS AND TERRESTRIAL RADIO-RELAY SYSTEMS Propagation considerations (Question 2/IV) (1966) 1. Introduction This Report discusses propagation topics relevant to the estimation of interference between space systems and terrestrial radio-relay systems. Propagation considerations involved in point-to-point interference between ground-based stations are emphasized. 2. Coordination distance and interference probability As a practical matter, recognized in the distinction between §§ 2 and 4 of Recommenda tion 355-1, the control of mutual interference between space stations and terrestrial stations involves few propagation uncertainties, while the control of mutual interference between earth stations and terrestrial stations involves serious propagation uncertainties. The first case emphasizes the geometry of the problem, and in the second case, the time variability and path-to-path variability of propagation and of antenna directivities may be more impor tant. For the first case, we have several useful C.C.I.R. Recommendations, but for the second case the state of technique has progressed only as far as Recommendation 355-1. There are rules for determining coordination distances and an associated coordination area, sometimes very large, within which it is essential to estimate interference probabilities to achieve satis factory coordination. 3. Estimation of interference probabilities Study Group V has been asked to provide sufficient propagation information about mechanisms such as diffraction, scattering or reflections from tropospheric layers or from precipitation, and tropospheric ducting, with a description of regional, diurnal, seasonal, and climatic differences. The statistics of available wanted signal-to-noise or wanted-to- unwanted signal ratios must be compared with the ratios required for satisfactory grades of service. Agreed methods do not yet exist for simplifying these problems and putting them in a proper perspective. Accordingly, this Report can only refer to propagation studies which may be found useful, with the following guidelines : — the estimation of interference probabilities for the control of mutual interference between earth stations and terrestrial stations requires a detailed knowledge of the characteristics of tropospheric propagation; — it is essential to estimate the percentage of time that high unwanted signal levels will be present. Especially for a long radio link, short-term measurements are of little value if not compared with summaries of available information such as those given in, or referred to, in Report 241-1 ; — to determine accurately the propagation losses over irregular terrain, a combination of measurement and theory is needed; * This Report was adopted unanimously. — 405 — Rep. 389, 390 general tropospheric propagation considerations are discussed in various Reports of Study Group V and a summary of useful prediction methods is given in Report 244-1 and in [1,2, 3,4]. Scatter from precipitation is discussed in Report 339. The following paragraph concerns two documents submitted to the Xlth Plenary Assembly which were not summarized in the Reports of Study Group V. Diffraction losses measured at 4 and 6 GHz over multiple mountain ridges for a number of paths in Japan [5] gave median values relative to free space loss as high as 80 dB. A method for evaluating median values of the diffraction losses and their variability in time is given in [1, 5]. High losses at 31 GHz were observed in Italy [6 ] over a 60 km path involving a diffraction angle of 2° 14' over a mountain ridge with a profile having a large radius of curvature. A smooth layer of snow increased the diffraction loss con siderably. Bibliography 1. National Bureau of Standards Technical Note 101, Revised May 1 (1966). 2. Note technique No. PR 3104 du Centre National d’Etudes des Telecommunications (1962). 3. K a l in in , A. I. Calculation of radio-relay links, SVIAZ (1964). 4. Over-the-horizon propagation at YHF. Sovietskoye Radio (1965). 5. C.C.I.R. Doc. IV/221 (Japan), 1963-1966. 6 . C.C.I.R. Doc. IV/243 (Italy), 1963-1966. REPORT 390 * EARTH-STATION ANTENNAE FOR THE COMMUNICATION- SATELLITE SERVICE (Question 1 /IV) 1. Introduction (1966) Question 1/IV is in four parts : — what limitations in antenna beamwidth result from atmospheric and ionospheric effects ; — what is the state of development in the design and fabrication of antennae ; — what is the state of development in side and back lobe suppression; — what pointing accuracy is reasonably attainable with antennae of various sizes and shapes ? * This Report was adopted unanimously. Rep. 390 — 406 — The first part of the Question is mainly concerned with propagation effects and considerable information is already contained in Report 205-1. This Report is concerned with §§ 2, 3 and 4 of Question 1 /IV, as applied to communi cation-satellite earth stations in terms of the following functions, where antenna design is a critical factor: — search and acquisition, — tracking, — command, control and telemetry, — transmission, — reception: rejection of noise, rejection of other unwanted signals. The earth stations of a communication-satellite system usually require large, steerable, microwave antennae, high radiated power, and receiving systems with a superior capacity for rejecting noise and other unwanted signals. They must share spectrum space with low- power terrestrial systems that also use narrow-beam antennae. Communication-satellite services depend mainly upon physical separation, shielding by terrain, and elevated antenna beams to avoid interference from or into other services. 2. General requirements The following requirements for earth-station antennae may be listed : — high gain in the direction of wanted signals, — fairly narrow beamwidth, — high efficiency, — low effective noise-temperature for the entire receiving system, — low gain in the direction of unwanted signals, — steerability over as much as a hemisphere in certain cases, — automatic steering regulated by a programme or by a satellite beacon signal. 3. The state of development in the design and fabrication of antennae 3.1 Construction o f reflectors Three methods of construction which have been used for reflectors of current antennae for use in communication-satellite systems are : — tubular steel spider, to which are attached shaped and reinforced aluminium sectors ; — steel frame strong-back with pre-shaped aluminium plates, of sandwich construction with honeycomb core. Each plate is individually adjusted to a best fit and joints are covered with aluminium faced tape; — a steel “ring and spider” strong-back, to which pre-shaped plates of stainless steel are attached for individual best-fit adjustment and finally welded. The assembly of the reflec tor and counterbalance structure about the elevation bearing is such, as to tend to correct distortion of the reflector, due to sagging, when the aperture plane is turned away from the horizontal position. — 407 — Rep. 390 At microwave frequencies, the maximum permissible size of the holes in the mesh is so small that there is no advantage in using a mesh in place of sheeting for the reflecting surface. 3.2 Profile accuracy The accuracy achieved in present antenna reflector-profiles varies from better than 1 mm r.m.s. to 5 mm r.m.s. relative to the ideal surface corresponding at a frequency of 4 GHz to A/75 and A/15 respectively. 3.3 Use o f radomes The provision of a radome for weather protection of a steerable antenna will permit some reduction in weight and simplification of drive and servo requirements. However, there are also disadvantages such as the occasional degradation in receiving system performance resulting from the use of a radome. For an antenna with radome having a system noise tem perature at 4 GHz of about 40°K in dry weather, the system noise temperature may increase by as much as 90°K due to rain, and as much as 120°K due to wet snow. 3.4 Steering The direction of a medium-altitude satellite, as viewed from an earth station, may be predicted to an accuracy of between about 0 -0 1 ° and 0 -1°, depending on such factors as the type of orbit and the method of computation used. The predicted data, computed for suitable intervals of time, may be used to steer an earth-station antenna, although for large antennae with narrow-beams some automatic correction, derived from scanning a satellite beacon trans mission may be desirable ; such correction can ensure that the pointing accuracy is sufficiently good to maintain the effective antenna gain within 0-1 dB or 0-2 dB of the maximum gain of the main beam. If predicted steering data is used, the search for, and “acquisition” of, a satellite at the beginning of each “pass”, does not necessitate the use of elaborate and complicated auxiliary equipment and antennae, except at earth stations which are used to support launches or for system re-orientation. Tracking accuracy depends on the accuracy, rigidity and stability of the antenna structure and its main components, and the accuracy of azimuth and elevation (or X—Y) direction determinations, the reduction of backlash or uncontrolled motion in any drive mechanism, stable, accurate and rapid response of servo-controls, and reliable and accurate servo-control signals. For medium-altitude orbits, tracking and slewing rates may have to be up to 50° per minute, or even 100° per minute, depending on the type of antenna mount and on the satellite orbit; however, by avoiding the use of particular orbital configura tions, or of particular satellite “passes”, the maximum Tracking and slewing rates may be reduced. If an elevation-over-azimuth mount is used (as is the case at a number of communication- satellite earth stations), there is a limitation on tracking near the zenith, owing to the high azimuthal velocities and accelerations which would be required. It is usual to consider a range of elevation angles between about 5° and about 85° for operational purposes, with the low angles below 5° available for acquisition of the satellite. Systems now installed, and in course of installation, for achieving adequate pointing accuracy, under all conditions of weather and of strain, on the large antenna structure for angles of elevation of medium-altitude satellites of up to about 80° or 85°, include : 3.4.1 radome-protected antennae, steered by information computed from predicted orbital data, with alternative autotrack modes of steering; Rep. 390 — 408 — 3.4.2 antennae without radome protection, steered by information computed from pre dicted orbital data, and including also automatic corrections acting sufficiently fast to maintain the pointing accuracy during strong gusts of wind, and for other adverse conditions. The servo-systems used in the control of steerable antennae may be such, that the angular velocity of the antenna is approximately proportional to the input signal used to demand a given movement; it may be advantageous for the input signal to be a relatively complex function of the required angular position of the antenna, the optimum velocity to achieve this position, and other correcting factors. 4. The state of development in the performance of earth-station antennae 4.1 Types o f antenna \ The three main categories of earth-station antenna now in use are : — parabolic reflectors with focal feed; — parabolic reflectors with Cassegrain feed; — horn-paraboloid antennae. The performance of present day earth-station antennae is discussed in the following sections. 4.2 Gain in the main lobe The gain achievable in the main lobe is largely a function of the uniformity of illumination of the antenna aperture. It is also a function of the accuracy of the reflector profile. Com pletely uniform illumination is difficult to achieve and is in any case generally undesirable, because it can result in excessive side-lobe levels due to spillover at the edges of the aperture. Present day antenna designs are compromises in this respect and aperture efficiencies of 50 to 75% are achieved. 4.3 Suppression o f side and back lobes The suppression of side and back lobes of an antenna depends on such factors as the smoothness and accuracy of the reflector surface, the amount of energy which spills over the edges of primary and secondary reflectors, and the amount of energy obstructed by and reflected by various parts of the structure. Inaccuracies of the reflector surface considered over a large correlation interval such as one metre, tend to cause large side lobes at small angles from the axis of the main beam, while inaccuracies considered over intervals such as a few centimetres, tend to cause large side lobes at angles of the order of 10° to 90° from the axis of the main beam. It appears that for para bolic reflectors, a profile accuracy (r.m.s.) of about A/50 over correlation intervals of 1 m, and several times better than this over correlation intervals of a few centimetres, is desirable for adequate suppression of side lobes. In this type of antenna, very careful attention to the design of structures in front of the reflector is necessary, to reduce side and back lobes due to reflections and spill-over; in horn-paraboloid antennae also the problem of spill-over is not entirely avoided. 4.4 Noise temperature All side and back lobes of an antenna, as well as the main beam contribute to its noise temperature. For most purposes it is sufficient to assume that side-lobes at elevations less than —10° “see” ground at a temperature of 300°K. Between —10° and 0° the temperature may be taken to be 150°K, between 0° and 10° to be 50°K, and between 10° and 90° to be 10°K. The power in the side-lobes may be expressed as a percentage of the total power and divided between these four regions. — 409 — Rep. 390 For example, for a Cassegrain antenna designed for 4 GHz, the noise temperature has been estimated to be : °K Main beam (3° elevation)...... 40-0 Near side-lobes...... 2-0 Sub-reflector spill-over...... 7-4 Main reflector spill-over...... 4-8 Total noise (4 G H z ) ...... 54-2 The value given above is not necessarily typical and is intended only as an illustrative example. The side-lobe pattern of the antenna itself is only one of a number of sources of noise in a satellite-communication receiving system. Other likely sources and representative noise temperature are: Waveguide between antenna and receiver = 6 °K to 20°K for a horn antenna and up to 40°K for a centre-feed paraboloid; Casse grain and offset feed antennae are likely to require slightly longer waveguide feeders than a horn antenna but less than a centre-feed antenna and will therefore have noise tem perature contributions from this source which fall between the figures quoted ; Radome (if provided) = 5°K when dry, about 100°K in heavy rain; First amplifying stage in receiver = 4°K if Maser, about 10°K or more with a cooled parametric amplifier. 4.5 Figure-of-merit o f the system The performance of a low-noise receiving system may be assessed by reference to its figure-of-merit, defined as : / antenna power gain \ °Sio \SyStem noise temperature (°K)/ Antenna power gain and system noise temperature are referred to the input of the low- noise receiver, noting that the noise contributions of the receiver stages following the reference point are included. The figure of merit is a very useful indication of the performance of an earth station. By way of example, consider a parabolic antenna in which the illumination of the reflector is tapered by 16 dB at the edges of the reflector to reduce the side lobes caused by spill-over or pick-up at the edges. In this case, only 1 dB of gain is sacrificed, while the figure-of-merit may be improved by 4 dB. Representative figures of merit have been calculated for several types of antenna having aperture diameters exceeding about 20 m. The values range from about 40 to 43 dB for an angle of elevation of 5°, and from about 42 dB to 45 dB for high angles of elevation. 4.6 Pointing accuracy The pointing accuracy achieved by large steerable antennae, using the steering systems described in § 3.4, is of the order of ± 0-02°, subject to the limitations in angle of elevation, etc., discussed in § 3.4. 15 Rep. 390 — 410 — 5. Characteristics of existing installations The main characteristics of the antennae of a number of earth stations are given in the Annexes to this Report. 6. Possible future improvements in design 6.1 Improvements to existing types o f antenna The cost of large antennae which have been built has been proportional, roughly, to the cube of the larger aperture dimension, so that an additional decibel of gain would cost half as much as the entire antenna. Moreover, the effect of atmospheric inhomogeneity is to reduce the achievable gain of large antennae. This effect depends on the amount of atmosphere traversed, and hence on the angle of elevation. It also depends on the aperture dimensions and the operating frequency. It has been calculated, for example, that a 20 m diameter antenna, operating at 8-4 GHz, would suffer a loss of gain of more than 0-5 dB at an angle of elevation of 5°. It is therefore unlikely that aperture sizes will be increased much beyond 26 m (85 ft). Some improvement in r.m.s. surface profile tolerance may be expected. Some improvements may be expected in the design of the primary radiator. For example, over-moded feed-horns, and horns divided into sections by septum plates provide improved control of illumination taper, and greater similarity between E- and /7-plane radiation pat terns. Such developments should apply to all types of antenna. It may be noted that a Casse- grain arrangement already possesses an increased measure of control of illumination through the possibility of making use of sub-reflector curves of different types. Some consideration is being given to the possible advantages of folded horn antennae ; the advantages include the location of all equipment in fixed accommodation at ground level, and the incorporation of wind-deflecting arrangements which might avoid the necessity for radomes. 6.2 Signal-processing arrays Looking to the future, it is possible that various types of signal processing or adaptive arrays may find a place in the space service. For instance, independent beam-forming in a linear or adaptive array may make it possible to follow several satellites at once, with hand over switching at electronic speeds. Signal-processing antennae include individual elements which are connected to electronic circuits that can respond quickly and automatically to changes in time of the phase, polariza tion, and amplitude of an incident radio wave at various points distributed over the aperture of the array. In general, a signal-processing array may improve on one feature, such as beamwidth or scanning speed, at the sacrifice of something else, such as gain or noise rejec tion. Its elements may be quite small, or they may be large antennae in their own right. Over a narrow band, an adaptive array can maintain its gain in the presence of phase incoherence over the aperture, can correct for differential Doppler, effects, can defocus to increase the zone of search and acquisition and can refocus for tracking, command, control and relaying and thus is able to optimize itself for different functions. Alternatively, one antenna or array may be,designed especially for the reception of weak downcoming signals and the rejection of noise and other unwanted signals. Other arrays allow for random phase variations or for differential Doppler shifts over a two-way path by reversing the radio — 411 — Rep. 390 frequency phasor or every re-radiating element to restore coherence at the other antenna. Arrays that allow for random variations in signal amplitude and polarization, using ordinary diversity combiners, are also adaptive arrays. The limitations of present-day technology indicate that some of these ideas may have restricted application for wideband high-capacity systems. Particularly for the functions of search, acquisition, tracking, command, control and telemetry, a signal-processing antenna should not respond to an incorrectly coded signal and should not depend on a continuous signal, and should be of a sufficiently flexible design so that antenna gain, angular coverage, and frequency of operation can be varied over a wide range. One disadvantage of such antennae for earth stations may be the increase in the effective noise temperature of the receiving system from transmission line and matching networks. This comparatively new field should be followed closely by engineers interested in advances in space-communication technology. ANNEX I SUMMARY OF CHARACTERISTICS OF THE HORN-REFLECTOR ANTENNA AT ANDOVER EARTH STATION The communication antenna at Andover is a much enlarged version of similar antennae used on Bell System microwave routes. For structural reasons, the horn at Andover is conical rather than pyramidal, as was the case in the smaller versions. The antenna rotates in azimuth on two concentric rails and in elevation about the axis of the conical feed horn on two large bearings. Two equipment rooms are carried on the structure. This configuration has several advantages over other possible forms. It is very broad band, presents an excellent impedance to the transmitter, and the parabolic surface is efficiently illuminated. Most important, however, for the space-communication application, the antenna has very low side and back lobes and may be connected to the receiver with short low-loss con nections resulting in a low system noise-temperature. The physical characteristics of the antenna are given in Table I. Radiation patterns on the antenna were obtained by using pulse measuring techniques. Pointing calibration of the antenna was made by tracking radio stars. The structural distortions indicated by these calibrations are corrected in the electronic antenna direction system. The corrections are known with sufficient accuracy that, with good ephermeris data, the antenna beam may be pointed at the satellite within a small fraction of a beamwidth. Measurement of tracking errors in the auto-track mode shows that the auto-track system maintains the null axis of the antenna within 0-005° of the actual satellite direction, limited only by the slowing rate capabilities of the drive motors. The noise sources that contribute to the system noise-temperature for several angles of eleva tion are shown in Table II. The Table illustrates some of the exchanges and choices available to the system designer that may be important in determining the type and size of antenna used at an earth-station. Increases in system noise-temperature at the Andover earth station due to accumu lations of rain or snow on the radome have been measured as high as 135°K for rain and up to 160°K for snow. It should be noted that similar measurements of noise during rain or snow, made without a radome at other test locations, show a completely different character. Rep. 390 — 412 — T able I Structural characteristics A perture...... 332-8 m2 (3600 sq. ft.) L ength...... 53-8 m (177 ft.) W eight...... 344 tons Reflector accuracy...... 1-5 mm (0-060 in.) 0 0) The precision achieved on the critical parabolic surface is such, that operation at frequencies considerably higher than 6 GHz is possible. Tracking and slewing Azimuth Elevation Maximum tracking velocity (d e g ./s ) ...... - . . . 1-5 1-5 Maximum slewing velocity (deg./s)...... 1-5 1-5 Maximum acceleration (deg./s/s)...... 1-3 3-0 Error during acceleration (d eg ./d eg ./s/s)...... 0-26 0-26 T able II Contributors to system noise temperature Angle of elevation (degrees) 90 30 15 7-5 Dry atmosphere (°K )...... 2-4 4-8 9-2 18-4 Antenna side-lobes (°K)...... 1-0 1 Waveguide circuits (°K )...... Maser ( ° K ) ...... ■ if *»•* Second stage (°K) : ...... 0-5 ) Absorption by dry radome ( ° K ) ...... 3 3 3 3 Scattering by dry radome (°K )...... 7-4 6-0 4-1 1-4 Total (dry, measured) ( ° K ) ...... 32-0 33-0 35-5 42-0 Bibliography 1. D o l l in g , J. C., B la ck m o re, R. W ., K in d er m a n , W . J. and W o o d a rd , K . B. The mechanical design of the horn-reflector antenna and radome. B.S.T.J., 1137-1186 (July, 1963). H in es, J.N., T in g -Y e Li and T u r r in , R.H. The electrical characteristics of the conical horn-reflector antenna. B.S.T.J., 1187-1212 (July, 1963). G ith en s, J.A., K elly , H.P., L o z ie r , J.C. and L u n d stro m , N.A. Antenna pointing system : Organ ization and performance. B.S.T.J., 1213-1223 (July, 1963). 2. B.S.T.J., Vol. 40, 975-1233 (July, 1961). 3. G ig e r , A. J., P ardee, S. Jr. and W ic k l if f e , P.R., Jr. The ground transmitter and receiver. B.S.T.J., 1063-1108 (July, 1963). 4. IEEE Trans, on Antennae and Propagation (March, 1964). — 413 — Rep. 390 ANNEX II SUMMARY OF ANTENNA CHARACTERISTICS FOR AN EARTH-STATION UNDER STUDY IN FRANCE This is a! Cassegrain antenna, comprising a primary source, an ancillary reflector and a main reflector. The primary source is a multimode conical horn; the half-angle at the apex is equal to 8 ° and the aperture diameter is 0-30 m ; the horn has a single phase centre and very weak secondary lobes. The ancillary reflector consists of a centre piece in the form of a revolution hyperboloid and a peripheral part in the form of a parabolic torus ; the diameter of the ancillary reflector is equal to 2 -6 m. The main reflector is a revolution paraboloid with a diameter equal to 25 m ; the focal distance/diameter ratio is equal to 0-25 and illumination at the rim is at a level 10 dB lower than the maximum illumination. Radio characteristics (F = 4 GHz) These were determined from measurements using a 1 : 9 scale model. Gain of the main lobe (0 = 0°): 58-5 dB ± 0-5 dB Gain of the first secondary lobe (0 = 0-35°): 38-5 dB Maximum gain of the side-lobes 0 > 10°: 0 dB Maximum gain of the back-lobes 0 > 90°: —20 dB Temperature o f noise from the ground T ^ 18°K for an elevation of 5°. T ^ 3°K for an elevation of over 30°. Overall receiver noise-temperature (without radome) Angle of e le v a tio n ...... 5° 30° Noise temperature ...... 70°K 33°K Figure o f merit With an angle of inclination of 30°: 43-3 dB ± 0-5 dB With an angle of inclination of 5°: 40-0 dB ±0-5 dB ANNEX III SUMMARY OF ANTENNA CHARACTERISTICS AT GOONHILLY EARTH STATION, UNITED KINGDOM (November, 1965) Type of antenna : Centre-fed paraboloidal reflector Type of mount: Elevation and azimuth bearings Reflector diameter : 85 feet (26 m) Primary feed : Coaxial, with arrangements for shaping the radiation pattern and for alternative circular or linear polarizations Ratio of focal length-to- diameter : 0-36 Angle at focus subtended by aperture: 140° Rep. 390 — 414 — Reflector construction: Circular centre portion and 24 adjustable radial petals Accuracy of reflector profile : 99% of the surface is within + 2-4 mm, -2-2 mm (peak values) of the true profile Gain after deducting losses in feeders and due to aperture blocking and other causes: 4 GHz 58-2 dB 6 GHz 59 dB Near side-lobes (peak levels) relative to main beam : Displacement from main beam 4 GHz 6 GHz > ± 0 -5 ° < -2 7 dB < -3 1 dB > ± 1-0° <-32 dB <-35 dB > ± 5 0° < - 5 2 dB < - 5 0 dB Far sidelobes (peak levels) at 4 GHz relative to main beam : at 20° off axis < —57 dB at 110° off axis < —70 dB Overall system noise temperature at 4 GHz : Elevation Mean (°K) Max. (°K) Min. (°K) Spread (°K) 5° 85-2 93-3 78-6 14-7 28° ' 68-3 77-6 61-5 16-1 90° 68-3 77-6 61-5 16 1 No significant correlation between system noise temperature and weather conditions has been observed. Figure of merit at 4 GHz for an angle of elevation of 30° is 40 dB. Figure of merit is defined as the gain of the antenna (expressed in dB relative to isotropic), less the ratio of the system noise power to the available noise power from a source at 1°K (expressed in dB). Modes of steering: — by predicted data, with electronic correction for velocity and acceleration effects, structural deformation, atmospheric refraction, etc.; — as the above but with additional automatic correction of the direction of the beam ; — vernier auto-track. Steering accuracy : ± 0 02° Weather protection : none necessary ANNEX IV SUMMARY OF ANTENNA CHARACTERISTICS AT RAISTING EARTH STATION (FEDERAL REPUBLIC OF GERMANY) 1. General characteristics of the antenna of the earth station at Raisting Type of antenna: Near-field type, with Cassegrain feed and radome. Main reflector : Diameter 25 m. Focal length/diameter ratio / /D = 0-26. Feed : Horn-reflector antenna, comprising a rotary joint to allow the tech nical installations in the operating room to be turned only in azimuth. Distance between feed point and centre of beam aperture 6-44 m. Angle of aperture of horn 16°, //D of horn = 0-25. — 415 — Rep. 390 Ancillary reflector: Paraboloid, 2-3 m in diameter, phase corrected. Radome : Diameter 48-8 m. Diameter of base 44-8 m. Centre of antenna 4-2 m above centre of radome. 2. Electrical characteristics 2.1 Circular polarization 417 GH z: Gain : 58-1 ± 0-5 dB Aperture efficiency : 54% for 58-1 dB Beamwidth (between 1 dB points): 0-13° — (between 3 dB points): 0-20° — (between 10 dB points): 0-33° 6-4 G H z: Gain: 61-5 ± 1 dB Aperture efficiency : 50% for 61-5 dB Beamwidth (between 3 dB points): 0-13° — (between 10 dB points): 0-22° Angle between main lobe at 4 GHz and 6 GHz: 0 01° Losses in directional filters and waveguides between antenna and maser at 4-17 GHz : . 0-15 dB 2.2 Linear polarization 416 G H z: Gain for vertical polarization: 58-4 ± 0 6 dB Aperture efficiency : 58% for 58-4 dB 6-3 G Hz: Gain for horizontal polarization : 61-5 ± 0-8 dB Aperture efficiency: 52% for 61-5 dB Reflection coefficient for linear polarization: (horizontal) <5-5% 3-7 to 4-2 GHz (vertical) < 2-5% 5-925 to 6-425 GHz Losses in directional filters and waveguides between antenna and maser at 4-16 GHz : 0-17 dB 3. System noise-temperature at reception During a period extending from autumn 1964 to spring 1965, the noise temperatures of the system were measured as a function of antenna elevation, and are shown in Table I, The values shown were measured with a directional filter arrangement for circular pola rization as used for the Telstar and Relay experiments, including a 19-5 dB cross-guide coupler. When receiving from the HS 303 satellite, the increase of filter noise-temperature (linear polarization) is 1-3°K. Rep. 390 — 416 — T a b l e I Elevation System noise-temperature (degrees) with dry (cloudless) weather (°K) 3 62 5 51 10 42 20 33 30 31 60 27 90 29 0 0) For elevations greater than 70°, the approach of the antenna beam aperture to the radome envelope, and the increased number of joints therein, result in an increase in noise temperature. The system noise-temperature of 29°K at 90° elevation is made up as follows : T a b l e II (°K ) Atmosphere in dry weather at 560 m above sea-level...... 2-0 Reflection on dry radome and antennae side-lobes...... 9-0 Absorption in dry radome...... 3-0 Antenna l o s s ...... 0-5 Wave-guide loss ...... 6-9 Cross-guide coupler l o s s ...... 3-3 M a s e r ...... 4-0 Second stage of receiver...... 0-3 29-0°K 4. Mechanical characteristics Weight of mass to be moved : in azimuth: 290 tons in elevation 1 2 0 tons Effect on the beam aperture of the shadow of the ancillary reflector : 0-85% Effect on the beam aperture of the shadow of the ancillary reflector supports : 0-65% Ancillary reflector supports : 4 elliptical supports, 13 cm x 37 cm Azimuth orientation : ± 380° Elevation orientation: -1° to + 115° T a b l e II I Tolerance on main reflector profiles with main reflector at 40° elevation Elevation (2) (degrees) (l) (3) 22-5 ± 0-38 mm ± 0-48 mm ± 0-46 mm 45 ± 0-49 ± 0-67 ± 0-63 67-5 ± 0-31 ± 0-53 ± 0-49 90 ±0-64 ± 0-73 ± 0-71 0) Mean geometric value of deviations from the ideal paraboloid (880 measurement points) referred toithe area of the ancillary reflector supports (30% of the 560 m2 of the reflector area). (2) Area outside ancillary reflector supports. (3) Total area of reflector. — 417 — Rep. 390 5. Dynamic characteristics Azimuth (°/s2) Elevation C/s2) Maximum initial acceleration: 2-7 3-1 Mean acceleration : 2*1 3-1 Maximum speed: 3*6 2-0 Braking angle at maximum speed (hydraulic control) (degrees): 1-9 0-72 6. Methods of antenna orientation control for circular polarization Programmed control Adjustment accuracy: ± 0 -01° Precision automatic tracking Adjustment accuracy: ± 0-003° Programmed control with precision automatic tracking 7. Figure of merit At 4 GHz, 5° elevation, linear polarization and referred to the maser input : GIT= 40-7 ± 0 -8 dB ANNEX V SUMMARY OF THE ANTENNA CHARACTERISTICS AT THE MILL VILLAGE EARTH STATION, NOVA SCOTIA, CANADA 1. General characteristics Type of antenna : Full-steerable far-field Cassegrainian antenna under an inflated type of radome. Main reflector: Solid paraboloid surface of revolution, with stretch-formed alumi nium surface panels. Diameter (D) 25-9 m, focal length (/) 11m; f/D = 0-4235; r.m.s. surface accuracy of paraboloid for worst conditions is 0-9 mm. Sub-reflector : Solid hyperboloid surface of revolution, spun aluminium with back- frame and positioning mechanism. Diameter 2-6 m, focal length 11m; r.m.s. surface accuracy 0-284 mm. F eed: Five-horn multimode monopulse system with communication-, azimuth- and elevation-difference channels, right or left circular, or adjustable linear polarization, simultaneously handling the trans mitted and received signals. Radom e: Hypalon-coated Dacron, diameter 36-6 m, base diameter 27-4 m, height 32-4 m, average wall thickness 1-4 mm. Temperature con trol, inside temperature 21-1°C ± 5-5°C. Rep. 390 — 418 — 2. Electrical characteristics 2.1 Useful frequency bands with present filters: 3850-4200 MHz 6000-6425 MHz 2.2 Gain For linear or circular polarization at 41 GHz, including radome and all circuit losses up to the input of low noise amplifier : 58-9 ± 0-3 dB. For linear or circular polarization at 6-3 GHz, including radome and all circuit losses after the output of the waveguide from the high power transmitter : 60-2 ± 1 dB. Average over Average over 2.3 Characteristics o f patterns receiver frequency transmitter frequency band band Beamwidth (between 3 dB points) (communication channel)...... 0-175° 0-15° First side-lobe level (communication channel) . . . - 2 0 dB - 2 2 dB Back-lobe level (communication c h a n n e l)...... -7 5 dB - 8 0 dB Difference mode null depth (elevation)...... -3 5 dB Difference mode null depth (azim uth)...... - 4 0 dB Difference mode slope at —30 dB level, at 3-9 GHz 0-0025°/dB Difference mode slope at —30 dB level, at 4-2 GHz 0-0012°/dB 2.4 Axial ratio Linear polarization Average over 3-8-4-2 GHz . > 24 dB Average over 6-0-6-425 GHz >23 dB 2.5 System noise temperature For dry weather conditions, using a cooled-parametric amplifier and tunnel-diode ampli fier in the following stage, giving a resultant 14°K receiver noise-temperature, the system noise-temperature was measured as follows : Angle of elevation 4160 MHz (°K) 90° 49 ± 3 7-5° 65 ± 4 5° 70 ± 4 2.6 Gain-to-noise temperature or G /T ratio (dB) All losses are included. Angle of elevation 4160 MHz (dB) 90° 42-1 ± 0-6 7-5° 40-8 ± 0-6 5° 40-5 ± 0-6 2.7 Reduction in carrier-to-noise temperature ratio as a function o f rainfall rate The effect of rainfall on the carrier-to-noise temperature ratio at 4-1 GHz was determined from measurements of the signal-to-noise ratio with the Intelsat-I satellite and is shown in Fig. 1. 2.8 Loss Receive band Transmit band Radome (absorption and reflection) . . . (dB) 0-20 0-30 Radiating source ...... (dB) 0-12 0-10 Duplexer ...... (dB) 0-12 0-15 2.9 Tracking modes The antenna-tracking modes include manual, programme track and auto-track for both linear and circular polarization. — 419 — Rep. 390 3. Mechanical characteristics Weight of mass to be moved : At elevation bearings 104 metric tons At azimuth bearings 196 metric tons Total weight, including equipment at top of concrete t o w e r 246 metric tons Elevation Azimuth Maximum velocity . . . l-5°/s 3 0°/s Maximum acceleration...... 0-5°/s2 l-0°/s2 Locked rotor frequency...... 2-2 Hz 21 Hz Weight of sub-reflector ...... 250 kg Scattered power by sub-reflector...... 1-08% Scattered power by sub-reflector support...... 2-61% Type of sub-reflector s u p p o r t ...... 4 legs at 45° to axis of elevation Weight of feed cone (all equipment installed)...... 2500 kg 0,05 0,1 0,2 0,5 1 2 5 10 Rainfall rate (mm/hour) F ig u r e 1 Reduction in carrier-to-noise temperature ratio as a function of rainfall rate (Angle of elevation of antenna : 28°) Rep. 391 — 420 — REPORT 391 * RADIATION DIAGRAMS OF ANTENNAE AT COMMUNICATION-SATELLITE EARTH STATIONS, FOR USE IN INTERFERENCE STUDIES (Question 1/IV) (1966) 1. Introduction Report 382 describes a procedure for calculating the mutual interference effects between radio-relay stations and the earth stations of communication-satellite systems sharing the same frequency bands. These calculations require that the gain of the earth-station antenna be known in the relevant directions. When the performance of the earth-station antenna is not known in detail, it may be necessary to assume antenna characteristics for the purpose of international consultation. In addition, the radiation patterns of earth-station antennae are required for interference calculations, when a number of communication-satellite systems share the same frequencies. 2. Characteristic radiation patterns of antennae It is considered that, for the assessment of mutual interference between communication- satellite earth stations and other services sharing the same frequency bands, the radiation pattern of the earth-station antenna at beam offsets greater than 1° should be specified in terms of smoothed mean power levels of the side-lobes. The central region of ± 1° is occupied by the main beam and first few side-lobes ; in this region, it would seem desirable to base the pattern on peak rather than mean side-lobe levels. 2.1 Side-lobes more than 1° from the axis o f the main beam Examination of Figs. 1 and 2 shows that, except for the region close to the main beam, antennae of similar types in the range of aperture diameters 3-27 m (10 to 90 feet) have radia tion patterns relative to the isotropic antenna, which do not differ very greatly. It has been assumed that the diameters of earth-station antennae will, in fact, generally lie in the range 10-26 m (30 to 85 feet). Figs. 1 and 2, which are based on available published data, suggest that a radiation diagram following the characteristics given below would fit most of the available data in the range 1 ° ^ 9 ^ 60° at least for antenna gains between 45 and 60 dB : Gain (relative to isotropic antenna) = 32—25 log10 9 (dB)J where 9 is the angle (in degrees) between the axis of the main beam and the direction in ques tion. 2.2 Radiated power at angles less than 1° from the axis o f the main beam The near-axis radiation patterns of a number of antennae are given in Fig. 3, and it will be seen that the characteristic suggested in § 2 .1 may be considered as approximately applicable to any antenna, at angles from the main axis greater than those of the half-power points of the main beam. At angles closer than this, the level will be set by the gain in the main beam, which is determined by the ratio of diameter to wavelength of the antenna concerned. * This Report was adopted unanimously. — 421 — Rep. 391 2.3 A typical radiation pattern Fig. 4 shows a curve representing the characteristic: Gain (relative to isotropic antenna) = 32—25 log10cp (dB), together with calculated main-beam gain (assuming tapered illumination and an aperture efficiency of 0-75), for a number of typical values for the diameter of the antenna aperture (expressed in wavelengths). For each size of antenna, the main-beam gain (assumed to apply fully at 0 -0 1 °), has been joined by a horizontal line to the curve of the proposed characteristic. It is considered that this composite characteristic represents a reasonable approximation to the radiation pattern likely to be achieved by most antennae of current design in use at earth stations. For interference calculations it is recommended that, taking account of the likelihood of local.ground reflections, the minimum value of gain in any direction should be assumed to be —10 dB relative to isotropic. 3. Conclusion The typical radiation diagram (Fig. 4), represents approximately the radiation levels (smoothed over angles of a few degrees), to be expected from typical earth-station antennae of current design in the range of diameters from 9-27 m (30 to 90 feet). It could, therefore, be used as an approximate basis for mutual interference calculations in the absence of specific information concerning particular earth-station installations for communication-satellite systems. The r.m.s. error which might result from the use of the inclined curve 32—25 log10 ANNEX SOURCES OF INFORMATION ON THE PERFORMANCE OF CURRENT EARTH-STATION ANTENNAE Station Source Goldstone P o t t e r , P . Application of Cassegrain principles to ground antennae for space-communications. IRE Trans, on space electronics and tele metry (June, 1962). Holmdel Project E c h o . B.S.T.J. (July, 1961). West Ford Proc. IEEE (May, 1964). Andover C u r t is , H. E. Stellite system interference tests at Andover, Maine. B.S.T.J. (November, 1963). The Telstar experiment. B.S.T.J., Part 2 (July, 1963). Wallops Island Relay satellite system data book (NASA) Rep. 391Rep. Gain (dB relative to isotropic antenna) Angle, Angle, (The figures on the diagram refer to apertures expressed in wavelengths) in expressed apertures to refer diagram the on figures (The 9 , between axis of main beam and direction considered (degrees) considered direction and beam main of axis between , o : antennae with small apertures (up to 4 m) 4 to (up apertures small with antennae : o • : antennae with large apertures (9-27 m) (9-27 apertures large with antennae : Typical radiation diagrams for large antennae large for diagrams radiation Typical 42 — 422 — F gure r u ig 1 Gain (dB relative to isotropic antenna) ansd-oelvl frdaindarm o ag nene (1°-100°) antennae large for diagrams radiation of levels side-lobe Main with the proposed characteristic E characteristic the proposed with fmaueet 1 dB) —10 measurement of E onil oiid (limit modified Goonhilly Holmdal Raisting Ford West Andover Goonhilly Goldstone Mill Village Mill 43 — 423 — = 32- log 5 -2 2 3 = F gure r u ig 2 == 10 9 22 log10 32—25 9 Rep. 391Rep. Rep. 391Rep. Gain (dB relative to isotropic antenna) ensd-oelvl frdaindarm o ag nene (0-01°-10°) antennae large for diagrams radiation of levels side-lobe Mean ,1 ,2 ,5 , 02 , 1 5 10 5 2 1 0,5 0,2 0,1 0,05 0,02 0,01 Angle, Angle, 9 showing the proposed characteristic E — 32—25 log10 —32—25 E characteristic the proposed showing , between axis of main beam and direction considered (degrees) considered direction and beam main of axis between , •-— ------o Goonhilly o o o 0 ------— ------s -m- * ■ • • ■ * —0 0— ----- , N - • Goonhilly modified Goonhilly • - 44 — 424 — F Wallops Island Wallops Goldstone Village Mill Andover et Ford West gure r u ig ' \ E = E * $ 3 \ 22 log 32—25 1 ‘•s i i, \ * s. s ■ \ N 10 \ % 9 \ \ \ 9 V \ 16 Angle, Gain (dB relative to isotropic antenna) Note. ,1 , 10 0 100 10 1,0 0,1 0,01 — — A Proposed reference radiation diagram radiation reference Proposed represents the recommended minimum gain minimum recommended the represents 45 — 425 — F igure 4 Rep. 391 Rep. Rep. 392 — 426 — REPORT 392 * PERFORMANCE OF EARTH-STATION RECEIVING ANTENNAE Effects of rain on radomes and of solar and cosmic noise (Questions 1/IV and 13/IV) (1966) This Report presents a partial reply to Question 13/IV, which is concerned with contributions to the noise temperature of an earth-station receiving antenna. It deals, in particular, with the effects of rain on radomes and of solar and cosmic noise. Contributions to the noise temperature by atmospheric gases, clouds, and precipitation are dealt with in Report 234-1. In considering the performance of an earth-station receiving system, it is noted that the signal-to-noise degradation is more frequently of interest than the increase of antenna noise temperature alone. In the case of solar and cosmic noise, the degradation of the signal-to-noise ratio of an earth-station receiver results from the increased noise temperature, whereas, in the case of rain on radomes, the degradation results both from the attenuation of the signal and from the additional contribution to the antenna noise-temperature. Since these effects are interrelated, they will be treated together in this Report, which therefore relates also to Question 1/IV. Annex I discusses the effects on earth-station performance of rain on radomes. Annex II discusses the contributions to antenna noise-temperature by discrete radio sources, and, in particular, the sun. The effects of cosmic background radiation are negligible at frequences above about 1 GHz. Note. — Attention is drawn to Doc. IV/248 (Telespazio), 1963-1966, regarding the effects of wind and rain on an earth-station antenna having no radome. The results apply specifically to a particular situation and it is difficult to generalize on this basis. These effects, however, partic ularly that of wind gusts, are of great interest and Administrations are invited to submit further contributions on this subject. The effects of the earth on antenna noise-temperature are discussed in Report 390 and Doc. IV/236 (Denmark, Norway and Sweden), 1963-1966. ANNEX I EFFECTS ON EARTH-STATION PERFORMANCE DUE TO WATER ON RADOMES 1. Introduction The presence of water on a radome will result in a two-fold degradation of the overall signal-to-noise ratio at an earth-station receiver. Firstly, the water, like the radome, will attenuate signals transmitted through it by causing reflection, scattering, and absorption. Secondly, the water will contribute to the antenna noise-temperature by the scattering or reflection of ground noise into the antenna aperture, and by emission from the absorbing medium itself. The noise contribution arising from absorption in the water depends on both the absolute temperature of the water and its loss. While the attenuation may be generally neglected at frequencies in the UHF band and lower, the significance of these effects increases rapidly with increasing frequency. Signal-to-noise degradation, due to the additional noise resulting from the presence of water will, of course, also depend on the relative importance of other factors contributing to the system noise-temperature. * This Report was adopted unanimously. — 427 — Rep. 392 2. Theoretical considerations 2.1 Losses due to rain on radomes Theoretical calculations of the attenuation of signals by thin water layers on radomes have been described in the literature [1, 2]. These calculations have shown that the presence of water layers of appreciable thickness can result in significant attenuation at frequencies of about 4 GHz and greater. This attenuation is, of course, additional to that produced by rainfall in the lower atmosphere along the path of the antenna beam. Using the method described in the references, Fig. 1 [3] has been constructed, showing the transmission loss in dB of a signal incident normally on a water layer as a function of water layer thickness for various frequencies (solid curves). The dashed curves show the effect of including in the calculations a radome which has a relative dielectric constant of 3 0 and a loss angle of 0-015°. 2.2 Contribution to antenna noise-temperature of water on radomes The losses due to water layers on radomes may also be used to obtain the contribution to antenna noise-temperature which results from absorption by the water. If it is assumed that this noise temperature contribution is given by the product of the thermodynamic tem perature of the water layer and its fractional absorption, calculated on the basis of a plane wave normally incident on a uniform layer of given thickness, the noise temperature contribution is as given in Fig. 2 [3]. The solid curves show the noise temperature contribution for various frequencies as a function of water layer thickness. The dashed curves again include the effects of a 0-060 in. (1-5 mm) radome, with a relative dielectric constant of 3-0 and a loss angle of 0-015 rad. The calculations apply to the case where the water layer is on the outside of the radome. Ground noise reflected or scattered into the antenna aperture by the wet radome will also contribute significantly to the system noise-temperature. The magnitude of this effect will depend on the antenna-radome geometry. In the case of very low noise systems, the contribution to system noise-temperature by water on radomes may be much more serious than the transmission loss in regard to the degradation of the system signal-to-noise ratio. 2.3 Losses due to water layers on reflecting surfaces In the absence of a radome, the occurrence of water layers of the same order of thickness on antenna reflecting surfaces will generally result in negligible signal attenuation. Water on the primary feed will have serious effects, however, and if no radome is used, apart from the necessity for the antenna to operate in a more severe environment, consideration must also be given to the use of antenna designs which prevent the wetting of surfaces through which energy is transmitted. 2.4 Other effects o f water layers In addition to the transmission loss in and noise temperature contribution by the water, thick non-uniform water layers, either on radomes or on reflecting surfaces, are also likely to give rise to other effects such as antenna defocussing, boresight error and directivity loss. 3. Formation of water on radomes Water may exist as a thin layer formed on the radome during rainfall or as droplets. It is also possible that, in some cases, water may be absorbed into the radome material, although this may be avoided by proper radome design. Rep. 392 — 428 — The actual nature of the deposition of water on, and run-off from, radomes is not well understood. One theoretical study [4], of the expected thickness of the water layers formed on spherical radomes, predicts that, in the absence of wind, the thickness will be fairly uniform over the upper hemisphere and is given approximately by: d 3 = (3/2) (\xrR/W) where r is the radius of the radome, R is the rainfall rate, p is the viscosity and W the specific gravity of water. The applicability of the model used is by no means confirmed. In all likeli hood, the problem is considerably more complex, particularly in the case of non-spheric'al or space-frame radomes. 4. Experimental results The theoretical predictions appear to be substantially corroborated by the results of measurements published recently on wet radomes. Direct measurements of the contributions of wet radomes to antenna noise-temperature have been carried out at Andover, U.S.A. [5] and at Raisting, Federal Republic of Germany [6 ], using 64 m and 48-8 m air-inflated radomes respectively. The Andover measure ments indicated an increase in antenna noise-temperature with increasing angle of elevation which was attributed to the increased scattering of ground noise into the antenna aperture at the higher angles. A detailed analysis of results in light rain showed that, of an overall system noise-temperature of 52°K at the zenith, 12°K was contributed by absorption by the wet radome, and 18-5°K by scattering. The corresponding contributions from the dry radome were 3°K for absorption and 7-4°K for scattering. Similar measurements at Raisting gave results which are in agreement with the theoretical prediction based on the theory in § 2 and assuming that the equation for water layer thickness given in § 3 is valid. An increase in the noise temperature contribution with increasing angle of elevation was also noted, which was particularly pronounced at angles of elevation greater than 70°. In moderate rain (1 -2-6 mm/h), the zenith noise-temperature was about 110°K compared to a dry radome value of 29°K. After cessation of the rain, recovery was quite rapid. For 2% of a year, temperatures of 75°K were obtained ; for 0-1% of the time, temperatures of up to 95°K were recorded. Measurements of excess transmission loss and signal-to-noise degradation in rain have also been made at Andover [7]. It was found that the excess transmission loss varied from about 1 dB at a rainfall rate of 0-1 mm/h to about 3 dB at 6 mm/h. The signal-to-noise degradation increased from about 3 dB to about 9 dB over the same range of rainfall rate. Measurements at Mill Village, Canada [8 ] at an angle of elevation 28°, where the dry system noise-temperature is somewhat greater than 50°K and the radome has a diameter of 36-6 m, gave a carrier-to-noise degradation which increased from less than 1 dB at a rainfall rate of 01 mm/h to about 5 dB at a rainfall rate of 10 mm/h. At Raisting [5], it was found that the formation of dew on the radome increased the antenna noise-temperature by about 30°K. In spite of the apparent agreement between the measured and expected values of trans mission loss and antenna noise-temperature in rain for systems using air-inflated radomes of varying diameters, some recent evidence [9] suggests that the equation given in § 3 predicts water layer thicknesses which are excessive. In addition, the evidence presented indicates that the use of specially treated radome surfaces will reduce water layer thicknesses during rain, and therefore losses and noise temperature contributions, to low values. — 429 — Rep. 392 Bibliography 1. B levis, B. C. Losses due to rain on radomes and antenna reflecting surfaces. IEEE Trans., Vol. AP-13, 175-176 (January, 1965). 2. R u z e , J. More on wet radomes. IEEE Trans., Vol. AP-13, 823-824 (September, 1965). 3. C.C.I.R. Doc. IV/227 (Canada), 1963-1966. 4. G ibble, D. Effects of rain on transmission performance of a satellite communications system. Paper presented at IEEE International Convention, New York (March, 1964) (not published). 5. G ig e r , A. J., P a r d e e, S. and W ic k l if f e , P. R. The ground transmitter and receiver. B.S.T.J., Vol. 42, 1063-1107 (July, 1963). 6 . C.C.I.R. Doc. IV/222 (Federal Republic of Germany), 1963-1966. 7. G ig e r , A. J. 4 Gc/s transmission degradation due to rain at the Andover, Maine, satellite station. B.S.T.J., Vol. 44, 1528-1533 (September, 1965). 8 . C.C.I.R. Doc. IV/254 (Canada), 1963-1966. 9. F a n n in g , W. R. and R u z e , J. Performance and design of metal space frame radomes. Paper presented at I.E.E. Conference on large steerable aerials, London (1966). Rep. 392 — 430 — F ig u r e 1 Transmission loss of water layers as a function of thickness of water layer (dashed curves include effects of 0-060 in. radome with £ = 3-0—j 0-045) — 431 Rep. 392 0 0,2 0,4 0,6 Thickness of water layer (in.) Thickness of water layer (mm) F ig u r e 2 Noise temperature contribution o f water layers as a function of thickness of water layer (dashed curves include effects of 0-060 in. radome with e = 3-0—j 0-045) Rep. 392 — 432 — ANNEX II CONTRIBUTION OF DISCRETE RADIO SOURCES TO ANTENNA NOISE-TEMPERATURE 1. Introduction At the frequencies of interest to the communication-satellite service, the contribution to antenna noise-temperature by the background component of cosmic noise may be neglected and only the discrete sources need be considered. These discrete sources are distributed over the celestial sphere, but have small angular dimensions and are only rarely intercepted by an earth-station receiving antenna. In practice, only the sun, moon and some of the more intense radio nebulae, such as Cassiopeia A, Taurus A and Cygnus A will give rise to a significant contribution to the antenna noise-temperature of an earth station. In the case of the sun, its apparent temperature (the antenna temperature depends on the apparent solar temperature and the fraction of the antenna beam included) is very high, and noise received in the side-lobes of the earth station antenna may also be important. The apparent temperature of the quiet sun at 4 GHz varies from 23 000° at sunspot minimum to 90 000° at sunspot maximum. In addition, solar radio bursts may give rise to an increase in the noise temperature. These occur most frequently at sunspot maximum, when for 1 % of the time the apparent noise temperature at 4 GHz will be about 50% greater than that of the quiet sun. For smaller percentages of the time, the increase in apparent noise temperature will be considerably greater. 2. Occurrence of solar noise interference A detailed study of the occurrence of solar interference in the receiving system of an earth-station antenna, in the case of equatorial satellite orbits, has been carried out in the Federal Republic of Germany [1]. A zone of interference is defined which depends on the angular width of the source, the antenna radiation diagram, and the permissible increase in noise temperature of the earth station receiving antenna. To a first approximation, this zone is circular. Interference occurs when the radio sun enters the zone of interference. 2.1 Time and frequency o f occurrence Assuming an angular radius of the zone of interference of the sun of 0-6° (which at Raisting corresponds to a noise increase in the baseband of 16 dB in overcast conditions at 4 GHz), an analysis was performed of the frequency and duration of interference occurring during the course of a year for a number of stations varying in latitude from 50°N to 26°S. Fig. 3 shows the elevation and azimuth as observed from Raisting of equatorial satellites at various heights (solid curves). Also indicated on the diagram (dashed curves) are the azimuth and elevation of the sun for several selected days in October and November, 1965. For equatorial orbits at altitudes greater than or equal to 10 400 km, no earth station is free from solar interference. For stations located in the northern hemisphere, interference occurs only in the six-month period between the autumnal and vernal equinoxes, and for stations in the southern hemisphere it occurs only in the remaining six months. At any station, there are two periods in the particular six-month interval in which interference occurs ; each of these periods may include several consecutive days. In the case of sub-synchronous equatorial satellites at heights of 13 900 and 10 400 km corresponding to orbital periods of 8 and 6 hours respectively) interference may occur twice in any one day. For satellites at heights of 35 800 and 20 300 km (orbital periods of 24 and 1 2 hours respectively), interference occurs only once in a given day. — 433 — Rep. 392 The difference in time between the occurrence of interference at two different earth stations may be calculated from a knowledge of the satellite position and the earth-station co-ordinates. The time difference in occurrence between Andover and Raisting is approxi mately 35 minutes for synchronous satellites and shows little variation with satellite longitude over a wide range of values. 2.2 Duration o f occurrence The duration of any individual occurrence of interference can be deduced from the size of the zone of interference and the angular velocities of the sun and satellite relative to the earth station. If the sun passes through the centre of the zone of interference at a small angle to the direction of satellite motion—this corresponds to maximum duration—the duration (for an angular radius of the zone of interference of 0 -6 °) varies from about five minutes for a synchronous equatorial satellite to about one minute for a sub-synchronous satellite at a height of 10 400 km. In the latter case, however, the period over which interference occurs will include approximately twice the number of days. B ibliography 1. C.C.I.R. Doc. IV/223 (Federal Republic of Germany), 1963-1966. Rep. 392 — 434 — Azimuth F ig u r e 3 Elevation and azimuth from Raisting of equatorial satellites at indicated heights (solid curves) and of sun for selected dates in 1965 (dashed curves) — 435 — Rep. 393 REPORT 393 * EXPOSURES OF THE ANTENNAE OF RADIO-RELAY SYSTEMS TO EMISSIONS FROM COMMUNICATION-SATELLITES (Question 2/IV) (1966) 1. Introduction The exposure of the antenna beams of radio-relay systems to emissions from communica tion satellites is geometrically predictable when such satellites have circular orbits with recurrent earth-tracks (see Report 206-1, § 4.2), but is only predictable statistically for inclined circular orbits of arbitrary periods. A phased system of these recurrent earth-track satellites can be made to follow a single earth-track and such systems are of increasing interest for communication. Stationary satellites are a special case, since the equator constitutes the earth-track of all equatorial orbits. At any earth location from which the satellites of a single-earth-track system could be seen, successive (non-stationary) satellites would follow a fixed arc through the sky, from horizon to horizon. Moreover, except for inclined orbits, this arc would be independent of longitude and be symmetrical relative to North/South. Subsequent portions of this Report consider exposure conditions relative to a circular equatorial orbit (including the special case of the orbit of a stationary satellite) and also the probability of exposure to unphased satellites (non-recurrent earth-track). Details of the exposure of radio-relay system antennae to the orbit of a stationary satellite are given in the Annex. 2. Some characteristics of the antenna beams of terrestrial radio-relay systems Line-of-sight radio-relay systems use antennae with gains of the order of 40 dB and half power beamwidths of the order of 2°. Trans-horizon systems generally use antennae with higher gain and narrower beams, say 50 dB and 0-5°. In either case, path inclinations are less than 0-5° on the average and rarely in excess of 5°. When all of a negatively inclined beam strikes the Earth, there would be no exposure to an orbit. For horizon-centred beams, the upper half could have exposure. When passive reflectors are used, spill-over also should be considered. Since the beams are close to the earth and traverse a considerable thickness of atmo sphere, diffraction and refraction should be taken into account in making precise calculations of exposure. 3. Directions to circular equatorial orbits It is well known that the azimuth angle, A (measured clockwise from North) and angle of elevation, e to a satellite in a circular equatorial orbit can be expressed by A = arc tan (± tan A/sin 9 ) (1) e = arc sin [(AT cos 9 cos \ - \ ) l s j K 2 + 1 —2 K cos 9 cos A] (2) * This Report was adopted unanimously. Rep. 393 — 436 — where K = orbit radius/earth radius, 9 = earth latitude of the terrestrial station, A = difference in longitude between the terrestrial station and the satellite. Eliminating A between these two equations leads to tan e + K~l \/ tan2 e + (1 —K~2) A = arc cos l - K - 4 ] , tan 9 [ (3) If necessary, azimuths and elevations to any single-earth-track inclined orbit system, of given height, inclination and equatorial crossings could be determined by an extension of this analysis. For such systems, however, the orbit directions would depend both on latitude and longitude of the terrestrial station. An antenna directed at the orbit of a non-stationary satellite (or other single earth-track orbit) will be certain to have intermittent exposure. For a circular equatorial orbit (other than the orbit of the stationary satellite) with m satellites, antennae having an interference beamwidth of 0 radians will have interference for a fraction of the time given approximately by: P — m 0/(2 it ) (4) For the special case of the orbit of a stationary satellite, P will equal either zero or unity. 4. Unphased satellite systems In this case it is possible to derive only an average probability of exposure to a satellite. Thus, for a system of n orbits of equal height and equal inclination angle, i, it can be shown that the average probability of exposure is given by P = (mnQI8 tt cos y) { arc cos [(sin ( y —0/2))/sin /] —arc cos [(sin (y + 0/2))/sin /]} (5) when y ^ (i—0 /2 ) and where m = number of satellites in each orbit, y = latitude of intersection between the antenna beam and the orbital sphere. For the particular case of the polar orbit, i = tt/2, and the above expression reduces to P — m n 02/(8 ttcos 9 /) 5. Geometrical relations between the directions of the antennae of radio-relay systems and the orbit of a stationary satellite Fig. 1 shows the azimuth angles of the directions of the antenna-beams of radio-relay systems which intersect the orbit of the stationary satellite, as a function of latitude and eleva tion. The angles of azimuth referring to stations located in the northern hemisphere are measured from the South and to stations in the southern hemisphere from the North. The range of latitudes covered by Fig. 1 extends from 30° to 56° North and South. Such diagrams may be constructed for the latitude ranges of interest to any particular Admi nistration by following the method outlined in § 5.1. Such diagrams are intended to provide a simple and practical aid by which to ascertain whether a particular radio-relay antenna is directed near a critical azimuth angle. If the azimuthal direction appears to be within say 4° or 5° of the critical directions, an accurate calculation using the standard formulae of spherical trigonometry should be made. — 437 — Rep. 393 The sample diagram in Fig. 1 covers the range of elevation angles of the radio-relay antenna from 0° (horizontal) to + 3°. Thus, as an example, if the antenna of the radio-relay system is located at a latitude of 46° North, the azimuthal directions to be avoided are in the range 76° to 82° East and West of South. Similarly, at latitude 46° South, the same angular range applies East and West of North. The appropriate azimuthal angle in the range depends on the angular elevation of the antenna. 5.1 Method to determine exposure curves for the general case The basic geometric relations are shown in Fig. 2. It will be seen that the orbit of an equatorial satellite SB intersects the geographical horizon (reckoned without flattening of the earth) HT at the points Px and P2. It is assumed that the antenna of the radio-relay system (with an interference half-beamwidth of 0/2) is located at latitude Bx. Therefore, the possibility of interference between radio-relay transmitters and satellite receivers exists when the satel lite orbit is 0 /2 , or less, distant from the main radiation direction (represented on the horizon by the point Pa). The position of Px is characterized by the angle of azimuth Azx and by the angle of elevation Elx (= 0°). The position of Pa is defined by Aza and Ela. Only a limited part of the total orbit will be subject to interference from the antennae of radio-relay systems of any one country. On the basis of §§ 5.1.1 and 5.1.2, the azimuth angle at which the radio-relay antenna looks exactly towards the orbit can be found from equation (6 ). 5.1.1 The angle of elevation of the antennae of a radio-relay system is, in the majority of cases, less than 3°. 5.1.2 The boundary of the area of exposure runs in parallel to the orbit at a separation of 0/2. (6) where: R = radius of the earth (6320 km for a mean latitude of 51°) ; r = radius of the circular orbit of the satellite (42 160 km for the orbit of a stationary satellite); B = latitude of the site of the antenna of the radio-relay system; El — angle of elevation of the antenna of the radio-relay system. Due to refraction, the orbit close to the horizon appears to be at a higher point than is actually the case. Allowance is made for this, as shown in Fig. 3, by lowering the geographical horizon down to the line of the radio horizon. This causes a displace ment of Pa to a new point as P'a. Fig. 3 also illustrates the effect of refraction on the angular deviation d between Pa and the orbit. Table I lists values of AEl for several angles of elevation (in degrees). The angular deviation, d, is found from equation (7). d = \f E l2 + (AA zx)2 sin ((3-a ) (7) The magnitude of a, (3 and AAzx may be obtained from the following formulae: tan a ^ | Aza—Azx \ jEla (8) AAzx = j Aza—Azx | + A El tan a (9) tan (3 = A Azx/Ela (10) Rep. 393 — 438 — The azimuthal directions (Azcrit), which are to be avoided in planning new radio relay routes, result finally from the following consideration: e Azcrit = Az (of equation (6 )) ± ^ + d (in degrees). T a b l e I Ela (degrees) A El (degrees) 0 0-59 0-5 0-51 i- o 0 -44 1-5 0-38 2-0 0-34 2-5 0-31 3-0 0-28 — 439 — Rep. 393 Latitude of the antenna of the radio-relay system (degrees) F ig u r e 1 Azimuthal directions of the orbit of a stationary satellite E l: angle of elevation of the antenna Note. — The shaded area makes allowance for the interference beamwidth of the antenna Rep. 393 — 440 — M M ® West F ig u r e 2 Basic geometric relations to determine exposure curves for the general case — 441 — Rep. 393 F ig u r e 3 Effect o f refraction 1 7 Rep. 393 — 442 — ANNEX EXPOSURES OF THE ANTENNAE OF RADIO-RELAY SYSTEMS TO AND FROM THE ORBIT OF A STATIONARY SATELLITE Information about the extent to which existing antennae of radio-relay systems are directed towards the orbit of a stationary satellite has been received from U.S.A., United Kingdom, Federal Republic of Germany, Canada, Australia and Japan. The stations examined use in aggregate almost 6000 antenna-beam directions of which 127 are so oriented that the orbit of a stationary satellite is within 2° of the beam axes. The percentage of the total antenna-beam direc tions, which so intersect, varies between the networks considered, but it may be concluded from the totals that, if no special care is taken by radio-relay system designers to avoid directing antenna- beams towards the orbit of a stationary satellite, about 2 % of the antennae will be so oriented. Fig. 4 shows the distribution of intersections reported for the existing network against a base of geographical longitude. It can be seen, that those parts of the orbit over the ranges of longitude 10°E to 50°W; 65°W to 80°W; 140°W to 160°E and 90°E to 50°E are all subject to iUumination by the antennae of radio-relay systems. There are twelve intersections which occur outside this range of longitude. The largest range of longitude in which no intersections have been reported is from 80°W to 128°W. It must be emphasized that the information from which Fig. 4 has been compiled does not include all existing networks, nor is it up to date in the case of all the networks for which reports were submitted. It cannot therefore be inferred that substantial portions of the orbit in any range of longitude are in fact free from illumination by the antennae of existing radio-relay systems. 443 — Rep. 393 Li i J, Number of incident beams 150° 120" 90” L1U1U60" 30" F ig u re 4 Distribution in longitude of intersections o f radio-relay antenna beams with the orbit o f a stationary satellite Rep. 215-1 — 444 — L. 3 : Direct broadcasting from satellites REPORT 215-1 * FEASIBILITY OF DIRECT SOUND AND TELEVISION BROADCASTING FROM SATELLITES (Question 12/IV) (1963 — 1966) 1. Introduction The large coverage area possible from a satellite-borne radio transmitter raises the pos sibility of establishing a direct broadcasting service to the general public, despite the major technical problems that would need to be resolved. An earth-station transmitter would direct programme material to the satellite, which in turn would broadcast this over a wide area to individual home receivers, by either passive or active techniques. Communication-satellite systems used to relay programme material to earth stations for subsequent broadcast are not considered to be a space broadcasting system and are not, therefore, discussed in this Report. 2. Preferred satellite orbits Table I shows the amount of broadcasting time possible for different orbits, and the approximate coverage area possible from each of these orbits. T a b l e I Satellite altitude Coverage area for Passes per day Visibility per pass maximum broadcast period over a given point (min) (degrees of longitude (km) (statute miles) at the equator) 320 200 16 9 16° for 5-min programme 1600 1000 12 24 28° for 15-min programme 8000 5000 4 126 60° for 1-h programme 36 000 22 300 stationary continuous 160° continuous There are a variety of possible orbits for communication-satellite systems, but the desirability of relatively simple receiving antennae for home reception and of uninterrupted broadcasting, preclude consideration of all but stationary satellites using the synchronous, orbit, 36 000 km (22 300 miles) above the earth. Low orbits are considered to be unsatis factory for satellite broadcasting, because they do not permit uninterrupted broadcasting and would require complex tracking antennae with home receivers. More than a third of the surface of the earth (see Fig. 1) would be visible from a single stationary satellite, and this would permit the use of fixed antennae at home receiving installa tions, and at earth-station transmitters used to transmit programmes to the broadcasting satellite. * This Report was adopted unanimously. — 445 — Rep. 215-1 3. Preferred technical characteristics 3.1 Compatibility with existing terrestrial broadcasting systems and standards It appears that a major consideration in direct broadcasting from satellites would be the large number of earth-station receivers (home receivers), which might be used to receive such broadcasts. This would suggest that space broadcasts should be received on typical home receivers in preferably the same manner as conventional terrestrial broadcasts are, at present, received and that the technical standards for broadcasting-satellite systems should be compatible with existing standards for the terrestrial broadcasting service. Since technical standards vary in different areas of the world, additional circuitry may be required in home receivers to achieve compatibility, when more than one set of technical standards are in use in the area in which space broadcasts can be received from a satellite. Such circuitry should enable the selection of the appropriate technical standards to be made by simple adjustment of the receiving controls. 3.2 Frequencies for broadcasting-satellite systems Direct broadcasting to home receivers, with minimum modification to the latter, would require that space broadcasting take place in those bands already allocated to the broad casting service in the Radio Regulations, Geneva, 1959. It is considered that the effects of the ionosphere preclude uninterrupted direct broad casting from satellites in bands 5, 6 and 7. While the broadcasting service allocations in bands 8 and 9 are, from the point of view of propagation, suitable, their use for space broad casting would require the clearing of a sufficient number of channels or time and/or area sharing, to avoid severe mutual interference [11, 12, 13]. Since channels in these bands are at present heavily assigned on a planned basis, throughout most of the world, the clearing of existing assignments would be extremely difficult. Use of these bands for direct satellite broadcasting raises frequency-sharing problems which require further study by the C.C.I.R. While band 10 is technically suitable for space broadcasting, nevertheless, there is no terrestrial broadcasting in this band at present, and home receiving equipment is not available [4]. For greatest efficiency, it appears that the earth-station transmitter, sending programmes to the satellite, should operate in an appropriate communication-satellite service band between 1 and 10 GHz or higher. 4. Order of power 4.1 Passive stationary satellite Reference [1] considers a stationary satellite in the shape of a rectangular trihedron 100 m in diameter as a passive reflector of radio broadcasts. Such a satellite would provide a uniform received field of 1 mV/m throughout an area of the size of France, if the transmitted power directed to the satellite from the earth is 30 MW, and the diameter of the earth trans mitting antenna is 8400 A. If one were content with a received field of 0-1 mV/m, the trans mitted power required would be of the order of 300 kW. 4.2 Active stationary satellite for FM sound-broadcasting in band 8 Reference [2] contains calculations for the order of satellite primary power that would be required for FM sound broadcasting in band 8 from an active stationary satellite. Examples are given for the case of maximum coverage (approximately one-third the surface of the Earth), and for limited geographical coverage (approximately the size of Europe). A similar example for the coverage of North America is given in [3]. Table II summarizes the results given in [2] and [3]. The primary power requirements shown have been computed by the method given in Reports 322 and 112, and Doc. IV/57, Washington, 1962. The primary power requirements are those necessary for producing a level at the receiver terminals, equivalent to that obtained with a dipole in a received field of Rep. 215-1 — 446 — 50 pV/m as contained in Recommendation 412. They do not take into account ionospheric or atmospheric absorption, terrain effects, or power required for satellite equipment other than the broadcasts transmitters. T a b l e II Primary power requirements for FM sound broadcasting in band 8 Maximum Europe North America coverage Required pre-detection signal-to-noise ratio (dB).. . . 26 26 26 Required signal power (dBW )...... -1 1 5 -1 1 5 -1 1 5 Receiving antenna gain ( d B ) ...... 6 6 6 Half-power beamwidth of transmitting antenna .... 17-5° 5-5° 1 0 ° Diameter of parabolic transmitting antenna (m) .... 12-5 40 20 Gain of transmitting antenna (d B )...... 19-3 29-4 23-6 Required transmitter power (W )...... 550 54 205 Primary power required based on 50% efficiency (W) . 1100 108 410 4.3 Active stationary-satellite for television broadcasting in bands 8, 9 or 10 Reference [2] contains calculations for the order of satellite primary power that would be required for television broadcasting in bands 8 , 9 or 10 from an active stationary-satellite. Table III summarizes the primary power required for maximum visible coverage (approxim ately one third of the surface of the earth), while Table IV summarizes the requirements for coverage of a limited geographical area (approximately the size of Europe). The power requirements shown do not take into account ionospheric or atmospheric absorption, terrain effects, or power required for satellite equipment other than the broadcast transmitters. The specific parameters assumed in estimating the power requirements are given in Appendices 1 and 2 of [2] and the required powers have been computed by the method in Reports 322 and 112 and Doc. IV/57, Washington, 1962. T a b l e III Primary power requirements for maximum coverage, television broadcasting in bands 8, 9 or 10 Band 8 9 10 Frequency (M H z )...... 70 650 11 800 Assumed ratio of peak carrier to r.m.s. noise (dB) . . . 30 30 30 Required signal power (dBW )...... -9 7 -2 -94-1 - 102-8 0 Receiving antenna gain (dB) ...... 6 15-2 38-8 Half-power beamwidth of transmitting antenna .... 17-5° 17-5° 17-5° Diameter of parabolic transmitting antenna (m) .... 17-9 1-9 0-11 Gain of transmitting antenna (d B )...... 19-3 19-3 19-3 Required vision transmitter power (kW )...... 17-4 427 74 Required sound transmitter power ( k W ) ...... 4-3 107 18-5 Primary power required based on 50% efficiency (kW) . 43-4 1068 185 0) Based on the assumption that the front end of the receiver will be designed as an integral part of the receiving antenna. — 447 Rep. 215-1 T a b le IV Primary power requirements for limited geographical coverage (approximately the size of Europe) for television broadcasting in bands 8, 9 or 10 Band 8 9 10 Frequency (M H z )...... 70 650 11 800 Assumed ratio of peak carrier to r.m.s. noise (dB) . . . 30 30 30 Required signal power (dBW )...... -9 7 -2 -94-1 - 102-8 0 Receiving antenna gain ( d B ) ...... 6 15-2 38-8 Half-power beamwidth of transmitting antenna .... 7-8° 6-7° 5-0° Diameter of parabolic transmitting antenna (m) .... 40 6 0-37 Gain of transmitting antenna (d B )...... 26-3 30-2 30-2 Required vision transmitter power ( k W ) ...... 3-5 35 6-0 Required sound transmitter power ( k W ) ...... 0-8 6 8-7 1-5 Primary power required based on 50% efficiency (kW) . 8-6 87 15-1 (0 Based on the assumption that the front end of the receiver will be designed as an integral part of the receiving antenna. 5. Major problems Formidable technical problems remain to be solved before high-quality broadcasting from satellites can be considered feasible. Among these are : — the development of high-capacity power supplies capable of providing continuous service over a reasonably long period of time ; — the dissipation of heat resulting from large power losses ; — the development of precise stabilization, orientation and station-keeping systems; — the development of system components of such size, weight and reliability, as will permit operation of a high-power broadcasting station for a reasonable period of survival; — the accommodation, if necessary, of broadcasting-satellite space stations capable of wide-spread reception in a portion of the spectrum (bands 8 and 9), already assigned heavily throughout most of the world on a planned basis, and/or the development of home receiving equipment for space broadcast reception in band 1 0 . In order that broadcasting satellite programmes can be received by large numbers of viewers with low-cost home receivers designed for conventional broadcast services, a number of problems dealing with technical compatibility require solution. 6. Conclusions This Report is intended to serve as an interim reply to Question 12/IV. Formidable technical problems remain to be solved, before high quality broadcasting from satellites can be established. In view of the above, this Question requires continued study. Rep. 215-1 — 448 — Elevation : Pole 0° 5° Equator F ig u r e 1 Geometry of stationary satellite system Bibliography 1. C.C.I.R. Doc. IV/21 (France), Washington, 1962. Feasibility of direct broadcasting from earth satellites. 2. C.C.I.R. Doc. IV/73-Rev. (U.S.A.), Washington, 1962. Technical factors affecting the feasibility of direct broadcasting from earth satellites. 3. C.C.I.R. Doc. 242 (Canada), Geneva, 1963. Active earth satellites for broadcasting. The feasibility of direct F.M. broadcasting from earth satellites. 4. C.C.I.R. Doc. 48 (U.S.A.), Geneva, 1963. Feasibility of direct broadcasting from earth satellites; sharing considerations. 5. C r a in , C. M. Broadcasting from satellites. International Symposium on Space Communications, U.R.S.I., Paris (September, 1961). 6 . C r a v e n , T. A. M. International broadcasting by means of space satellite relays. Signal (March, 1962). 7. D e c k e r , M. T. Airborne television coverage in the presence of co-channel interference. NBS Tech nical Note No. 134. U.S. Dept, of Commerce, Washington, D.C. (March, 1962). 8 . G o u l d , R. C. TV broadcasting from an earth satellite. Report NA-55-586. National Aeronautics and Space Administration, Washington, D.C. (August, 1961). 9. I ves, G. M. Television broadcasting from satellites. Paper 61-185-1879. Convention of the American Rocket Society, New York (1961). 10. J acobs, G. and M a r t in , E. T. Direct broadcasting from earth satellites. Telecommunication Journal 1, 11-18 (1963). 11. C.C.I.R. Doc. IV/5 (United Kingdom), 1963-1966. Feasibility of direct broadcasting from artificial earth satellites. Considerations of frequency sharing. 12. C.C.I.R. Doc. IV/82 (United States), 1963-1966. Technical feasibility of direct broadcasting from earth satellites. Sharing considerations for band 8 frequency-modulation sound broadcasting. 13. C.C.I.R. Doc. IV/83 (United States), 1963-1966. Technical feasibility of direct broadcasting from earth satellites. Sharing considerations for bands 8 and 9 television broadcasting. — 449 — Rep. 216-1 L. 4: Radionavigation by satellites REPORT 216-1 * USE OF SATELLITES FOR TERRESTRIAL NAVIGATION (Question 8/IV) (1963 — 1966) 1. Introduction 1.1 Terrestrial systems for radio-navigation aid have been developed capable of providing contin uously available services whose accuracy, reliability and simplicity of operation, under ideal conditions, are comparable to that of astronavigation. However, the coverage provided by existing radionavigational aid systems is restricted, due either to line-of-sight considerations or to the difficulties of discriminating against unwanted sky-wave components of the received signal. (The latter components are not, in general, of sufficient inherent stability for normal navigational application at long ranges, except in the case of transmissions at very low fre quencies.) Experience gained in the reception of radio transmissions from artificial earth satellites indicates that the problem of coverage might be overcome by the development of a world-wide system based on such transmissions. Such a system might achieve both the accuracy of astronavigation, through precise knowledge of the satellite orbits involved, and the reliability of existing terrestrial radio aids, through the use of radio frequencies sensibly unaffected by the propagation medium. 1.2 Satellite technology is of particular interest because of the possibility of combining, for the first time, a number of related functions, such as communications, emergency operations, traffic control, and weather reporting, as well as navigation for ships, aircraft, and other mobile craft. Among the navigational features that make satellites an attractive possibility are the following: — global coverage by one system ; — versatility to meet individual service requirements ; — economy of spectrum utilization. 2. Requirements for a satellite navigation system 2.1 The primary purpose of a satellite navigational aid system should be to provide adequate position-fix accuracy at relatively long range from traffic terminal areas, or high-density traffic zones, with a degree of service availability determined by the needs of the type of mobile unit involved. In general, it is considered that a navigational aid service should be available to an unlimited number of independent observers essentially simultaneously. In general, ships require a position-fixing service of only moderate accuracy (say a few kilo metres) at relatively long intervals of time. Whilst a similar position-fixing accuracy would suffice for long-range aircraft, an effectively continuous navigational service is required in view of the high speeds involved. * This Report was adopted unanimously. Rep. 216-1 — 450 — 2.2 It is important that the user equipment for a satellite navigational aid system be comparable in cost, complexity and size with equipment now employed in conventional radionavigational aid systems, to minimize the economic burden of accommodating a new system, and to facilitate its use in aircraft, where space, weight and power are at a premium. 3. General techniques 3.1 Basically, positioning techniques can be grouped under four classifications, measurement o f: — distance; — distance-rate; — angle; — angle-rate. Within each classification a large number of variations are theoretically possible. Any given system may incorporate some combination of the four basic techniques. 3.2 The most direct method of measuring distance is in terms of the propagation time of the signals between the satellite and the observer. For this purpose it is necessary either : 3.2.1 to carry a clock in the satellite and another in the ship or aircraft, accurate to within about 3 ps for an error of 1 km ; 3.2.2 to carry a CW or pulse transponder in the satellite, activated by a signal from the ship or aircraft; 3.2.3 to carry a CW or pulse transponder in the user craft, activated by a signal from the satellite (which may originate at a master control earth station and be relayed by the satellite) ; 3.2.4 to radiate signals from several satellites, in such a manner that the user craft can measure two path differences from three satellites. 3.3 From the measurement of distance rate, the observer’s position can be established with respect to the satellite. The measurement can be made either by radiating pulses from the satellite, generating pulses at the same pulse-repetition frequency in the ship or aircraft, and measuring the time intervals between corresponding pulses over a period of seconds or even minutes ; or by radiating CW from the satellite and measuring the received frequency over a comparable period of time. Provided the time standards and frequency standards are of the same accuracy, then the manner of using the information and the accuracy of the final result are the same whether pulse or CW methods are used. Since pulse methods have no funda mental advantage over CW, and require much more bandwidth, CW systems may be pre ferred. The simplest technique for determining distance is thus based on the measurement of Doppler frequency-shift in CW transmissions from the satellite. Use of Doppler frequency- shift is most suitable for satellite orbits relatively close to the earth, since as the radii of the orbits increase, the Doppler effect is reduced and it becomes less sensitive to the position of the observer. 3.4 Measurements of angle [1] can be made either at the user craft or at the satellite. 3.4.1 If measurements were made at the user craft, each satellite would transmit a narrow band CW signal, and a modified radio sextant, of the type used with the Sun and Moon, might be employed. — 451 — Rep. 216-1 3.4.2 If the angle were measured at the satellite, a directional antenna or interferometer would receive signals transmitted by the user craft and measure the angle or angles. This information might then be transmitted either back to the user craft or to an earth station for computation of position of the user. Each angle measured would provide one line of position. A good directional reference would be needed in the satellite. Its position and attitude, to high accuracy, would need to be known at the point of computation. A variation of the technique would be for the satellite to transmit signals from a directional antenna, phased array, or interferometer for interpretation and computation aboard the user craft. In this variation, the system would be non-cooperative and not subject to saturation. 3.5 While various forms for the measurement of angle-rate are theoretically possible, this approach appears to add complications to angle measurement, without adequately compensating advantages. Consequently, it is not being actively considered at the present time. 4. Orbits for satellite navigation systems A requirement for such orbits is that the position of the satellite in orbit shall be known to the appropriate accuracy at all times. This may involve prediction of up to 24 hours or more ahead. This standard of predictability can be achieved a few days after launch, provided only that the height of perigee exceeds about 300 km ; at lower heights, unpredictable effects are caused by the variability of atmospheric drag. At heights above 300 km, changes in the orbit occur slowly and the nature of these changes is being established with increasing accuracy, as the study of satellite orbits continues. The choice of satellite orbit and number of satellites for a navigation system depend on the type of service involved, and the degree of availability required. For shipborne use, solar navigation could be replaced more than adequately in its contin uity by a single satellite. Aircraft operations, however, require more continuous navigation facilities and, for this purpose, it will be necessary to employ station-keeping navigational satellites. A practical Doppler system must have low orbits resulting in long periods of non availability of a single satellite. The important parameter is not the percentage of time during which no observation is possible, but the possible interval of time during which no observa tion is possible, which can most effectively be reduced by employing station-keeping satellites. Coverage up to latitudes of about 70° could be achieved at all times in the radio sextant application by a system of about 3 equally-spaced satellites in synchronous equatorial orbits, provided that a North reference is available at the sextant. In the system involving two simultaneous measurements of path difference, in which 3 stationary satellites must be visible simultaneously, 12 satellites, spaced by 30° are necessary, to provide coverage up to at least latitude 70°. In some satellite navigation systems, particularly those involving synchronous equatorial orbits, attitude stabilization of the satellite will be required, to facilitate orbit adjust ment corrections. Ultimately, it should prove practicable for the control station to issue command signals to a satellite equipped with orbit correction devices to minimize the influence of departures from the published ephemeris. The availability of attitude stabilization would permit exploitation of antenna gain on the satellite. The radio link used for attitude stabiliza tion and orbit correction could also be employed for relative phase control of the transmis sions from the satellites. 5. Particular techniques 5.1 Measurement o f distance As indicated in § 3.2, the distance measurements might be made at the mobile craft, at the satellite, or at fixed earth-stations. Ephemeridal information would be needed at the point at which computation of position was made. Rep. 216-1 — 452 — 5.1.1 If synchronized time-standards of adequate accuracy were available in both the satellite and the user craft, passive (non-cooperative) mode operation would be possible by making the measurement and computation at the user craft. Such a system could not be saturated. However, it would be difficult to realize the required accuracy of time standards. 5.1.2 To assess the feasibility of distance measurements involving a transponder in the satel lite, use might be made of existing transponders aboard satellites now in orbit, or the transponders in satellites which will be launched for other purposes. 5.1.3 There has been considerable study of a distance-measuring technique involving a transponder on the user craft, activated by a signal transmitted from an earth station and relayed by the satellite. In this system, the earth station would interrogate each user, at an interval determined by the requirements of the user craft (at frequent intervals for aircraft, at longer intervals for ships), employing a unique address code. The message would be relayed by the satellite to the user craft and at the same time repeated back to the earth station. The user craft would repeat the message back to the satellite, from which it would be relayed to the earth station. Distance would be measured from the delay between direct and delayed repeat-back. Essentially simultaneous measurements of distance from each of two satellites to the user craft would be made by the earth-station, where the position of the craft would be computed and transmitted via one of the satellites. The earth-station com puter would also maintain an ephemeris for each satellite, keeping it up to date from time to time by distance measurements on beacons at known locations on the earth. The satellites used for any given position fix would be selected on the basis of favour- ability of position with respect to the user. Sixteen to twenty-four satellites in four high-inclination orbits of 7000 to 10000 km, would provide continuous world-wide coverage, with considerable redundancy, with six earth stations suitably located throughout the world. Lower orbits could be used satisfactorily, but for continuous coverage, an excessive number of satellites and earth stations would be needed. Use of very low orbits would produce gaps in the coverage. The technique is also suitable for use in synchronous orbits at 36 000 km to provide continuous area coverage, two such satellites being adequate for coverage of nearly all of the Atlantic Ocean. It is expected that, with satellites in medium altitude orbits, a time block of about 0-25 s per user would be adequate, and might even provide enough time for transmission, in either direction, of brief messages relating to traffic control, weather information, emergencies, etc. A passive (non-cooperative) mode of operation would be possible if each ground station periodically, such as once each 30 s, transmitted to a pair of satellites pulse signals so timed as to be re-transmitted by the satellite at the same instant or at a known interval. The difference in time of reception at the user craft would define a hyperbolic position line on the earth if the positions of the two satellites were known. The computa tion would be made aboard the user craft. In a pulse-ranging system of this type, a wide bandwidth of the order of 1 MHz would be needed for adequate resolution of range. If simple, low-gain antennae were used, high peak power would be needed. However, because of low information rate, the average power would be low. Two radio-frequency channels would be needed, because of the essentially simul taneous reception and transmission of signals at the satellite. The optimum separation between these frequency channels is about 1 0 %, to facilitate using a single antenna. Frequencies between about 400 and 2000 MHz are suitable for these two channels, with frequencies near 500 MHz being optimum. Below 400 MHz, the frequencies rapidly — 453 — Rep. 216-1 become unusable, because of propagation anomalies which severely degrade the accuracy of the system. The power requirement increases as the square of the frequency, unless higher gain antennae are employed by the users. The use of either higher peak power or directional antennae is practical for experimental work, but would be a serious restriction in an operational system. 5.1.4 Path-dijference CW measuring techniques based on stationary satellites The path-difference measuring system requires three stationary satellites to be visible simultaneously from a given observing position. Hence, to provide world-wide coverage, about 1 2 satellites would be required, equally-spaced in the equatorial orbit. The modulation of the transmissions from all satellites in the system would require to be phase-locked to a common reference source and could be used to provide a means for positional discrimination. Use of frequencies in the range 100 to 2000 MHz appears suitable. Synchronous (phase-locked) transmissions from the satellites simplifies the craft receiver, the measurement of path difference being accomplished preferably by phase difference techniques. Two possible positions are defined by the conic intersection of hyperbolic surfaces, having the satellites as foci, with the surface of the earth. The ambiguity would be resolved by dead-reckoning. 5.2 Measurement o f distance-rate The measurement of a single Doppler frequency-shift characteristic from a satellite transmitter in a known orbit enables a navigation fix to be obtained every time a satellite passes within receiving range of the observer. In a system of this type, each user would compare the frequency of signals from a satellite with a frequency standard aboard the user craft during a period of several minutes, noting the Doppler frequency shift. Each satellite would be tracked and its orbital parameters kept up to date at a computing centre on the ground. The orbital information would be trans mitted to the satellite at appropriate intervals, where it would be stored and transmitted at precise intervals. Computation of positions of the satellite and user would be made by the user. The simplest Doppler navigation system [2] depends on the use of a satellite in nearly circular orbit and containing a long-life radio beacon, having a known stable frequency (within one part in 108) during the observational period. It is assumed that the Doppler shift in the received CW radiation at a given point on the earth, due to the relative velocity between transmitter and receiver, can be measured accurately as a function of time to 2 parts in 1 0 8. From the observations, the instant at which the satellite is closest to the observer and the minimum slant range can be deduced. From a known satellite orbit, the observer’s position can now be computed directly, with one ambiguity which in most cases can be resolved by dead-reckoning. Calculations needed to find the maximum rate of change of frequency can be made easier by more complicated apparatus in the ship or aircraft. Because of the need for a time derivative, this technique is limited to orbits on the order of 500-2000 km in altitude if accuracy of position fix is required. Somewhat higher orbits could be used with reduced accuracy; medium and synchronous altitudes are not suitable. The choice of operating frequency for the satellite CW transmitter for a Doppler naviga tion system is a compromise between the avoidance of ionospheric influences and the provision of an adequate signal level at the receiving station. In a Doppler system, omni-directional antennae must be used for reception. Hence, the required transmitter power increases rapidly as the wavelength is decreased. Having regard to the variation with operating frequency of galactic emissions, receiver input noise level, and of the efficiency of available transmitting oscillators and amplifiers, it appears that a choice of frequency between 100 and 1000 MHz would provide the minimum required transmitter power. The satellite transmitter antenna Rep. 216-1 — 454 — should be as non-directive as possible. The use of circularly polarized antennae for transmis sion will minimize, but not eliminate, signal fading due to rotation of the satellite. Under these circumstances, a transmitter power output in the range 100 mW to 1 W should prove adequate. Frequencies near the low end of the range are less desirable because of ionospheric effects, which include fading due to the Faraday effect, deviations of the wave trajectory due to refraction, and variations in both of these effects due to irregularities in the density and distribution of ionization [3], [4]. If the Doppler shift is received simultaneously on two frequencies, preferably harmonic ally related, appropriate corrections can be made for ionospheric refraction effects that would otherwise influence the navigational accuracy achievable, without prior knowledge of the prevailing electron density distribution. The positional accuracy, achievable with satellite Doppler navigation systems employing two frequencies, is expected to be better than 1 km, which is more precise than that required for normal navigational functions. 5.3 Measurement o f angles 5.3.1 The radio sextant In a system of this type, the computation of the position of the user craft would be made by the user. Existing sight reduction tables could be used if desired. The system would be non-cooperative and non-saturable. Complex, expensive, bulky equipment would probably preclude its use by aircraft, and also its wide use by non-military ships. However, a highly accurate directional (North) reference would , be available, and the same user equipment might be used with the Sun and possibly the Moon to increase the coverage. Due to the problems of acquisition in position and frequency, angular measure ments on fixed or slowly moving bodies are not easy, and the difficulty is much greater in the case of rapidly moving bodies such as a satellite at a height of the order of 2 0 0 0 km. Because of the difficulty in making angular measurements on moving satellites, unless the earth station is “locked on” the satellite from a known start position of the earth stations, it is preferred that the radio sextant be used only with stationary satellites and slowly moving celestial objects such as the Sun and Moon. High directivity is required in the receiving antenna of the sextant, so that the use of frequencies in the range 10 to 300 GHz (e.g., the window at 35 GHz), for the trans mitter frequency of the satellite beacon appears desirable if the aperture of the antenna is to be kept to reasonable proportions. (Even so, the use of split-beam or similar techniques for improving antenna discrimination may be necessary for a high-precision system.) The use of such frequencies would eliminate propagation difficulties due to refraction in the ionosphere, but difficulties could arise as a result of absorption and scattering in the troposphere due to cloud, rain, etc. In this system, the transmitted power requirement could be minimized by the use of a very narrow bandwidth in a phase-lock type of receiver. If the system is also to be used for sun tracking, a very Wide radio-frequency bandwidth is involved. Hence, a receiver, designed for operation with both artificial satellites and the Sun, should be able to operate in both the narrow-bandwidth and wide-band width modes. A number of components could be common to both modes of operation. The use of the phase-lock system provides the optimum method of extracting the Doppler signal if this is desired. One disadvantage of the radio-sextant system is the requirement for a highly- stable platform, necessary to provide an accurate vertical reference. With a system — 455 — Rep. 216-1 operating at frequencies in the range 3 to 300 GHz, this might only be practicable in the larger ships due to equipment size and weight. It is possible that the size and cost of the receiving installation might be much reduced by operating this type of system in the infra-red band. Many problems, includ ing the design of a suitable receiving element and infra-red transmitter, have yet to be solved. This subject is being intensively studied and it is possible that the use of infra red will eventually become practicable. Thus, in time, radio sextants might profitably be employed in the 3 to 43 THz band. (It is to be noted that the Radio Regulations, Geneva, 1959, Art. 1, Section 1, No. 7, at present define radio waves as “electromagnetic waves of frequencies lower than 3000 Gc/s propagated in space without artificial guide” .) The small size of such units should make it possible to fit them in both small ships and aircraft. 5.3.2 Measurement o f angles at the satellite One method of measuring angles from the satellite to the user craft would involve carrying aboard the satellite a radio-interferometer. The interferometer would be essentially horizontal, but would not require precise stabilization because earth stations at known locations could monitor the orientation of the interferometer array by com paring indicated angles with known true angles, derived from the known positions of the satellite and the monitoring earth stations. The length of the baseline of the interferometer would depend on altitude of the satellite and required accuracy of fixes. For satellites at synchronous altitude, a baseline of 1 0 0 wavelengths is considered adequate for “fine” measurements of angle, with a second pair of interferometer receivers on a shorter baseline to resolve ambiguities. Angular information derived from the interferometers could be transmitted back to the user craft, or to an earth station, for computing the position of the user. The latter is considered preferable, both to simplify equipment aboard the user craft and to facilitate application of the attitude information obtained at the monitoring earth stations. Frequencies in the range 800 to 8000 MHz are potentially suitable for such a system. If the gain of the antennae on the satellite and aboard the user craft were held constant, a relatively low frequency would be desirable to minimize the power requirements. However, because of ionospheric distortion in the measurement of angle, 800 MHz is considered a minimum. If the gain were constant at one end of the link and the effective aperture fixed at the other end, the required power would be independent of frequency. If the aperture of the antenna were fixed at both ends of the link, the required power would be inversely proportional to the square of the frequency. In either of the second or third alternatives, higher frequencies would be desirable, because the beam width of a constant aperture antenna decreases with higher frequency, and hence the discrimination accuracy improves. Above 8 GHz, however, attenuation due to rain becomes increasingly significant. The optimum frequency is, therefore, 8 GHz. The accuracy of measurement of an angle would decrease with height of the satel lite, but at low altitudes the ephemeris problem would be greater, and the coverage would be limited unless a large number of satellites were used or intermittent fixing were acceptable. High-inclination orbits at 7000-10 000 km would be suitable for world-wide coverage with eight satellites. Continuous coverage of the entire Atlantic Ocean could be provided by one equatorial satellite at the synchronous height of 36 000 km. Rep. 216-1 — 456 — 5.4 Measurement o f distance and angle at the earth This approach would be similar to that described in § 5.1.3, with the important distinction that essentially simultaneous measurement of distance of the user craft from two satellites would be replaced by measurement of distance and two angles, from one satellite. Angles are measured by means of essentially horizontal, orthogonal interferometers. In a typical system of this type, five radio-frequency channels are envisaged. Two channels would be required between the earth station and the satellite. Each of these would require about 250 kHz bandwidth, located between 800 and 8000 MHz for reasons given in § 5.3.2. Two additional 250 kHz channels, located between 800 and 2000 MHz, would be required between the satellite and the user craft. The upper limit of 2000 MHz is due to the use of a wide beamwidth antenna aboard the user craft. One additional channel of 15 kHz bandwidth would be needed, in the 100 to 150 MHz range (in the interest of compatibility with existing aeronautical radionavigation services), for data transmission from the satellite to the user craft. 6. Conclusions 6.1 There are four basic techniques for radionavigation by satellite : distance, rate of change of distance, angle, or rate of change of angle. Combinations of these techniques may be used. 6.2 A system using the rate of change of distance technique (Doppler), has been in limited opera tion. 6.3 Angle systems (radio sextant) have been constructed experimentally for use on the Sun and the Moon. 6.4 Neither of these two systems is considered an optimum utilization of satellites for all commer cial radionavigation applications. 6.5 Studies and analyses have been, and are being, made to assess the feasibility of systems employ ing most of the techniques discussed above. 6 .6 It seems probable that these studies will lead to experiments to demonstrate the feasibility of other techniques, and possibly operational systems. The frequency ranges appropriate to each technique are broadly indicated in the foregoing discussion. B ibliography 1. M o o d y , A. B. Navigation using signals from high-altitude satellites. Proc. IRE, Vol. 48, 4, 500 (1960). 2. R y le, M . a n d Sm it h , F. G. Radio-navigation system using earth satellites. Journal I.E.E., Vol. 5, 168 (1959). 3. G u ie r , W . H . and W e iffe n b a c h , G . C . A satellite Doppler navigation system. Proc. IRE, Vol. 48, 4 (1960). 4. Kitchen, F. A. and J o y , W . R. Some effects of the fine structure of the ionosphere on transmissions received from the Russian Earth Satellite 1958 Delta. Nature, Vol. 181, 1759 (1958). — 457 — Rep. 394 REPORT 394 * FEASIBILITY OF FREQUENCY SHARING BETWEEN THE RADIONAVIGATION-SATELLITE SERVICE AND TERRESTRIAL SERVICES (Question 8/IV) (1966) 1. Introduction This Report refers to Question 8 /IV, § 3, i.e., “is the sharing of frequencies with other services feasible, and if so, with what other services and under what conditions ?” The question is treated in terms of a typical radionavigation-satellite system of the type described in Report 216-1. Three potential situations for interference are examined: — interference from terrestrial stations to an airborne or shipborne receiving station in the radionavigation-satellite service; — interference from terrestrial stations to a satellite-borne receiving station in the radio navigation-satellite service; — interference to terrestrial stations resulting from radionavigation-satellite space station transmissions. These are treated below, sequentially. 2. Interference from terrestrial stations to users of the service 2.1 If the aircraft or ship uses a directional antenna to receive signals from the space station, it will always be pointed at the space station and never below a nominal minimum elevation. Interfering signals from terrestrial stations will therefore be received only in the minor lobes where the gain is 20 dB or more below the gain of the main lobe, reaching a practical maximum horizontal value of 0 dB for the antennae contemplated. 2.2 As at present conceived, systems requiring frequencies of the order of 1 GHz would use omnidirectional antennae on aircraft or ships. 2.3 In view of §§ 2.1 and 2.2, the gain of any aircraft or shipboard antenna used for reception in the radionavigation-satellite service can be assumed to be 0 dB, insofar as interfering signals from terrestrial stations are concerned. 2.4 With present day techniques, the receiver noise temperature suitable for radionavigation- satellite systems is approximately 1000°K for frequencies above 800 MHz. 2.5 The noise-power spectral-density kT, referred to the input of a 1000°K receiver on board aircraft or ships, is —198 dBW/Hz. 2.6 To ensure a minimum of interference, the level of an interfering signal should be less than the thermal noise by at least 6 dB, or a maximum average of —204 dBW/Hz. ( — 6 dB is an assumed value, pending determination of the interference susceptibility of an actual system.) 2.7 The effective aperture of an isotropic antenna ranges from approximately 10-2/m2 at 900 MHz to 10-4/m2 at 9 GHz. * This Report was adopted unanimously. 18 Rep. 394 — 458 — 2.8 The peak power-flux spectral-density of an interfering signal from a terrestrial transmitter in the passband of the airborne or shipborne receiver should be less than —184 dBW/m2/Hz at 900 MHz, rising to -164 dBW/m2/Hz at 9 GHz. 2.9 These parameters have been used in Annex I to calculate the coordination distance for a typical situation using propagation curves from Recommendation No. 1A of the Extra ordinary Administrative Radio Conference, Geneva, 1963. 3. Interference from terrestrial stations to the satellite-borne receiver 3.1 Antennae used for receiving aboard one typical class of radionavigation-satellites are designed to cover the whole of the earth’s surface in the most efficient manner. For earth-oriented conical beam antennae, the sum of the basic transmission loss and spacecraft antenna gain is nearly independent of altitude, the value being : L— —153 + 20 log10A (dB) where A is the wavelength in m. 3.2 A typical system noise temperature for radionavigation-satellite space station receivers is 600°K. 3.3 The interfering signal should be 6 dB below the space station receiver noise, which is -201 dBW/Hz. 3.4 The maximum tolerable interfering signal level is, therefore, approximately —207 dBW/Hz. 3.5 The maximum value of the terrestrial transmitter power spectral density, PJB, terrestrial antenna gain, Gt in the direction of the navigational satellite is derived from the expressions : (PJB ) + Gt = -1 0 4 + 20 lo g /MHz (dBW/Hz) [for 600°K] (PJB) + Gt = -101 + 20 l o g / M H z (dBW/Hz) [for 1000°K] These equations are plotted in Fig. 1. To determine the maximum tolerable transmitter power, Pt, of the terrestrial station, Pt = (PJB) + Gt + 10 log B —Gt where B is the bandwidth (Hz) of the radiated power spectrum (here assumed to be uniform). 3.6 The maximum radiated power of such a terrestrial transmitter (Pt + Gt), in the direction of a navigational satellite, which can be tolerated, is approximately —44 dBW/Hz at 1000 MHz and —24 dBW/Hz at 10 000 MHz. (This assumes a navigational satellite with a receiver temperature of 600°K.) 3.7 Most of the services which are candidates for sharing, exhibit higher effective radiated power in a direction which could illuminate a navigational satellite. 3.8 For a radionavigation-satellite service which provides world-wide coverage, sharing with other services is not feasible. — 459 — Rep. 394 4. Interference to terrestrial receivers from space stations 4.1 Radionavigation-satellite space stations as conceived at present may employ either continuous wave or low duty-cycle pulsed-transmissions, either in synchronous or non-synchronous orbits. 4.2 The spectral density of interfering power flux in the antenna beam, which would result in interference equal to the inherent thermal noise in a terrestrial system, would be, typically, about —190 dBW/m2 per hertz for an airborne weather radar and about —235 dBW/m2 per hertz for large communication-satellite earth stations. 4.3 The spectral density of power flux at the earth surface from radionavigation-satellite space stations may exceed —175 dBW/m2 per hertz (based on the receiver described in the Annex). 4.4 In view of §§ 4.2 and 4.3, interference to terrestrial system receivers of all types, in frequency bands shared with radionavigation-satellite space stations, would be intolerably high. Such sharing, therefore, is not considered feasible. ANNEX Example : A shipborne receiver in the radionavigation-satellite service has a noise temperature of 1000°K, a bandwidth of 500 kHz, a path antenna gain of 0 dB in the direction of the interfering station, and an operating frequency of 1500 MHz. What is the coordination distance over sea water for a co-channel terrestrial transmitting station of 20 W power (uniform power spectrum) and 30 dB antenna gain ? Lb = (Pt + Gt—Fs—Pr + Gr) dB where Lb = required minimum basic transmission loss; Pt = power (dBW), supplied by the interfering transmitter to the transmission line input; Gt = isotropic gain (dB), of the transmitting antenna of the interfering station in the direction of the shipborne antenna ; Fs = site shielding-factor ; Pr = maximum permissible interference level (dBW), at the receiver in p u t; Gr = isotropic gain (dB), of the receiving antenna in the direction of the interfering station; Lb = 13 + 3 0 -0 + 147-6 + 0 - 190-6 dB From § 2.6, the level of an interfering signal at the receiver input should not exceed —204 dBW/Hz. (Actually, -204-6 dBW/Hz.) The total permissible interfering power in 500 kHz is, then, -204-6 + 57, or —147-6 dBW. Lb = Lb + correction factor to permit use of 4 GHz curves = 190-6 + 9-4 = 200 dB. From Fig. 2 of Recommendation No. 1A of the Extraordinary Administrative Radio Conference, Geneva, 1963, the distance at which this basic transmission loss can be expected, for all but 0-1% of the time, is 600 km, in Zone B. For the same conditions in Zone C, between 23-5°N and 23-5°S inclusive, the distance would be about 1200 km. Rep. 394 — 460 — Frequency (MHz) F ig u r e 1 Maximum spectral density o f the radiated power from a terrestrial transmitter in the direction o f a navigational satellite, as a function o f frequency T : noise temperature of the navigational-satellite receiver (°K) i — 461 — Rep. 395 L. 5 : Meteorological satellites REPORT 395 * RADIOCOMMUNICATIONS FOR METEOROLOGICAL SATELLITE SYSTEMS (Question 9/IV, Study Programme 9A/IV) (1963 — 1966) 1. Introduction The information given in this Report is based mainly on the t i r o s and n im b u s meteoro logical satellite systems, described in the Annex, which are considered to be representative of the present stage reached in this technique. In addition to the television cameras and radiation sensors as used in these systems, future meteorological satellites may well lend themselves to further and more refined weather data collection. Predictions of future develop ments in meteorological satellites are necessarily extrapolations from the experience gained from the initial satellites. 2. Television Weather data have very short-lived usefulness and delivery via television links, satellite to the earth station and on to the user, must be as rapid as possible. This can be achieved by using more than one satellite, by frequent transmission of the essential information, or by a combination of both techniques. Operations of this kind require a larger bandwidth than is available in the bands used at present for research. To discern features like ice or clouds and to separate desert areas from clouds is sometimes difficult and an increase in resolution may be required. Cloud types and other significant meteorological features are also recogniz able when resolution is increased. Although present cameras offer no convenient way to improve resolution, techniques under development, such as the electrostatic tape camera, offer a great advance. Similar to the generation of a charge pattern in a vidicon television camera tube, this latter pattern is stored by means of a suitable tape with dielectric properties. This tape carrying the charge pattern can be rolled on reels like magnetic tape in magnetic tape recorders. Transmission of this greater detail will require greater television bandwidths. 3. Weather radar Radar is being used extensively by aviation and the weather forecasting services for the detection of three-dimensional precipitation patterns. The combination of precipitation information with television cloud pictures would be of inestimable value, particularly in regard to furnishing data from uninhabited areas. Radar not only detects precipitation in itself, but also permits an estimate of intensity, volume, freezing level, extent of vertical convec tion, etc. Other types of radar can yield information about clouds not easily detected by pictures. Intensive research has been conducted to determine suitable frequencies for the detection of these phenomena. Considerations as to the possible size of antenna reflector, desirable ground resolution, beamwidth, and the resolution of precipitation volume have strong influence on the choice of frequency. These questions have been analysed but are still * This Report, which was adopted unanimously, replaces Report 217. Rep. 395 — 462 — subject to intensive study. The present state of knowledge, as represented by the published literature, indicates the desirability to design radars in the 10 GHz band, as this is roughly the centre of the good radar return from precipitation. The 34-35 GHz band appears to be the most desirable for cloud detection radars. Large bandwidths are needed because of the stringent resolution requirements. 4. Microwave spectroscopy Infra-red spectroscopy is performed to measure the energy relationship in the atmosphere. This is under further development. Other techniques can be developed to determine the vertical temperature profile of the atmosphere, such as measuring narrow spectral intervals in the 15 p carbon dioxide absorption band. Although considerable meteorological knowledge has been accumulated in the application of the optical properties of the atmosphere, its properties in the microwave region are not being applied presently. Intensive research is being conducted, to enable application of the microwave absorption lines to the measurement of atmospheric phenomena. It is, however, necessary to know that no measurable interference exists at the absorption lines of interest so the measurements are not distorted. In view of the preceding, some degree of protection may be required in the future in the following bands : water vapour absorption lines at 22-2,- 68-4, 80-6, 121, 138, 144, 183, 242, 255 and 267 GHz. Likewise, oxygen has several lines between 53-67 GHz and at 120 GHz. 5. Sferics Thunderstorms and their frequently associated ancillaries, such as hail, turbulence, precipitation, strong winds, etc., represent severe weather hazards to life and property and are especially important in aircraft operations. At the present time, radar and ground-based sferics networks provide meteorologists with the locations and movement of thunderstorms over only a small fraction of the surface of the earth. These observations have proved most valuable in weather analysis and forecasting. Satellites, potentially, provide the means by which observations of thunderstorm activity can be obtained on a global basis. In addition, a sferic sensor aboard a meteorological satellite is probably the most economical method of extending the sferics network to a global basis. Supporting studies have been made to deter mine the feasibility of a satellite sferics sensor. Results from these studies are encouraging. A prototype satellite sferics sensor is under development leading to the flight model to be flown on an early n im b u s satellite. Frequencies between 30 MHz and 600 MHz appear to be the most desirable for this application. Sferics data are envisioned as useful when employed in conjunction with television cloud photographs. Thunderstorms, or areas of thunderstorms, cannot be definitely discerned from the cloud photograph because of resolution limitations, masking by cirrus clouds, etc. An independent sensor seems to be the best approach to this problem. Even with better resolu tion, cirrus clouds will frequently obscure the thunderstorms. A sferic sensor aboard a satellite would not be limited in this manner and would provide additional information on the location of thunderstorms over both the sunlit and dark portions of the earth. In other words, the sferics data would complement the television cloud photographs and infra-red data. Sferics data are useful: — to locate cold fronts and squall lines ; — to locate areas of thunderstorm activity ; — 463 — Rep. 395 —■ to obtain a measure of the severity of thunderstorm activity from the frequency of discharges recorded; — to locate the active portions of the equatorial zone of convergence ; — to provide a better climatology of thunderstorms on a global basis. Ground-based sferics networks use the signal emitted by a lightning flash in the 10 kHz region of the spectrum where maximum energy is emitted, with the exception of those signals propagated by the Whistler mode. Although the spectral density of a lightning flash decreases with increasing radio fre quency, the results of experimental research show that sufficient energy is emitted up to 400 MHz and possibly higher up to the 1700 MHz band to permit detection of the flash at orbital altitudes. Band 9 offers the advantages of higher resolutions, easier antenna design and better directivity. The bandwidth required for these measurements is fairly small, being of the order of voice communication bandwidths. The measurement of a natural phenomenon from a satellite requires freedom from man- made interference on a global scale. Since narrow bandwidths are sufficient for the task, receivers designed for the internationally agreed guard bands of the standard-frequencies or radioastronomy frequencies will satisfy the need. 6. Telecommand The t i r o s and n im b u s meteorological satellites are using a telecommand frequency in band 8 . However, the meteorological community has been using ground transmitters in a meteorological aids band in the neighbourhood of 400 MHz for a ranging system with “weather balloons”, so it appears to be good engineering to consider a similar employment of this band for telecommand functions of later meteorological satellites. This is within the concept of using, as far as practical, the present meteorological bands for meteorological satellites. It would, however, be necessary to avoid that part of the band which is allocated to space telemetering, because of the possibilities of interference. 7. Data collection and location The collection of scientific meteorological and oceanographic data from fixed or mobile data sources by a meteorological satellite will provide important inputs into the weather prediction process. A system called IRLS (Interrogation, Recording, Location System) is under development to be flown on the n im b u s spacecraft in the near future. This system automatically interrogates platforms floating on the ocean near the satellite-ground track, collects the data, and measures the distance from the spacecraft to the platform. The data accumulated during each satellite orbit are relayed to a Command and Data Acquisition (CDA) station for final processing and dissemination. Normally, two or more interrogations are made to each platform while it is within sight of the spacecraft, so the location of the platform can be determined later at the CDA site by solving for the common intersection of the multiple range spheres with the surface of the earth. A brief description of the system and communication parameters follows. A memory in the spacecraft is loaded from the CDA site with the address and time of interrogation of each platform to be contacted during the next orbit or orbits. The CDA site will be at Fair banks, Alaska (or Rosman, North Carolina). The transmitter has been assigned the frequency of 466 MHz, and will frequency-modulate the carrier at a rate of 12000 bits a second deviating plus and minus 6-5 kHz using a PCM-NRZ-FSK (pulse code modulation-non- return to zero-frequency shift keying) modulation. The CDA transmitter power will be 15 W, with a nominal antenna gain of 15 dB. Rep. 395 — 464 — At the specified time the spacecraft will transmit an address code to activate the selected platform. Transmission will be via frequency modulation of the carrier in the 400 05 to 401 MHz band at the 12 kHz bit rate using the same PCM-NRZ-FSK system described above. The transmitter will have a power of 15 W with a maximum antenna gain of 6 dB looking straight down. The information rate of the system in either direction is 1000 bits per second. Twelve bits are used per information unit to provide the high bit rate needed for accurate ranging, and to provide error detection and correction capabilities, word synchronization, and a long unambiguous range code. A 16-bit address word is used for activation of the transmitter of the selected platform. The same transmitting and receiving frequencies are used for the satellite-to-CDA station links and the satellite-to-platform links. The automatic weather and oceanographic platform transmitter, modulation, antenna, and power level will be identical to that on the spacecraft, except that it will operate in the 460 to 470 MHz band. The platform will operate in a transceiver mode, with the spacecraft continuously transmitting the PCM range code to the selected platform which is received, demodulated, and then remodulated onto the transmitter with a specific 8 bits of the 1 2 -bit range code, complemented if the up-link data are to be a binary zero and not complemented for a binary one. Thus the up-link information data rate is 1000 bits a second. The trans mitter on any specific platform will be powered for a total of approximately 3 s on each interrogation. A given platform will be interrogated approximately 4 times a day, and thus its transmitter will be on for a total of about 1 2 s a day. The spacecraft transmitter is active during the time that any specific platform is being interrogated. The initial experimental system will have about 15 platforms, so that the trans mitter in the spacecraft will be on for about 180 s a day. The system design is such, that it could service a great many more platforms, so that if as many as 1 0 0 0 platforms were to be in use, the spacecraft transmitter would be on for about 2 0 0 minutes a day. The transmitter emission presently authorized for this system development is 100F9, with a modulation index of approximately 1. The centre frequency stability of the transmitter will be plus or minus 0 0015% under all conditions. The receiver will have a noise figure of 4 0 dB and an intermediate-frequency bandwidth of 100 kHz, with a limiter-discriminator detector with a frequency stability of plus or minus 0-0015%. A single antenna is used on the spacecraft and the platform for both transmitting and receiving with a diplexer to isolate one from another. The use of a single antenna is especially important on the spacecraft because of the limited space available for mounting antennae. For.efficient operation of the antenna, the two frequencies should be close together, but the need for a good diplexer to isolate the transmitter from the receiver requires that the two frequencies must differ by at least 10 to 15%. Thus the selection of 400-05-401 MHz and 466 MHz as the two bands for this first experimental application. 8. Synchronous satellites Operational systems using the t o s and n im b u s satellites will receive at least two sets of data a day. Most weather conditions do not change so rapidly that more frequent measure ments (in time) are needed, however, there is enough change that gross continuous coverage is needed. Studies are being performed to investigate the possibilities of obtaining such coverage continuously from synchronous satellites having 24-hour orbital periods. Since this space vehicle will be launched into an equatorial orbit at 35 871 km altitude, it will move at the spin rate of the earth and thus observe the same large area continuously. A series of three or four satellites spaced, respectively, 120° or 90° apart around the equator, can observe all the Earth with the exception of the polar caps. No study results on this are available at present, so no details with regard to experimental resolution and telemetering can be stated. — 465 — Rep. 395 But referring to the fundamental laws of communication of information for the case of tele vision, it can be seen that similar angular resolution requirements, dynamic ranges, and signal- to-noise ratios must result in the requirement for similar bandwidths, if the time interval between observations is kept the same. Consequently, faster data read-out to shorten observa tion time must result in more required bandwidth. There are plans for meteorological experi ments to be carried out late in 1966 on a non-synchronous satellite at an altitude of 6000 miles and on synchronous satellites in 1967 and 1968, using space communication research bands for telemetering the data. These experiments will aid in the design of an operational syn chronous meteorological satellite. 9. Discussion Meteorological satellites, under development and being planned, will serve research and spacecraft development and lead to the evolution of operational satellite systems. For the present half of this decennium, it is planned to launch both research and operational satellites. The research meteorological satellite system is designed to be compatible with space research frequency allocations and to share these allocations with other space research users. Realization of the present plans point very clearly toward a situation where mutual inter ference cannot be avoided. In addition, operational functions are not intended to be served by research frequency allocations, showing the necessity to conduct operations in other than the space research bands. The tracking requirement of meteorological satellite systems can be satisfied by narrow emission channels, in the 136-137 MHz research tracking band in the case of research meteoro logical satellites, and adjacent to this band for operational meteorological satellites. The functional or maintenance space telemetering, which might be accomplished in this same frequency area, could, in the case of the research and development satellites, be in the space research band. However, the possible several satellites of an operational system would need approximately one megacycle of bandwidth for this basic telemetering in this same frequency area, preferably adjacent to the research band. In these two cases, adjacency is suggested because it is good engineering practice, self-evident, to put the operational systems imme diately adjacent to the development frequency areas. Secondly, frequencies of this order are dictated for these purposes principally to facilitate antenna pointing and acquisition of the spacecraft signals. The same adjacency philosophy applies to the case of the television requirements. Tele vision places the maximum demand on the space research band, 1700-1710 MHz, and with two satellites the entire band will be occupied. From the design information reported previ ously, it is shown that conservative narrow-band techniques are employed and further reduc tion in bandwidth cannot be accomplished without further research in automatic on-board information processing and optimum encoding. The 1700-1710 MHz space research band has been selected for meteorological research satellites, because of the proximity to the meteoro logical aids band immediately below. Review of the system design features of the n im b u s satellite (for example) shows that sharing in the meteorological aids band is not detrimental to the operation of the meteoro logical satellite. Use in the earth station of a narrow-beam directional antenna facilitates the use of low-power transmitters in the spacecraft, so that very low-power density results on the Earth. A more and more pressing need is rising for faster data distribution. More receiving stations will be needed, thereby increasing the burden on terrestrial communication facilities. These stations cannot be built as very high-gain receiving sites for economical reasons, so that an increase in meteorological satellite transmitter power in both bands 8 and 9 can be foreseen. Under these conditions, interference to terrestrial users by Rep. 395 — 466 — meteorological satellites could occur where bands are shared. The ability to share will be severely restricted geographically relative to receiving sites with the evolution of a continuous operation system. ANNEX 1. Introduction The purpose of this Annex is to delineate U.S. plans of interest to Study Group IV in the field of meteorological-satellites ; and to describe the systems of such satellites and their telemetering and telecommand functions. The t i r o s satellite (1960B) was orbited by the U.S. on 1 April, 1960. It delivered a large quantity of pictures of the cloud cover of the Earth. Later t i r o s satellites have telemetered data on the terrestrial and reflected solar radiation in the visible and infra-red parts of the spectrum. An automatic picture transmission (APT) system was test-flown on tiros viii. This system represents a first step in using the satellite itself to convey weather information directly to weather forecast centres, wherever they may be. This slow-scan television system is designed to take, and immediately transmit, cloud pictures for reception by a relatively inexpensive earth station. These satellites serve primarily research and development purposes, although the pictures have been, and are being, used in operations when available. However, the present t i r o s do not fulfil the requirements for an operational satellite. Work has begun on an elaborate meteorological satellite, n im b u s, to fulfil the operational requirements and two test vehicles have been launched. However, to implement the operational programme at an early date, the t i r o s satellite has been modified into a wheel configuration and launched into a quasi-polar orbit to provide greater observa tional capability. These satellites are called tos (tiros Operational Satellite), and are also known as ess a satellites. Research versions of the t i r o s satellite will be used as test beds, to aid in improving the operational system and to provide data for other meteorological research purposes. The n im b u s satellite is being used to provide research and development of advanced sensory systems, power supplies, stabilization systems, and to develop a more reliable, longer- lived satellite with greater sensor carrying capacity, to become, in due course, an operational satellite. Nature uses the clouds to draw its own weather map ; the satellites permit us, by means of television, to record and use this map. Thus, they should provide global coverage, giving man a powerful tool to supplement conventional meteorological data by filling voids where they exist. Among such voids are vast oceanic areas and sparsely populated areas, where economic considerations make prohibitive an increase in the present observational network. 2. The TIROS operational satellite (TOS) Weather observations from spacecraft are most meaningful when the cameras and sensors are aligned in a position normal to the surface of the earth. To accomplish this, the t o s system utilizes spin-stabilization in a “cart wheel” configuration. Fig. 1 shows a typical orbit with the alignment of sensors normal to the Earth. The satellite itself (Fig. 2) is constructed as an 18-sided polyhedron, 57 cm high and 107 cm in diameter. The telecommand and transmitter antennae are located at the bottom and top of the structure. Aligned along the sides will be the television and infra-red sensors. All other sub-systems are contained internally and therefore cannot be seen in this sketch. — 467 — Rep. 395 2.1 Orbit and coverage The spacecraft will be placed in a 1330 km altitude sub-synchronous polar orbit. Every point on the surface of the Earth will be observed at least twice a day by each satellite, once in daylight and once at night. This is a significant advance over the present t i r o s coverage. Reception of telemetering and issuing of telecommands will be performed from Com mand and Data Acquisition (CDA) stations located at Fairbanks, Alaska, and Wallops Station, Virginia. Although one orbit out of the 14 daily passes will not be interrogated each day by these stations, on-board storage will ensure acquisition of data acquired by the satellite on all orbital passes. 2.2 Launch programme Current plans call for the launch of two satellites approximately 6 months apart. These two will be for research and development purposes and will also provide the initial operational spacecraft for the t o s programme. The first of this group of two was launched 22 January, 1965. It is expected that the operational satellites will be launched at the rate of 4 per year, with the first launch scheduled for early 1966. This plan will provide two operating t i r o s spacecraft in orbit most of the time. The useful life of each spacecraft is expected to be an average of 6 months. 2.3 The basic spacecraft system The t o s spacecraft will be able to support any one of several television and sensor con figurations, thereby providing flexibility as an operational tool. The control and stabilization system will provide for spin-stabilization through magnetic torquing. The camera systems will be controlled through triggering systems, which will activate the cameras when the plane of the spin axis and the optical axis of the cameras con tains the local vertical. This trigger will be a horizon sensor which will be geometrically displaced, so that the camera triggering occurs on the earth-sky transition of the sensor. A common power supply will deliver regulated voltage (—24V) ± 2% and will be able to supply 350 mA continuously. 2.4 Tracking The t o s system will utilize redundant 250 mW FM /FM beacon transmitters in the 136-137 MHz frequency range. The beacon will be employed for transmitting continuously two channels of attitude information and, on command, one channel of telemetry data. Provision has been made to turn the beacon off after 18 months of operation. 2.5 Maintenance telemetering Telemetry sampling on the t o s is accomplished through a redundant 90 point commutator at a 10 point per second rate. Timing for the sampling rate is determined by a tuning fork. The output signal frequency modulates one sub-carrier of the 136 MHz frequency-modulation beacon transmitter. Transmission of the telemetry information will occur on command from the CDA station. 2.6 Telemetering antenna The beacon antenna is circularly polarized and located on one side of the spacecraft. This antenna provides a nominal antenna gain of -5-0 dB. 2.7 Telecommand Digital tone frequency shift keyed commands will be used to issue instructions to the spacecraft for activating or deactivating the various sub-systems. An internal logic system can be loaded from the CDA stations, and thereby used to command on and off the television sub-systems. This programming permits the systems to be turned on and off at the proper time to give optimum coverage and will be used to conserve the useful life of the television sub-systems. The telecommand antenna is a whip antenna with a nominal -5-0 dB located on the side of the spacecraft opposite that of the beacon antenna. Rep. 395 — 468 — 2.8 Television subsystem The t i r o s operational satellites will be in two configurations, one using two 800-line advanced vidicon cameras (for redundancy) with 108° lenses. These will provide pictures of about 3200 km on a side. The other configuration will be a redundant APT camera system, using two 800-line storage vidicons with 108° lenses. These will also provide pictures of about 3200 km on a side. Only one camera will be in operation at a time. 2.8.1 AVCS (Advanced vidicon camera system) The AVCS camera will use a 2-5 cm (1 in.) vidicon with a resolution of 800 tele vision lines. After a 1 - 5 ms exposure time, the information will be read out electronically in 6-5 s per frame and stored on a tape recorder for playback upon command from either CDA station. A maximum of 48 pictures may be stored on each tape recorder, which provides the ability to store up to three orbits. A second mode of operation will permit direct read-out of pictures when the spacecraft is within range of a CDA station. Each camera has a 96 kHz local oscillator, which is frequency modulated (72- 120 kHz) and recorded on its assigned track of the tape recorders. A second tape recorder is provided for redundancy. The third track of the recorders is used for flutter and wow information; at present the fourth track is not being used. A circularly polarized set of antennae, for transmission of the AVCS signal, is mounted on the side of the spacecraft opposite the whip antenna. The frequency-modulation transmitter develops a peak deviation of 125 kHz, presently operating at 235 MHz. The system will playback the AVCS information (48 pictures) in approximately 5 min. This allows all picture information to be transmitted during CDA station acquisition time. 2.8.2 A P T (Automatic picture transmission) The APT camera in the t o s system is the same basic system as used in the tiros viii spacecraft and described in NASA Technical Note D-1915. In the APT t o s , a whip antenna on one side of the spacecraft is used with APT video transmitter. The beacon and command antennae are circularly polarized and are located on the side of the spacecraft opposite that of the whip antenna. 2.9 Infra-red (IR) subsystem The t o s has provisions to accommodate a heat balance infra-red sub-system, by providing space, power and suitable thermal environment. Data transmission will be provided through the existing video or beacon transmitter on a time sharing basis. 3. The NIMBUS satellite Weather observation from a spacecraft is best performed from an attitude-controlled platform. Television cameras and radiation sensors can then be arranged on a side which constantly faces the earth, enhancing the possibility of continuous global coverage. N im b u s carries a stabilization and attitude-control system which orientate the axis of symmetry of the vehicle towards the centre of the Earth. At the top are the telecommand antenna and the stabili zation and control system container which will power the entire spacecraft. Connected through the triangular truss is a toroidal container for the electrical and mechanical instru mentation, and the telemetering and telecommand components. Cameras and infra-red scanners can be seen at the bottom. — 469 — Rep. 395 3.1 Orbit and coverage Complexity in the stabilization system is greatly reduced if the satellite is launched at an inclination of 80° into a retrograde orbit. At this angle of inclination, the drift of the orbital plane around the Earth will have the correct magnitude and direction to compensate for the orbital movement of the Earth around the Sun, thereby keeping the earth-sun line always in the orbital plane. A rotation motion of the solar power collectors keep them at a right angle with the sun rays, giving an optimum input of solar energy to the collectors. Reception of telemetering and issuing of telecommands will be performed from a number of receiving sites, the earliest one being located at Fairbanks, Alaska. Approximately ten out of the 14 orbits can be received by this station. A second command and data acquisition site is located near Rosman, North Carolina and used on a research basis. This site can be used to acquire approximately four of the n im b u s orbits per day, two of which cannot be acquired at the Fairbanks CDA station. Engineering considerations, in the choice of television camera angles and the desired complete picture coverage, suggest the launching of the space vehicles into a circular orbit at an altitude of 800 to 1200 km. Simple geometrical relationships and a maximum sun angle with respect to the surface of the Earth are obtained if the launch time is chosen to be local midnight or noon. This is because the solar collector plane is always normal to the orbital plane ; thus rotation, once per orbit, is only necessary around an axis normal to the velocity vector. The need may arise for one or more of the n im b u s satellites to be launched in other than a retrograde, local noon-midnight orbit. Different arrangements in the control system can be made to permit operation for such cases, satisfying the need for faster data coverage or different sun angles. 3.2 Launch programme The first n im b u s was successfully launched in August, 1964. A second launch, with a spacecraft differing from the first primarily in the sensory systems, was made at the beginning of 1966. A system with significant change and improvements in sensors, stabilization and power systems, but with only minor changes in the telecommand and telemetering system, is scheduled to be launched in 1967. 3.3 The basic spacecraft system Fundamental service-type sub-systems are provided to promote flexibility and early employment of the n im b u s spacecraft as an operational tool. Sensory sub-systems are inde pendent of each other but use the basic spacecraft components. A common power supply delivers regulated voltage (—24-5V) and is able to supply over 200 W continuously. A series of frequencies derived from a common 800 kHz source is provided for use through the spacecraft along with an unambiguous time code with a one second resolution. The stability is chosen so that only a few seconds discrepancy will be found after 6 months. In addition, the clock can be reset by telecommand from ground stations. Accurate time is important, because it serves as a relative reference for the sensory sub-systems and allows geographic correlation of the satellite when used in conjunction with the satellite ephemeris. 3.3.1 Tracking Tracking systems used by the U.S. require a stable low-power beacon with low sideband interference close to the carrier. N im b u s, like all U.S. research satellites, uses a frequency in the 136 to 137 MHz band. In n im b u s, a beacon transmitter develops 300 mW of power for this purpose. This transmitter will be employed also for trans mission of time signals and for one type of various telemetering channels. The coded clock signal modulates the amplitude of a 10 kHz coherent sub-carrier oscillator 50% Rep. 395 — 470 — and it then amplitude modulates the beacon 80% (non-coherent beacon frequency). Provision has been made to turn the beacon off when the spacecraft has ceased to function or after one year. 3.3.2 Maintenance telemetering Large and complex spacecraft require monitoring of the performance of all sub systems to assess the status of their operation. This operation has been called main tenance telemetry. For reasons of accuracy, flexibility, and ease of automatic data handling, pulse code modulation (PCM) is used on n im b u s as on most other major spacecraft. Two systems are provided; one records and stores information over the entire orbit, transmitting it to earth stations upon command; the other provides only instant aneous data upon command. A seven-bit code and a maximum frame rate of one frame per second were chosen. In the recorded telemeter information, a frame consists of 64 words and each word of seven-bits plus a word-sync. bit. The sync, word is all “ones” and the word-sync. is a “zero”. The first 32 words are not sub-commutated; words 33 to 48 are sub-commutated into 16 columns, so that 256 channels are available at a data rate of one per 16 s. The remaining 16 channels of the first row are sub commutated into 16 columns also having a sampling rate of one per 16 s. Arrangement was also included for some further sub-commutation for the latter 16 channels. Thus, a total of 544 channels is available with the facility for some extension. Two of the channels are required for frame- and sub-commutation synchronization. A bit rate of 500 Hz was chosen and is supplied by the master clock. A coherent 500 Hz sub-carrier is modulated by the coder output and recorded on an endless loop recorder. Upon telecommand for playback, the data are received thirty times faster than they were recorded. The 500 Hz sub-carrier is converted to 15 kHz, covering a spectral bandwidth from very low-frequency components up to 30 kHz and the informa tion from one orbital period is compressed into approximately four minutes. This output signal modulates the amplitude of the tracking beacon (PCM /AM /AM). Only one signal is applied at a time, i.e., the time-code signal is removed when telemetering is performed. Many data points are required only once per orbit and at an arbitrary time. Such data are transmitted through the second telemeter which transmits three sync, words and 125 channels in sequence. Again a seven-bit code is used with a word-sync. bit. The bit rate, derived from the master clock, is 10 Hz, so that the entire pulse train is trans mitted in 104-8 s. It modulates a 5000 Hz coherent sub-carrier in its phase 180° (phase- shift keying), which in turn is transmitted through the beacon by 80% amplitude modula tion (PCM/FSK/AM). 3.3.3 Telemetering antenna and range Because of the importance of analysing the performance of the spacecraft, partic ularly if malfunctions of the stabilization system occur, a nearly isotropic radiation pattern of the antenna is desired. Because of the size of the spacecraft, this is difficult .to achieve ; under normal conditions, a stabilized spacecraft will emit a fairly uniform field-strength towards all possible receiving directions. Range calculations for orbits — 471 — Rep. 395 from 800 to 1200 km reveal, that a high-gain receiving antenna (26 dB or better), and low-noise receiving equipment, yield an adequate signal-to-noise ratio for both tracking and telemetering. Because of the use of low power transmitters and narrow-beam directional receiving antennae, interference caused by the satellite to other terrestrial communication installations is negligible, and the possibility of interference from terrestrial communication systems with the receiving site is localized to the immediate vicinity of the directional receiving antenna. 3.4 Telecommand Instructions must be issued to the spacecraft to activate or deactivate sub-systems for data readout or to analyse the satellite trouble, in case of malfunctions. A narrow-band channel in the VHF region is employed to give telecommands using PCM as the modulation technique. To offer security against powerful terrestrial stations, the code contains an address which is checked in the spacecraft and upon verification the “execute” telecommand is trans mitted. 3.5 Television sub-system The combination of the near-polar orbit and earth stabilization provides ideal conditions for the television camera system. This system consists of three cameras ; one faces straight down and the other two are angled at 35° to the vertical in the plane normal to the velocity vector. Thus, a three-array picture, covering 2600 by 800 km, is provided when the spacecraft is at approximately 1100 km altitude (Fig. 3). Successive frames overlap by roughly 10% and are spaced at time intervals of 108 s. Complete daylight orbital coverage is obtained with 32 picture frames. An orbit will be displaced about 25° in longitude westward at the equator from the one that precedes it, and will provide a picture adjacent to the preceding one. Overlap increases with distance from the equator. The three cameras use one inch vidicons with 800 lines resolution, which are driven by a common timer providing the picture sequence. Vidicons are exposed for 40 ms and the entire frame signal is scanned by their electron beams for 6-5 s. Separate local oscillators for each camera are frequency-modulated and recorded on separate tracks of a double-reel recorder. A further track records a continuous timing signal so that picture exposure-time can be identified. Sufficient tape is provided for recording 64 pictures (two orbits), which can be played back in 10 min. By elemental analysis, one finds that a 60 kHz video bandwidth is needed, which deviates the 96 kHz local oscillators by ± 24 kHz. As is well known, narrow-band frequency-modulation is particularly suitable for tape recording. A directional antenna was chosen to cover approximately the terrestrial viewing angle underneath the satellite (1 2 2 °), and to accommodate the longest possible trans mission time. Upon earth-station command, the tape recorder plays back, and local oscillators convert the four sub-carrier modulated signals to other bands, thus forming a frequency multiplexing spectrum. The composite signal modulates the 1707-5 MHz, 5 W transmitter ±1-5 MHz. The centre frequency is held stable to 10 parts in the 10-6 (FM/FM). A multiplexing schedule is shown in Table I. Rep. 395 — 472 — T able I Multiplexing schedule for the NIMBUS spacecraft Television video (k H z )...... 0-60 Television frequency-modulation sub-carrier output (each) (k H z)...... 96 ± 24 Television playback camera 1 ( k H z ) ...... 235-329 Television playback camera 2 ( k H z ) ...... 400-494 Television playback camera 3 ( k H z ) ...... 565-659 Television reference-time ( k H z ) ...... 200 ± 0*2 Infra-red scanner video (H z)...... 0-300 Scanner FM sub-carrier output (kHz)...... 7-5-10 Playback scanner Channel 1 (k H z)...... 137-5 ± 22 Playback scanner Channel 2 (k H z)...... 65-5 ± 22 Scanner reference-time Channel 1 ( H z ) ...... 27-5 ± 800 Scanner reference-time Channel 2 ( H z ) ...... 14-5 ± 800 Television transmitter (frequency-modulation) (M H z)...... 1707-5 ±1-5 3.6 High resolution infra-red scanner Television will show the complete meteorological cloud-cover over the light side of the globe, measured at local noon. This will not be true over the midnight half of the orbit, because the vidicons lack sufficient sensitivity to televise under star illumination. Terrestrial thermal radiation permits another mechanism to be used for night-time cloud picture genera tion. Analysis of the physical phenomenon shows that the use of a lead selenide detector in the 4 p window yields an acceptable signal-to-noise ratio. In contrast to television, infra-red detectors form no image ; the detector only integrates the energy received from the target. Composition of a picture is achieved by circular scanning with a mirror. The optical axis prescribes a plane that is normal to the instantaneous velocity vector of the satellite. To obtain a contiguous picture, the satellite must advance the width of one picture element during the time it takes the mirror to scan one revolution. The optical angle is thus determined by this method of scan. The n im b u s high-resolution infra-red radiometer (HRIR), is designed for viewing angles of 8-4x 10~3 radians and a 260 Hz video bandwidth. It scans at 0-67 r.p.s., yielding 230 ele ments over the 1 2 2 ° from horizon to horizon, thus achieving a linear resolution of about 8 km. The 260 Hz video bandwidth of the HRIR output modulates the frequency of a 10 kHz voltage-controlled oscillator by 2-5 kHz. The sky level corresponds to 10 kHz and the hottest signal deviates to 7-5 kHz. A tape recorder, almost identical in design to the television recorder, records the signal at 3-75 Hz. A four-track head similar to the television camera recorder is used. One track receives the radiometer signal; another records the 10 kHz timing signal from the master clock. When one tape reel is fully unwound, the movement is reversed and the signals are switched to the remaining two tracks. The recorder continues to record until the reel is empty again and then is stopped by a limit switch. For transmission, the tape speed is increased eight-fold and simultaneously all tracks are applied to the four heads. Local oscillators and mixers generate a frequency-multiplex signal, which is added as part of the composite television signal and transmitted through the same transmitter (FM/FM). 3.7 Television and infra-red telemetering Bandwidth economy is very important in the transmission of video signals, because of the large quantity of data. A good signal-to-noise ratio must be available at the receiver, making the use of techniques offering modulation improvement mandatory. Some of these techniques demand bandwidths many times as large as the basic information bandwidth, making their application less attractive from the point of frequency economy. Particularly, — 473 — Rep. 395 PCM designed for n im b u s television may place excessive bandwidth demands on the available space-research telemetering bands. Those considerations make the use of narrow-band FM/FM techniques very attractive, from the standpoint of both spectrum economy and modulation improvement. The choice of the antenna pattern and gain for the spacecraft is guided by the desire for technological simplicity and a long read-out time for a receiving site. Because of the complexity and proneness to failure of antenna pointing mechanisms in spacecraft, the advantages in gain to be obtained by using them do not justify their use. Fixed directional antennae on the spacecraft, which avoid this complication, offer an inverse relationship between gain and available transmission time from the vehicle, i.e., the narrower the antenna beam, the higher the gain, but the shorter the time the receiving antenna beam intercepts the transmitting antenna beam. Since a fixed amount of information is gathered during a satellite orbit, shorter transmitting times demand proportionally more bandwidth. It is useful to consider the relationship between the antenna beamwidth on the spacecraft, the transmission time available during a satellite pass through the zenith, and the maximum communications distance (Fig. 4). By making use of known relationships, antenna beamwidth and communication distances, the transmitter power can be derived. Inspection of the curves in Fig. 4 shows, that the penalty to be paid in transmitter power by using a wider beamwidth rather than q narrow beamwidth is small; longer transmission time is available, but longer communication range is to be covered. This set of conditions has been applied for the n im b u s television telemetering. The disadvantage of somewhat higher transmitter power is outweighed by the simplicity inherent in narrow-band recorders and electronic equipment. 3.8 Low data-rate experiments A stabilized platform is a convenient vehicle for the conduction of scientific geophysical experiments. Some of these experiments produce only low data-rates and can use the 300 mW PCM telemetering. Other experiments, which require higher data-rates, make use of a 2 W FM telemetering channel in the 136 to 137 MHz space research band. This transmitter is equipped with a broad-beam directional antenna. N im b u s satellites may carry an instrument designed to measure the reflected solar and terrestrial radiation in five optical bands. Five channels have a video output of 0 to 8 Hz, are digitized with a 7-bit precision and recorded on a digital recorder, which will be played back at a higher speed when the spacecraft passes over a CDA station. The data are transmitted as PCM/FM, with a direct modulation of the carrier at a bit rate of 6 6 0 0 0 bits per second. 3.9 The automatic picture transmission system (APT) This system makes available to users throughout the world, pictures of the cloud cover 1 in their vicinity using a moderately priced, receiving only, earth station. The camera uses a 108° lens which, with a 1330 km altitude circular orbit, provides a picture about 3200 km on a side. The system is designed for automatic and continuous operation during the daylight portion of the orbit. The camera uses an electrostatic storage vidicon, which is exposed for 40 ms and read-out during the succeeding 200 s. The 0-25 s scanning time per line is compat ible with standard 240 r.p.m. facsimile equipment, which can be used for ground display. Full compatibility is achieved by amplitude-modulation of a 2400 Hz sub-carrier and by transmission of turn-on and phasing signals during the eight seconds preceding actual picture transmission. The sub-carrier frequency modulates a 5 W transmitter in the 136-137 MHz 19 Rep. 395 — 474 — band, deviating it by ±10 kHz, so that receivers with intermediate-frequency bandwidths of 30 kHz may be used. The spacecraft transmitting antenna has a maximum gain of unity and is designed to be primarily right-handed circularly polarized. Both the spacecraft and the earth systems are simple. Neither command links nor storage are required. An antenna with a gain of at least 10 dB, a commercially available receiver and facsimile recorder are the only equipment required for an APT earth receiving station BIBLIOGRAPHY 1. J o h n so n , D. S. and M o o k , C. P. A proposed weather radar and beacon system for use with meteoro logical earth satellites. Proceedings o f the 3rd National Convention on Military Electronics IRE, 206 (June, 1959). 2. B a t t a n , L. J. Radar meteorology. University of Chicago Press, Chicago, Illinois (1959). 3. G r a n t , C. R. and Y a plee, B. S. Back-scattering from water and land at centimetre and millimetre wavelengths. Proc. IRE, Vol. 45, 7, 976-982 (June, 1959). 4. R y d e , J. W. The attenuation and radar echoes produced at centimetre wavelengths by various meteorological phenomena. Meteorological factors in radio wave propagation, 169-188, London : Physical Society (1946). 5. G u n n , K. L. S. and E a st, T. W. R. The microwave properties of precipitation particles. Quarterly Journal o f the Royal Meteorology Society, Vol. 80, 522-45 (June, 1954). 6 . A tl a s, D. and M ossop, S. C. Calibration of a weather radar using a standard target. Bulletin of the American Meteorological Society, Vol. 41, 7, 337-382 (June, 1960). 7. B yers, H. R. and B r a ha m , R. R. The Thunderstorm. U.S. Weather Bureau, Washington, D.C. (1949). 8 . Sh e r h a g , R. Stratospheric temperature changes and the associated changes in pressure distribution. J. Meteorol., Vol. 17, 6 , 575-582 (December, 1960). 9. S a ie d y , F. Nature and properties of the atmospheric continuum (in preparation). 10. M a l a n , D. J. Radiation of lightning discharges and its relation on the discharge process. Recent advances in Atmospheric Electricity, 557, Pergamon Press. 11. H e w il l , Dr. F. J. Radar echoes inter-stroke processes in lightning. Proc. Phys. Soc., Vol. 70. Part 10, 691 (October, 1957). 12. A tl a s, D. Radar and a Sferics detector. Proceedings of the 7th Weather Radar Conference, Miami Beach, Florida (November, 1958). 13. H o l z e r , R. E. Simultaneous measurement of sferics signals and thunderstorms activity. Thunder storm Electricity, 267, University of Chicago Press. 14. St a m pfl , H a n e l . An earth satellite instrumentation for cloud measurements ; IRE National Conven tion Record, Part 5 (1958). 15. B a n d een , H a n el, L ig h t , Sta m pfl and St r o u d . Infra-red and reflected solar radiation from the “Tiros II” meteorological satellite. Jour. Geophys. Res., Vol. 6 6 , 10, 3169-3185 (October, 1961). 16. S t a m p f l . The Nimbus satellite and its communication system as of September 1961. NASA Technical Note, D1422 {January, 1963). 17. St a m p f l . The Nimbus spacecraft system Press. Presented at American Meteorological Society in New York (January 22-26, 1962). 18. St a m p f l . The automatic picture transmission (APT) TV camera system for meteorological satellites. NASA Technical Note D-1915 (November, 1963). — 475 - Rep. 395 North Pole Orbit of satellite F ig u r e 1 Orientation of TIROS wheel Rep. 395 — 476 — Axis of spin Orbit Equator (a) Path of orbit on the Earth Equator (b) Earth-oriented picture F i g u r e 2 TIROS wheel satellite, the basic spacecraft for TOS — All — Rep. 395 Rotation of the earth F ig u r e 3 Television coverage by the NIMBUS spacecraft Ny N2 : N im b u s spacecraft during first and second orbits respectively S2 : Sun’s rays during first and second orbits Oy 02: First and second orbits of N im b u s E : Terrestrial equator Rep. 395Rep. aiu aalbeprps (i) aiu cmuiaindsac (km) distance communication Maximum (min) pass per available Maximum Beamwidth of antenna (degrees) Beamwidth o f antenna as a function oj the telemetry time and the communication distance, communication the and time telemetry the oj a function as f antenna o Beamwidth Curve A F E C H G D B with the height o f the satellite as as parameter satellite the f o height the with 48 — 478 — F gure r u ig 16090 8045 3218 1609 965-4 804-5 312-8 160-9 4 m miles km Height of satellite satellite of Height 10 2000 5000 1000 "600 00 0 00 2 500 100 — 479 — Rep. 396 L. 6: Maintenance telemetering, tracking and telecommand REPORT 396 * MAINTENANCE TELEMETERING, TRACKING AND TELECOMMAND FOR DEVELOPMENTAL AND OPERATIONAL SATELLITES Frequency sharing between earth-satellite telemetering or telecommand links and terrestrial services (Question 4/IV) (1966) 1. Introduction 1.1 The Extraordinary Administrative Radio Conference, Geneva, 1963, has allocated certain frequency bands between 100 MHz and 10 GHz to space telemetering, tracking and telecom mand, taking account of C.C.I.R. Recommendation 363. In most cases, the bands allocated are also shared by terrestrial fixed and mobile services, and this Report considers in general terms, the interference problems that might arise. Since the potential interference is largely independent of frequency over fairly broad frequency ranges, the results have been calculated only for the representative frequencies of 400 MHz and 4 GHz. 1.2 Estimates have been made of the interference problems that can arise with co-channel working of various systems : co-channel working here implies the operation of two systems at frequencies which are near enough for the spectrum of the transmission of one system to overlap the whole or part of the receiving bandwidth of the other system. Such overlapping of frequencies must be contemplated, since there seems little likelihood of interleaving satellite services, channel by channel, with existing services. Since the magnitude of interference is usually a function of the particular relationship between the frequencies of the two systems involved, an endeavour is made in each assessment to consider the worse case. A comprehen sive treatment of the interference would require detailed statistical analyses of the percentages of time that the interference occurs. The present discussion is, however, concerned primarily with relative levels of interference in the various cases, to determine whether there is an a priori basis for sharing. 1.3 The possibilities of interference with respect to the following services and systems have been examined: — earth-satellite telemetering; — earth-satellite telecommand; — communication-satellite service; — line-of-sight radio-relay systems ; — frequency-modulation broadcasting; — television broadcasting; — land mobile services ; — aeronautical mobile service ; — tropospheric-scatter systems; — ground radar systems. * This Report was adopted unanimously. Rep. 396 — 480 — 2. The assessments involved 2.1 There are four typical modes of interference involved : 2.1.1 Interference between satellite-borne transmitters and satellite-borne receivers. 2.1.2 Interference between satellite-borne transmitters and ground receivers. 2.1.3 Interference between ground transmitters and satellite-borne receivers. 2.1.4 Interference between ground transmitters and ground receivers. Note. — In this Report, the term “ground station” refers to either terrestrial or earth stations. 2.2 Interference of the first type can be considered without the need for calculation. In any case, it would not seem wise to permit frequencies to be transmitted by satellites which are the same as those which have to be received, for other services, by other satellites. Unless specific cases only are concerned, the possibility must always be considered of the transmitting and receiving satellites reaching quite close spacings as their relative motions continue. Thus, such sharing cannot be proposed in any general allocation of bands. 2.3 Interference between satellite-borne transmitters and ground receivers or vice-versa, can be quantitatively assessed. Assumptions must, however, be made as regards frequency, trans mitter power, receiver sensitivity and susceptibility to interference, antenna gains inside and outside their beams (if beamed) and also the altitudes of the satellites involved. 3. Calculation of interference between satellite-borne transmitters and ground stations 3.1 Consider interference from a satellite transmitter to a ground receiver. If the interfering transmitter power within the input bandwidth of the receiver is PT (dBW), the transmitter antenna gain GT (dB), the free-space basic transmission loss L (dB) and the receiver antenna gain Gr (dB), then the level of the interference entering the receiver is : Pt +Gt- L + G r (dBW) The maximum permissible level of interference (Pj) within the receiver input bandwidth can be expressed as follows : Pj= X-Y where X is the minimum permissible median level (dBW) of the wanted signal at the input to the receiver which may experience interference, and Y is the required protection ratio (dB), i.e. the minimum permissible ratio of wanted- to-interfering signal powers at the receiver input and within the receiver input bandwidth. The values of Y are derived so as to give a prescribed minimum acceptable, signal-to- interference ratio at the system output. An “interference safety margin” M can now be defined by the equation : M = :P 7 - ( P r + Gr - L + Gr ) (dB) (1) L is calculated for the range over which interference takes place with the assumed satellite orbit; if the value of L is assumed to be varied until M = 0, then a minimum safe slant-range can be determined. For satellites at less range than this there will be significant interference. The above equation for interference safety margin is evidently applicable to interference from ground to satellite or vice-versa. — 481 — Rep. 396 4. Calculation of interference between ground stations 4.1 Equation (1) is valid for interference between the earth stations for satellite systems and those for terrestrial services, if L is now taken as the tropospheric basic transmission-loss. In this case, however, it is more useful to determine the safe spacing of the ground stations, for which M — 0. From (1) it follows that, if L0 is the basic transmission loss for which M = 0: L0 = Pt +Gt + Gr —Pj (dB) (2) From calculated values of L 0, safe spacings can be determined by the use of the tropospheric propagation data given in Reports 239-1 and 243. The present calculations are based on 1% propagation data, but in evolving specific sharing criteria, more detailed considerations would need to be given to the percentage time to be adopted and other factors would need to be taken into account. 4.2 In the conclusions in regard to such spacings, it has been taken that, where the safe spacing is of the order of 500 km or more, frequency sharing is not likely to be generally practicable. Where more practicable spacings are effective, these may be still further reduced if natural screening of hills etc. can be exploited : such reduction will, however, make aircraft-reflected interference relatively more im portant; for the cases where frequency sharing seems possible, the effect of aircraft-reflected interference has been estimated. 5. General parameters of the telemetering and telecommand systems involved 5.1 Since either telemetering or telecommand is common to all the assessments, much care has been taken to decide the most realistic single values possible for the various parameters of these systems ; it is inevitable, however, that some practical systems may depart appreciably from the figures given. It is assumed, that telemetering and telecommand are pulse-coded systems and that factors X and Y therefore relate to simple systems of this sort. 5.2 Two classes of telemetering system have been considered, according to the bit-rate of the system and the complication of the receiver. The first class of system is representative of relatively low-altitude satellites for developmental purposes ; the bit-rate may be a few kilobits per second, with a radiated power of the order of a watt and omnidirectional satellite antennae. It is found in these circumstances that relatively simple receiving equipment will give adequate sensitivity. It is assumed, that for a typical system of this sort, the intermediate-frequency bandwidth can be as wide as 100 kHz ; this is a compromise value between possible lower values at the lower end of the range of frequencies considered and higher values at the higher end. The second class of telemetering is representative of operational communication satellites in higher altitude orbits. Although such satellites would have earth-stabilized communica tion antennae, it is assumed that omnidirectional telemetering and telecommand antennae are a requirement arising from the need to assure these services, even if the satellite should be unstabilised. Only very low bit-rates are required in this case and an output bandwidth of about 10 Hz would be adequate. By properly taking advantage of this narrow data bandwidth in the receiving system, the extra attenuation due to the greater range of this second class of satellite can be overcome whilst still retaining modest transmitted power. To do this requires the use of coherent detection systems for bi-phase phase-modulation or feed-back systems for frequency-modulation. The intermediate-frequency bandwidth can be quite narrow, of the order of a few kHz, but its exact value does not affect the theoretical assessment of thermal Rep. 396 — 482 — noise, since in these systems, the effective input noise bandwidth has a low limit value of about twice the output bandwidth. The value of intermediate-frequency bandwidth does, however, affect the value of the protection ratio Y for noise-like interference: it has, therefore, been necessary to select an intermediate-frequency bandwidth ; 10 kHz has been taken as a reason able and practicable value. The two telemetering systems outlined above are referred to as, “wide-band” and “narrow band” systems respectively, in the assessments which follow. 5.3 For radio-frequency equipment, it is assumed that the earth-station receivers use good para metric amplifiers (not masers), and that ground antennae have apertures such as are provided by paraboloids of up to 4-5 m in diameter ; this aperture can provide the gains specified using circularly polarized feed. Similar sizes of ground antennae are assumed for telecommand. 5.4 For telecommand also, “wide-band” and “narrow-band” receivers have been considered. However, for this service, the receiver must always be simple and the difference between the two classes depends only on the data-rate assumed and the Doppler effect, according to the orbit. For wide-band receivers, the input bandwidth is taken as 100 kHz. This can accom modate considerable Doppler shift as well as a large bit-rate. The narrow-band design is only materially different for a low bit-rate receiver operating at high altitude but at low frequency. For the one case considered the input bandwidth is taken as 5 kHz. 5.5 The minimum value of wanted-signal X for telemetering has been taken as 3 dB higher than the threshold value for satisfactory operation determined solely by system noise considera tions. For telecommand, X is taken as well above the noise level of a simple receiver ; it is assumed that such receivers would employ, in effect, some artificial threshold to combat the effects of noise and interference. 5.6 Protection ratios have been estimated for the typical telemetering and telecommand receivers, in respect of typical types of interference. The resulting values discussed in Annex I differ from those which can be derived from Recommendation 364-1 and Report 219-1 applying to links for space research. 5.7 The typical characteristics chosen must not only represent important classes of system, but must also be numerically self-consistent in terms of quantities such as receiver sensitivity and actual received power under representative conditions. By such consistency, it is assured that the interference potentials in both directions are properly related, with the implication that by “trading” transmitter power for receiver sensitivity, within limits, the balance of interference in the two directions can be adjusted for the best compromise. The assumed characteristics of the various systems are given in Table IV of Annex II. 6. Conclusions 6.1 Eleven cases of frequency sharing for telemetering and eleven for telecommand have been considered. A list of these cases with a brief note on the conclusions is given below. It has been mentioned in § 2 .2 , that frequency sharing which involves the use of the same frequency for transmitting in some satellites and receiving in others, is considered as unacceptable; this applies to telemetering cases 2 and 3 and telecommand cases 2 and 4. Calculations and com ments for one important case are given in Annex III. — 483 — Rep. 396 T a b l e I Possibilities of sharing between telemetering and other systems Telemetering case No. Other system considered Possibility of frequency sharing 1 Other similar satellite telemetering Limited sharing is possible. 2 Satellite telecommand (systems as Unlikely to be practicable. Satellites of the described in this Report) two systems may approach. 3 Communication-satellite (earth-to-space Unlikely to be practicable. Satellites of the link) two systems may approach. 4 Communication-satellite (space-to-earth Limited sharing is possible. link) 5 Line-of-sight radio-relay systems Limited sharing is possible with acceptable separation distances. There is interference with line-of-sight service in some circumstances. 6 Frequency-modulation broadcasting Unlikely to be practicable. Severe interference with telemetering reception. 7 Television broadcasting Unlikely to be practicable. Severe interference with telemetering reception. 8 Land mobile services Limited sharing is possible. 9 Aeronautical mobile service Unlikely to be practicable. Severe interference with telemetering reception can be produced by aircraft emissions. 10 Tropospheric-scatter systems Unlikely to be practicable. Severe interference to the scatter service. 11 Ground radar systems Unlikely to be practicable. Severe interference to telemetering reception. T a b l e II Possibilities of sharing between telecommand and other systems Telecommand case No. Other system considered Possibility of frequency sharing 1 Other similar satellite telecommand Limited sharing is possible. 2 Satellite telemetering (systems as Unlikely to be practicable. Satellites of the described in this Report) two systems may approach. 3 Communication-satellite (earth-to-space Appropriate frequency separation will be needed link) to avoid interference to reception in the com munication satellite. 4 Communication-satellite (space-to-earth Unlikely to be practicable. Satellites of the link) two systems may approach. 5 Line-of-sight radio-relay systems Limited sharing is possible with reasonable separation distances between stations. There is interference to telecommand in some circum stances. 6 Frequency-modulation broadcasting Unlikely to be practicable. Severe interference to telecommand. 7 Television broadcasting Unlikely to be practicable. Severe interference to telecommand. 8 Land mobile services Limited sharing is possible but interference to telecommand can occur in some circumstances. 9 Aeronautical mobile service Unlikely to be practicable. Ground stations of the aeronautical service can interfere with tele command. Telecommand can produce severe interference in aircraft receivers. 10 Tropospheric-scatter systems Unlikely to be practicable. Severe interference to telecommand. 11 Ground radar systems Unlikely to be practicable. Severe interference to telecommand. Rep. 396 — 484 — ANNEX I FACTORS X AND Y FOR TELEMETERING AND TELECOMMAND RECEIVERS 1. The most likely systems to be encountered for telemetering and telecommand are simple pulse-code modulation (PCM) systems; frequency-modulation (FM) or bi-phase phase- modulation (PM) is assumed, the modulation waveform being rounded pulses. The types of interference considered are : — noise-like interference, such as may arise when a part of a wide-band transmission falls in the comparatively narrow band of the telemetering or telecommand receiver, or when many unrelated narrow-band transmissions combine to give interference of fairly uniform power spectrum; — CW-type interference, e.g. carriers and sub-carriers of other transmissions ; — interference from narrow-band AM and FM transmissions. 2. Minimum permissible signal at receiver input, X 2.1 In all PCM systems the decision circuit produces a threshold, above which increasing noise inputs rapidly give intolerable noise or error rate. This threshold corresponds to a signal- to-noise ratio in the input to the decision circuit of about 12 dB. (In an FM system there will be a similar threshold at the input to the discriminator.) For telemetering, the minimum permissible level of wanted signal at the receiver input X (dBW) is here taken as 15 dB above the thermal noise, to allow for the presence of interference in addition to thermal noise. 2.2 For telecommand, the factor X usually includes a greater margin over noise and interference than for telemetering. This follows from the greater importance usually attached to reliability of telecommand as compared to telemetering and also the greater possibility of providing increased transmitter powers in the case of telecommand. Thus, the philosophy in telecommand is to use receivers which are intentionally less sensitive than signal-to-noise design would dictate : in effect, an artificial threshold is produced somewhere in the receiving chain, which eliminates the effects of wanted signals or interference below a prescribed level. The level at which such a threshold is set is arbitrary, but a typical procedure would be to add an allowance of 6 to 12 dB to the minimum signal, which would be considered appropriate to telemetering. Thus, the value of X for telecommand would be about 15 + 6 to 15 + 12 dB above thermal noise. In this Report, the mean of these values is taken so that the minimum permissible signal level X for telecommand is 24 dB above the thermal noise level. 3. Protection ratio, Y 3.1 A PCM system will fail in the presence of noise-like interference when the ratio of signal to thermal noise plus noise-like interference reaches the threshold value of 12 dB. As thermal noise has been set at 15 dB below signal (see § 2.1) it follows from this limiting condition, that the ratio of permissible signal-to-noise-like interference is also 15 dB. Coherent systems for bi-phase PM or frequency-modulation feedback (FMFB) systems cause this threshold to be effective, in the limit, only over a radio-frequency bandwidth equal to twice the post-detector bandwidth. Thus, in the narrow-band telemetering system employing the above techniques, as the output bandwidth is 10 Hz, the 15 dB criterion has only to apply to any 20 Hz of the input bandwidth, assuming that the major part of the wanted signal power is in its carrier. — 485 — Rep. 396 If the input bandwidth is, say, 10 kHz, then the noise in it will be 27 dB greater than that which provides threshold. The value of Y for noise-like interference is therefore given by 15—27 — -12 dB. 3.2 CW, narrow-band FM or narrow-band AM interference, with the receivers considered, can be estimated by considering the interference as a vector which combines with the wanted- signal vector to phase- or frequency-modulate it. Any wanted modulation of the signal vector is completely distorted when the interference vector is about the same magnitude ; any further increase of interference then “captures” the receiver. For the telemetering cases for which the simultaneous effects of noise and CW-type interference on the decision circuit have been estimated, the value of the protection ratio Y has been found to be about 14 dB. For the narrow-band receiver, this form of interference is confined to that in a very narrow frequency band near the wanted carrier; CW-type interference outside these limits, but within the intermediate-frequency band, can only interfere by capture effects, for which the factor Y can be assumed to lie between 0 dB and the above value of 14 dB. For narrow-band AM interference, allowance must be made for the fact that the critical level may be reached on the peaks of the modulated wave. Values of Y for AM interference are therefore taken as 6 dB greater than those for CW or narrow-band FM interference. The various values of the protection ratio Y are collected in Table III. T a b l e III Protection ratio Y (dB) Type of interference Telemetering Telecommand Wide-band or narrow-band Wide-band Narrow-band N oise-lik e...... 15 - 1 2 15 CW-type or narrow-band FM . . 14 14 14 Narrow-band A M ...... 20 20 20 3.3 The protection ratios for telemetering tabulated above are different from the values which can be derived from Recommandation 364-1 and Report 219-1. For noise-like interference in a wide-band receiver, it will be noted that the permissible interference level quoted, —15 dB relative to the signal level, is equal to the noise value in the .whole input band not — 6 dB relative to the thermal-noise level as advocated in Recommandation 364-1. The difference arises because the designs given here specifically allow increased transmitter power to enable the assumed signal-to-interference and thermal-noise ratio to be achieved (see § 2 .1). Rep. 396 — 486 — For CW-type interference, the protection ratio is 14 dB for the typical PCM system that has been evaluated, so that the permitted CW interference is + 1 dB relative to the input noise which is effective in producing output noise (this effective noise is that of the full input band width for the wide-band receiver, but only that of 20 Hz of the input bandwidth for this particular narrow-band design described). ANNEX II TELEMETERING AND TELECOMMAND SYSTEMS, DESIGN VALUES ASSUMED The following Table gives summarized calculations for the various telemetering and telecom mand systems considered. The aim is to show, for each case, a set of compatible characteristics with, however, a general safety factor included. It is considered that a target value for this factor of safety should be 3 dB for telemetering and 10 dB for telecommand. The higher value for tele command is because of the greater importance attached to the achievement of maximum reliability in this service. Calculations have been made, as appropriate, at one or both of two typical fre quencies, which have been taken as 400 MHz and 4 GHz respectively. T a b l e IV Characteristics o f telemetering and telecommand systems Receiving system Transmitting system Maximum Sys Bandwidth Attenuation L System Altitude slant- Frequency tem (Hz) Antenna Radiated Antenna (dB) Received power safety Service Factor factor (km) range (GHz) WB X gain power gain p r = p t + g t G r Max. Gr -L (dB) (2) (km) or Post p T G t + NB Pre (dBW) (dB) (dBW) (dB) Zenith slant- (dBW) o detector detector range Up to the -1 3 7 20 0 0 139 500 2600 0-4 WB 105 order of 153 -1 3 3 + 4 105 (3) (4) (5) Tele 000 0 4 NB -1 7 4 164 10 15 000 104 10 20 -1 0 0 metering 168 -1 5 8 + 16 (6) NB -1 7 7 40 184 4 104 10 -1 0 0 (4) 188 -1 5 8 + 19 (6) Up to the 139 500 2600 0-4 WB 105 order of -122 0 20 20 105 (5) (4) 153 -1 1 3 + 9 Up to the Tele 10 000 15 000 0-4 NB 5x10® order of -1 3 5 0 23 20 164 command 5 x10s 168 -1 2 5 + 10 Up to the 37 40 184 4 WB 105 order of -120 0 105 (4) 188 -111 + 9 (0 The systems are classed as wide-band (WB) or narrow-band (NB) as explained (4) Antenna gains of 20 dB at 400 MHz and 40 dB at 4 GHz have been assumed. in §§ 5.2 and 5.4 of the main Report. These correspond to a paraboloid of 4-5 m diameter with circularly polarized (2) Safety factors are estimated for maximum slant-range. feed. The beamwidth is 12° at 400 MHz and 1-2° at 4 GHz. (3) In estimating factor X for telemetering a total input temperature of 460° K (5) It is assumed that the satellite antennae are omnidirectional. is assumed at 400 MHz, of which 280°K is due to galactic noise ; at 4 GHz, (6) These safety factors are unnecessarily great although PT has been reduced to the total temperature has been taken as 220°K of which 40°K is galactic. a practical low limit. However, for a synchronous satellite, the attenuation Factor X is then taken as 15 dB above the noise value produced, at the for maximum slant-range is 9 dB.greater so that factor X is more nearly of appropriate temperature, in the radio-frequency band which is effective for the order required. Therefore, the low values of factor X are retained. How noise calculations (only 20 Hz for the narrow band case). ever, for satellites at 10 000 km altitude it would seem reasonable to permit For telecommand, a noise factor of 8 dB has been assumed at 400 MHz interference inputs say 10 dB greater than would follow from these values of and 10 dB at 4 GHz. As discussed in § 2 of Annex I, the minimum per factor X : this point is given attention in particular cases in the Tables which missible signal levels have been taken as 24 dB above these noise values. follow. Rep. 396 — 488 — ANNEX III TYPICAL INTERFERENCE CALCULATIONS AND DISCUSSION All the cases referred to in Table I have been calculated. For brevity in this Report, only the tabulated calculations and the discussion for the case of interference with respect to the land- mobile service (case 8 ), are given in this Annex. The following abbreviations have been used in these Tables: TM Telemetering TC Telecommand TX Transmitter RX Receiver NB Narrow-band WB Wide-band of S Line-of-sight A/C Aircraft T a b l e V Telemetering — Case 8 Interference between telemetering and land-mobile service Y X Permitted pt gt Gr L Interference Interference level of level margin Case Frequency of safety No. Type of interference Altitude (GHz) interference produced pi p t + G t + g r —l M (dB) (dB) (dB) (dB) (dB) (dB) (dB) (dB) (dB) AM land-mobile service TM transmitter Satellite to receiver of at zenith 14 -116 -130 0 0 - 3 139 -142 + 12 mobile service Low 0-4 Satellite at horizon 14 -116 -130 0 0 + 2 153 -151 + 21 Satellite at zenith 14 -116 -130 - 1 0 0 - 3 164 - 1 1 1 + 47 High 0-4 Satellite at horizon 14 -116 -130 - 1 0 0 + 2 168 -176 + 46 TM 8(a) FM land-mobile service TM transmitter Satellite to receiver of at zenith 10 -130 -140 0 0 - 3 139 -142 + 2 . mobile service Low 0-4 * 396 Rep. Satellite at horizon 10 -1 3 0 -140 0 0 + 2 153 -151 + 11 e. 9 — 490 — 396Rep. T a b l e V (Continued) Telemetering — Case 8 ( Continued) Interference between telemetering and lend-mobile service Pj. Safe spacing r X g t Gr Required (km) Case Frequency Lo No. Type of interference (GHz) (dB) (dB) (dB) (dB) (dB) (dB) Overland Oversea AM land-mobile service Mobile ground In TM beam 20 -137 20 (0 + 2 20 199 420 500 transmitter to TM receiver (WB) 0-4 Out of TM beam 20 -137 20 (0 + 2 0 179 250 500 Mobile ground In TM beam 20 -174 20 (0 + 2 20 236 > 500 > 500 transmitter to TM receiver (NB) 0-4 Out of TM beam 20 -174 20 0 ) + 216 > 500 > 500 TM 2 0 8 (b) FM land-mobile set vice Mobile ground In TM beam 14 -137 + 20 + 2 20 193 360 >500 transmitter to TM receiver (WB) 0-4 Out of TM beam 14 -137 + 20 + 2 0 173 200 > 500 Mobile ground In TM beam 14 -174 + 20 + 2 20 230 > 500 > 500 transmitter to TM receiver (NB) 0-4 Out of TM beam 14 -174 + 20 + 2 20 210 , > 500 > 500 0) 20 dBW is taken as the possible maximum value for the base station of a mobile service. See following page for Notes. — 491 — Rep. 396 Note 1. — The values shown in the Table are those for mobile or base stations of the land-mobile service. Note 2. — The interference from the telemetering transmitter of a low- or high-altitude satellite when above the horizon would be acceptable in AM and FM mobile services. The interference produced in single-channel FM fixed services would also be acceptable. Note 3. — Interference with a wide-band telemetering receiver, out of its beam, would be accept able if the receiver were spaced (overland) at 200 km from an FM mobile or base transmitter or 250 km from an AM transmitter. For a narrow-band telemetering receiver, these spacings would become greater than 500 km. FM fixed service transmitters would give the same order of inter ference as the mobiles services. Note 4. — Interference with a wide-band telemetering receiver, in the beam, would be acceptable if the spacings were 360 km (FM) and 420 km (AM). The corresponding spacing for an FM fixed service transmitter is of the same order. With a narrow-band telemetering receiver, the spacings to permit tolerable in-beam interference, would be impracticably great. Note 5. — For interference to narrow-band telemetering receivers, except where the satellite is synchronous, some 10 dB increase of interference input could be permitted with consequent decrease of safe spacings. Note 6. — In any specific case of interference from mobile or fixed services, the possibility of simultaneous interference from a number of transmitters must be estimated. Note 7. — Aircraft would not reflect significant interference in any of the cases. e. 9 — 492 — 396Rep. T a ble V I Telecommand — Case 8 Interference between telecommand and land-mobile service Safe spacing Y X p T g t Gr Required (km) L o Frequency Case Type of interference No. (GHz) Overland Oversea (dB) (dB) (dB) (dB) (dB) (dB) AM land-mobile service TC transmitter In TC beam 14 -116 23 20 2 175 230 > 500 to receiver of mobile service 0-4 Out of TC beam 14 -116 23 0 2 155 100 > 500 TC 8 (a) FM land-mobile service TC transmitter In TC beam 10 -130 23 20 2 185 280 >500 to receiver of mobile service 0-4 Out of TC beam 10 -130 23 o 2 165 120 > 500 T a ble VI (Continued) Telecommand Case 8 ( Continued) Y X Permitted p t g t Gr L Interference Interference Case Frequency level of level margin No. Type of interference Altitude (GHz) interference produced of safety P i P T + G t + Gr -L M (dB) (dB) (dB) (dB) (dB) (dB) (dB) (dB) (dB) AM land-mobile service Mobile or base Satellite transmitter to at zenith 20 - 1 2 2 -142 20 0 ) - 3 0 139 - 1 2 2 - 2 0 TC receiver (WB) Low 0-4 in satellite Satellite at horizon 20 - 1 2 2 -142 20 (0 + 2 0 153 -131 - 1 1 Mobile or base Satellite 396 Rep. —493 transmitter to at zenith 20 -135 20 (l) - 3 0 164 TC receiver (NB) High 0-4 -155 -147 - 8 in satellite Satellite at horizon 20 -135 -155 20 0 ) + 2 0 168 -146 - 9 TC 8 ( b ) FM land-mobile service Mobile or base Satellite transmitter to at zenith 14 - 1 2 2 -136 20 - 3 0 139 - 1 2 2 -1 4 TC receiver (WB) Low 0-4 in satellite Satellite at horizon 14 - 1 2 2 -136 20 2 0 153 -131 - 5 Mobile or base Satellite transmitter to at zenith 14 -135 -149 20 - 3 0 164 -147 - 2 TC receiver (NB) High 0-4 in satellite Satellite at horizon 14 -135 -149 20 2 0 168 -146 - 3 (0 20 dBW is taken as the possible value for the base station of a mobile service. See following page for Notes. Rep. 396 — 494 — Note 1. — The values shown in the Table are those for mobile or base stations of the land-mobile service. Note 2. — Interference from the telecommand transmitter, out of its beam, would be tolerable if the mobile or base station receiver were spaced from the telecommand transmitter by an overland distance of 120 km for FM mobile services and by 100 km for AM services. The corresponding spacing for FM fixed service receivers is of the same order. The interference from telecommand is, in any case, likely to be of short duration. Note 3. — Interference from the telecommand transmitter, in its beam, would be tolerable if the spacings were 280 km and 230 km for FM and AM services respectively. The corresponding spacings for FM fixed service receivers is of the same order. Note 4. — As regards interference to the telecommand receiver in the satellite, the practicability of frequency sharing will also depend on the number of mobile or base stations within the coverage area of the satellite, which may contribute to the total interference power received. The value shown above for a single transmitter will thus in practice be exceeded. Comparisons between Tables III and IV show that there would be possibilities of sharing if telecommand transmitter powers were increased and telecommand transmitters made correspondingly less sensitive. The possibilities of sharing are greater for high altitude satellites. — 495 — Rep. 218 L. 7 : Space research REPORT 218 * TECHNICAL CHARACTERISTICS OF TELECOMMUNICATION LINKS BETWEEN EARTH STATIONS AND SPACECRAFT FOR RESEARCH PURPOSES (Question 4/IV) (1963) 1. Introduction This Report presents a summary of the technical characteristics and system parameters of the telecommunication systems used for the purposes of space research. The discussions and conclusions provide a foundation for establishing the radio-frequency spectrum requirements for space research purposes, and for ensuring that the maximum practical use is made of this spectrum space. 2. General system considerations There are three basic telecommunication systems which are required by nearly all space craft. These are: — a telemetering system for transmitting to the earth station, data obtained from the sensor systems in the spacecraft, or from its human occupants; — a tracking system to provide information regarding the position and velocity of the spacecraft necessary for computing its orbit; — a telecommand system to enable the ground experimenter to guide or control the space craft. In addition, some spacecraft will carry television equipment and voice-communication equipment. The detailed technical characteristics of these telecommunication links differ widely from one system to another, making standardization impracticable at this time, particularly for research spacecraft. However, sufficient experience has been gained to make possible a compilation of typical characteristics for several types of spacecraft systems. A list of these characteristics is given in Table I. Because of severe limitations on size and weight, many spacecraft use combined tracking- telemetering-telecommand-video-voice systems similar to those illustrated in Fig. 1. Such systems have the advantage of allowing one receiver and one transmitter to perform several functions simultaneously, thus providing for efficient spectrum usage. More detailed information regarding the technical characteristics of telecommunication links between earth stations and spacecraft, pertinent to the determination of suitable oper ating frequencies and bandwidth is given in the following sections of this Report. Additional technical information, pertinent to telecommunication links between earth stations and space craft, is presented in [1 to 9]. 3. Telemetering systems 3.1 Modulation techniques In a general way, the modulation forms employed for telemetering can be classified in two major categories : — time-division multiplex systems ; — frequency-division multiplex systems. * This Report, which was adopted unanimously, has been brought to the attention of the Extraordinary Administrative Radio Conference, Geneva, 1963. Rep. 218 — 496 — T a b l e I Illustrative experimental communication links between earth stations and spacecraft Probe to Typical Lunar orbiter Lunar lander Mars orbiter edge of solar Parameter near-earth, with with with system with satellites television television television cosmic-ray counter Range ( k m ) ...... 103 to 104 • 4 X 105 4 x 105 4 X 108 4 x 1010 gain (dB) . . 25 50 60 60 60 Earth station antenna diameter (m). 12 26 76 76 76 (ft)- 40 85 250 250 250 area (m2) . . 018 3-5 3-3 25 25 Spacecraft antenna Omni beamwidth . 3° 4-7° - ° directional 1 2 1 -2 ° Temperature of earth system (°K). 300 to 400 125 225 25 25 Radiated power of spacecraft (W ). 0 1 to 10 20 10 150 150 Frequency (G H z)...... 0-2 2-3 2-3 2-3 2-3 Video bandwidth for a signal-to- 1 x 103 to 10® 10® 2-5 X 103 noise ratio of 30 dB (Hz). . . . 2 X 10® Video bandwidth for a signal-to- noise ratio of 20 dB (coded trans Not used 107 107 2-5 X 104 2-5 mission) (H z)...... 6 years Time for spacecraft to reach destin 3 days 3 days 200 days (electric ation ...... propulsion) The performances of various specific modulation systems are described in considerable detail [10 to 22] and [26 to 28] and especially in [20]. In general, time-division multiplex systems make more efficient use of the transmitted energy for highly accurate measurements (1% to 3% accuracy, or better). For less accurate measurements, frequency-division multi plex systems are simpler and will, therefore, remain desirable for certain applications. The radiated power per bit of information in the presence of noise may be reduced by increasing the occupancy of the radio-frequency spectrum. Conversely, bandwidth occupancy may be contracted by expending more energy per bit of information. Limitations in the power capabilities of spacecraft will thus tend to make modulation system design lean toward a smaller amount of energy per bit of information at the expense of bandwidth. Frequency- — 497 — Rep. 218 or phase-modulation systems are attractive, because they can exchange bandwidth for trans mitter power fairly easily. Pulse-code modulation systems also accomplish this exchange by proper selection of the code structure. Extensive use can safely be predicted in the immediate future for suitable orthogonal coding techniques in space telemetering. 3.2 Storage and processing o f data aboard the spacecraft The use of on-board data storage and processing techniques provides a possibility for significant reduction in the required bandwidths for the information to be transmitted and of transmitter power. F ig u r e 1 Typical block diagram of a spacecraft for a system of integrated tracking, telemetering and telecommand Data storage can provide a step-down conversion between real-time and actual trans mission rates to reduce transmission bandwidth. This technique is particularly effective when the source produces information at a high rate, but for short periods of time. Also, the use of on-board data storage permits several spacecraft to share common earth-station facilities, since the information stored by the spacecraft may be transmitted only when com manded by a ground station. On-board data processing has not as yet been extensively employed because the weight, power, and complexity of the equipment required have been incompatible with the require ments of the spacecraft. Wider use of this technique in the future depends greatly on component reliability and miniaturization. Rep. 218 — 498 — 3.3 Required bandwidths for spacecraft to earth station telemetering links The estimates of radio-frequency bandwidth in Table II refer to a single link of each type contemplated; in consequence, it will be necessary to estimate the total number of spacecraft in operation at any one time, the number of links per spacecraft and the extent to which frequencies can be shared to obtain final estimates for the total bandwidth required. The estimates in Table II allow for the width of the radiated radio-frequency spectrum, typical guard bands, Doppler shifts and transmitter frequency-drifts. T a b l e II Bandwidths for data transmission for typical spacecraft Frequency range Radio-frequency (MHz) Type of signal bandwidth per link Telemetering 10 to 100 kHz Less than 1000 Wide-band data 50 to 500 kHz Telemetering 500 to 800 kHz 1000 to 5000 Wide-band data and television 500 kHz to 4 MHz Telemetering 500 kHz to 4 MHz 5000 to 10 000 Wide-band data and 4 to 20 MHz television 4. Tracking systems 4.1 Introduction Reliable radio tracking of spacecraft is one of the basic requirements of any experi mental space programme. In addition to providing information necessary to determine the location in space of the spacecraft at any instant (past, present, and future), tracking is also necessary for evaluation of launch performance, for vernier corrections to trajectories, for determining precise timing for critical manoeuvres such as retro-rocket firing, and for pointing highly directional ground receiving antennae. 4.2 Characteristics of interferometer tracking systems A simple type of spacecraft tracking system may use an unmodulated low-power trans mitter on board the spacecraft and an interferometric receiving system on the earth, to provide a measure of the angular position of the spacecraft with respect to the antenna of the earth station. (In more common combined systems, however, the transmitter in the spacecraft is modulated by telemetering data.) The power transmitted from the spacecraft will range from a fraction of a watt to a few watts ; the fixed-array, receiving antenna at the earth station produces a fan-shaped beam of approximately 10°x75°. A typical operating temperature for an earth station is 300°K to 3000°K, determined largely by the manner in which the system temperature depends on cosmic noise. The optimum frequency range for the interferometer system is determined from consid eration of the necessity for a good omnidirectional antenna on the spacecraft, transmitter efficiency in the spacecraft, and a sufficiently wide beamwidth for the earth antenna. Consid eration of such factors usually favours a frequency below 1 GHz. More elaborate, moving — 499 — Rep. 218 antenna interferometers have been built for frequencies greater than 5 GHz, but atmo spheric attenuation and noise usually limit their performance at frequencies greater than about 6 GHz. 4.3 Characteristics o f two-way coherent Doppler and ranging systems An interferometric system such as that described above only provides angular-bearing information and several observations must be made over an appreciable period of time before the orbit of a satellite can be computed. When more precise data are required in a shorter time (e.g., manned space experiments), more elaborate tracking systems, which provide a measurement of range and range-rate as well as angular bearing, are required. One system of this type uses a coherent turn-around transponder. In this type, a signal is transmitted from the earth to the spacecraft, where it is multiplied coherently by a rational fraction of the received frequency and retransmitted to the earth station. A measurement of range is also obtained from turn-around systems by appropriate modulation of the carrier-frequencies of the earth station and the spacecraft. The main factor which dictates the maximum bandwidth needed per one-way channel is the range resolution required. Range resolutions of 15 to 50 m can be obtained by using appropriate modulations with bandwidths of about 1 to 3 MHz. The ranging systems are “duplex” systems and require equal transmitting and receiving bandwidths. Consideration of the design of antenna diplexers and the frequency multiplying circuitry in the spacecraft dictate that the “up frequency”, transmitted from the earth, be spaced between 6 % and 1 0 % from the “down frequency” transmitted from the spacecraft. The principal limitations imposed by tropospheric attenuation and ionospheric refraction [23] define a useful frequency range of 1 to 8 GHz for these precision tracking systems, which must operate during all weather conditions, and in which the error in incremental position is limited to between 1 0 -5 and 10-4. 4.4 Radar systems Tracking information for short periods of time can also be obtained by the use of con ventional pulse and CW radars. Such systems have proved particularly useful in the real time tracking of manned space flights of short duration. Current plans indicate that fre quencies for conventional radar tracking systems will generally be available in the bands already assigned to the radio-location service. 5. Telecommand systems A telecommand system is necessary to control functions within the spacecraft (manned or unmanned), from the earth station. It may use its own radio-frequency link or be com bined in an integrated system, in which the radio-frequency link is shared with voice, ground data and tracking signals. Commands, transmitted to the spacecraft when it is in radio visibility of the earth station, can either be executed immediately as real-time commands, or stored in a memory to be extracted later and executed as stored-programme commands. The telecommand signals are coded to provide a capacity for a number of discrete com mands and to protect the spacecraft from spurious signals. Commands may be transmitted, either by the use of various combinations of sub-carriers modulating the main carrier, or in the form of a serially pulse-coded transmission. Typically, the operating noise-temperature of the telecommand receiver in the spacecraft is from 600°K to 3000°K and a design margin of 6 to 10 dB is allowed for fading, nulls in the antenna pattern and degradation in equipment performance. Typical information rates Rep. 218 — 500 — range from as low as 1 to 1 0 bits per second for a deep-space probe to more than 1 0 0 0 bits per second for some of the more complex near-earth satellites. The rates for manned spacecraft have attained as many as 32 000 bits per second. The problem of interference for the telecommand receivers in spacecraft is difficult, in that the “wanted signal” is not always present—the interfering signal must not trigger the telecommand receiver even in the complete absence of the “wanted signal”. However, this factor is partly offset by the rejection by the spacecraft of signals which are not properly coded. 6. Spacecraft voice links The requirements for the transmission of voice, to and from a manned spacecraft, differ from more conventional mobile voice links in the high degree of reliability required during critical periods and the necessity for communications with a given spacecraft from points widely separated on the surface of the earth. For normal speech transmission, an information bandwidth of 3 kHz and a speech-to- noise ratio (for peak speech-power as read on a VU meter and r.m.s. noise power for flat spectrum noise) of 20 dB is recommended. 6.1 Speech compression techniques There is some promise that future requirements for power and bandwidth for voice transmission from spacecraft might be reduced by the use of the techniques of speech band width compression. The results obtained to date, with experimental voice compression systems, suggest that word intelligibility of the order of 80% and 85% can be achieved with analogue bands of 200-800 Hz or with digital transmission rates of 1200 to 2000 bits per second. Assuming equal speech-to-noise ratios for uncompressed and compressed speech, analogue transmission of compressed bands can be expected to result in a transmission improvement of 6 to 12 dB. Digital transmission at a rate of 1500 bits per second would be possible with a received power 4 dB above the noise in a 6 kHz band. Speech transmitted over such channels would not be comparable in quality to that transmitted by uncompressed analogue methods, but the potential exists for communicating by voice beyond the range of conventional voice transmission. In considering the use of speech compression devices, the reduction in transmitter power they afford must be weighed against the penalties incurred by the additional weight and complexity of the equipment and by power requirements. At their present state of develop ment, most of these equipments are rather bulky. The non-digital devices require consider able numbers of audio-frequency filters. Digital equipment requires, in addition, hundreds of transistors. It remains to be established that careful design and further development effort can produce speech compression equipment, with physical characteristics and reliability suitable for use in manned spacecraft. A more quantitative evaluation of such equipment will be essential, but is beyond the scope of this Report. 6.2 Power and spectrum conservation by peak clipping techniques Peaks some 15 dB above the average speech power can be expected for about 0-1% of the time ; smaller peaks occur more frequently. The peak factor (ratio of peak to average power) can be reduced by clipping the peaks above a specified level, thereby permitting higher average modulation and a higher average received signal. Clipping of peaks above 7 or 8 dB permits — 501 — Rep. 218 a 5 to 1 reduction in transmitter power compared with that required for essentially unclipped speech, at a moderate sacrifice of speech quality and appears to be a reasonable compromise between quality and power. 6.3 Modulation characteristics In spacecraft-to-earth voice links, emphasis is placed on minimizing the transmitter power requirements of the spacecraft. This is an important consideration, both from the stand point of primary power and of peak-power rating of the radio transmitting equipment. There is not a great difference in the size, complexity and weight of the output stages of radio transmitting equipment per watt of peak radiated-power for the various methods of modulation. Thus, a comparison of different modulations on the basis of peak-power requirements seems reasonable, but other equipment comparison must also be made. For example, both double-sideband (DSB) and single-sideband (SSB) equipment, transmitting and receiving, will involve additional complexity relative to AM and FM equipment. Furthermore, from the viewpoint of primary power in the spacecraft, average transmitter power may be of more importance than peak power, and average power will be substantially lower than peak power for DSB and SSB transmitters. Pulse-code modulation does not appear to offer transmission advantages in a single-link voice transmission system; however, it may be attractive for time-sharing a digital trans mission channel with voice and telemetry. 6.4 Carrier frequency In most circumstances, it appears that the optimum frequency ranges for other earth station-to-spacecraft emissions, will also be the best for voice links. However, there are special situations where frequencies outside the normally recommended 100 MHz to 10 GHz will be required. Voice frequencies in the HF range are needed to extend the range of the voice link beyond the normal line-of-sight. Frequencies higher than 10 GHz appear to be useful for voice communications with the pilot of a spacecraft, in conditions of blackout during atmospheric re-entry. 7. Television systems 7.1 Bandwidth The video bandwidth of a television system can be expressed as [24]: Bandwidth - 0-35mL2FR (1 —V) / (1 -H ) (Hz) where L = number of lines per frame, F = number of frames per second, R — width-to-height ratio of picture, V = fraction of total vertical sweep-time, during which the trace is blank, H — fraction of total horizontal sweep-time, during which the trace is blank, m = ratio of horizontal-to-vertical resolution. The values of R, V and H of the above equation will remain approximately the same for all applications. The important parameters are, therefore, L and F. The choice of the value of L is a matter of the degree of resolution required for the mission. Present-day television systems for spacecraft, planned or in use, have from 200 to 800 lines per picture. With new pick-up devices under development, and the higher powers which will be available in future spacecraft, it is expected that the number of lines will be increased substantially. Rep. 218 — 502 — In some applications, the time needed to read-out each frame for transmission to the earth may differ by orders of magnitude from the time used initially to view each frame. For example, in a recent series of meteorological-satellites, each picture is taken with an exposure time of 1-5 ms, while the time taken to transmit the picture is 2 s. For applications where a given number of pictures are taken, to be played back later, when the satellite is in view of a suitable earth station, the time needed to read out each frame is chosen to permit that number of pictures to be scanned during the time in view. 7.2 Modulation In television broadcasting, vestigial sideband transmission is used to conserve spectrum. For space applications, the modulation method must be chosen primarily on the basis of minimizing weight and complexity. Given the video-bandwidth and the wanted signal-to- noise ratio, the designer may use a minimum radio-frequency bandwidth system, such as vestigial-sideband transmission and a corresponding transmitter power output, or a wider band system such as FM, FM with feedback, or pulse-code modulation, and a lesser power output. Commercial quality television, using pulse-code modulation, would require a radio frequency bandwidth of approximately 70 MHz. In general, it does not appear that pulse- code modulation offers any advantages in a single link video transmission system; however, slow speed television pulse-code modulation may be attractive for time-sharing a digital channel with telemetering. 7.3 Carrier frequency For normal conditions, it appears that frequencies between 0-1 GHz and 10 GHz will be suitable for television between earth stations and spacecraft, with the centre of the range usually being most suitable. 7.4 Interference The problem of interference on the space-to-earth circuit is somewhat different for tele vision than for other types of signal. The wider bandwidth of the former makes it corres pondingly more susceptible to outside interference. On the other hand, the damaging effects due to the loss of some television signals on a mission, are usually much less than a com parable loss of tracking, telemetering or telecommand signals. The error rates can therefore be somewhat higher. 8 . Transmitters in spacecraft The development of solid-state transmitters is advancing very rapidly. Moderate power may be generated by amplification techniques up to about 100 MHz, and at frequencies above this, efficient methods of paranietric frequency-multiplication may be employed. The powers and the efficiency of conversion from d.c. to radio-frequencies achievable with solid- state devices (some experimental) (January, 1963), are given in Table I I I : — 503 — Rep. 218 T able II I Frequency Power output Efficiency (MHz) (W) (%) 100 20 50 400 11 27 1000 6 15 2000 3 11 5000 1 4 10 0 00 0-25 1 Substantial increases may be expected during the period 1965-1970, in the powers and efficiencies achievable with solid-state transmitters, due to the rapid rate of progress. Solid-state transmitters are inherently well suited to relatively wide-band applications. Due to the small size of solid-state devices, the low voltages involved (which render pressuri- zation unnecessary), and the simplicity of the problem of heat transfer, the weight of this type of transmitter can be much less than its tube counterpart. Powers in excess of those tabulated can be obtained with various types of vacuum tube, such as high trans-conductance triodes, tetrodes, beam power-tubes, klystron oscillators, klystron amplifiers, backward-wave oscillators, voltage-tuned magnetrons and travelling- wave tubes. Triodes, tetrodes and beam power-tubes are most efficiently used in medium bandwidth systems in the UHF region and below. Powers in excess of 150 W are possible in push-pull configurations with conversion efficiencies from d.c. to radio-frequencies, of approx imately 15%. High-level output above the UHF region is best obtained with voltage tuned magnetrons, klystron oscillators, klystron amplifiers and travelling-wave tubes. 9. Antennae for spacecraft For many applications, essentially isotropic antennae are required for spacecraft. Such antennae can usually be designed much more readily at frequencies toward the low end of the spectrum, below 1 GHz, than above. However, some antennae for spacecraft have been designed, which produce omnidirectional radiation pattern characteristics at frequencies as high as 10 GHz, with a loss of efficiency not exceeding a few decibels. Considerations of pointing accuracy and surface precision for a directional antenna on a spacecraft are essentially the same as those for a directional antenna at an earth station. However, some of the problems encountered with high-gain antennae on the Earth will be absent in space. For example, the mechanical design of an antenna might be simpler, since it will not involve loadings due to gravity, wind and ice; on the other hand, problems of erecting antennae with suitable surface accuracy and protection against micrometeorites will be important. Isotropic or small broad-beam antennae are usually desirable for spacecraft, v to minimize the requirements for stabilization and attitude control, but for communication at very great distances high-gain spacecraft antennae are essential. Precision in attitude control, and the other factors mentioned above, will limit the antenna beamwidth on the spacecraft. For a gain degradation of 1 dB due to pointing errors of ± 0-2°, ±1° and ± 3°, the minimum beamwidths at the 3 dB points are approximately 0-8°, 4°, and 12° respectively. Assuming typical antenna diameters of 1 foot (30.cm), 3 feet (98 cm), 10 feet (3 m) and 30 feet (9-8 m), the corresponding frequency limits are given in Table IV. Rep. 218 — 504 — T a b le IV Frequency (GHz) for antenna size (m) Beamwidth (degrees) 0-3 0-9 3 9 0-8 87-5 O 29-2 0) 8-75 2-92 4 17-0 0) 5-7 1-70 0-57 12 5-7 2-0 0-57 0-2 (0 The upper frequency limits are determined by atmospheric absorption rather than by minimum usable beam widths. 10. Receivers for spacecraft On the spacecraft, a low-noise receiver would be desirable, but this might not be worth a large weight penalty, since a more powerful transmitter at the earth station could compen sate for less receiver sensitivity. If a narrow-beam antenna is used on a spacecraft, it would “see” the earth-station transmitter against a background at 300°K (temperature of the surface of the earth), so there would be no advantage in a receiver temperature much lower than 300°K. The noise-temperature of typical spacecraft receivers range from 600°K to 3000°K. A deep-space probe, however, would see a far lower temperature, particularly at fre quencies above a few hundred megahertz, since the earth will not fill the antenna beam. Therefore, a low-noise receiver could advantageously be used, space and weight consid erations permitting. The sensitive receivers, however, are more susceptible to man-made interference and thus demand better spectrum control. 11. Earth-station antennae The type of antenna at an earth station used for space research, tracking and data acquisi tion, depends to a large degree, on the particular mission requirements and spacecraft performance including orbital characteristics. It can, therefore, be expected that earth- station antennae will have a number of configurations for experimental telecommunication links between earth stations and spacecraft. 11.1 Fixed-beam, low-to-medium gain antenna at earth stations for automatic tracking Many scientific near-earth satellites will operate at frequencies below 1 GHz and at altitudes where angular rates are high. For these applications, a number of fixed fan-beam antennae or low-to-medium gain, automatic-tracking, parabolic reflectors or arrays will be used. 11.2 High-gain antennae at earth stations for automatic-tracking For deep-space missions and for high-information rate near-earth satellites, earth-station antennae, with gains and effective aperture areas as large as practicable, will be required. The antenna gain and effective area of earth-station antennae are limited by the mobility of massive structures for tracking spacecraft, attainable manufacturing tolerances, and meteoro logical conditions. 11.2.1 Tracking The tracking errors of antennae, which use automatic tracking circuitry to control the pointing of the main beam, must be less than one-half the beamwidth at maximum gain. This limitation is closely related to the tracking rates involved ; for near-earth — 505 — Rep. 218 orbital vehicles, angular rates can be as high as several degrees per second, but may be as low as sidereal rates (0-004 deg/s), or even essentially zero with stationary satellites. 11.2.2 Surface tolerance The accuracy of the surface contour, which can be fabricated and maintained with loads due to gravity, heat, wind and ice, decreases as the size of the antenna increases. Current manufacturing practices result in an r.m.s. deviation from a true parabola, ranging from approximately 1-Ox 1CM to 2-5 xl(H times the antenna diameter. Fig. 2, based on a method developed by Ruze [25], shows the frequency at which maximum gain occurs as a function of antenna diameter for an r.m.s. devia tion from a true parabola of ICHx diameter. Fig. 3 shows antenna gain as a function of frequency for several parabolic antennae of different diameters, with an r.m.s. surface deviation equal to KHx diameter. 11.2.3 Meteorological conditions Insufficient information is at present available regarding the phase instability of the earth’s atmosphere during periods of adverse meteorological conditions, to predict accurately the degradation in gain which can be expected with large paraboloidal antennae. Present estimates indicate that the combination of limitations on structural tolerance and the inhomogeneities of the earth’s atmosphere will probably limit the gain of large aperture antennae to between 60 and 70 dB. 12. Receivers at earth stations The smallest signal that a radio receiver can detect is limited by the ambient noise gener ated in the receiver and that contributed by external sources. At the earth station receiver, the noise is kept as low as possible, to minimize the power requirements of the spacecraft trans mitter. The effective input noise temperature of earth-station receivers, due to the receiving equipment alone, should be capable, by 1965, of being reduced to the range 5°K to 10°K over the part of the spectrum concerned. Except for possible man-made interference, the major contribution to the total system-noise will be the antenna background-noise, which is a function of the operating frequency, angle of elevation of the antenna, existing meteoro logical conditions, and ground thermal radiation into the antenna side- and back-lobes. Sky background noise, due to the galaxy and sun bursts, decreases with frequency (approxi mately as f ~ 2 5), while noise due to the earth’s atmosphere increases above 1 GHz. For fre quencies above approximately 4 GHz, heavy rainfall can increase the noise contribution due to the atmosphere to more than 100°K. Using narrow-beam antennae at the earth station down to angles of elevation of 5°, under adverse weather conditions, the minimum overall operating noise temperature which can be expected is 30° to 40°K at approximately 2-5 GHz. This temperature is almost lower by a factor of 100 than that of conventional terrestrial service systems. 13. Transmitters at earth stations The transmitter powers and stabilities required in the foreseeable future at earth-stations, for telecommunication links between earth-stations and spacecraft, should not present any serious technical problems. It may sometimes be necessary to programme the Doppler frequency-shift into the earth-station transmitter, to reduce the bandwidth or complexity of spacebome receivers and antennae. By 1965, the powers of earth-station transmitters greater 21 Rep. 218 Rep. Diameter of antenna (m) Frequency at which maximum gain occurs as a function of antenna diameter diameter antenna of a function as occurs gain which maximum at Frequency rqec (GHz) Frequency 56 — 506 — F gure r u ig 2 (a/D 10-4) = Power gain Gain/frequency characteristic for antennae of various diameters (based on u/D on (based diameters various of antennae for characteristic Gain/frequency A : Antenna diameter : 750'ft (230 m) (230 : diameter Antenna 750'ft : A : nen imtr: 5 t(2 m) ( ft 26 85 : diameter Antenna m) :( ft 76C : diameter Antenna 250 : B Atnadaee 6 f 8 m) ( ft 18 60 : diameter Antenna : D rqec (MHz) Frequency 57 — 507 — F gure r u ig 3 = 10-4) Rep. 218 Rep. Rep. 218 — 508 — than 100 kW, will be needed in the frequency range under consideration, and should be realizable*. Occasionally, the geographical location, antenna characteristics, and distance from earth stations of other users sharing the same frequency, may limit the amount of radiated power of specific earth-based transmitters. 14. Radio-frequency spectrum 14.1 Introduction The subject of the choice of frequency bands, suitable for telecommunication links between earth stations and spacecraft, is covered in considerable detail in Report 205-1. On the basis of the information contained therein, and in the preceding sections of this Report, it is possible to specify those frequency ranges in the radio-frequency spectrum for which allocations are required, for various links between earth stations and spacecraft, for research. 14.2 Below 1 GHz Frequencies below 1 GHz, particularly in the range from 100 MHz to 500 MHz, are suited primarily for transmission from near-earth spacecraft. Frequencies in the region from 100 MHz to 300 MHz are desirable for use by existing tracking networks and the extensive VHF networks, which have been established for reception of telemetering data. The use of frequencies in the region from 100 MHz to 500 MHz is especially attractive for spacecraft and earth stations, which have very broad or isotropic antenna patterns. Such patterns are normally required for spin-stabilized or unstabilized spacecraft and simple earth stations without facilities for antenna tracking. 14.3 Between 1 and 6 GHz The radio-frequency spectrum from 1 to 6 GHz is expected to be the region which will be most heavily used for links between earth stations and spacecraft for research. In this range, the galactic noise density is low, atmospheric absorption is low, low-noise parametric and maser pre-amplifiers are feasible, directional spacecraft antennae, whose gains are compatible with the capabilities for attitude stabilization and beam-steering of the spacecraft, are practical, and both surface and pointing accuracy requirements for large earth-station antennae can be met. 14.4 Between 6 GHz and 10 GHz Frequencies in the range from 6 to 10 GHz will be required to accommodate very wide band (10 to 100 MHz) precision tracking and communication systems. Some re-entry links may also use frequencies in this range. Low-noise parametric and maser pre-amplifiers are feasible in this region and galactic noise is even lower than that in the 1 to 6 GHz region; however, atmospheric absorption is somewhat higher and atmospheric noise can be con siderably higher. 14.5 Above 10 GHz ' A t present, the solution of the problem of re-entry “black-out” can only be solved by the use of frequencies above 10 GHz. It is unlikely that operation will be attempted above the partial atmospheric window at about 34 GHz, because of the heavy atmospheric absorp * It should be noted that these high powers will be used with high-gain antennae with beamwidths less than 1° and angles of elevation greater than 5°. — 509 — Rep. 218 tion, although, with very high-speed re-entry, operation at even higher frequencies may possibly be required. Practical considerations will, in general, force the frequency of operation as close as possible to the lower end of this range. 15. Conclusions 15.1 Tracking, telemetering and telecommand systems will be required on essentially all experi mental spacecraft; in addition, many spacecraft will also carry voice and video telecommuni cation systems. These systems will normally be combined to reduce the number of transmitters required aboard the spacecraft and to conserve radio-frequency spectrum. 15.2Because of differing research mission requirements, the same links between earth stations and spacecraft in current or planned use do not all depend on the same parameters. It is, therefore, premature at this time to attempt establishment of system or performance standards. 15.3 The choice of frequencies most suitable for telecommunication links between earth stations and spacecraft are determined by consideration of several factors, including: propagation, noise, and interference ; gains and patterns of spacecraft and earth-station antennae; availa bility of electrical power, transmitter efficiency, complexity and reliability of the system in the spacecraft; weight and volume of the spacecraft; and the nature and required bandwidth of the data to be transmitted. Most useful frequencies are those between 100 MHz and 10 GHz, although some frequencies below 100 MHz, particularly in the HF range, will be required for voice communication from manned spacecraft, and some frequencies above 10 GHz will be required for communications with spacecraft re-entering the earth’s atmo sphere. 15.4 Frequencies from 100 MHz to 1 GHz will be used primarily for telecommunications with near-earth spacecraft, especially where the use of wide-beam or non-directional antennae are required on the spacecraft or on the earth. t 15.5 The range from 1 GHz to 10 GHz will be used for applications involving directional antennae and wide bandwidth links, such as precision tracking systems and video transmission links. 15.6 The bandwidths, required for the transmission of telemetering data, range from a few kilo hertz in the region below 1 GHz to many megahertz in the regions above 1 GHz. 15.7 The bandwidths required for tracking range from a few kilohertz for simple interferometer systems operating below 1 GHz to several megahertz for precision, coherent turn-around systems operating at higher frequencies. 15.8 The widths of the basebands required for typical telecommand systems are of the order of a few kilohertz, but Doppler frequency-shift, being much larger, usually determines the required radio-frequency bandwidths. 15.9 Voice transmission between earth stations and spacecraft does not differ greatly from other mobile voice links, except in the high degree of reliability required and the necessity for com munications with a given spacecraft from points widely separated on the surface of the earth. There is some promise, that future requirements for voice bandwidth might be reduced by the use of the techniques of speech bandwidth compression. 15.10 For normal conditions, it appears that frequencies between 0-1 and 10 GHz will be suitable for television between earth stations and spacecraft, the centre of this band usually being most suitable. Rep. 218 — 510 — B ibliography 1. R e c h t in , E. Deep-space communications. Astronautics, Part I (April, 1961); Part II (May, 1961). 2. R e c h t in , E. Space communications. Jet Propulsion Laboratory. Technical Release No. 34, 68 (May, 1960). 3. P ie r c e , J. R. and C u t l e r , C. C. Interplanetary communications. Advances in space science. Vol. I, Academic Press (1959). 4. M c C o y , C. T. Space communications. Philco Corporation Report No. 279. 5. M u e l l e r , G. E. A pragmatic approach to space communication. Proc. IRE, 557-566 (April, 1960). 6 . V o g e l m a n , J. H. Propagation and communications problems in space. Proc. IRE, 567-574 (April, 1960). 7. S m it h , A. G. Extraterrestrial noise as a factor in space communications. Proc. IRE, 593-599 (April, 1960). 8 . S w e r Li n g , P. Space communications. Rand Corporation Report P. 1443. 9. S h a n n o n , C. E. Communication in the presence of noise. Proc. IRE, 10-21 (January, 1949). 10. I n t e r -R a n g e I nstrumentation G r o u p , I.R.I.G. Telemetry standards. Document No. 106-60 (November, 1960). 11. O l iv ie r , B. M. and P ie r c e , J. R. The philosophy of PCM. Proc. IRE. 1324-1331 (November, 1948). 12. B alakrishnan , A. B. and A b r a m s , I. J. Detection levels and error rates in PCM telemetry systems. IRE Convention Records (March, 1960). 13. C a h n , C. R. Performance of digital phase-modulation communication systems. IRE Transactions on Communication Systems (May, 1959). 14. C a h n , C. R. Performance of digital phase-modulation communication systems. Ramo Wooldridge Report M110-9U5 (April, 1959). 15. D e v e l e t , J. A., Jr. A note on demodulation of PCM/PM signals with switching-type phase detectors. S.T.L. Report No. 8616-0001-NU-000 (June, 1961). 16. V it e r b i, A. J. On coded phase-coherent communications. J.P.L. Report No. 32-25 (August, 1960). 17. D e v e l e t , J. A ., Jr. A note on product demodulation of binary PCM/PM signals. S.T.L. Report No4 8949-0002-NU-000 (February, 1961). 18. L a w t o n , J. G. Theoretical comparison of binary data transmission systems. Cornell Aeronautical Laboratory (May, 1958). 19. S t u m p e r s , F. L. H. M. Theory of frequency-modulation noise. Proc. IRE 1081-1092 (September, 1948). 20; N ic h o l s , M. H. and R a u c h , L. L. Radio telemetry. John Wiley and Sons, Inc. (June, 1957). 21. B a g h d a d y , E. J. Lectures on communication system theory. McGraw-Hill (1961). 22. S a n d e r s , R. W. Communication efficiency comparison of several systems. Proc. IRE, 575-588 (April, 1960). 23. B a r To n , D. K. and S h e r m a n , S. M. Pulse radar for trajectory instrumentation. Proceedings of Sixth National Flight Test Instrumentation Symposium, Instrument Society o f America, San Diego, California (3 May, 1960). 24. F in k , D. G. Television engineering. McGraw-Hill, New York, 31-32 (1952). 25. R u z e , J. Physical limitations on antennae. Technical Report No. 248, Research Lab. for Electronics, M.I.T. (October, 1952). 26. B a t t a il , G. Etude gen6rale comparee des syst&mes de modulation. Colloque International de V U.R.S.I. sur les recherches en telecommunications spatiales (Paris, September 1961). Space Radio Communi cations, Elsevier (1962). 27. B a t t a il , G. Determination approximative de la position extreme du seuil de reception en modulation de frequence. International Symposium on Information Theory, Brussels (September, 1962) IRE Transactions on Information Theory, 8 , 5. 28. B a t t a il , G. Un demodulateur pour modulation de frequence, utilisant une estimation prealable de la frequence instantan6e. International Conference on satellite communication (London, November, 1962), published in Proc. I.E.E. — 511 — Rep. 219-1 REPORT 219-1 * INTERFERENCE AND OTHER SPECIAL CONSIDERATIONS FOR TELECOMMUNICATION LINKS FOR MANNED AND UNMANNED SPACECRAFT IN THE SPACE-RESEARCH SERVICE (Questions 3/IV, 4/IV, Study Programmes 2B/IV, 3A/IV) (1963 — 1966) 1. Introduction This Report presents technical information, which is primarily applicable to considera tions of mutual interference between telecommunication links in the space-research service and various other services. Special considerations and examples of system designs and functions are given, followed by a discussion of the appropriate interference criteria. 2. Illustrative design considerations The technical characteristics and factors affecting communication with and between research spacecraft for both manned and unmanned missions are covered in considerable detail in Reports 205-1 and 218. In general, these Reports present the selection of optimum frequencies, types of modulation, and the characteristics of antennae, transmitters, and receivers. 2.1 Deep-space research The most practical approach for communication to and from the Earth and spacecraft journeying to the Moon and beyond, is now generally agreed to be radio-frequency links using directional antennae operating at decimetric wavelengths (see Report 218). Typical antennae for space stations have diameters between 1 m and 10 m. The radiated powers from space stations are of the order of tens to hundreds of watts. Practical earth-station antennae have diameters between 20 m and 80 m. Equivalent isotropically radiated powers (e.i.r.p.) from the Earth are of the order of 10 kW, during the early portion of flight, to more than 1000 MW for flight near the planets. The operational noise-temperatures of the receivers are typically 30°K at frequencies greater than 1 GHz (except for lunar landings, when the noise temperature of the Moon increases the system temperature by about 100°K), and 600°K to 3000°K for the space station. At frequencies lower than 1 GHz, the system noise temperatures are increased by cosmic noise (see Report 205-1, Fig. 6 ). For example, for reception at 100 MHz, one may expect a system temperature greater than 500°K for the earth station and greater than 1000°K for the space station. Earth stations for deep-space links are usually located in terrain providing a natural horizon several degrees above the horizontal. Full performance communication and tracking is seldom planned for angles of elevation less than about 1 0 °; however, initial acquisition of the signals is attempted at angles as low as 3°. Operations are characterized by critical periods immediately after earth launching, during mid-course (vernier) manoeuvres, and during the terminal phases at the Moon or planets. The critical periods last from a few minutes to several hours, depending upon the technique. For lunar operations, the critical periods occur approximately once a day. * This Report, which was adopted unanimously, replaces Reports 219, 220 and 221. Rep. 219-1 — 512 — 2.2 Near-earth research satellites The tasks assigned to near-earth satellites and the time available for read-out of large amounts of data, result in larger requirements for radio-frequency bandwidth than those associated with similar functions on deep-space missions. Earth terminals are located to provide optimum coverage of satellite orbits. Because of the relatively short periods that a near-earth satellite may be within view of any instrumentation site, time of the acquisition is of the utmost importance, and while effective operation is difficult below an elevation of 3°, acquisition is attempted at lower angles. Critical periods for near-earth missions occur during those times when the satellite is within viewing range of a tracking site. Thus, for any one site, the duty-cycle per satellite over a 24-hour period may be quite low. 2.3 Manned space-research The most over-riding requirement of manned space flight is that of safety of the astronaut. Since satisfactory communications have a direct bearing on his safety, it is essential that only the most reliable communication techniques be utilized. To this end, critical manned space-flight communications must utilize those portions of the spectrum where techniques of proven reliability exist and where the most beneficial propagation characteristics may be realized. Furthermore, it is essential that sufficient pro tection against interference be afforded, so that the safety of the astronaut is not compromised inadvertently. 2.3.1 Telecommand Remote control of the spacecraft is desirable, to ensure the safety of the spacecraft crew in the event of incapacity of the astronaut, as well as to take advantage of the greater diagnostic capabilities which exist on earth. Such a capability for command is not intended to replace control of the spacecraft by the astronaut, but rather to sup plement it. Additional inter-spacecraft telecommand may also be required between spacecraft for some missions to effect rendezvous and/or rescue. Near-earth telecommand communications will utilize tone-codes, and modulation schemes employing frequency-shift keying, and phase-shift keying to obtain the highest degree of protection from interfering signals. Earth-station transmitters having power outputs of 500 to 10 000 W, and directive antennae will be utilized. The frequency range for such functions will be between 200 and 600 MHz, 600 MHz and 1 GHz and 1 and 10 GHz. The additional capability for telecommand between spacecraft will also utilize this frequency region. In either case, bandwidths of approximately 50 kHz per link will be required. 2.3.2 Telemetering In addition to voice communications, telemetering will be relied upon to monitor the astronauts’ safety and ensure their well-being. Functions of the telemetering sub system are to monitor the astronauts, to monitor critical environment, to monitor critical sub-system in the spacecraft, and to aid in in-flight checkout of the spacecraft. The latter item is unique to manned space-flight. Since the safety of the astronaut is of utmost importance, extensive checkouts of critical spacecraft sub-systems, both earth- based and on board the spacecraft, must be accomplished before human lives are com mitted to the mission, e.g. checkout of landing module in lunar parking orbit before descent to moon. Space-station transmitters will normally use frequency or phase modulation of the carrier to conserve power and to provide the high signal-to-noise ratios required for — 513 — Rep. 219-1 data transmission. Typical power outputs are 1 to 20 W, and directional antennae will be used to provide extended range. Typical telemetering bandwidths will range from 100 to 200 kHz within the frequency bands between 200 and 600 MHz, 600 MHz and 1 GHz and 1 to 10 GHz. 2.3.3 Voice communications In manned space-flight research, voice communication, with its inherent flexibility in the transmission of information, is an essential factor in guaranteeing successful missions with maximum safety of the astronaut. The technical considerations involved in the choice of carrier frequencies for this function lead to requirements in the bands between 10 and 600 MHz, 600 MHz and 1 GHz and 1 to 10 GHz. For communications between the space station and earth station, reliable, line-of- sight two-way links are needed. Low powered (approximately 20 W), selectable fre quency, voice modulated transceiver equipment is most suitable for this application. This same equipment may be utilized at the time of initiating the descent to the surface of the earth. The sequence of events that occur at this time will govern the place on the earth at which the spacecraft will land. To deploy rescue and recovery forces, it is mandatory that the sequence of events be reported by the spacecraft personnel to the earth stations in a highly reliable manner. It is necessary, therefore, that no interfering signals be present on the channels in use. Accordingly, minimum interference with voice frequencies should be provided. While in low earth-orbit and as the spacecraft approaches the surface of the earth, communication ranges at VHF gradually diminish, due to contracting line-of-sight conditions. It is therefore necessary to provide HF communications between the space station and the earth stations and recovery forces. Primary power as well as limitations on space and weight aboard the space station dictate the use of a low powered (approxi mately 5 W) HF transceiver. The actual radiated power from this equipment will be decreased by the type of antenna that can be provided on the spacecraft. It therefore becomes necessary to provide interference-free voice frequencies in the HF region below 20 MHz for use by manned space-flight research missions. Additionally, the frequencies provided should be spaced within the HF region so that reliable communica tions can be achieved under various conditions of propagation. 2.3.4 Combination o f telecommunication functions on a single radio-frequency link In manned space-flight research, it is practical and desirable to effect telemetering data transmission, tracking functions (range and range-rate), telecommand and voice communications on the same earth-space link and the same space-earth link. Pairs of coherently related frequencies with a separation ranging from 6 % to 20% of the higher frequency are required, but preference should be given to the range between 6 % and 1 0 %. The technical considerations involved in the choice of carrier-frequencies for this function lead to requirements in the 1 to 10 GHz band. Typical bandwidths will range from 500 kHz per link near 1 GHz to 20 MHz per link near 10 GHz. 3. Interference criteria The degree of protection from interference required by any receiving system depends upon the sensitivity of the system to interference and the consequences of degraded reception upon the overall operations. Rep. 219-1 — 514 — 3.1 Protection for the earth station Radio-frequencies from below 20 MHz up to 10 GHz have been shown to be suitable for various telecommunication links used in space research (see Reports 205-1 and 218). The need for highly efficient communications, especially for deep-space missions, has resulted in the development of fairly complicated earth stations. As an example, maser receivers operating at microwave frequencies are available which have an effective noise temperature of about 10°K. This internal thermal noise, plus the addition of 6 ° to 8 ° from radiation of the earth into the back and side-lobes of the antenna, and 3° to 13° (depending upon the angle of elevation) from atmospheric and galactic background, would constitute a system operating at —214 dBW per hertz (30°K). In addition, there is a contribution from atmospheric water vapour which is a function of the frequency, the angle of elevation and weather conditions. Experimental data on the contribution from water vapour to the temperature are at present limited. At frequencies near 6 GHz, the contribution due to water vapour, during and pre ceding heavy rainfall, can occasionally increase to between 50°K and 100°K *. At frequencies below about 4 GHz, there are relatively small effects due to water vapour. At frequencies below about 1 GHz, cosmic noise increases the operating noise-temperatures of the system at the rate of about 20 dB per decade of decreasing frequency. Utilizing present techniques, the following operating noise-temperatures are typical for the frequencies indicated: T a b le I Frequency Operation noise-temperature of system (°K) 100 MHz 3000 500 MHz 300 1 to 10 GHz 30 Interference which is considerably less than the level of background noise is of no consequence. The permissible ratio of interference-to-background noise is determined by the design margins customarily used in the particular communications link. In any spacecraft there is a great incentive to minimise design margins because of the severe penalties in weight and cost of producing electrical power. It is possible to work with design margins as low as 3 to 6 dB, if the utmost care is exercised in equipment design. Design of a communication link may become much more difficult when man-made interference increases the total noise level by more than 1 dB. Complete disruption of communications and tracking could occur if the interference level were two or three times that of the background noise. A study of the effects of interference on narrow-band phase lock loops which supports the above state ments is summarized in Annex I. It should be noted that in certain types of satellite it may temporarily be possible to increase the radiated radio-frequency power sufficiently to combat anticipated interference. It is necessary to distinguish between the effects of interference which is “noise-like”, i.e. which has approximately the same spectral characteristics as the noise in the system and that concentrated in narrow frequency bands, or “CW-type”. The latter can be relatively more * H o g g , D. C. and Se m pla k , R. A. The effect of rain on the noise level of a microwave receiving system. Proc. IRE, 48, 2024-2025 (December, 1960). — 515 — Rep. 219-1 harmful for a given total interference power at the input of the receiver to Doppler, narrow band telemetry, telecommand, synchronization of television, and certain types of range modula tion, the bandwidths of which are typically of the order of 1 to 100 Hz. Because signals sweep through a considerable frequency range by Doppler frequency-shift and carrier instabi lities, an interfering carrier would successively harm each sub-carrier or sideband (disrupting the corresponding telemetry channel or range tone), as the Doppler frequency-shift changes. Interference of the noise-like type can conveniently be considered and specified in terms of its power density, but for interference of CW-type the power level of every major component is significant and the effects of these component's upon the wanted signal must be estimated. A significant increase in background noise-level is not likely to occur for prolonged periods of time. However, sporadic interference from man-made sources can be expected, due to the dependence of trans-horizon propagation on fluctuating weather conditions, the changing gain in the link between the interfering station and the receiving station due to relative motions of the antennae, etc. Therefore, any established criterion of interference must be stringent enough to minimize the possibility of this type of interference. Loss of more than 5 min of communication during critical periods would seriously affect a deep-space or manned research mission and loss of more than 15 min could easily result in failure of the mission. The following criterion is, therefore, the most directly appropriate for protection of earth stations : For frequencies greater than 1 GHz, the total time during which the power density of noise-like interference or the total power of CW-type interference in any single and all sets of bands 1 Hz wide, is greater than —220 dBW per hertz, shall not exceed five minutes per day for deep-space and manned research missions. (0 -1 % of the time being permissible for other near-earth research missions); for frequencies below 1 GHz, the permissible interference may be increased at the rate of 20 dB per decreasing frequency decade. Specifying the power or power density at the receiver input terminals, rather than the spatial power-density (W/m2), has two particular advantages for the present discussion. It is, within a broad frequency range, independent of frequency, and it permits advantage to be taken of the exact siting of the earth-station antenna and its pattern characteristics. These characteristics are generally in the direction of reducing the interference level at the receiver inputs. As mentioned, the directional antenna of the earth station will seldom be used below angles of elevation of 3° for normal full performance communications. Therefore, many of the earth antennae have an effective area of capture in the horizontal direction equivalent to the gain of an isotropic antenna. A more refined estimate, based on specific characteristics of the antenna pattern, might be expected to lead to a ± 10 dB modification to the isotropic assumption. Typical beamwidths of earth antennae will be between 0-03° and 6 ° between the half-power points, with the near side-lobes (5 to 20 beamwidths) averaging 10 dB above isotropic and with side- and back-lobes averaging zero to 12 dB below isotropic. Beamwidths of the less elaborate antennae used at the lower frequencies may be as great as 20°. Using the assumption of isotropic gain as a first approximation, the interference power-density (dBW per hertz) can be converted to an equivalent interference spatial power-density by multiplying by the effective area of an antenna with unity gain. For example, at 1-7 GHz, the effective area of the isotropic antenna is 2-5 x 10~3 m2 and the interference spatial power- density is —194 dBW per Hz/m2 (corresponding to —220 dBW per hertz). Rep. 219-1 — 516 — 3.2 Protection for the spacecraft The derivation of a criterion for protection of the space station is similar to that of the earth station, but some modification is necessary. 3.2.1 The factors which make space stations more vulnerable than earth stations are: — space stations are in the direct line-of-sight from large areas of the Earth; — antennae on space stations are necessarily pointed directly at the Earth and poten tial interfering sources; — critical periods may occur when a space station is over any point on the Earth. 3.2.2 The factors which reduce the vulnerability of the space-station are : — the operating noise temperature of the receiver in the space-station is usually limited (600°K or —201 dBW per hertz) by the necessity of viewing the warm earth; — the detection-bandwidth on the space-station is greater (1 kHz), due to the need for rapid, automatic acquisition of signals from the earth; — the angular motion of the space-station in the heavens keeps the space-station from staying very long within the (fixed) main beam of an interfering station (angular motions are 0-25°/min or faster). Typical beams are 1° wide; — the high velocity of the space-station keeps it from staying within 1 0 0 0 km of any one station for longer than 5 min ; — for deep-space missions, the design margins permit the interference spectral- density to equal that of the noise background ; — for near-earth missions, the vulnerability of the command-receiver in the space station can be further reduced by raising its threshold of sensitivity, permitting the satisfactory operation under interference levels 10 dB greater than the noise temperature of the receiver. The appropriate criterion for protection of the space-station is therefore: For frequencies greater than 300 MHz, the total time, during which the power density of noise-like interference or the total power of CW-type interference in any single and all sets of bands 1 kHz wide, is greater than —171 dBW per kHz at the input terminals of the receiver in the space station, shall not exceed five minutes per day for deep-space and manned research missions (—161 dBW per kHz not to exceed 0-1% of the time being permissible for other near-earth research missions); for frequencies less than 300 MHz, the permissible interference may be increased at the rate of 20 dB per decreasing frequency decade. 4. Examples of mutual interference The following examples are intended to show the order of magnitude of the mutual interference in various situations for single line-of-sight transmission : 4.1 Other stations within direct line-of-sight o f an earth station in the space-research service A power density of —220 dBW per hertz is produced by the sources described below, radiating into the 0 dB side-lobes of an earth station operating at 2 GHz. — 517 — Rep. 219-1 T a b l e II Distance Bandwidth E.i.r.p. needed to Source (kHz) O equal -220 dBW (km) per hertz Fixed, mobile, aircraft...... 10 5 0*5 pW 5000 0-5 mW A ircraft...... 100 5 50 pW 50 0-5 mW Aircraft, satellites...... 1000 5 5 mW 5000 0-5 W 0) A uniform spectral density is assumed. The above examples show that typical line-of-sight sources will produce harmful inter ference to an earth station for space-research, because each type of source in Table II normally radiates a considerably higher e.i.r.p. (and often in narrow-band form) than would be permit ted by the interference criterion. This interference can be expected to. be mutual because space-research earth stations often transmit 100 kW through their 0 dB side-lobes. 4.2 Other stations within the line-of-sight o f the spacecraft 4.2.1 Deep-space research A power density of —171 dBW per kHz is produced by the sources described below, radiating into an omnidirectional antenna on the space-station receiving at 2-1 GHz. As discussed in previous texts, distances less than 1000 km need not be considered. T a b l e III Distance Bandwidth E.i.r.p. needed to Source produce —171 dBW (km) (kHz) C) per kHz Fixed, mobile, aircraft and satellites . . 1000 1 0 1 w 50 5 W 1000 100 W 3000 1 1 W 50 50 W 1000 1 kW greater 1 1 W than 50 50 W 3000 (2) 1000 1 kW 0) A uniform spectral density is assumed. (') Assumes that the antenna gain of the spacecraft is adjusted for altitude. The above examples show that a variety of line-of-sight sources can exist within the criterion, provided that the space-station does not remain too long in the main beam of the interfering station, because the e.i.r.p. via the side-lobes of such sources is typically less than that in Table III. There are some possible exclusions. Terrestrial Rep. 219-1 — 518 — 4 radars, whether aimed at the spacecraft or not, will be harmful and, in turn, will probably be harmed if receiving at the transmitted-frequency of the space station. Care should be taken to determine the possible presence of sources of narrow-band radiation below the space station during the near-earth phase of flight. On the other hand, without illustrating the calculations here, it can be stated that harmful interference from the space station into other terrestrial circuits is unlikely. The space station, because of its angular motion in the sky, will seldom be in the main beam of the other circuit. Also, the distance from the space station to the other service will be at least ten times as great as the length of the service circuit. 4.2.2 Near-earth satellites A power density of —161 dBW per kHz is produced by the sources described below radiating into an omnidirectional antenna on a satellite receiving at 500 MHz and 1-5 GHz. T a b le IV Distance Bandwidth E.i.r.p. to produce E.i.r.p. to produce Source (kHz) O -161 dBW per kHz -161 dBW per kHz (km) at 500 MHz at 1-5 GHz Fixed, mobile or aircraft 500 1 10 mW 100 mW 50 500 mW 5 W 1000 10 W 100 w 1000 1 40 mW 400 mW 50 2 W 20 W 1000 40 W 400 W 3000 1 400 mW 4 W 50 20 W 200 W 1000 400 W 4 mW (l) Uniform spectral density assumed. The above examples show that some line-of-sight sources can operate within the criterion, provided that the satellite remains at a high altitude (above 1 0 0 0 km) and does not remain in the main beam of the interfering station for too long, since the e.i.r.p. via the side-lobes of such sources is typically less than that in the Table, partic ularly for frequencies in the region above 1 GHz. There are some exclusions : terrestrial radars, whether aimed at the satellite or not, can be harmful and, in turn will probably be harmed if receiving on the satellite transmitting frequency. In general, harmful interference from a satellite into other terrestrial circuits is unlikely. The satellites will be in the main beam of the other circuit only occasionally and for short periods of time, because of the angular motion of the satellite in the sky. 4.3 Other stations over the horizon from a space-research earth station The permissible distance between ah interfering source and the affected trans-horizon receiving system is theoretically calculable, providing the interference criteria and sufficient — 519 — Rep. 219-1 knowledge of the propagation conditions are known. Minimum permissible distances between typical earth stations and terrestrial stations with the parameters listed below have been calculated using the methods outlined in Reports 243 and 244-1 and in Annex II to this Report. T a b l e V Deep-space Earth-satellite Terrestrial earth station earth station Transmitter power ...... 10 W 100 kW 1 kW Bandwidth of emission...... 10 MHz 1 kHz 4 kHz Coupling loss ( d B ) ...... 4 4 4 Sum of antenna gains in pertinent direction ( d B ) ...... 0 0 0 Operating noise temperature (°K). 900 30 30 Unit receiver bandwidth...... 4 kHz 1 Hz 1 Hz Noise power in unit bandwidth . . -1 6 3 dBW/4 kHz -2 1 4 dBW/1 Hz -2 1 4 dBW/1 Hz Maximum tolerable interfering power per unit bandwidth . . . -1 6 3 dBW/4 kHz -2 2 0 dBW/1 Hz -2 2 0 dBW/1 Hz Available transmitter power per unit bandwidth...... - 6 4 dBW/1 Hz 46 dBW/4 kHz 26 dBW/4 kHz From these calculations, which indicate that interference from the space-research earth stations to the terrestrial station receivers will be the limiting factor, it can be shown that the minimum separation ranges from approximately 300 km at 1 GHz for earth-satellite stations to approximately 440 km at 2 GHz for deep-space earth stations. Caution should be exercised in applying these illustrative computations to specific situa tions : for example, if naturally shielded sites are available, it may be possible to obtain 1 0 to 50 dB of additional attenuation between the sites. On the other hand, an aircraft in mutual view of the two stations (interfering and interfered) can produce harmful interference by reflection of the signals from one station to the other, particularly if the aircraft is in the beam of one of the stations. Terrestrial radars over the horizon from the earth station can produce harmful inter ference if they illuminate aircraft, space objects, or the Moon, which are above the horizon of the earth station. Depending upon the circumstances, the illuminated target may or may not have to be in the main beam of the earth station. For this reason, it is highly desirable that the space-research service does not share frequencies with terrestrial radars, regardless of point-to-point distance separation along the Earth. The interference potential can be shown to be mutual if the radar is receiving at the space-research earth transmitting frequency and both stations are viewing the same target. 4.4 Other stations over the horizon from a space station From the previous discussion, stations over-the-horizon from the space station are unlikely to create mutual interference problems. Rep. 219-1 — 520 — 4.5 Sharing within the space-research service The sharing of frequencies among near-earth satellite telecommunication links will in general be feasible, because of the use of directional ground receiving antennae, on-board data-storage, on-board data processing, and the increasing use of ground commands for the initiation and cessation of transmission. However, to avoid mutual interference problems, considerable prior planning and close coordination will be required. Sharing of frequencies among near-earth satellites and deep-space research spacecraft is not desirable, since satellites will seriously interfere with reception at a deep-space earth station, whenever the satellites are within line-of-sight of the earth station. Different spacecraft for deep-space research at the same target will interfere with each other’s transmission to the earth and consequently must use different frequencies. Spacecraft at different planets, however, must be followed by different narrow-beam earth antennae and consequently mutual interference is unlikely. 5. Conclusions It is concluded that telecommunication links for manned and unmanned space-research can share frequencies with terrestrial services only in relatively remote areas, where terrain- shielding or large geographical separation (several hundreds of kilometres) are possible. If space research is to be undertaken in less remote areas, frequency sharing with other services does not appear practical. For the near-earth and recovery phases of manned space-research missions, it is necessary to provide interference-free voice frequencies in the HF region below 20 MHz. Additionally, the frequencies provided should be spaced within the high-frequency region, so that reliable communications can be achieved under various propagation conditions. It is urged that proper type propagation measurements be made to permit calculations of permissible interference on the basis of 5 min per day. ANNEX I THE EFFECTS OF INTERFERENCE ON NARROW-BAND PHASE-LOCK LOOPS 1. Introduction An experimental investigation of the effects of interference on narrow-band phase-lock loops has been conducted to determine the degree of performance degradation caused by interfering signals on the ground-station receivers of space-research communication sys tems [1]. A summary of the results of this investigation is presented below. 2. Summary of results Experimental data were taken with narrow-band (1, 12, 48 and 152 Hz effective noise bandwidths) phase-lock circuits which had the same transfer characteristics as those currently used in space-research receivers. These circuits were subjected to two types of interfering signal: — varying levels of CW interfering signals which were swept through the loop passband at varying rates; — 521 — Rep. 219-1 — varying levels of a noise-modulated FM interfering signal of low modulation index which simulated the power spectral density of an FM radio-relay system with 1800 loaded voice channels. The degradation of loop performance due to interference under those conditions that did not cause cycle skipping or loss of lock was measured by defining an “equivalent signal- to-noise ratio” (R0) for the loop signal-to-noise ratio, as the inverse of the loop mean-square phase error: R 0 = 1/cre2, where ae2 is the mean-square phase error. This definition is a natural one, since an analysis of a linearized phase-lock loop yields a value for loop mean-square error due to random receiver noise o f : ere2 = Bl9 J S where S is the wanted signal power, BL is the equivalent noise bandwidth of the phase-lock loop and cp„ is the power spectral density of the noise at the input of the loop. The term y nBL is thus the noise power referred to the loop bandwidth. Use of the mean-square phase error as a measure of system performance led to a difficulty when dealing with slowly sweeping interfering signals because of the transient nature of the interference. This difficulty was overcome by averaging squared error only over the period of time during which the interference was within the passband of the loop. In the experiments, the squared values of phase error were averaged over a time period corresponding to the time the sweeping interfering signal was within the — 6 dB points of the loop amplitude- frequency response curves. Repeated trials were used to obtain sufficient data for fast sweep cases. During periods when a phase-locked loop is out of lock, its usefulness is lost. Therefore certain noise-interference environments can be designated as definitely placing the loop beyond its limit of operation. However, because a loop can often be caused to “skip cycles” without losing lock for extended periods, it is difficult to define the boundary of this destructive environment from phase-error records alone. In particular, it is usually impossible to deter mine whether the phase error has simply dwelled near 90°, or whether the loop has skipped one or more cycles before the phase detector responds; simple phase detectors do not give unambiguous outputs except over a range of ± 90°. To define the threshold of useful loop operation, the average cycles skipped were counted for various interference-to-signal ratios. A good indication of cycle skipping activity was obtained by totalling net cycles skipped over many counting periods. The usual signals were recorded during the skipped cycle counts, so that counts could be correlated with phase-error measurements. For CW interference sweeping across the input signal, a rather simple programme was set up for treating the different possibilities. The ranges of experimental parameters were primarily determined by the fact that lock was nearly always lost in the following cases: — with no interference present and a loop signal-to-noise ratio (R) below 6 dB ; — with no additive noise present and a loop input signal-to-interference ratio (Rt) of about — 3 dB. The experiments were conducted with values of R of 3, 6 , 9, 12 and oodB and Rt of —3, 0, 3, 6 , 9 and oo dB. A complete test sequence was used for the 12 Hz bandwidth loop and interference signal sweep rates of 1, 10, 100, 500 and 1000 Hz/s. Sample data were taken 22 Rep. 219-1 — 522 — at the other loop bandwidths to determine that the data at 12 Hz were typical. The raw data was interpolated and smoothed to obtain the best fitting curves by subjecting the data to a step-wise multiple regression procedure. A summary of the experimental data taken with interfering CW signals, having a sweep rate of 1 Hz/s and with a loop bandwidth of 12 Hz, is presented in Figs. 1 and 2. Fig. 1 shows the variation in loop threshold as a function of interfering CW signal-to-noise power ratio, where loop threshold is defined as the point where the numbers of cycles skipped per second by the loop equals 0-025 BL (loop bandwidth). Fig. 2 shows the decrease of equivalent loop signal-to-noise ratio as a function of the interference-to-noise ratio. This increase in mean- square error due to the interference signal may be considered a measure of the degradation of the loop. Experimental data indicates that these figures may be scaled to other loop band widths. Tests with slowly swept simulated FM radio-relay signals indicated that the effect on the loop of signals of this type is essentially the same as a CW signal. For cases where frequency separation places the carrier well outside the IF passband, the FM sidebands falling within the loop bandwidth have an effect on the loop that is similar to additive noise of the same power level. 3. Conclusions For CW interfering signals at levels within several dB of the level of the receiver thermal noise power in the loop noise bandwidth, interference effects are negligible if the frequency of the interfering signal is separated from that of the desired signal by several multiples of the loop noise bandwidth of the phase-lock receiver, or if the interfering signal sweeps through this bandwidth at a sweep rate of the order of 1 0 times the square of the loop bandwidth. However, when the interfering signal sweeps slowly through, or remains within the bandwidth of the loop, it can cause large phase errors, cycle skipping, or cause the receiver to break lock, in which case it will often transfer lock to the interfering signal. This latter effect can occur if the interference level exceeds the signal level. Slowly sweeping CW interference will cause a decrease in effective signal-to-noise ratio in the loop of as much as 1 dB for interference to noise levels of —3 dB and as much as 0-5 dB for interference to noise levels of — 6 dB. These levels of interference can cause cycle skipping or loss of lock to occur if the R in the absence of interference is of the order of 6 dB. FM signals with low modulation index have essentially the same effect as CW signals when the carrier component sweeps through the loop passband. The sidebands of a noise modulated FM signal have an effect similar to white additive noise at the same power level. B ibliography 1. B r it t , C. L ., P a l m e r , D. F . and S m it h , P . G. A study of the effects of interference on narrow-band phase-lock loops. N.A.S.A. Contractor Report CR-66036, National Aeronautics and Space Administration, Washington, D.C. (October, 1965). — 523 — Rep. 219-1 Interference-to-noise power ratio (dB) F ig u r e 1 Variation in loop threshold as a function of interfering CW signal-to-noise power ratio Loop threshold is defined as the point where the number of cycles skipped per second by the loop equals 0-025 Bl (loop bandwidth) (From data taken with Bl ph 12 Hz and interfering sweep rate 1 Hz/s) Rep. 219-1 — 524 — Equivalent signal-to-noise ratio with no interference Q -16 -1 2 -8 - 4 0 4 8 Interference-to-noise power ratio (dB) F ig u r e 2 Degradation of equivalent loop signal-to-noise ratio as a function o f CW interference-to-noise power ratio (From data taken with Bl ra 12 Hz and interfering sweep rate 1 Hz/s) — 525 — Rep. 219-1 ANNEX II PROCEDURES FOR PREDICTING INTERFERENCE LEVELS AT SPACE-RESEARCH EARTH STATIONS 1. Introduction Methods for predicting the interference between space-research earth stations and ter restrial services, when the protection criteria are expressed as percentages of the time, are discussed in Reports 243 and 244-1 and in [1]. This Annex sets forth procedures to be used when protection from interfering signals must be in accordance with criteria established in the main body of this Report and Recommendations 365-1 and 366-1 for deep-space and manned research missions. These criteria dictate that interfering signal levels shall not exceed specified values for more than five minutes per day rather than for a percentage of time. This Annex contains: — a discussion of the factors involved when utilizing standard propagation data to limit harmful interference to less than five minutes per day ; — curves derived from propagation data contained in Reports 243 and 244-1 and [1]; — sample calculations to demonstrate the use of the procedures and curves. 2. Discussion The final step in estimating the interference at space-research earth stations may require point-to-point prediction and measurements which are not covered in this Annex. Only propagation factors are considered here. Propagation data are generally available in terms of hourly medians of path signal attenuation over a given period of time. In providing pro tection for an earth station from interference by an unwanted signal, consideration must be given to the probability that the path signal attenuation between stations will be less than an acceptable (tolerable) value of attenuation for more than five minutes in any day. The acceptable value of attenuation is determined from calculations utilizing the transmitted power within the passband of the receiver sensitivity, radiation patterns and pointing direc tions of both antennae, polarizations, the effects of phase interference, modulation and inter modulation characteristics and a maximum allowable interference level of —220 dBW per hertz for each and every band 1 Hz wide. An analysis of available propagation data (as yet unpublished) shows, that the interference criteria of five minutes per day can be considered to be met if the predicted minimum attenuation for the worst hour in a year * has a probability of 95% of being greater than the acceptable value of attenuation. 3. Sample propagation curves The curves shown in Figs. 1, 2 and 3 are based upon the following conditions: p = 0 -0 1 % (i.e., the hourly median attenuation of the unwanted signal will not be less than the indicated values for more than 0 -0 1 % of a typical year), P = 5-0% (i.e., the probability of interference for more than 0-01% of time is 5%), hx — 15, structural height of transmitting antenna in metres, h2 = 15, structural height of receiving antenna in metres, Be = 0° and 5°, where Qe is as defined in Fig. 4. * Minimum hourly median attenuation, corresponding to 0-01% of a year, since one hour is approximately 0 -0 1 % of a year. Rep. 219-1 — 526 — 4. Sample calculations The following sample data are taken from the examples in Table V of § 4.3 of the Report. Interfering station Deep-space Earth-satellite Terrestrial earth station earth station Deep-space or Interfered-with station earth-satellite Terrestrial station Terrestrial station earth station Available transmitter power per unit band - 6 4 dBW/Hz 46 dBW/4 kHz 26 dBW/4 kHz width Maximum tolerable in terfering power per -220 dBW/Hz -1 6 3 dBW/4 kHz -163 dBW/4 kHz unit bandwidth Minimum acceptable transmission loss 156 dB 209 dB 189 dB Consider the case where the interfering and interfered-with stations are terrestrial and deep-space earth stations respectively. Referring to Fig. 1, draw a horizontal line correspond ing to 156 dB transmission loss. Note that the line intersects the 5% probability, Qe = 0° curve at a range of 150 km. This distance represents the minimum distance allowable between the earth station and a terrestrial station, which will provide protection to the earth station in accordance with the interference criteria established in this Report and Recommendations 365-1 and 366-1. Should additional shielding to the earth station be available due to terrain conditions, the distance between the stations may be reduced. Methods described in [1] may be used to estimate the amount of such reduction. B ibliography 1. R ic e , P. L ., L o n g l e y , A. G., N o r t o n , K. A. and B a r s is , A. P. Transmission loss predictions for tropospheric communication circuits. NBS Technical Note 101 (1965). Basic transmission loss (dB) 140 120 180 200 220 240 160 260 10 1 / , 20 Predicted values o f basic transmission loss transmission f basic o values Predicted y (p • irglrtran 0 = 5° = 0e terrain, irregular : A C : free-space loss free-space : C terrain, smooth : B : 0*01% ; Frequency: 2 GHz) 2 Frequency: 0*01%:; 0 0 200 100 50 v Distance (km) Distance 57 — 527 — F gure r u ig : : C / ‘ > V / . A 3 __ Qe p p = 0° = 5% = 50% = ' X ' / ' / / / y / 500 4 ' / / 4 r f > / / / r / f ' / i 1000 f 1 / i Rep. 219-1 Rep. Rep. 219-1 Rep. Basic transmission loss (dB) 0 0 0 0 20 0 1000 500 200 100 50 20 10 Predicted values o f basis transmission loss transmission f basis o values Predicted (p : (p A : irregular terrain, terrain, irregular : A C : free-space loss free-space :C B : smooth terrain, terrain, smooth : B 00% Feuny: GHz) 5: Frequency ; 0-01% Distance (km) Distance 58 — 528 — F gure r u ig : : 4 Qe = Qe = p = p 50% = 5° 5% 0° Basic transmission loss (dB) Predicted values of basic transmission loss transmission basic of values Predicted (p : (p A : irregular terrain, terrain, irregular : A B : smooth terrain, terrain, smooth : B C : free-space loss. free-space :C 00% Feuny = Frequency ; 0-01% Distance (km) Distance 59 - 529 — : : — F gure r u ig 5 Qe Qe = p p = 0° = 5° = 8 GHz) 50% % 5 Rep. 219-1Rep. Rep. 219-1 -530- FIGURE 6 Definition of angular distance, 6, and the sum of horizon elevation angles, 6e = 6e1 +ea (a : effective radius of the Earth) — 531 — Rep. 222-1 REPORT 222-1 * FACTORS AFFECTING THE SELECTION OF FREQUENCIES FOR TELECOMMUNICATIONS WITH SPACECRAFT RE-ENTERING THE EARTH’S ATMOSPHERE (Question 6/IV) (1963 — 1966) The communications problem encountered by spacecraft re-entering the earth’s atmosphere is’due to the conversion of kinetic energy into thermal energy during the deceleration process, causing a layer of ionized air around the vehicle, which temporarily prevents the passage of some frequencies. This layer is commonly called the “plasma sheath” , and its effect is called “re-entry radio blackout”. The vehicular plasma sheath, which is analogous in many respects to the ionosphere of the earth, contains free electrons in high concentration. These electrons are created by thermal ioniza tion processes in the vehicle flow field. The resulting plasma interferes with the propagation of electromagnetic energy. The degree of interference depends upon the frequency of the electro magnetic energy, the number density or concentration of electrons, and the frequency of collision between electrons and air molecules. In the same way that a layer of the ionosphere has a critical frequency which must be exceeded for signal penetration, the plasma sheath surrounding a re-entering vehicle also has a critical frequency. However, since the plasma sheath electron density will often be extremely high, the critical frequency will be correspondingly high. The theory of electromagnetic propagation in an ionized layer is applicable to propagation through a plasma sheath [1, 2, 3]. In computing expected signal loss, the simplest approach is to consider plane-wave propagation through a relatively uniform plasma model. More exact compu tations, especially those which allow for appreciable variations in plasma properties within a wave length of the incident radiation, require involved mathematical techniques [4, 5, 7, 8 ]. The description of the plasma sheath must be obtained from computations which take into considera tion the aerodynamic, thermodynamic and chemical kinetic processes for a specific spacecraft shape and trajectory. In general, sharp nosed spacecraft exhibit thin plasma sheaths while blunt bodies exhibit much thicker ones. The electron density in the sheath is a complex function of velocity and altitude, generally increasing with velocity and decreasing with altitude, while the collision frequency depends principally on altitude. In addition, the possible effect of the plasma sheath on the impedance, and thus the efficiency of an antenna, must be considered. This effect will normally be appreciable even at frequencies higher than the critical frequency of the sheath. At least one experiment, with a specific space craft antenna, has shown only a small effect [1 0 ], but larger effects may exist under different condi tions. For a re-entering spacecraft, estimates can be made of the effect of the plasma sheath on signal transmission using the theory of electromagnetic propagation, in conjunction with the aerodynamic and thermodynamic characteristics of the flow field of the spacecraft. These predictions must take into consideration such factors as the configuration of the spacecraft, the velocity, altitude, operat ing frequency, etc., as well as the effect of detuning of the antenna. The propagation computations result, quite generally, in a transmission curve giving high loss at frequencies lower than the critical frequency. The extent of the transition region depends on the collision frequency and, for thin layers, on the thickness of the layer as well. Provisional estimates and a few experimental results indicate that the critical frequency is often as high as 1 to 10 GHz and may sometimes be even higher. It is concluded that frequencies of 10 GHz or * This Report was adopted unanimously. Rep. 222-1 — 532 — higher are technically required for some re-entry communications, especially for re-entry from lunar or planetary missions [6 ]. At these frequencies, absorption in the earth’s atmosphere can be very important. Report 205-1 gives data on this point. Report 223-1 shows that there are several “windows” above 60 GHz where the absorption in atmospheric gases may be acceptably low. The data in Report 205-1, however, indicate that absorption in precipitation could be prohib itively high, but that frequencies near 90 GHz and perhaps those near 140 GHz might be usable in a relatively dry climate. It is possible on theoretical grounds, that frequencies in band 7 (3-30 MHz) or lower could be used, if the plasma sheath was thin and the collision frequency high. Moreover, the effects of the plasma sheath could be reduced by modifying the plasma itself; for example, by aerodynamic shaping (sharp nosed or spike configurations), to reduce the plasma thickness, by injecting into it a material with a strong affinity for electrons, and by the use of a strong magnetic field to provide a propagation window. Possibly, a combination of techniques, such as body shape design, choice of frequency, careful siting of antenna location, and material injection may be used to reduce plasma densities. These possibilities are being studied [11, 12, 13, 14]. The attention of the U.R.S.I. is drawn to this Report. B ibliography 1. B a c h y n s k i, M. P., J o h n s o n , T. W. and S h k a r o f s k y , I. P. Electromagnetic properties of high temperature air. Proc. IRE, 48, 347 (March, 1960). 2. H o l l is , R. Reciprocal relations in an N-slab dielectric. Proc. IRE, 49, 1579 (October, 1961). 3. D ir s a , E. F. The telemetry and communication problem of re-entrant space vehicles. Proc. IRE, 48, 703 (April, 1960). 4. R a w e r , K. Elektromagnetische Wellen in einem geschichteten medium. (Electromagnetic waves in a stratified medium.) Annalen der Physik (1939). See also correction, ibid., 1942. 5. B u d d e n , K. G. Radio wave propagation in the ionosphere. Cambridge (1961). 6 . C.C.I.R. Doc. IV/14 (U.S.A.), Washington, 1962. 7. K l e in , M. M., G r e y b e r , H. D ., K i n g , J. I. F. and B r u e c k n e r , K. A. Interaction of a non-uniform plasma with microwave radiation. Electromagnetic effects of re-entry. Selected papers from the Symposium on Plasma Sheath, its effect on communication and detection. Boston, Mass. (1959), Pergamon Press, 105 (1961). 8 . C.C.I.R. Doc. IV/8 (Federal Republic of Germany), 1963-1966. 9. C.C.I.R. Doc. IV/70 (U.R.S.I.), 1963-1966. 10. C l o u t ie r , G. C. and B a c h y n s k i, M. P. Antenna characteristics in the presence of a plasma sheath. In electromagnetic theory and antennae, Proceedings of a symposium held at Copenhagen, Den mark (June, 1962), edited by E. C. Jordan. Pergamon Press (1963). 11. Electromagnetic effects of re-entry. Selected papers from Symposium on Plasma Sheath, its effects on communication and detection. Boston, Mass. (1959), edited by Walter Rotman and Gerald Meltz. Pergamon Press, 1961. 12. Electromagnetic aspects of hypersonic flight. Second Symposium on Plasma Sheath. Boston, Mass. (1962), edited by Walter Rotman, Howard K. Moore and Robert Papa. Cleaver Hume Press, London (1964). 13. Transactions of the Eighth Symposium on Ballistic Missile and Space Technology, Vol. II, Air Force Systems Command and Aerospace Corporation, U.S. (October, 1963). 14. Sim s , T. E. Re-entry communications research at the Langley Research Center. IEEE Convention Record PT IV Aerospace and Military Electronics (1965). — 533 — Rep. 223-1 L. 8: Radioastronomy REPORT 223-1 * LINE FREQUENCIES OR BANDS OF INTEREST TO RADIO ASTRONOMY AND RELATED SCIENCES, IN THE 30 TO 300 GHZ RANGE ARISING FROM NATURAL PHENOMENA (Question 10/IV) (1963 — 1966) Radioastronomical observations at millimetre wavelengths have increased rapidly in recent years. Technological improvements in receivers and antennae have been largely responsible for this increase, and it is clear that more use of the millimetre spectrum can be expected, both for radioastronomy and other purposes. Astronomical observations using radio techniques have been made at wavelengths as short as 3 mm in the U.S.A. and 1 mm in the U.S.S.R. Furthermore, infra-red or bolometric techniques have been applied to astronomical observations in the same general range and in shorter wavelengths. Therefore, from the point of view of availability of equipment, the entire millimetre and sub-millimetre spectrum can now be employed for astronom ical measurements. Scientific interest in this portion of the electromagnetic spectrum stems not only from radioastronomy but also from the potential for studies of the terrestrial atmosphere. Many atmospheric constituents have resonance lines in the millimetre spectrum, and the study of these resonances for geophysical purposes is becoming quite feasible. In both cases, adequate protection from man-made interference is highly desirable. The choice of the wavelength of observation for astronomical observations is dictated by the transmission properties of the earth’s atmosphere, i.e., the location of the so-called “windows” in the atmosphere. On the other hand, the choice of the wavelength of observation for geophysical studies of the atmosphere itself is dictated by the wavelength of the particular resonance line sought. The presently-known regions of interest are summarized in Table I, and shown schema tically in Figs. 1 and 2. In general, the windows are defined by the resonance of molecular oxygen and water vapour. The abundance of other atmospheric constituents, for example, CO, NO, N 0 2, etc., is sufficiently small that their presence in a window will not significantly alter the use fulness of this window for astronomical observations. Therefore, bands in this region of interest to radioastronomy include, rather than exclude, the resonance lines of these minor constituents. T a b l e I Band suggested Frequency for scientific Nature of phenomenon (GHz) programme (GHz) Window in earth’s atm osp h ere...... 30-35 31-3-31-5 OH ...... 36-983-36-994 \ 36-38 o s ...... 36-023 and 37-832/ Window in earth’s atm osp h ere...... 80-90 88-90 n 2o ...... 100-492 98-102 CO ...... 115-271 ) o 3 ...... 118-364 J 110-120 o 2 ...... 118-746 j Window in earth’s atm osp h ere...... 130-150 0 N O ...... 150-176-150-644 148-152 h 2o ...... 183-311 178-188 Window in earth’s atm osp h ere...... 200-300 0 C) Band recommendation must await further study. * This Report was adopted unanimously. Rep. 223-1 — 534 — B ibliography 1. A a r o n s , J., B a r r o n , W. R. and C a s t e l l i, J. P. Radioastronomy measurements at VHF and micro waves. Proc. IRE, 46, 122 (1958). 2. B a r r e t t , A. H. Microwave spectral lines as proofs of planetary atmospheres. Mem. Soc. Roy. des Sci., Ltege, 5e, 7, 197 (1962). 3. C o a t e s , R. J. Measurements of solar radiation and atmospheric attenuation at 4-3 millimetres wave length. Proc. IRE, 46, 122 (1958). 4. Handbook of Geophysics (revised). Macmillan Co., New York (1960). 5. Hogg, D. C. Effective antenna temperatures due to oxygen and water vapour in the atmosphere. J. Appl. Phys., 30, 1417 (1959). 6 . H o g g , D. C. and C r a w f o r d , A. B . Measurement of atmospheric attenuation at millimetre wave lengths. B.S.T.J., 35, 907 (1956). 7. K a m e n s k a y a , S. A., K is l y a k o v , A. G., K r o t ik o v , V. D ., N a u m o v , A. I., N ik o n o v , V . N., P o r f ir ie v , V. A., P l e c h k o v , V. M., S t r e z h n y o v a , K. M., T r o it s k ii, V. S., F e d o see v , L. I., L u b y a k o , L. V. and S o r o k in a , E. P. Observations of radio eclipse of the Moon at millimetre waves. Izv. Vyss. Ucebn. Zaved. Radiofizika, 8, 219 (1965). 8 . M e e k s, M. L. Atmospheric emission and opacity at millimetre wavelengths due to oxygen, J. Geop. Res. 6 6 , 3749 (1961). 9. R o s e n b l u m , E. S. Atmospheric absorption of 10-400 GHz radiation. Microwave J. 91 (1961). 10. V a n V l e c k , J. H. The absorption of microwaves by oxygen. Phys. Rev. 71, 413 (1947). 11. V a n V l e c k , J. H. The absorption of microwaves by uncondensed water vapour. Phys. Rev., 71,425. 12. W h it e h u r s t , R. N ., C o p e l a n d , J . and M it c h e l l , F. H. Solar radiation and atmospheric attenua tion at 6-mm wavelength, J. Appl. Phys., 28, 295 (1957). Attenuation (dB/km) A : sea-level; P = 760 mm H g; T = 20°C ; p (HaO) = 7-5gm/m 20°C (HaO)p;= = T g; 760H mm = :sea-level;P A B : 4 km above sea-level; T = 0°C, p (H p0°C, = T abovesea-level;B :km 4 Attenuation per kilometre for horizontal propagation horizontal for kilometre per Attenuation Frequency(GHz) Wavelength (cm) 55 — 535 — F igure 1 2 0) = 1*0 gm/m = 0) 3 3 Rep. 223-1 Rep. Rep. 223-1 Rep. Attenuation (dB/km) : T W : W A : C : C D : : A a r o n s s n o r a A D ick e e ick D T exas exas T oates hitehurst et al et 1960 1960 1958 Total attenuation for a one-way transmission through the atmosphere the through transmission one-way a for attenuation Total 1958 1958 1946 1946 1957 1957 H : Handbook Geoph. Handbook : H R ------: Frequency(GHz) Wavelength(cm) ng ( g g o (H g in R 56 — 536 — F igure T h eissin g g eissin h T H ogg 2 1959, 1960 1960 1959, 1960) 1960) and and 1960 1960 an n la p a K 1958 X mean :Y Z Z dry : humid : — 537 — Rep. 224-1 REPORT 224-1* RADIOASTRONOMY Characteristics and factors affecting frequency sharing with other services (Question 10/IV) (1963 — 1966) 1. Introduction Radioastronomy and the radioastronomy service are defined in Article 1, Nos. 74, 75, and 75A of the Radio Regulations, Geneva, 1959 (as revised in the Final Acts of the Extra ordinary Administrative Radio Conference, Geneva, 1963), as being astronomy based upon the reception of radio waves of cosmic origin. Since it uses receiving techniques only, the radioastronomy service does not cause interference to any other service. “ Radar” astronomy, which involves the transmission of a signal at a high power-level and the detection of that signal after reflection from celestial bodies, man-made satellites, or meteor trails, is a quite different service, and is defined in Question 11/IV. Radioastronomy began with the discovery in 1932, by Karl Jansky [1], of radio waves of extra-terrestrial origin. The cosmic emissions with which the radioastronomy service is concerned constitute the “cosmic background noise” of communications engineering. Since Jansky’s original observations, remarkable progress has been made in identifying the nature of these emissions, and radioastronomy is now firmly established as an important branch of astronomy. It is a new field of science, but it has already made important con tributions, for example, to the measurement of atmospheric absorption at radio frequencies; to our knowledge of the composition and nature of the Sun, the planets, and interplanetary space and, in particular, of major disturbances in the solar atmosphere which are often fore runners of interruptions to radiocommunication circuits and of radiation hazards to man in space. Further afield, studies of individual sources over a range of frequencies, and of the “line” emissions at precise frequencies which result from transitions within certain atoms, are providing information basic to our understanding of the physical processes responsible for the emissions of the physics of plasmas, and of the structure and evolution of galaxies and of the Universe as a whole. The radioastronomy service offers means for studying magnetic fields in distant regions of the Universe, and much of the information it provides is unique in that it is unobtainable by optical or other methods ; one of its most spectacular characteristics is the ability to probe even further into the depths of space than is possible with the largest optical telescopes. In addition to providing new knowledge and understanding of great significance to astrophysics and cosmology, radioastronomy is repaying, in a practical way, some of the investment of specialized radio techniques that helped to bring it into being. It supplied a major stimulus to the development of maser and parametric amplifier techniques, and hence to an increase, by orders of magnitude, in the sensitivity attainable in radio receivers. It has also made, and is continuing to make, significant contributions to the design of large steerable antennae and feed systems. The cosmic emissions with which the radioastronomy service is concerned are character ized by low power-flux levels at the earth and by the absence of modulation, other than random noise. For many sources the best times for observation are dictated by natural phenomena, * This Report, which was adopted unanimously, replaces Report 225. 23 Rep. 224-1 — 538 — over which the observer has no control, and radioastronomers are not generally able to observe over any chosen limited time interval. Further, the radioastronomer is unable to change the character of the “signal” he wishes to receive ; he cannot increase the transmitter power nor code the transmitted signal to increase its detectability. The radioastronomy service is thus extremely susceptible to interference, and to ensure its continued progress there is need for the I.T.U. to provide it with the greatest practicable degree of protection. 2. Origin and nature of the emissions 2.1 The radio waves with which the radioastronomy service is concerned are generated in extra terrestrial sources by three distinct mechanisms : — thermal emission from hot ionized gas and from solid bodies ; — non-thermal processes, mainly synchrotron emission from electrons spiralling in a magnetic field, but including also emission from plasmas (as in the solar atmosphere); — “line” emission resulting from transitions within individual atoms. These combine to produce : A continuum o f radiation, which extends relatively smoothly over the whole frequency range accessible to observation; upper and lower limits of observation are imposed by the earth’s atmosphere at roughly 50 GHz and 1 MHz respectively. The continuum is composed of a background together with numerous small “bright” regions, the discrete radio sources. The background shows a general distribution over the whole sky with a broad maximum in the direction of the galactic centre, together with a ridge of intense emission around the galactic equator (the Milky Way), showing a marked maximum in the direction of the centre. The discrete sources, often referred to as radio stars, are, with a few exceptions, notably the Sun, not stars but radio “nebulae”. They are of two kinds, those of extra-galactic origin and those originating within our galaxy. The extra galactic sources are, in general, distributed randomly over the sky while the galactic sources are for the most part confined to within a few degrees of the galactic equator. “Line” emission which, though occurring at the source at one or more precise frequencies determined by the transitions involved, is observable over a band of frequencies as a result of Doppler shifts due to relative motions in the line of sight. Spectral lines are also observable in absorption when a strong source of continuum emission is viewed through an intervening gaseous medium. Intermittent emission (“bursts”), of durations which may vary from seconds to hours. They are most intense in the HF and VHF bands, and those from some disturbances in the solar atmosphere may vary progressively in frequency, from high to low, during their lifetime. Those so far detected originate in localized areas on the Sun, a few other stars, and the planet Jupiter. 2.2 Continuum radiation and discrete sources The discovery of radio sources and the bulk of current knowledge about their nature and distribution, and of the processes responsible for the radio emission from them, has come through observations of the continuum radiation, made at a limited number of frequencies at the lower end of the band transmitted by the ionosphere. Observations of intensity need to be made at a number of frequencies to determine the characteristic “spectra” of sources, but, because the distribution of continuum radiation with frequency is relatively smooth, — 539 — Rep. 224-1 observations of this kind do not need to be made at specific or closely adjacent frequencies. Bands, spaced at intervals of about an octave of the radio-frequency spectrum, are normally satisfactory. However, some sources have spectral features requiring observation at closer spacings. Certain radio sources are known to have complex structures on a very small angular scale. Resolution of these structures has been attempted by special interferometers and by the use of occultation by the Moon [2, 3], Both techniques are necessary and provide comple mentary information, the occultation technique being particularly valuable at frequencies below 1000 MHz. The existence of “quasi-stellar” objects, which are extremely powerful radio emitters situated at enormous distances from our galaxy, has opened up a completely new chapter of cosmology. Here again the observations require a series of bands separated at intervals of about one octave, with protection on a worldwide scale because of the reflecting power of the Moon. The continuum radiation from many discrete sources and from parts of the background is partially plane-polarized. A study of this polarization over a range of frequencies is useful in determining the magnetic field conditions in the source of the radiation, and along the radiation path, including the ionosphere [4]. The bands made available to the radioastronomy service, in accordance with the final acts of the Extraordinary Administrative Radio Conference, Geneva, 1963, represent a partial fulfilment of the requirements of the service, and are a significant improvement over the international allocations made to the service in 1959. However, many of the allocated bands have insufficient bandwidths ; they are in most cases shared with other services ; many apply to limited areas of the world; and there are large intervals between some of the allocated bands. 2.3 “Line” radiation Observations of “line” radiation came somewhat later in the history of radioastronomy, the first to be discovered (1951) being that due to neutral atomic hydrogen [5], at a rest fre quency of 1420-4 MHz. In 1963 a second line (a doublet) was discovered due to the hydroxyl ion OH, when absorption was observed in the spectrum of the radio source Cassiopeia A [6 ] from lines whose rest frequencies were determined at 1665-4 and 1667-4 MHz. Absorption due to OH has since been studied in some detail in the spectrum of the radiation from the galactic centre, and intense but very narrow band emission has also been observed from some regions in the galaxy. Studies of the neutral hydrogen line at 1420 MHz have provided, and are continuing to provide, new knowledge about the distribution and motion of interstellar hydrogen, and thus about the mass distribution and the relative proportions of gas and stars in different parts of our own galaxy, and in external galaxies. A series of spectral lines exists at close intervals over a wide range of frequencies, due to hydrogen in excited states in galactic nebulae. Some of these have now been detected. Observations of OH are at a preliminary stage but indicate that the proportion OH to H is unexpectedly high in the vicinity of the centre of our galaxy. Studies of “line” emissions thus provide information, which is often unique and of the most basic significance to astrophysics and cosmology; for example, the detailed distribution of neutral hydrogen and OH cannot be observed by optical methods. The investigation of line emission (or absorption) is one of the most difficult and chal lenging fields in radioastronomy. Reception techniques of the highest attainable sensitivity, often involving long integration times, are required; consequently, freedom from harmful interference is necessary. It is required over a band of frequencies wide enough to include broadening of the original emissions due to Doppler effects, together with a band of compa rable width for comparison or reference purposes adjacent to that containing the line. 2.4 Bursts The Sun is the outstanding source of short-period emissions ; at metre wavelengths, the level of some bursts may exceed that of the thermal radiation from the Sun’s disk by a factor Rep. 224-1 — 540 — of at least 106 [7]. This has led to a search for “burst” emission from some relatively nearby stars of a class known as “flare” stars, which show occasional short-period variations in brightness : in several of them coincident increases in radio and optical brightness have been detected. High intensity bursts are also received from the planet Jupiter [8,9], These are observed sporadically in the frequency range 5 to 30 MHz. 3. Classes of observations 3.1 Radioastronomy observations can be broadly divided into two classes: 3.1.1 Class A observations are those in which the sensitivity of the equipment is not a primary factor. They are often used in the study of those cosmic emissions which are of relatively high intensity. Many of the solar, Jupiter, riometer, and scintillation observations fall into this class ; continuity is a primary factor for these observations. 3.1.2 Class B observations are of such nature that they can be made only with advanced low- noise receivers using the best available techniques : long integration times and wide receiver bandwidths are usually involved. The significance of these observations is critically dependent upon the sensitivity of the equipment used in making them. 3.2 The sensitivity of receivers used for class B observations and the levels of harmful interference are discussed in Annex I. Some of the basic measurements in the radioastronomy service are those of the absolute intensity of emissions originating in the outer fringes of the Universe; for these and many other radioastronomy observations the presence of unwanted signals can produce misleading or erroneous results. Because the levels of interference that can be tolerated are so very low it is often impracticable to measure them with conventional monitor ing equipment; it is difficult to measure them with a very sensitive radioastronomical receiver unless it is used with a high-gain antenna. Some typical calculations are given below. It is noted from Recommendation No. 11A of the Final Acts of the Extraordinary Administrative Radio Conference, Geneva, 1963, that further consideration to the provision of improved frequency allocation for radioastronomy is to be given at the next Ordinary Administrative Radio Conference. 4. Details of radioastronomy observatories Appendix 1A (Section F) of the Radio Regulations (as revised in the Final Acts of the Extraordinary Administrative Radio Conference, Geneva, 1963) describes the information on observatories and on the observations in progress or planned, which Administrations should furnish to the I.F.R.B. for incorporation in the Master International Frequency Register. It is understood that this information will be published by the I.T.U., from time to time, in the form outlined in the revision of Appendix 9 of the Regulations (List VIIIA). 5. Sources of interference The following notes describe some of the more probable causes of interference which may occur, particularly to Class B observations, even at an observing site chosen to minimize interference. Some of these will be significant, even in bands allocated exclusively to the radioastronomy service on either a local or a more extensive basis. 5.1 Terrestrial stations within about 100 km are very likely to cause interference if operated in a band shared with radioastronomy. If they operate at lower frequencies, harmonic radiation can cause interference if it is not suppressed at the source. Unwanted sideband radiation — 541 — Rep. 224-1 from high-power transmitters can have appreciable energy in an adjacent radioastronomy band. Adequate suppression of both harmonic and unwanted sideband radiation at the transmitter is technically possible, provided that sites are chosen with care. In meteorological conditions leading to anomalous tropospheric propagation, the distance from which inter ference can be caused by ground transmitters is likely to be increased to several hundred kilometres, but these conditions will occur only infrequently in most parts of the world. At VHF, interfering signals can be received occasionally by ionospheric propagation. 5.2 Some transmitters carried by aircraft and spacecraft are likely to cause interference at funda mental or harmonic frequencies, but adequate suppression of harmonic radiation is tech nically possible. 5.3 Reflections from aircraft are likely causes of harmful interference in a shared band even when the terrestrial transmitter is distant, and the possibility of interference by reflections from low-orbit satellites also exists. A single reflecting body will be effective for only a short time and the interference problem will depend on the density of the air or space traffic. 5.4 For certain types of radioastronomical measurement in shared bands, reflections of terrestrial transmissions by the Moon can cause serious interference. The Moon is of great importance to radioastronomy for two main reasons. The first, and more important, is that because both the shape and the motion of the Moon are known, the observation of lunar occultations of radio sources provides the most accurate available method for determining their angular positions and, in some instances, their sizes. Occultation of a particular radio source by the Moon will occur only at long intervals and it is important that there should be no harmful interference at these times. The second use of the Moon is as a calibration source, because its effective temperature over a range of frequencies is accurately known. jH In both applications the main beam of the radio telescope is directed at the Moon and the observations are, therefore, particularly susceptible to interference by signals reflected from the lunar surface. Illumination of the Moon by terrestrial transmitters, either inten tionally or otherwise, in frequency bands used by the radioastronomy service can thus cause harmful interference; occultation measurements have already been prevented on many occasions by radar pulses reflected from the Moon. 5.5 It has been reported [10] that belts of orbiting dipoles, if sufficiently dense to be'used as reflectors for an operational communications service, may lead to harmful interference to radioastronomy in bands shared with services using high-power transmitters, even if these bands are well separated from the resonant frequency of the dipoles. 5.6 Incoherent scattering of high-power transmissions by free electrons in the ionosphere is a further potential source of interference, but is at present less important than the sources already discussed. 6. Levels of harmful interference and factors affecting frequency sharing 6.1 Interference to the radioastronomy service may be of two kinds : one is readily apparent and may interrupt or prevent the observations; the other, perhaps even more harmful to the progress of radioastronomy, involves interfering signals at such low flux levels that they cannot be detected at the time of the observation, nor, in many cases, during the analysis of the data. This latter type of interference can cause misleading or invalid conclusions. 6.2 Because of the noise-like character of cosmic emissions, the inherent uncertainty in a radio- astronomy measurement is set by fluctuations of the total noise, both desired and undesired. These fluctuations can be expressed in terms of an equivalent r.m.s. temperature fluctuation, Rep. 224-1 — 542 — ATe, defined in the Annex. A level of harmful interference to a Class B radioastronomy measurement is reached when the unwanted signal flux incident upon the antenna is sufficient to increase the operating noise temperature by a significant fraction of ATe. If the radio- astronomy service is to secure the advantage of low-noise receivers, the level of interference must not introduce an error of more than 10% in the measurement of flux density. For example, the flux density of a continuous unwanted signal must contribute less than 0 -1 ATe ; alternatively, an unwanted signal equivalent to ATe must not exist for more than 1 0 % of the observation time. 6.3 The levels of co-channel interference which are harmful to a Class B radioastronomy observa tion can be obtained from Table I of the Annex, by taking account of the gain of the receiving antenna in the direction of the interference. To calculate the level of interference from a specific unwanted signal, we require knowledge of the antenna pattern and the direction of arrival of the signal. Many radioastronomical observations are performed by using a single antenna, usually a parabolic reflector having characteristics similar to those of earth- station antennae used in communication-satellite systems when the diameters, expressed in wavelengths, are similar. However, at the longer wavelengths used in radioastronomy it is, in most cases, impracticable to use antennae with correspondingly large diameters, and consequently these are more susceptible to interference received through the sidelobes. Furthermore, some experiments involve the use of interferometers in which the individual antenna elements have relatively low gain. Such antenna assemblies may, nevertheless, have high gain in a number of areas forming a coherent pattern over the whole sky. For example, in a direction in which each antenna of an assembly is effectively isotropic, the combined gain may be equal to the number of antennae. The examples which follow refer to a highly directive parabolic antenna, but it should be borne in mind that an interferometer will require a greater degree of protection unless its individual elements are highly directive. 6.4 For the usual case of unwanted signals entering via the far sidelobes of the antenna, we may assume that the gain of the best receiving antennae at centimetre wavelengths is —10 dB relative to an isotropic antenna. Since the gain of the main beam and the nearby sidelobes will normally be more than 30 or 40 dB, unwanted signals from these directions will be harmful at correspondingly lower levels. As a numerical example, Table I shows that, for typical continuum observations at 2-7 GHz, radioastronomy measurements may be rendered invalid by unwanted flux levels above —167 dBW/m2 incident upon the far sidelobes of the radio telescope. It is immediately evident that, for line-of-sight paths, interference from a single transmitter could be harmful at enormous distances, even for transmitters of moderate power. In this example, a 1 W transmitter with an omnidirectional antenna could interfere at a distance of 70 000 km. 6.5 The curvature of the Earth provides some protection against such levels of interference if the transmitter is on the earth. However, energy will be propagated over large distances by tropospheric scatter and other mechanisms. For example, according to Report 243, a 2-7 GHz signal propagated along a 500 km path over relatively smooth earth will be only 70 dB below free space intensity for about 1% of the time; in such conditions a 700 W transmitter with an omnidirectional antenna could cause harmful interference. Other propagation mechan isms, though more erratic than tropospheric scatter, may result in even more severe inter ference. These include reflections from sporadic E ionization and from meteor trails at meter wavelengths and scattering of microwaves by heavy rain. Reflections from aircraft and artificial satellites may be effective over even greater distances, their importance increasing with the number of such objects. As an illustration we may take 100 m2 as the scattering cross-section of a large aircraft, and assume that the aircraft is equi-distant from the transmitter and radioastronomy observ atory. With the aircraft flying at a height of 12 km the horizon distance is 450 km and one may ask under what conditions can harmful interference be propagated to the horizon. At 408 MHz, for example, it is assumed for the present purpose that the gain of the far lobes — 543 — Rep. 224-1 of the receiving antenna are approximately 10 dB below that of an isotropic antenna; the minimum effective radiated power to cause harmful interference is then 60 kW. The range at which aircraft-reflected interference can be harmful is thus limited primarily by the horizon distance, i.e. 450 km for an aircraft at a height of 12 km. This can mean that interference from a high-power transmitter, as far away as 900 km from the radioastronomy observatory, could be harmful. We must also consider scattering from artificial satellites. Here the horizon distance is greatly increased and the satellite will be within the simultaneous line of sight of a vast area of the earth. The scattering cross-section is perhaps a factor of 100 below that of an aircraft, but this will be more than counterbalanced if, because of the high angle of elevation of the satellite, it appears in the near side lobes of the radioastronomy antenna. As mentioned earlier, these may have gains greater than 30 dB. However, with a narrow-beam antenna, a satellite would be in the beam or near side lobes for a very short time and, as in other inter ference problems, this would need to be taken into account in determining the severity of the interference. Reflections from the Moon are especially likely to be a source of serious interference to occultation and calibration observations, in which the main beam of the antenna is necessarily directed at the Moon. For purposes of illustration, the effective cross-section of the Moon may be taken to be 1011 m2. Then, an occultation observation at 408 MHz made with a radio telescope with a gain of 45 dB (beamwidth approximately 1°), using a bandwidth of 2 MHz and an integration time of 2 s, will suffer harmful interference if a total power of only 3400 W is radiated isotropically from one or many antennae located on the moonlit hemisphere of the Earth. 6 .6 Harmonics Up to this point, it has been assumed that the transmitter is operating at a frequency within the receiver bandwidth. It is well known that the technical ability to control the transmitter frequency and to design receivers with great selectivity has provided a fundamental basis for present radiocommunications. To explore the protection this offers to radioastron omy, let us assume that the transmitter is operating at a sub-harmonic of the radioas tronomy receiver and that the harmonic suppression is 80 dB. From the illustrative example given for the line-of-sight situation, even with this degree of suppression at the transmitter, there may be harmful interference at separation distances of the order of 1 0 0 km, for the conservative case of isotropic antennae at both transmitter and receiver. 6.7 Some conclusions on frequency sharing In view of the very low levels at which interference has been shown to be harmful to the radioastronomy service, it is concluded th a t: 6.7.1 harmful interference may be readily apparent and may then interrupt or prevent radio astronomical measurements; at lower intensities it may not be apparent at the time of the observations but may, nevertheless, lead to erroneous results ; 6.7.2 because of the nature of the phenomena observed in radioastronomy, only under special conditions will it be feasible to devise time-sharing programmes between radio- astronomy and other services operating on the same frequencies; 6.7.3 if relatively high power is beamed at the Moon, or at artificial satellites, or if lower power is beamed at aircraft closer to the ground stations, the signal picked up through side-lobes of the radioastronomy antenna may cause interference, if the transmitter frequency corresponds to the reception band in use by the radioastronomy service; Rep. 224-1 — 544 — 6.7.4 when the Moon is used for occultation experiments, harmful interference may be caused by reflection of signals from earth transmitters even when these are not beamed at the Moon; 6.7.5 considering the large areas of the Earth visible to a satellite or the Moon, administrative actions to protect the radioastronomy service will need to be on a world-wide (or, at least, regional) basis ; a single Administration acting independently cannot cope with the general problem; 6.7.6 it is not feasible for the radioastronomy service to share frequencies with any other services in which direct line-of-sight paths from the transmitters to the observatories are involved; 6.7.7 dense belts of orbiting dipoles could lead to harmful interference by reflecting signals from transmitters sharing a radioastronomy band, even if the resonance frequency of the dipoles were well removed from this b an d ; 6.7.8 unless harmonic suppression much greater than 80 dB is provided in transmitter designs, services employing high transmitter powers will cause interference, if assigned to operate within the line-of-sight at frequencies sub-harmonically related to those employed by the radioastronomy service; 6.7.9 if high power transmitters in adjacent frequency bands operate within line-of-sight distances, harmful interference may be caused by the far sidebands of the emissions or by superious signals generated by intermodulation; 6.7.10 to obtain maximum usefulness of his equipment, the radioastronomer should place his observatory at a site remote from centres of population and protected from unwanted radiations to the greatest extent possible by the surrounding terrain. While such a site will in no way provide protection from interfering radiation at the desired frequency reflected or scattered into the antenna beam, it will provide a significant degree of protection, particularly from radiation of frequencies other than those intended for reception, to which the receiver may nonetheless be sensitive. Bibliography 1. J a n sk y , K . G . Proc. IRE, 23, 1158 (1935). 2. P a lm er, H . P. Resolution of radio sources. Contemporary Physics, Vol. VI, 401 (1965). 3. H a z a r d , C. The method of lunar occultation and its application to a survey of the radio source 3C 212. Monthly Notes, Royal Astronomical Society, Vol. 124, 343 (1962). 4 . G a r d n e r , F . F . and W h it c o c k , J. B. Polarization of 20 cm wavelength radiation for radio sources. Physical Review Letters, Vol. 9, 197 (1962). 5. E w e n , H . I. and P u r c e l l , E. M. Nature, 168, 356 (1951). 6. W ein reb, S., B a r re tt , A. H ., M eeks, M. L . and H en r y , J. C. Nature, 200, 829 (1963). 7. W il d , J. P., Sm er d , S. F . and W eiss, A. A. Solar Bursts, Annual [Review o f Astronomy and Astro physics, 1, 291 (1963). 8. Bu r k e , B. F ., and F r a n k l in , K . L . Nature, 175, 1074 (1955). 9. R oberts, J. A. “Radio emission from the planets.” Planetary and Space Science, 11, 221 (1963). 10. F in d l a y , J. W. and R y le, M. (In course of publication.) — 545 — Rep. 224-1 ANNEX LEVELS OF HARMFUL INTERFERENCE TO RADIOASTRONOMY OBSERVATIONS OF CLASS B 1. Sensitivity criteria The extra-terrestrial “signal” detected in any radioastronomical observation is a broad band noise, and is superimposed on other noise which is unavoidably produced in the receiver system itself, or is collected by the antenna from the sky, the ground and the earth’s atmos phere. These different noise sources are incoherent, and therefore add to give a total power P which fluctuates in time in accordance with the normal statistical properties of random noise. The root mean square fluctuation of P about its mean value is given by : A P = P / sJTBt (1) where B is the receiver bandwidth and t is the total time of the observation, called the integra tion time. The power is usually expressed in terms of an operating noise temperature Te according to the equation: P = k Te B (W) (2) where k is Boltzmann’sconstant (l-38x 10~23 J/°K),Te isin degrees absolute (°K) and B is in Hz. The root mean square fluctuation of Te about its mean value is given by : a t ; = t J \i 2 Bt (3) where t is in seconds. From (1) and (3), it follows that AP or ATe may be taken as a measure of the sensitivity of the power or temperature determination in Class B observations. The equations show that improved sensitivity is obtained by using wide bandwidths and long integration times. The bandwidths used for continuum measurements are usually as wide as is practicable (e.g. 10 to 20 MHz), the main limitation being the need to avoid interference from transmitters in adjacent bands. For the study of the fine structure of lines in the spectrum, arising from molecular emission or absorption in various parts of the galaxy, receivers of much narrower bandwidth maybe used, down to a few kilohertz. However, to allow for Doppler frequency- shifts the receiver frequency must either be swept over a much wider bandwidth, or alterna tively must include many narrow-band channels. Furthermore, provision must be made for simultaneous reference measurements in an adjacent band and, for the reasons given above, this should be wide enough to provide an accurate standard of reference. The operating noise temperature is made up from a number of components, which correspond to the several sources of noise which contribute to P. It can be expressed as : T ,= Ta + T eff (4) where Teff is the component due to the receiving system referred to the antenna [terminals and Ta that entering via the antenna. The value of Teff for many current receiving systems is about 200°K or lower, depending on frequency. In the most advanced apparatus in use at a few observatories it is of the order of 20°K, and the use of such highly sensitive receivers is increasing. The minimum antenna temperature Ta (in the absence of solar noise), results from the collection of noise from cosmic sources and the terrestrial environment. Rep. 224-1 — 546 — 2. Cosmic noise The flux density of radio-frequency radiation from the celestial sphere is dependent on frequency and on direction or position in the sky. In Tables I and II of this Annex, minimum antenna noise temperatures are listed for the series of frequencies at which there are alloca tions for the radioastronomy service. Cosmic noise is the main factor leading to the large variations in antenna noise temperature as a function of frequency. 3. Noise from the terrestrial environment Contributions to the antenna noise temperature from the relatively warm environment are unavoidable. They are not significant at the lowest frequencies in the range considered, by comparison with that from cosmic noise. At the higher frequencies, present knowledge suggests that by good design the contribution of thermal noise from the ground to the tem perature of a paraboloid antenna can be reduced to the order of 1 0 °K, except when the antenna is directed at low angles of elevation, and this value has been adopted provisionally, where significant, in compiling the Tables. Figures in column 2 in each Table therefore include this allowance for ground noise. At the two highest frequencies an allowance has also been made for noise associated with absorption in the atmosphere. 4. Sensitivity From (1) and (3) it is seen that the accuracy of measurement is increased by using large integration times and these may range up to several hours or even days or weeks. In the calculations leading to Tables I and II, an integration time of 2000 s has been assumed. Then from (3) and (4) : _____ ATe = (Ta + Teff)/ sj4000B For example, if Teff — 200°K and B = 10 MHz (continuum measurements) : ATe = (5x 10-6) Ta + 0-001 (°K) While with the same receiver noise temperature and B — 10 kHz (line-frequency measure ments) : ATe = (1-6 x 10-4) Ta + 0-032 (°K) Values of the sensitivity ATe are given in the Tables. These values are representative of the potential (interference-free) sensitivity of the best observations currently being made. 5. Levels of harmful interference Harmful interference occurs when the unwanted signal flux impinging on the antenna is so great that the operating noise temperature is increased by an amount comparable with ATe. If radioastronomy service is to secure the advantages of low-noise receivers, the interference should not introduce an error of more than 10% in the measurement of ATe. The level of harmful interference can therefore be expressed as a power input APH, measured at the antenna terminals, given by : APR = 0-\kB(AT) (W) or if expressed in dB rel. 1 W APH = -238-6 + 10 log B + 10 log (AT,) (dBW) — 547 — Rep. 224-1 Values of APH are given in the Tables. The harmful interference can also be expressed in terms of the power flux incident at the antenna, either in the total bandwidth or as a flux density SH per 1 Hz of bandwidth. For convenience, the values are given for an antenna having a gain, in the direction of arrival of the interference, equal to that of an isotropic antenna (which has an effective area of c2/4 tt/2, where c is the speed of propagation and/the frequency). Appropriate allowance must be made if the gain has a different value. Values of SHB, in dB rel. 1 W/m2, are derived from APH by adding : 20 l o g / - 38-6 (dB) where / is in MHz. SH is then derived by subtracting 10 log B to allow for the bandwidth. Finally the interference may be expressed as a field strength Eh in the total bandwidth. This is given by : Eh = ( Z 0 ShB)V* Where Z 0 is the impedance of free space, 377 O. In dB rel. 1 pV/m, EH is derived from SHB by adding 145*8 dB. T able I Sensitivity tsTe of typical continuum measurements and flux density SH which causes harmful interference Level of signal causing harmful Minimum interference (isotropic antenna) (3) Minimum Receiver harmful Freuuency noise Typical Sensitivity antenna noise bandwidth power Power flux f temperature B A T e Field strength (M H z) temperature (°K) input Ta O (MHz) A PH S ffB e h SH (dB(nV/m)) (°K) w (dBW) (dBW/m2) (dBW/m2/Hz) 2 0 32 000 2 00 0-1 1-6 -1 8 6 -1 9 9 —249 — 53 40 6200 200 0 1 0-32 -1 9 3 - 2 0 0 —250 —54 80 1000 200 1 0 -0 2 -1 9 6 -1 9 6 -2 5 6 -5 1 150 200 200 2 0-0045 -1 9 9 -1 9 4 -2 5 7 - 4 8 327 40 100 2 0-0016 -2 0 4 -1 9 2 -2 5 5 - 4 6 408 26 100 2 0-0014 -2 0 4 -1 9 0 -2 5 3 - 4 5 610 16 100 8 0-00065 - 2 0 1 -1 8 4 -2 5 3 - 3 9 1420 10 20 27 0-00009 -2 0 5 -1 8 0 —254 - 3 5 1665 10 20 4 0-00024 -2 0 9 -1 8 3 -2 4 9 - 3 7 2700 10 20 10 0-00015 -2 0 7 -1 7 7 -2 4 7 -3 1 5000 10 20 10 0-00015 -2 0 7 -1 7 1 -2 4 1 - 2 6 10 680 12 20 2 0 0-00011 -2 0 5 -1 6 3 -2 3 6 - 1 7 15 350 18 100 50 0-00026 -1 9 7 -1 5 2 -2 2 9 - 6 0), (2) and (3) see page 548. Rep. 224-1, 397 — 548 — T a b le II Sensitivity A Te of typical line measurements and flux density S g which causes harmful interference Bandwidth 0-01 M Hz (4) Level of signal causing harmful Minimum Receiver Minimum interference (isotropic antenna) (*) Frequency antenna noise Sensitivity harmful noise temperature A T e power Power flux f temperature input (MHz) T e ffO (°K) Field strength TaC) A PH Eh (°K) (°K) sgB Sh (dBW) (dBW/m*) (dBW/m*/Hz) (dB(|fV/m)) 327 40 100 0-0 2 -2 1 6 -2 0 4 — 244 — 58 1420 10 20 0-005 - 2 2 2 -1 9 7 —237 — 51 1665 10 20 0-005 - 2 2 2 -1 9 6 -2 3 6 - 5 0 (0 Noise from the ground has, provisionally, been assumed to increase the antenna temperature by 10°K. (2) Referred to antenna terminals. C) For an antenna of power gain G (dB) in the direction of any unwanted signal, reduce all values by G. (*) Typical for a single channel of a multi-channel or tunable receiver. Total band reauired is much greater (see § 1 of this Annex). Note. — An integration time (or total time of observation) of 2000 s is used throughout. For longer integration times, the minimum detectable power flux will be lower and the unwanted signal will be harmful at correspondingly lower levels. For example, with a time of observa tion of ten hours the relevant figures in the Tables should be reduced by 6 dB. REPORT 397 * THE OH-LINES IN RADIOASTRONOMY f Question 10/IY) (1966) Radio-frequency spectral lines due to the hydroxyl molecule (OH) were first detected and measured in the laboratory in 1959 [1], and in interstellar space in 1963 [2], when absorption of the radio-frequency radiation from the radio source Cassiopeia A was observed at frequencies ** corresponding to those of the two principal lines, at 1665-402 and 1667-357 MHz. Shortly after wards, even stronger absorption bands were found from the region of the galactic centre [3,4, 5], and two expected subsidiary lines, arising from alternative configurations within the OH molecule, were detected and their rest frequencies determined at 1612-231 and 1720-533 MHz [6 ]. More recently, investigation of narrow-band emissions from regions of ionized hydrogen in the galaxy [7, 8 ] has led to the detection of a similar emission from OH [9,10]. This emission is unexpectedly intense, producing increases in antenna temperature of as much as 30°K, and it has, surprisingly, been found to have a circularly polarized component [11]. Further sensitive measurements are needed to investigate this phenomenon. Prior to 1963, the only line that had been detected in radioastronomy observations was that due to atomic hydrogen at a frequency of 1420-4 MHz. The discovery of lines due to OH is of great astrophysical significance. The extent by which the observed frequencies are displaced from their rest frequencies by Doppler effects provides direct information about the motions of the gas clouds in which the OH occurs [4, 5, 12]. Furthermore, the fact that the relative intensities * This Report was adopted unanimously. ** The frequencies in this Report are the rest frequencies for the radiation concerned. — 549 — Rep. 397 observed at the four frequencies, both in absorption and emission [9], vary widely from the values expected on theoretical grounds, throws a new and unexpected light on the physical conditions which must occur in particular regions of our galaxy. Lastly, observations of OH are providing new information on galactic structure and magnetic fields, since some gas clouds, from which no hydrogen-line radiation is detectable, can be readily studied using the OH lines. The continuation of this research, and particularly the investigation of the mechanism by which the OH-molecule is formed in interstellar space, will involve further detailed observations using the greatest attainable sensitivity and freedom from harmful interference. Receivers used to study the OH-lines need to have narrower bandwidths than those used in the observation of the 1420 MHz line of hydrogen, because the OH molecule, being heavier, has a lower thermal velocity and, correspondingly sharper line features : bandwidths of 1 to 10 kHz are typical. They need, however, to be tunable over an appropriate range, since the lines observed are broadened and displaced in frequency up to several MHz, as a result of Doppler effects due to relative motions in the line of sight; furthermore, accurate measurement of the shape of the spectral lines requires comparison measurements at adjacent frequencies which are free from the effects of OH-absorption or emission. The overall frequency band technically necessary for detailed study of the two principal lines at 1665-4 and 1667-4 MHz, taking into account the require ments for comparison observations and Doppler shifts, is at least 5 MHz and preferably about 10 MHz. Fig. 1 shows the profiles of absorption due to the OH-lines at 1665 and 1667 MHz in the direction of the galactic centre, as observed with the Australian 64-m diameter radio telescope. The various features of the curve are due to absorption by individual clouds of OH moving with different velocities, and are reproduced generally, but not necessarily in fine detail, at the two frequencies. With the smaller telescopes more generally used, the changes in antenna temperature are smaller and therefore more difficult to detect. B ibliography 1. Ehrenstein, G., T o w n e s, C. H . and Stevenson, M. J. Phys. Rev. Letters, 3, 40 (1959). 2. W e in re b , S., B a r r e t t , A. H ., M eeks, M. L . and Henry, J. C. Nature, 200, 829 (1963). 3. B o lto n , J. G., Van Damme, K. J., G a r d n e r , F. F. and Robinson, B. J. Nature, 201, 279 (1964). 4. Robinson, B. J., G a r d n e r , F. F., Van Damme, K. J. and Bolton, J. G. Nature, 202, 989 (1964). 5. Goldstein, S. J., Gundermann, E. J., P e n z ia s , A. A. and Lilley, A. E. Nature, 203, 65 (1964). 6. G a r d n e r , F. F., R o b in so n , B. J., B o lto n , J. G. and Van Damme, K. J. Phys. Rev. Letters, 13, 3 (1964). 7. W eaver, H ., W illiams, D . R . W., D ieter, N. H. and Lu m , W. T . Nature, 208, 29 (1965). 8. Zuckerman, B., L ille y , A. E. and Penfield, H . Nature, 208, 441 (1965). 9. M c G e e , R . X ., R o b in so n , B. J., G a r d n e r , F. F. and Bolton, J. G. Nature, 208, 1193 (1965). 10. W e in re b , S., M eeks, M. L ., C a r t e r , J. C., B arrett, A. H . and Rogers, A. E. E. Nature, 208, 440 (1965). 11. D av ies, R . D., de Jaeger, G. and Verschuur, G. L . Nature, 209, 974 (1966). 12. B o lto n , J.G., G a r d n e r , F.F., M cG ee, R .X . and Robinson, B.J. Nature, 204, 30 (1964). — oss — oss — Antenna temperature relative to arbitrary temperature (°K) Frequency (MHz) — 551 — Rep. 226-1 L. 9 : Radar astronomy REPORT 226-1 * FACTORS AFFECTING THE POSSIBILITY OF FREQUENCY SHARING BETWEEN RADAR ASTRONOMY AND OTHER SERVICES (Questions 3/IV, 11/IV) (1963 — 1966) 1. Introduction A radar-astronomy system has a paradoxical nature, with regard to factors affecting frequency sharing. Radar-astronomy receivers have sensitivities comparable with the best radioastronomy receivers and, when comparable bandwidths are considered, have similar susceptibility to interference. Radar-astronomy transmitters develop, and antennae radiate, high power, so that they are capable of interfering with other services over significant distances, which may be appreciably extended by scattering from space objects such as the Moon, spacecraft or scatterers such as the troposphere, and ionosphere. Although radar astronomy is a relatively new discipline, it is responsible for a number of notable achievements. For example, the accuracy of orbital information on the planet Venus has been improved by more than a factor of ten, and hence, by inference, an equal improvement applies to all solar system objects. It has been shown that Venus rotates on its axis in the direction opposite to the rotation of most solar system objects (retrograde rotation). Contrary to the inference drawn from optical observations, it has been shown by radar observations that the rotation of the planet Mercury is not synchronous with its orbital period. The radar reflectivity of the Sun varies with solar activitiy, and the spectrum of the echoes shows effects which are believed to be due to large mass motions of the solar plasma and outward flow of the solar wind. As with other advances made using radar astronomy, such information cannot at present be obtained using any other ground based technique. Radar-astronomy transmitters are generally outgrowths of component developments made for other services ; indeed, many systems put to radar astronomy use have some other primary purpose. However, for scientific reasons this may not be desirable in all cases in the future. Unlike a radioastronomy system detecting cosmic noise, the channel bandwidth required for radar astronomy can be much smaller, in general only that required to encompass the modulation band, Doppler spread, and Doppler shift encountered. Radio- and radar- astronomy systems are similar, in that they require large antennae, sensitive receivers “and low tracking rates. * This Report was adopted unanimously. Rep. 226-1 — 552 — Many similarities exist between radar-astronomy systems and communication-satellite earth stations. In addition, many similarities exist between radar-astronomy systems and deep-space tracking facilities. These or similar facilities on earth may also be used in a bistatic mode, where the other terminal (receiver or transmitter) is in a space probe, for important radar studies of the planets and the interplanetary medium. This mode of operation has been used to study the atmosphere and ionosphere of Mars using the Mariner IV spacecraft. 2. The problem of radar astronomy The salient problem in radar astronomy is the detection and study of targets at long ranges. These targets may be small in angular extent relative to the antenna beam, e.g. the planets, or extended, e.g., the terrestrial ionosphere. The detection range of a system, as defined in most radar text books, for a quasi-point target, at a given wavelength, follows the proportionality: R4 = Const. PA2T~l 0 ) where R is the range, P the transmitted power, A the effective area of the duplexed antenna, and T the operating noise-temperature. For extended targets, we have R2 = Const. PAT-1 (2) Both relations point to the fundamental importance of large powers, large antennae and sensitive receivers. The detectability of Venus by radar has been established by a number of observers. Venus, Mercury, Mars, or possibly Jupiter, provide a convenient point of reference for considering the detectability of other planets (Fig. 1) [1]. The problem of signal design for radar astronomy has been treated by Green [2], in a way that allows consideration of propagation, multipath, and reflection effects in the optim ization of the modulation, as well as the associated detection features. In general, the fre quency spectrum, occupied by such transmitted signals and those reflected from the target, is narrow compared with that desirable for observation of cosmic noise. The spectral width of radar-astronomy signals is more akin to that commonly used in spectral-line radioastro nomy. However, because of the coherent nature of radar signals, and because correlation with the transmitted wave-form is generally employed, several orders of magnitude of inter ference tolerance can be gained as compared with radioastronomy. Unwanted narrow-band emissions are less likely to cause trouble because there is little probability that they will fall in the receiver passband. Incoherent interference can be suppressed to a degree by correlation techniques in signal processing. For greater discrimination, the accumulated data of many hours of actual observation can be processed over many more hours by analogue and digital computers. An estimate of the channel bandwidth required to allow an adequate receiver offset for Doppler shift, can be obtained by considering a specific problem in planetary detection *. Beyond this, little can be said regarding susceptibility of the system to interference without some detailed discussion of a particular system. * The radar Doppler shift is given approximately by : fd = fo 2 v/c where fd is the Doppler frequency shift, fo the carrier frequency, v the radial velocity of the target and c the speed of light. Considering the entire orbit of some planets, for example, the fraction 2 v/c, can be about 2* 10-4 (Jupiter). The Doppler spread is a result of differing Doppler shifts from the various parts of a target reflecting area. — 553 — Rep. 226-1 Radar-astronomy receivers have many features in common with the radiometers used in radioastronomy. Receiver designs have progressed to the point where the sensitivity of the system is limited by environmental background noise. As a result, any generalized approach to the appraisal of the susceptibility to interference of radar-astronomy receiving systems would be along lines identical to those used for communications-satellite stations and for radioastronomy. Several reports [3, 4] on this subject have already been prepared. 3. Frequency characteristics of experimental radar-astronomy systems Most experimental radar-astronomy systems have operated at frequencies allocated to radiolocation services, because these services have borne the costs of transmitter component developments and radar astronomy makes use of the available equipment in the interest of economy. However, there are a few specific cases wherein the nature of the scientific study strongly affects the choice of frequency. For example, studies of the solar plasma to certain depths by radar-astronomy techniques requires the use of frequencies in the 20 to 60 MHz range. 4. Susceptibility of radar-astronomy receiving systems to other signals Any signal that significantly increases the operating noise-temperature of the receiving system would be troublesome to the radar astronomer. This effect is primarily a function of the average power of the interfering signal. As an illustration, a 400 MHz system at the Lincoln Laboratory’s Millstone Hill site and airborne altimeters shared the same band in the frequency allocation table. Ideally, in this specific case, the unwanted signal level at the output terminals of the radar-astronomy antenna should not exceed 5 x 10~23 W/Hz. At higher frequencies, where the background noise is lower, the unwanted signal level should be reduced further. To achieve these low levels of protection will require time sharing or other special arrangements that may not always be possible on a local basis. 5. Indirect sources of interference Scattering of the transmitted signal (or harmonics thereof), from other sources, for example, from the Moon, troposphere, and orbiting objects, can, under certain conditions, be a hazard. In questions of sharing, these effects should be considered on a case-to-case basis. 6 . Interference by radar astronomy to other services High power is produced by radar-astronomy transmitters and this power is usually confined to a fairly narrow beam by the large antenna. In many instances, the antenna beam is directed at such high angles of elevation that interference is limited to that produced in the side lobes. The power in the first side lobes is usually 15 to 30 dB below that in the main lobe, and that in the remaining lobes, below isotropic. This holds also for systems operating in the 20 to 60 MHz range, where the high angle of elevation limits ionosphere forward- scatter primarily to the side lobes. Typical powers for radar-astronomy transmitters are given in Table I. 24 e. 2- — 554 — 226-1Rep. T a b l e I Comparison o f some systems o f radar astronomy 0 Antenna Effective area System Approximate Mean Peak Pulse noise- Organization Location frequency (dB power power length temperature (MHz) Diameter Gain (kW) (kW) (°K) (dB) (m2) rel. (ft.) 1 m2) Calif. Inst. Goldstone Lake, 2-4 XlO3 85 54-2 355 25-5 400 400 CW (3) 30 Tech. JPL Calif. Cornell Univ.] Arecibo, P.R. 4-3 X102 1000 57-0 (2) 20 000 43-0 (2) 150 2500 0-03-10-0 400 ms Mass. Inst. Tech. Westford, Mass. 1-3 XlO3 84 46-5 190 22-8 150 5000 0-04-4 80 Lincoln Lab. (Millstone) ms Mass. Inst. Tech. Tyngsboro, Mass. 7-8 XlO3 120 66-1 482 26-8 (100) 0 (100) 0 CW 145 Lincoln Lab. (Haystack) Institute for Tele Jicamarca Radar 50 940 (sq.) 42-6 84 000 49-2 250 6000 0 -0 1 -1 0 0 0 2 0 0 0 - communication Observatory ms 3000 Science and (Lima) Peru Aeronomy Cosmic 24 ft. array 25 6880 38-4 300 600 10 ps Stanford Univ. Stanford, Calif. 1200 noise 48 log periodic Research Inst. Cosmic 50 150 parab. 25 920 29-6 300 600 to noise 4-2 XlO2 150 parab. 44 920 29-6 30 200 CW 100 U.S. Naval Res. Washington, D.C. 8-4 XlO3 50 60 • 92 19-6 20 20 CW 145 Lab. (0 Data supplied for U.S. systems only ; for completeness, other Administrations may wish to supply additional data. (2) These are initial values and improvements are expected shortly. (*) CW means continuous-wave. (*) Estimated values. — 555 — Rep. 226-1 7. Conclusions 7.1 Adequate management of interference, involving high-power radars, is normally affected on a local basis. 7.2 Many functions of radar-astronomy installations can be carried out on a shared basis. There are instances, however, where a channel of modest bandwidth within the radiolocation bands concerned may be cleared or protected on a local or regional area basis for certain radar astronomy experiments. 7.3 For equivalent bandwidths, radar-astronomy receiving systems are as sensitive as satellite communication earth-stations and radioastronomy installations. 7.4 When the frequency range in use is dictated by natural phenomena (e.g., solar and other plasma investigations), local or regional arrangements may be required. 7.5 From the point of view of availability of equipment, it is desirable that radar-astronomy systems be operated in or near frequency bands for which high power transmitting technology has reached a suitable degree of development. 7.6 As with other high-power operations, radiolocation stations used for radar-astronomy observations should be sited with great care, to minimize mutual interference problems with stations operating in the same and adjacent bands. B ibliography 1. P e t t e n g il l , G. H. Radar studies of the planets. Hagfors and Evans, Radar Astronomy, McGraw- Hill (1966). 2. G r ee n , P . E., Jr. M .I.T. Lincoln Laboratory Report 34-81 (May, 1959). M .I.T. Lincoln Laboratory Report 34-84 (January, 1960). 3. C.C.I.R. Report 224-1 : Radioastronomy. Characteristics and factors affecting frequency sharing with other services. 4. P e t te n g ill, G. H. and Sh a pir o , I. Radar astronomy. Annual Review of Astrophysics Academic Press (1964). Rep. 226-1 Rep. Echo level relative to Venus at closest approach (dB) Detectability of the planets by radar by the planets of Detectability Round-trip time (minutes) time Round-trip 56 — 556 — F gure r u ig 1 — 557 — STUDY GROUP IV (Space systems and radioastronomy) Terms of reference: To study technical questions regarding systems of telecommunication with and between locations in space and radioastronomy. Chairman : Professor I. R a n z i (Italy) Vice-Chairman : Mr. W. K l e in (Switzerland) INTRODUCTION BY THE CHAIRMAN, STUDY GROUP'IV 1. During the period 1963-1966, the first experimental programme, on a commercial basis, of a communication-satellite system was realized. The results of this experiment, as well as the results of the previous ones, essentially confirmed the correctness and the validity of the fundamental principles of such communica tion systems, which were established, on theoretical grounds only, since the first meeting of the Study Group IV, Washington, 1962. In the following, the main results of the work performed by the Study Group IV, during the period 1963-1966, are summarized. 2. Terminology The list of terms, concerned with space telecommunications in general, was completed and some definitions, especially regarding the orbit parameters, were improved. The use of the term Equivalent isotropically radiated power was recommended, when the antenna gain must be referred to an isotropic radiator. 3. Communication-satellite systems 3.1 Technical characteristics The main Reports on “General considerations relating to the choice of orbit, satellite and type of system”, on “A comparative study of possible methods of modulation”, on “Factors affecting multiple access”, and on “The results of tests and demonstrations in the field of active communication-satellite experiments”, were improved on the ground of the results of new studies and of recent experimental and operational results. Also the Reports on “The use of pre-emphasis by FM-systems” , on “The effects of transmission delay” , on “The effects of Doppler frequency shifts and switching discontinui ties” , were reviewed and improved. In connection with Question 234 (IV), a new Report was adopted, in which the state of development in the design and fabrication of earth-station antennae, and possible future improvements in their design and performance, are pointed o u t; also the main characteristics of the antennae in existing earth stations are described. Another new Report was adopted, which gives a radiation diagram representing, with satisfactory approximation, the radiation to be expected from typical earth-station antennae of current design; in the range of diameters from 9 to 27 m, such a typical diagram is very useful for the solution of sharing problems. — 558 — 3.2 Interference and sharing problems The group of basic Recommendations (with their respective supporting Reports), con cerned with the general conditions and principles for frequency sharing between communica tion-satellite systems and terrestrial services, the maximum allowable power of interference in a telephone channel of a communication-satellite system, were reviewed jointly with Study Group IX, and a better definition of the values of the noise, for various percentages of the time, was given. As regards to the maximum allowable values of the power flux-density at the surface of the earth produced by communication satellites, it was agreed to express it as a function of the angle of arrival, 0, of the signal above the horizontal (it must not exceed: (—152+0/15) dBW/m2 in any 4 kHz band, 0 being expressed in degrees). On the other hand, Recommendation 406, originating from Study Group IX, was discussed jointly, and a revised version was adopted, which takes account of the possibility of reducing the interference probability, by a suitable orientation of the antennae in the terrestrial services, with respect to the orbit of the stationary satellite. New Reports were prepared on the two important subjects—techniques for calculating interference noise in communication-satellite receivers and terrestrial radio-relay receivers, and of methods for the determination of the coordination distances. In these Reports and in a separate Report on “Propagation considerations in the general case of interference between space and terrestrial systems”, the need for more experimental data on propagation losses (especially for small percentages of the time), on scatter by rain and on the screening effect from ground reliefs, was stressed. The Report on frequency sharing within and between communication-satellite systems was completed and improved. Also the problem of frequency-selection and carrier-energy dispersal for communication- satellite systems was studied and a Recommendation, accompanied by an exhaustive Report, was adopted. 4. Direct broadcasting from satellites Report 215, with some amendments, which were agreed during the Interim Meeting, was maintained. Further studies and, particularly, experimental and operational data will be necessary, before discussing the possibility of adopting a Recommendation on this subject. 5. Propagation and noise The fundamental Report, on the factors affecting the selection of frequencies for tele communications with and between spacecraft, was reviewed and some amendments were introduced. A new Report was prepared on the important subject of the effect of rain on radomes and of solar and cosmic noise, when the performance of the earth-station receiving antennae is considered. 6 . Radionavigation by satellites Report 216 was considerably improved during the Interim Meeting and the resulting document was, in substance, approved. A new Report was also adopted, on the feasibility of frequency sharing with terrestrial services. — 559 — 7. Meteorological satellites As a result of the work performed during the Interim Meeting, the Report on radiocom munications for meteorological satellites presents a complete description of the main technical characteristics of the systems realized until now, and a discussion on the frequency require ments for such systems. 8. Maintenance telemetry, tracking and telecommand. Telecommunication links for manned and unmanned spacecraft in the space research service. Telecommunication links for deep-space research. Telecommunication with spacecraft re-entering the Earth’s atmosphere All the Reports and Recommendations were modified according to the decisions of the Extraordinary Administrative Radio Conference, Geneva, 1963, and in the light of recent experimental results. 9. Radioastronomy and radar-astronomy The basic Recommendation on the protection of frequencies used for radioastronomical measurements was modified, on the ground of the provisions adopted by the Extraordinary Administrative Radio Conference, Geneva, 1963, and the protection of other frequency bands (Deuterium and OH-lines, bands above 30 MHz) was recommended. The need was also stressed that the bands already allocated for standard-frequency and time-signal emis sions do not effectively include any other emission. A new Report on the importance of the OH-lines in radioastronomy was adopted and a single Report was prepared on the characteristics and factors affecting frequency sharing with other services. The difficult problems of frequency sharing between radar astronomy and other services were presented in a clear and up to date revision of the old Report. Op. 3, Q. 1/IV — 560 — OPINION 3 DATA ON TRAFFIC LOADING AND ROUTING FOR USE IN DEVELOPING COMMUNICATION-SATELLITE SYSTEM FACILITIES (Question 2/IV) (1963) The C.C.I.R., CONSIDERING (a) that communication-satellite systems promise to provide a means of communication between countries on a global scale ; (b) that the design and operation of a global communication-satellite system or systems should depend, amongst other factors, upon the traffic density and its characteristics and locations ; (c) that in planning global communication-satellite facilities, it will be necessary to examine the world’s regional and inter-regional traffic patterns to forecast the volume and routing of traffic, which might be accommodated via such facilities ; and (d) that the Plan Committee (Joint C.C.I.T.T./C.C.I.R. Committee) is preparing a question naire for submission to the Administrations to collect data on global traffic in preparation for its meeting in Rome in November-December, 1963 ; IS UNANIMOUSLY OF THE OPINION that the Plan Committee should provide, for use by interested Administrations at the Extra ordinary Administrative Radio Conference, Geneva, 1963, and subsequently: 1. information as to the nature, volume and routing of international, regional and global traffic, from or to the different countries, for 1962 and for such subsequent years for which it might assemble such information; 2. forecasts for 1968, and, if possible, up to 1975, of the volume of global traffic, from or to the different countries, and its routing which might be accommodated via communication- satellite systems, the forecasts being revised as opportunities offer. QUESTION 1/IV* ANTENNAE FOR SPACE SYSTEMS The C.C.I.R., (1961 — 1963) CONSIDERING (a) that the limitations on the physical size and beamwidth of antennae for earth and space stations are important factors in determining the useful frequency range for space systems ; * Formerly Question 234(IV). — 561 — Q 1/IV, 2/IV (b) that atmospheric effect, ionospheric effect, and techniques of fabrication provide limitations on the size of antennae and on the minimum permissible beam width; and (c) that interference is an important problem ; unanimously d e c id e s that the following question should be studied: 1. what limitations in antenna beamwidth result from atmospheric and ionospheric effects ; 2. what is the state of development in antenna design and fabrication ; 3. what is the state of development of side- and back-lobe suppression ; 4. what pointing accuracy is reasonably attainable with antennae of various sizes and types ? QUESTION 2/IV * TECHNICAL CHARACTERISTICS OF COMMUNICATION-SATELLITE SYSTEMS The C.C.I.R., (1963) CONSIDERING (a) that active or passive communication-satellite systems may well be an important means in the future for fixed and mobile communication, both regional and global; (b) that the frequency bands to be used by such systems should be the subject of international agreement, not only to facilitate the setting up of communication links between earth stations in different countries, but also to avoid interference to and from other satellite systems and terrestrial services which may share the same frequency bands; (c) that the choice of preferred frequency bands for such systems is determined by a number of factors including the characteristics of wave propagation, radio noise levels, the feasibility of frequency sharing with terrestrial services, the beamwidths and size of antennae, tracking considerations and pay-load limitations ; (d) that the scope for development and future application of such systems will depend to a great extent upon the feasibility of sharing frequency bands used by terrestrial services, without mutual interference; (e) that the total frequency space required by such systems will be determined in part by the technical characteristics employed, including the arrangement and method of modulation of the radio-frequency carriers, having regard to spectrum economy; (f) that the establishment of such systems would require international agreement on the technical characteristics to be employed, including the baseband, modulation and radio-frequency char acteristics ; * Formerly Question 235(IV). Q. 2/IV, S.P. 2A/IV — 562 — unanimously d e c id e s that the following question shall be studied: 1. what are the preferred types of orbit and characteristics for active and passive com munication- satellite systems for the following applications : 1.1 fixed services for multi-channel telegraphy and telephony, television and data transm ission; 1.2 mobile services providing telegraphy, telephony and data transmission, between fixed earth stations and/or mobile stations ; 2. what are the preferred frequency-bands for these applications; 3. under what conditions and to what extent is it feasible for communication-satellites, operat ing in the same system or operating in different systems, to share these preferred frequency b a n d s ; 4. under what conditions and to what extent would it be feasible for communication-satellite systems to share these preferred frequency bands with terrestrial services ; 5. should radio-frequency channelling or preferred reference-frequency arrangements be employed in all or a portion of the preferred bands by communication-satellite systems used for the applications referred to in § 1, and if so, what channelling arrangements are preferred; 6. what are the preferred baseband and modulation characteristics for these applications ? STUDY PROGRAMME 2A/IV * FEASIBILITY OF FREQUENCY SHARING BETWEEN COMMUNICATION-SATELLITE SYSTEMS AND TERRESTRIAL RADIO SERVICES T h e C.C.LR., (1965 — 1966) considering (a) that use of communication-satellite systems will require extensive occupation of the radio frequency spectrum ; (b) that for communication-satellite systems, the spectrum should be shared with terrestrial services to the extent practicable, in the interest of spectrum conservation; and (c) that the feasibility of sharing spectrum space with line-of-sight radio-relay systems should be investigated; unanimously d e c id e s that the following studies should be carried o u t: 1. the criteria which affect the selection of sites for earth stations in the communication-satellite system, taking into account the various portions of the radio-frequency spectrum ; 2. determination of the preferred technical characteristics of transmitting and receiving antennae for earth stations at fixed locations, from the standpoint of spectrum sharing with other radio s e r v ic e s ; * Formerly Study Programme 235F(IV). — 563 — S.P. 2A/IV, 2B/IV 3. the criteria which affect the determination of the maximum power per 4 kHz which may be radiated in the horizontal plane by an earth station; 4. the criteria which affect the determination of the minimum angle of elevation, which should be employed at the locations of the earth stations ; 5 . the degree to which physical modification of terminal sites will provide electromagnetic shielding between earth stations and stations in other radio services ; 6 . the criteria which affect the selection of satellite power in frequency bands shared with other radio services; 7 . the criteria which affect the determination of the minimum practicable separation between the transmitting and receiving locations of line-of-sight radio-relay systems and the receiving and transmitting locations of earth stations in the communication-satellite systems, either or both systems using frequency-modulation, pulse-code modulation or other forms of modula tion. STUDY PROGRAMME 2B/IV * FREQUENCY SHARING BETWEEN COMMUNICATION-SATELLITE SYSTEMS AND TERRESTRIAL RADIO SERVICES Wanted-to-unwanted signal ratios The C.C.I.R., (1965 — 1966) CONSIDERING (a) that methods are described in Reports 209-1 and 382 for determining the conditions under which frequency sharing is feasible between communication-satellite systems and terrestrial services; (b) that a precise determination depends upon the availability of appropriate values for the acceptance ratios between wanted and unwanted signal powers at the receiver input, for specified grades of service; (c) that acceptance ratios are required between each type of wanted signal and each type of unwanted signal, for appropriate modulation and fading conditions, for which a test of the feasibility of sharing is desired; (d) that frequency-modulation is at present extensively used for terrestrial radio-relay systems and this use is likely to increase in the future; (e) that pulse-code modulation (PCM) may, in future, be applied to terrestrial radio-relay systems and communication-satellite systems, and such systems using PCM may have advantages from the point of view of interference reduction and may facilitate frequency sharing; (f) that the Recommendation No. 4A of the Extraordinary Administrative Radio Conference, Geneva, 1963, invited the C.C.I.R. to make a special study of modulation methods such as pulse-code modulation (PCM), using phase- or frequency-modulation in particular for radio relay systems in relation to frequency sharing with communication-satellite systems. * Formerly Study Programme 235G(IV). S.P. 2B/IV, 2C/IV — 564 — unanimously d e c id e s that the following studies should be carried o u t: 1. theoretical and experimental determination of the acceptance ratios required for specified grades of service for various types of wanted and unwanted signal, for appropriate m odulation conditions and for various kinds of fading; 2. investigation of those techniques of transmission, reception and modulation which will minimize the acceptance ratios required for a specified grade of service. STUDY PROGRAMME 2C/IV * COMMUNICATION-SATELLITE SYSTEMS Feasibility of frequency sharing among communication-satellite systems The C.C.I.R., (1963) CONSIDERING (a) that the use of communication-satellite systems will require extensive occupancy of the radio- frequency spectrum; (b) that the feasibility of frequency sharing among communication-satellites, operating in the same system or operating in different systems, should be investigated; unanimously d e c id e s that the following studies should be carried out: 1. the criteria which affect interference among communication-satellites in a given system and between communication-satellite systems, taking into account the two directions of transmission, for: 1.1 systems using stationary satellites; 1.2 systems using station-keeping satellites; 1.3 systems using unphased satellites; 1.4 satellites operating in various orbits in the same system; 1.5 satellites operating in various orbits in different systems; 2 . the preferred technical characteristics of transmitting and receiving antennae for earth stations, from the standpoint of frequency sharing within the same system and with other communication-satellite systems; 3. the criteria which affect the determination of the minimum elevation angle which should be employed at the earth stations, from the standpoint of frequency sharing among communi cation-satellite systems; 4. the optimum range of powers to be employed by satellites and by earth-station transmitters, to facilitate frequency sharing among communication-satellite systems; 5. the effects of baseband and modulation characteristics on frequency sharing among commu nication-satellite systems; 6 . the extent to which the selection of preferred reference frequencies would facilitate frequency sharing among communication-satellite systems. * Formerly Study Programme 235C(IV). — 565 — S.P. 2D/IV, 2E/IV STUDY PROGRAMME 2D/IV * STUDY OF PREFERRED MODULATION CHARACTERISTICS FOR COMMUNICATION-SATELLITE SYSTEMS The C.C.I.R., (1961 — 1963) CONSIDERING (a) that earth satellites are expected to be used extensively for the relay of communication signals of various types; (b) that substantial use of communication satellites will place heavy demands upon the radio frequency spectrum; (c) that, in the interest of conservation of the radio-frequency spectrum, effort should be exerted to use the minimum feasible amount of spectrum space to convey the maximum amount of information; unanimously d e c id e s that the following studies should be carried out: 1. determination of the preferred characteristics for transmission from earth to satellite to earth in passive communication-satellite systems ; 2 . determination of the preferred modulation characteristics for transmission from earth to satellite and from satellite to earth in active communication-satellite systems; 3. determination of the extent to which signal compression or signal processing techniques can usefully be employed to conserve radio-frequency bandwidth, and the preferred charac teristics which should be employed when such techniques are used for satellite-communi- cation systems. STUDY PROGRAMME 2E/IV ** FACTORS AFFECTING FREEDOM OF ACCESS IN COMMUNICATION-SATELLITE SYSTEMS The C.C.I.R., (1965 — 1966) CONSIDERING (a) that communication-satellite systems may require simultaneous use by large numbers of earth stations at various locations, this being termed “freedom of access” ; (b) that this freedom of access may be affected by the orbital design of the system ; (c) that this freedom of access may be affected by the choice of modulation techniques used; (d) that this freedom of access may be affected by the interference characteristics of the system or systems used; (e) that multiple access requirements may dictate a system design, different from that which may be optimum for limited access systems ; * Formerly Study Programme 235DGV). ** Formerly Study Programme 235H(IV). S.P. 2E/IV, 2F/IV — 566 — unanimously d e c id e s that the following studies should be carried o u t: 1. the factors which determine the accessibility of a communication-satellite system to a number of earth stations simultaneously or in random order; 2. the extent to which the choice of orbital parameters affects this freedom of access, and the existence of preferred orbits for such freedom of access ; 3. the extent to which the type of modulation and channel arrangement employed affects free dom of access, and the existence of preferred types of modulation and channel arrangement for such freedom of access ; 4. the extent to which the accom m odation of earth stations of different sensitivities and different effective radiated transmitter powers would affect the choice of parameters for freedom of access in communication-satellite systems ; 5. the effects of the preferred choices resulting from §§ 2, 3 a n d 4 on the possibilities of sharing with terrestrial services and with other satellite systems of the same and of different type. STUDY PROGRAMME 2F/IV * ENERGY DISPERSAL IN COMMUNICATION-SATELLITE SYSTEMS The C.C.I.R., (1965 — 1966) CONSIDERING (a) that certain designs of communication-satellite systems involve multiple signals through com mon earth-station amplifiers and satellite repeaters with resulting high levels of intermodula tion products under light-loading conditions ; (b) that the necessary conditions for avoiding mutual interference between communication satellite systems using frequency-modulation and terrestrial radio-relay systems using fre quency-modulation are more difficult to satisfy when the energy density of the communication- satellite system is high, e.g. when it is lightly loaded with telephony signals ; (c) that the potential for mutual interference may be greatly reduced by ensuring that the energy density of the communication-satellite system is uniformly low over the occupied band ; (d) that, in communication-satellite systems carrying telephony, it may be desirable to employ energy-dispersal techniques to load the system artificially to simulate the busy hour conditions; (e) that there is also a need to disperse energy in the case of communication-satellite systems carr ying television, but that in this case the problem is more complex because of the specific nature of the signal energy spectrum, which differs for the various systems (monochrome or colour, with or without an associated sound channel); (f) that any such techniques of energy dispersal should be designed so as not to give rise to a significant increase in total system noise or distortion of the signal; * Formerly Study Programme 235J(IV). — 567 — S.P. 2F/IV, Q. 3/IV (g) that, as far as possible, the application of such energy-dispersal techniques should not add unduly to the complexity of communication-satellite system design and operation; unanimously d e c id e s that the following studies should be carried out: 1. a theoretical and experimental assessment of the manner in which the radio-frequency spectral energy density of a multi-channel telephone system using frequency modulation can be simply * maintained close to the busy hour condition irrespective of the volume of traffic handled, and an evaluation of the reduction in power flux-density in any 4 kHz band obtained by using this technique; 2 . a theoretical and experimental assessment of a simple way in which the radio-frequency spectral energy density of a frequency-modulated television transmission can be kept appre ciably constant, regardless of the signals transmitted (in the different systems — monochrome or colour — with or without an associated sound channel), and an evaluation of the reduction in power flux-density in a specified bandwidth obtained by using this technique. QUESTION 3/IV * SHARING OF RADIO-FREQUENCY BANDS BY LINKS BETWEEN EARTH STATIONS AND SPACECRAFT The C.C.I.R., (1961 — 1963) CONSIDERING (a) that sharing of the radio spectrum by links between earth stations and spacecraft with all other radio services may be necessary, because of the limited spectrum space available to support the world’s communication requirements ; and (b) that factors which determine the ability to share spectrum space are strongly interdependent; unanimously d e c id e s that the following question should be studied: 1. how do the following factors, among others, affect the practicability of sharing : 1.1 location of space and earth stations of a link and the resulting zones of mutual visibility; 1.2 time of use during period of mutual visibility; 1 .3 probability of occupancy of the zones of mutual visibility of links between earth stations and spacecraft by other operating stations during the required times of use of the link and the resulting power levels at all earth stations, as a consequence of this combined occupancy; 1.4 other system parameters, such as modulation techniques, antenna directivity, etc.; 1.5 natural (non man-made) interference; 2 . to what extent is spectrum sharing feasible for different links between earth stations and spacecraft; between these links and other space systems; and between these links and ter restrial radio services ? * Formerly Question 236(IV). S.P. 3A/IV, Q. 4/IV — 568 — STUDY PROGRAMME 3A/IV * SPACE RESEARCH, MAINTENANCE TELEMETERING. TRACKING AND TELECOMMAND SYSTEMS Possibilities of sharing and protection criteria The C.C.I.R., (1965 — 1966) CONSIDERING (a) that the frequency bands allocated to telemetering, tracking and telecommand for space research and for developmental and operational spacecraft may be shared with terrestrial services; (b) that suitable criteria should be established as a basis for the protection of telemetering, tracking and telecommand receivers against interference from other transmissions of the space and terrestrial services; (c) that such criteria should take into account the spectral characteristics of the interfering signals, for example, whether “CW-type” or “noise-like”, and the time pattern of the interference in relation to the time pattern of the system operation ; (d) that the protection criteria for space research discussed in Report 219-1 and embodied in Recommendations 364-1, 365-1 and 366-1 are not necessarily optimum, and that in addition no criteria have yet been recommended for the protection of maintenance telemetry; (e) that the requirements for maintenance-telemetering, for tracking; and for telecommand of developmental and operational satellites may be less stringent than those for research pur poses ; unanimously d e c id e s that the following studies should be carried out : 1. the minimum levels of wanted signal input for which receiving systems for telemetering, tracking and telecommand should be designed ; 2 . the permissible ratios of wanted signal level to interfering signal level for such receiving systems for appropriate modulation of wanted signals and for various time patterns and spectral characteristics of the interfering signals ; 3. the protection criteria applicable to space research and to maintenance-telemetering, tracking and telecommand systems in the frequency bands allocated for these purposes. QUESTION 4/IV * TECHNICAL CHARACTERISTICS OF LINKS BETWEEN EARTH STATIONS AND SPACECRAFT The C.C.I.R., (1961 — 1963) CONSIDERING (a) that the value of spacecraft will, in the future, depend almost entirely on the ability to use radio-frequency electromagnetic energy-for the transmission of all types of information over links between earth stations and spacecraft; * Formerly Study Programme 236A(IV). ** Formerly Question 237(IV). — 569 — Q. 4/IV, 5/IV (b) that available bandwidth in useful regions of the radio-frequency spectrum will be limited ; unanimously d e c id e s that the following question should be studied: what are the preferred technical characteristics and system parameters commensurate with technical feasibility, which will insure the maximum practical use of radio-frequency spectrum space for the following types of link between earth stations and spacecraft: 1 . maintenance telemetering; 2 . tracking; 3. telecommand; 4. communication and data transmission? Note 1. — This Question relates both to space research and to developmental and operational systems. Note 2. — The following factors should be taken into account in carrying out this study : — information rate and duty-cycle, as they affect bandwidth requirements; — required signal-to-noise ratio ; — required reliability and lifetime of links between earth stations and spacecraft; — gain and effective aperture area and pointing accuracy requirements of transmitting and receiving antennae, both at earth stations and at space stations ; — transmitter characteristics (including power, stability, efficiency, etc.); — receiver characteristics (including sensitivity, effective noise-temperature, etc.); — operating factors (including maximum distance between earth stations, size and weight limitations, vehicle orientation capability, relative velocities, etc.); — attenuation due to absorption of energy by the transmission medium; — interference with other links between earth stations and spacecraft and terrestrial services; — type of modulation. QUESTION 5/IV * ACTIVE COMMUNICATION-SATELLITE SYSTEMS FOR FREQUENCY-DIVISION MULTIPLEX TELEPHONY Transmission characteristics of audio-frequency channels The C.C.I.R., (1962 — 1963) CONSIDERING (a) that satellites at various altitudes may be used for communication purposes ; (b) that echoes, e.g. due to impedance mismatch at 4-wire/2-wire terminations external to the satellite link, may be present; * Formerly Question 23 8(IV). 25 Q. 5/IV, 6/IV — 570 — (c) that for telephony transmission, it may be necessary to incorporate echo suppressors; (d) that, for the specification of these echo suppressors and for other reasons, the C.C.I.T.T. will require information on certain transmission characteristics of the communication-satellite system, such as the expected noise value and attenuation variations; (e) that Recommendation 353-1 gives provisional values for the mean noise-power in any hour ; (f) that it may be necessary to correct the frequencies of frequency-division multiplex channels affected by Doppler shifts in communication-satellite systems; unanimously d e c id e s that the following question should be studied: 1 . what higher noise values are to be expected for small percentages of the time; 2 . what attenuation variations in the audio-frequency channels of the baseband are to be expected in the various types of communication-satellite system ; 3. what residual frequency variations due to Doppler and other effects are to be expected in audio frequency channels of the baseband for various types of communication-satellite systems ? QUESTION 6/IV * EFFECTS OF PLASMA ON COMMUNICATIONS WITH SPACECRAFT The C.C.I.R., (1963) CONSIDERING (a) that ionospheric plasma has been observed to have a considerable effect upon the operation of transmitting and receiving antennae mounted on rockets and spacecraft; (b) that the plasma produced by the shock wave resulting from the re-entry of a spacecraft into the terrestrial atmosphere, may have analogous effects ; unanimously d e c id e s that the following question should be studied: 1. what are the effects of the surrounding plasma on the operation of the transmitters and receivers, and in particular the antennae on board spacecraft, taking into account labo ratory experiments and direct measurements ; 2 . what factors determine the formation and structure of the induced plasma surrounding a spacecraft; 3 . what communication problems (wave propagation and noise), are represented (in particular during re-entry into the terrestrial atmosphere), as a result of the plasma ; 4. what influence do these effects exert on the choice of usable frequencies, especially during re-entry of a spacecraft into the terrestrial atmosphere ? Note. — The above Question should be brought to the attention of the U.R.S.I. by the Director, C.C.I.R. * Formerly Question 239(IV). — 571 — S.P. 6A/IV, Q. 7/IV STUDY PROGRAMME 6A/IV * FREQUENCY BANDS FOR RE-ENTRY COMMUNICATIONS The C.C.I.R., (1963) CONSIDERING (a) that communication with a spacecraft during the re-entry phase may be crucial during many missions; (b) that the optimum frequency is dependent upon the configuration of the spacecraft and its re-entry speed; (c) that Recommendation 367 considers only a limited portion of the radio-frequency spectrum ; (d) that, due to the increased transparency of the plasma sheath at higher frequencies, it appears desirable to consider atmospheric windows above the oxygen absorption band at about 60 G H z; (e) that Report 222-1 mentions the theoretical possibility of communicating at frequencies well below the critical frequency of the plasma sheath ; unanimously d e c id e s that the following studies should be carried ou t: 1. investigation of the technical suitability of frequencies above 60 GHz for re-entry com munication ; 2 . investigation into the feasibility of communicating at frequencies well below the critical frequency of the plasma sheath. QUESTION 7/IV ** TRANSMISSION DELAY, ECHOES AND SWITCHING DISCONTINUITIES IN COMMUNICATION-SATELLITE SYSTEMS The C.C.I.R., (1962 — 1963) CONSIDERING (a) that satellites at various altitudes may be used for communication purposes ; (b) that, due to the distances to be traversed by the signals and the finite velocity of radio waves, the use of earth satellites for communication purposes will introduce transmission delay which, if large, may be troublesome for public telephony; (c) that echoes, e.g. due to impedance mismatch at 4-wire/2-wire terminations external to the satellite link, may also be present; * Formerly Study Programme 239A(IV). ** Formerly Question 240(IV). Q. 7/IV, 8/IV — 572 — (d) that transmission discontinuities, due to the switching of signals from satellite to satellite in non- synchronous satellite systems, may cause difficulties for the transmission of telephony, tele graphy, television and other signals, if the discontinuities are excessive or too frequent; (e) that the permissible overall transmission delays, levels of echoes and switching discontinuities, are matters for the C.C.I.T.T. (in the case of television, for the C.M.T.T.) to decide; (f) that the permissible values of transmission delay may have a marked effect on the costs of establishing and maintaining communication-satellite systems ; (g) that, whereas high altitude satellites offer increased coverage with fewer satellites, the transmission delay would be greater than if low altitude satellites were used ; unanimously d e c id e s that the following question should be studied : 1. what transmission delays and switching discontinuities are to be expected in the various types of communication-satellite system ; 2 . what methods, within the satellite system itself, could be used to minimize or avoid transmis- sion-delay variations and switching discontinuities in non-synchronous satellite systems; 3. which orbits are most suitable for communication-satellite systems, as regards the maximum permissible values of the transmission delay, level of echo signals and switching discontinuities for telephony, telegraphy, television and other signals, taking account of the views of the C.C.I.T.T. and C.M.T.T., as appropriate? QUESTION 8/IV * TECHNICAL CHARACTERISTICS OF RADIONAVIGATION-SATELLITE SYSTEMS The C.C.I.R., (1963) CONSIDERING (a) that systems using satellites may well be an important means of world-wide radionavigation in the future; (b) that the frequencies to be used by such systems should be the subject of international agreement, not only to facilitate the setting up of radionavigation systems, but also to avoid interference to and from other satellite systems and stations of other services, which may share the same frequency bands; unanimously d e c id e s that the following question should be studied: 1. what are the preferred types and technical characteristics of radionavigation-satellite systems ; 2 . what are the preferred frequency bands for radionavigation-satellite systems; 3. is the sharing of frequencies with other services feasible, and if so, with what other services and under what conditions ? * Formerly Question 242(IV). — 573 — Q. 9/IV, S.P. 9A/IV QUESTION 9/IV * RADIOCOMMUNICATION FOR METEOROLOGICAL-SATELLITE SYSTEMS The C.C.I.R., (1963) CONSIDERING that the value of meteorological-satellites has been demonstrated and that they will soon be operating in a routine manner ; unanimously d e c id e s that the following question should be studied: what are the preferred characteristics of radiocommunication for meteorological-satellite systems ? STUDY PROGRAMME 9A/IV ** RADIOCOMMUNICATION ASPECTS OF METEOROLOGICAL-SATELLITE SYSTEMS The C.C.I.R., (1 9 6 3 ) CONSIDERING (a) that meteorological-satellite systems may well, in the future, be an important means of world wide weather forecasting (World Weather Watch); (b) that meteorological information is now gathered by meteorological-satellites and relayed to earth stations ; (c) that these satellites may employ different type orbits—polar, equatorial or at intermediate angles and altitudes up to and including the synchronous altitude (3 6 000 k m ); (d) that all of these orbits pass over, or are in view of, many different countries ; (e) that the international character of these systems dictates that the frequency bands employed to relay their collected meteorological data to earth should be subject to international agree ment ; (f) that this would facilitate the establishment of an international weather system and would minimize interference situations; (g) that the evolution of such systems would be facilitated if frequency sharing with other ser vices is practical; unanimously d e c id e s that the following studies should be carried out: 1. what parts of the radio-frequency spectrum are preferred for meteorological-satellite systems; 2 . what are the preferred types and characteristics of radiocommunication for such systems, both under development and in planning; 3. is the sharing of frequencies with other services practical, and if so, with what services and under what conditions ? * Formerly Question 243 (IV). ** Formerly Study Programme 243A(IV). Q. 10/IV — 574 — QUESTION 10/IV * RADIOASTRONOMY The C.C.I.R., (1961 — 1963) CONSIDERING (a) that radioastronomy is based on the reception of signals at much lower power levels than are generally employed in other radio services and, hence, is subject to harmful interference from sources that could be tolerated by many other services; (b) that, for an understanding of the naturally occurring phenomena that produce characteristic line frequencies and the physical processes that result in continuum emission, radioastro nomers must observe at both the immutable line frequencies and at several diverse points in the continuum; (c) that the C.C.I.R., in recognition of the above situation, adopted Recommendation 314-1 “Protection of frequencies used for radioastronomical measurements” at the Xlth Plenary Assembly, Oslo, 1966; (d) that the Administrative Radio Conference, Geneva, 1959, recognized the radioastronomy service and the cosmic origin of the signals used in this branch of astronomy; (e) that the Radio Regulations, Geneva, 1959, allocated a band at the hydrogen-line frequency (1400 to 1427 MHz) for the radioastronomy service on a world-wide basis, and provided some further protection for the radioastronomy service on a regional basis, either by other than primary allocation or by footnote ; (f) that the Administrative Radio Conference, Geneva, 1959, in its Recommendation No. 32, drew attention to the special case of the radioastronomy service and to the failure of the revised Table of Frequency Allocations to meet fully the stated requirements, urged that observatories be located as remotely as possible from sources of radio interference, and recommended that further consideration be given to the question of frequency allocations for radioastronomy; (g) that the expected expansion of space-communication programmes, noted by Recommendation No. 36 of the Administrative Radio Conference, Geneva, 1959, has occurred ; (h) that space communication signals may nullify the partial protection to radioastronomy observations afforded either by remotely located observatory sites or by other regional or more local administrative provisions ; unanimously d e c id e s that the following question should be studied: 1. what are the characteristics of the signal of interest to radioastronomy ? In addition to para meters such as intensity, frequency, bandwidth, polarization, and modulation, or frequency of occurrence, how do the characteristics vary depending on the celestial source, e.g. the Sun, planets, extended galactic sources, extragalactic sources with large Doppler shifts; 2. with reference to Article 1, No. 93 of the Radio Regulations, Geneva, 1959, which defines harmful interference as “Any emission, radiation, or induction which endangers the function ing of a radionavigation service or of other safety services, or seriously degrades, obstructs * Formerly Question 244(IV). — 575 — Q. 10/IV, 11/IV or repeatedly interrupts a radiocommunication service operating in accordance with these Regulations”, what is a practical interpretation of this definition for the radioastronomy service; 2.1 with reference to § 1 , what are the threshold values of the signal level for there to be harmful interference; 2 .2 how are these threshold values modified if reception is via the side lobes only, rather than via the main beam of the radioastronomy telescope ; 3. what interference levels would typical observatory sites (the site at Green Bank, W. Va. might be taken as a typical remote site, and the Observatory of the University of Michigan or Lincoln Laboratory might be typical of sites in more populated regions) experience in the VHF, SHF and EHF bands, as predicted by means of the best available experimental data and accepted theories on atmospheric scatter, diffraction, and related propagation phenomena, from the following types of source : 3.1 ground-based transmitters, assumed to operate in accordance with accepted standards; 3.2 airborne transmitters, including both fundamental and spurious emissions; 3.3 aircraft illuminated by ground-based transmitters ; 3.4 orbiting devices with active transmitters, considered both with and without directional an tenna systems; 3.5 orbiting reflectors, illuminated from the ground, including both “Echo” type satellites and unintentional reflectors such as active satellites, carrier rocket cases, and similar debris; 3.6 orbiting zones or bands of diffuse scattering elements, illuminated from the ground; 4. what are the general areas of interest in the radio-frequency spectrum to the radioastronomy service, in the light of: 4.1 the Table of Frequency Allocations, 1959 ; 4.2 the advent of operational space telecommunication systems; 4.3 the rapidly improving observational capabilities and techniques now in use and anticipated, such as larger antenna systems, masers, and other forms of improved receivers, and more sophisticated means of handling and analysing data; 4.4 possible use of higher frequencies than the present limit of 40 GHz in the Table of Frequency Allocations; 4.5 other line frequencies than those listed by the C.C.I.R. in Recommendation 314-1 ? QUESTION 11/IV* RADAR ASTRONOMY The C.C.I.R., (1963) CONSIDERING (a) that radar astronomy is a part of pure science, contributing to our knowledge through studies of the reflecting properties of natural and man-made targets, advancing the study of celestial mechanics by direct measurements with great precision, of the motions and distances of orbiting bodies, and through the study of the nature and effects of the propagating medium ; * Formerly Question 245(IV). f Q. 11/IV, 12/IV — 576 — (b) that the receiving techniques of radar astronomy require sensitivities equivalent to those of radioastronomy; (c) that the problems of detection, location, tracking and determination of ephemeris are common to radar astronomy and communication-satellite systems ; ( d) that radar astronomy transmitters are seldom developed solely for this application, but are ordinarily the outgrowths of the most advanced transmitter technology developed for other p u r p o s e s ; (e) that radar astronomy has immediate applications to space radiocommunication by providing basic data required for the computation of trajectories and ephemerides for space objects; (f) that there is a close parallel between the objectives of radar astronomy and some communica tion-satellite systems, because both depend upon radio scattering from objects in space; (g) that radar astronomy frequencies are not generally tied to the frequencies associated with natural phenomena, the exception being special experiments on the atmosphere of the earth and the planets; unanimously d e c id e s that the following question should be studied: 1. what are the performance characteristics of radar astronomy systems ; 2. what levels and durations of interfering signals are tolerable in radar astronomy reception ; 3. what factors, both technological and scientific, are fundamental in the selection of frequencies for radar astronomy experiments ? QUESTION 12/IV* FEASIBILITY OF DIRECT SOUND AND TELEVISION BROADCASTING FROM SATELLITES The C.C.I.R., (1966) CONSIDERING (a) that there are many parts of the world with little or no broadcasting service ; (b) that there is considerable interest in the possibility of broadcasting from satellites; unanimously d e c id e s that the following question should be studied: 1. what are the satellite orbits most satisfactory for direct broadcasting to the general public from satellites; 2 . what accuracy of positioning or station keeping can be achieved; * This Question, which replaces Question 283, shall be studied in connection with Questions 20/X and 5/XI; contribution to the study of this Question shall be brought to the attention of participants to the work of Study Groups X and XI. — 577 — Q. 12/IV, 13/IV 3. what maximum primary power is likely to be available to operate a transmitter in a satellite, and what other factors associated with the space environment operate to limit the power that could be developed in the transmitter at the various frequencies that might be used, up to 12-7 G H z; 4. what gain, directivity and stability of orientation are attainable for satellite transmitting antennae at various frequencies ; 5. what is the probable working life of a satellite, bearing in mind that failure in accurate position ing or antenna orientation may end the useful life ? QUESTION 13/IV* CONTRIBUTIONS TO THE NOISE TEMPERATURE OF AN EARTH-STATION RECEIVING ANTENNA The C.C.I.R., (1965) CONSIDERING (a) that the noise temperature of an earth-station receiving antenna includes contributions asso ciated with atmospheric constituents such as water vapour, clouds, and precipitation; (b) that the use of a radome can introduce additional components into the antenna noise tem perature ; (c) that these effects can be isolated and examined when proper account is taken of all sources of receiving system noise temperature, such as solar and cosmic noise, the ground and other features of the antenna environment, man-made noise and unwanted signals, and thermal noise generated by the receiving system or noise measuring system and referred back to the antenna terminals ; unanimously d e c id e s that the following question should be studied: 1. what contributions to the noise temperatures of typical earth-station receiving antennae, with or without radomes, are introduced by : 1.1 water vapour in the atmosphere, 1.2 other atmospheric constituents, 1.3 clouds, 1.4 precipitation, 1.5 the earth, 1.6 solar and cosmic noise ; 2 . how do these contributions vary diurnally and seasonally ; how do they vary with the receiving pattern of the antenna and with direction, specifically at the fixed values 1°, 2°, 3°, 5°, 10°, 30° and 90° of angles of elevation; 3. how can those variations of noise temperature, which are related to meteorological factors, be expressed statistically; 4. what additional contributions to the noise are caused, typically, by the use of a radome, when wet and when dry, taking into account both its absorbing and its scattering properties ; and also the possibility of ice accretion both with and without other matter ; 5. how are these contributions expected to depend on frequency ? * Formerly Question 284(IV). Q. 13/IV, 14/IV — 578 — Note. — It is expected that most data from earth stations will be at frequencies near 4 G H z; data from other types of antennae, such as those used in radioastronomy, may also be useful in this connection. QUESTION 14/IV* PROPAGATION FACTORS AFFECTING SHARING OF THE RADIO-FREQUENCY SPECTRUM AND COORDINATION BETWEEN SPACE AND TERRESTRIAL RADIO-RELAY SYSTEMS The C.C.I.R., (1966) CONSIDERING (a) that the calculation of coordination distance (Recommendation No. 1A of the E.A.R.C., Geneva, 1963), for the siting of earth stations for space systems in relation to stations for terrestrial services sharing the same frequency band, must take account of all mechanism of propagation that may lead to mutual interference ; (b) that the case of an earth station with a highly directional antenna shielded by local high ground from terrestrial stations needs particular study because of the possibility of increased propagation loss and reduced coordination distance due to such terrain; (c) that little is known of the extent to which scattering from precipitation may cause enhanced interference and therefore influence the coordination distances and siting of earth stations ; unanimously d e c id e s that the following question should be studied: 1. what are the modes of propagation over typical paths from 50 to 2000 km long (50-500 km for SHF), with no appreciable site shielding, and what are the values below which the trans mission loss over such paths falls for small percentages of the time, e.g. 0 -0 1 %, 0 *1% and 1% of a year, when highly directional earth-station antennae are used; 2 . in what way and to what extent are transmission losses affected by the presence of high ground near the earth station and what are the safe working values of site-shielding factor that can be used in calculating coordination distances ; 3. to what extent are transmission losses affected by scatter from precipitation, taking account of all expected orientations of the earth-station antenna and considering scattering into the main beam and also into side-lobes ? Note 1. — Frequencies of primary interest in this Question are 0*14, 0*4, 2, 4, 6 and 9 GHz. Note 2. — Data are required for different climatic regions of the Earth. Note 3. — The detailed work under this Question is more appropriate to the work of Study Group V under Study Programmes 5D/V, 5E/V and 5F/V. * This Question replaces Questions 285, 286 and 287. — 579 — Q. 15/IV, 16/IV QUESTION 15/IV* FREQUENCY UTILIZATION ABOVE THE IONOSPHERE AND ON THE FAR SIDE OF THE MOON The C.C.I.R., (1965 — 1966) CONSIDERING (a) that some radioastronomical and other scientific experiments are difficult, and perhaps impossible, to carry out on the surface of the earth; (b) that the advent of spacecraft has already permitted scientific observations to be made from vantage points above the ionosphere, and that further developments will enable experiments to be carried out in the relatively quiet environment on the far side of the M oon; (c) that in addition to the establishment of line-of-sight communication links for scientific and other purposes between the earth and spacecraft, it may be necessary to establish links between spacecraft above the ionosphere and also to establish links between stations on the far side of the Moon and other stations either on or visible from the Earth; (d) that at frequencies below the critical penetration frequency of the ionosphere, the region above the ionosphere is relatively isolated from terrestrial noise and communication signals; (e) that on the far side of the Moon an even greater degree of isolation from terrestrial radiation is provided at all radio frequencies; unanimously d e c id e s that the following question should be studied: 1. what are the preferred means and routes for communicating between: 1.1 a station on the far side of the Moon and a station just above the ionosphere, 1.2 a station on the far side of the Moon and an earth station, 1.3 two spacecraft above the ionosphere ; 2 . in what frequency bands would radioastronomical measurements carried out: 2.1 on a station above the ionosphere, 2.2 on the far side of the Moon, have marked advantages compared with observations from the surface of the Earth ? QUESTION 16/IV SHIELDING EFFECTS DUE TO THE IONOSPHERE The C.C.I.R., (1966) CONSIDERING Question 15/IV, and the fact that optimum utilization of frequencies above the ionosphere requires better understanding of the shielding effects due to passage through the ionosphere; * Formerly Question 288(IV). Q. 16/IV, 17/IV — 580 — unanimously d e c id e s that the following question should be studied: what attenuation of terrestrial radio signals occurs when they travel through the ionosphere to the region beyond, and how does this attenuation vary with frequency, zenith angle, time of day, season of year, phase of the sunspot cycle, and the location of the source of the signals ? Note. — The attention of Study Group VI is directed to this Question. QUESTION 17/IV SHIELDING EFFECTS DUE TO THE MOON The C.C.I.R., (1966) CONSIDERING Question 15/IV, and the fact that optimum utilization of frequencies on the far side of the Moon requires better understanding of the shielding effects due to the presence of the Moon ; unanimously d e c id e s that the following question should be studied: how does the shielding caused by the Moon vary as a function of frequency, angular distance from the limb of the Moon to the centre of the far side, and distance above the surface of the Moon ? Note. — The attention of Study Group V is directed to this Question. — 581 — LIST OF DOCUMENTS CONCERNING STUDY GROUP IV (Period 1963-1966) Doc. Origin Title Reference IV/1 C.C.I.R. Secretariat Telephone subscribers’ tolerance of delay times CCITT Docs. and echoes (CCITT Study Groups XII and COM XII - 62, XVI — June, 1963) COM XVI - 55 IV/2 United Kingdom Number of satellites required for specified zone S.P. 235E of coverage and quality of service in multiple- access communication-satellite systems using random satellites IV/3 United Kingdom Factors affecting the freedom of access in com S.P. 235E munication-satellite systems IV/4 United Kingdom Effect of the duration of calls on the outage time Q. 235 of random satellite systems IV/5 United Kingdom Feasibility of direct broadcasting from artificial Q. 241 and Corr. 1 earth satellites. Considerations of frequency sharing IV/6 Director, C.C.I.R. Recommendation concerning propagation-time CCITT (Illrd Plenary Assembly of the CCITT, Rec. G.114 Geneva, 1964) (P. 14) IV/7 Italy Summary of the activities of the Italian earth station with experimental communication satel lites IV/8 Federal Republic Effects of plasma on communications with Q. 239, §§1,3 of Germany spacecraft IV/9 C.C.I.R. Secretariat Report by Special Joint Study Group C CCITT (CCITT/CCIR) on circuit noise Doc. AP III/47 IV/10 United States Radioastronomy — Proposed modification to Rep. 224 of America Report 224 IV/11 United States Proposed modification to Report 225. The Rep. 225 of America possibility of frequency sharing between radio- astronomy and other services IV/12 United States The OH-lines in radioastronomy Q. 244 of America IV /13 Canada Time differences between the sound and vision Q. 240 (X/7) components of a television signal Q. 66(X) (CMTT/1) Q. 270(CMTT) Rec. 265 IV/14 Canada Sharing of radio-frequency bands. Computer Q. 236 (IX/7) study of the incidence of exposure of 4 GHz Q. 192(IX) radio-relay systems in Canada to communica tion satellites in various orbits IV/15 Canada Frequency sharing between communication- Q. 235 (IX/8) satellite services and terrestrial radio services Q. 192(IX) — 582 — Origin Title IV/16 Canada Draft Study Programme 241A(IV). Feasibility Q. 241 (X/8) of direct sound and television broadcasting (XI/3) from satellites IV/17 Canada Communication-satellite systems sharing fre Q. 235 quency bands with line-of-sight radio-relay sys S.P. 235B tems. Maximum allowable values of inter ference in a telephone channel of a radio-relay system IV/18 New Zealand Draft Question. Communication-satellite links Draft Q. for world-wide telephone coverage. Combina tions of systems and preferred characteristics IV/19 New Zealand Draft Recommendation. Active earth commu Q. 236 nication-satellite repeaters. Provision of mul tiple access for large and small users IV/20 United States Computation of receiver transfer characteristic Q. 235 of America between two phase-modulated carriers S.P. 235B Rep. 209 (Ann. I, § 3.2) IV/21 United States Laboratory measurements of the receiver trans Q. 235 of America fer characteristic, radio-relay system to com S.P. 235B munication-satellite system Rep. 209 (Ann. I, § 3.2) IV/22 United States An example of interference to radio-relay sys Q. 235 of America tems from a multiple-access set of frequency- S.P. 235B modulated carriers, as compared with inter ference from a single frequency-modulated car rier IV/23 United States Active communication-satellite systems for Q. 235 of America frequency-division multiplex telephony. Fre S.P. 235D quency deviation 1V/24 United States Proposed modifications to Report 210 Q. 235 of America S.P. 235C Rep. 210 IV/25 United States Feasibility of frequency sharing between the Q. 242 of America radionavigation-satellite service and terrestrial services IV/26 France Technical characteristics of communication- Q. 236 satellite systems. Statistical distribution of noise power IV/27 France Active communication-satellite systems for Rec. 353 telephony. Allowable noise power in the basic Rep. 208 hypothetical reference circuit IV/28 France Technical characteristics of communication- Q. 235 satellite systems. Pre-emphasis characteristics Rep. 212 for frequency-division multiplex telephony using frequency-modulation IV/29 France Radiocommunication for meteorological-satel Q. 243 lite systems. Satellite systems for the location S.P. 243A of constant-altitude balloons and for the retransmission of meteorological data (Project EOLE) — 583 — D oc. Origin Title Reference IV/30 France Technical characteristics of links between earth Q. 237 stations and spacecraft. C.N.E.S. tracking, telemetering and telecommand stations (DIANE and IRIS networks) IV/31 Federal Republic Technical characteristics of radionavigation- Q. 242, § 1 of Germany satellite systems IV/32 United States Number of antennae of A.T. & T. Co. radio- Q. 235 (IX/24) of America relay systems operating in the 4 GHz band, the S.P. 235A beams of which intersect the orbits of stationary satellites IV/33 United Kingdom Proposed modifications to Report 214. Com Rep. 214 munication-satellite systems. The effects of Doppler frequency-shifts, transmission time- delays and switching discontinuities IV/34 United Kingdom Proposed modifications to Report 224. Radio- Rep. 224 astronomy (Q. 244) IV/35 United Kingdom Proposed modifications to Recommendation Rec. 314 314. Protection of frequencies used for radio astronomical measurements IV/36 United Kingdom Proposed modifications to Recommendations Rees. 353, 355, (IX/27) 353, 355, 356 and 357 356 and 357 IV/37 United Kingdom Antenna characteristics for the earth stations Q. 234 (IX/28) of communication-satellite systems. Proposals S.P. 235A for a reference antenna radiation diagram for S.P. 235C use in interference studies IV/38 United States The effects of modulation and multiplexing Q. 235 of America upon freedom of access in active communica S.P. 235D tion-satellite systems S.P. 235E IV/39 United States Draft Recommendation. Telecommunication Rees. 364, 365 of America links for the manned and unmanned space and 366 research service. Frequencies, bandwidths and interference criteria IV/40 United States Draft Report. Interference and other special Reps. 219, 220 of America considerations for telecommunication links for and 221 the manned and unmanned space-research ser Q. 236 vice Q. 237 S.P. 205 IV/41 United States Draft Report. Use of earth-satellites for ter Rep. 216 of America restrial navigation Q. 242 IV/42 United States Draft Recommendation. Frequency require Rec. 361 of America ments for radionavigation-satellite systems IV/43 United States Meteorological satellites Q. 243 of America S.P. 243A IV/44 United States Draft Report. Radio spectrum requirements Q. 243 of America for meteorological satellites S.P. 243A IV/45 United Kingdom Antennae for space systems Q. 243 IV/46 United Kingdom Technical characteristics of radionavigation- Q. 242 satellite systems. Applications of earth satel lites for terrestrial radionavigation service IV/47 United States Draft new Question. Frequency utilization Draft Q. of America above the ionosphere and on the far side of the Moon — 584 — D oc. Origin Title Reference IV/48 United States Bandwidth requirements for communication- Q. 235, §5 of America satellite systems employing angle-modulation IV/49 United States Proposed modifications to Recommendation Rec. 360 o f America 360. Criteria for selection of preferred reference frequencies for communication-satellite systems sharing frequency bands with line-of-sight radio-relay systems IV/50 C.C.I.R. Secretariat List of documents issued (IV/1 to IV/50) IV/51 United States Proposed modifications to Report 214. Com Rep. 214 of America munication-satellite systems. The effects of Doppler frequency-shifts, transmission time- delays and switching discontinuities IV/52 United States Earth-station antennae for the communication- Q. 234 of America satellite service IV/53 United Kingdom Preferred reference frequencies for communica Q. 235 (IX/37) tion-satellite systems sharing frequency bands S.P. 235C with line-of-sight radio-relay systems IV/54 United Kingdom Proposed modifications to Recommendation Rec. 364 364. Telecommunication links for near-earth research satellites. Frequencies, bandwidths and interference criteria IV/55 United Kingdom Proposed modifications to Report 219. Inter Rep. 219 ference considerations for near-earth research satellite telecommunication links IV/56 Federal Republic Antennae for space systems Q. 234 of Germany IV/57 United Kingdom Maintenance telemetering, tracking and tele Q. 236, 237 command for developmental and operational satellites. Interference protection criteria IV/58 United Kingdom Sharing of radio-frequency bands by links Q. 236 between earth stations and spacecraft. Fre quency sharing between earth-satellite tele metering or telecommand links and terrestrial services IV/59 Japan Draft Study Programme. Radio-relay systems Q. 235, 236 for telephony using pulse-code modulation. Frequency sharing with communication-satel lite systems IV/60 United Kingdom Communication-satellite systems sharing the Q. 235 (IX/58) same frequency bands as line-of-sight radio- S.P. 235A relay systems. Maximum allowable values of Rec. 358 power flux-density at the surface of the earth produced by non-stationary communication satellites IV/61 United Kingdom Line-of-sight radio-relay systems sharing the Q. 235 (IX/59) same frequency bands as the satellite receivers S.P. 235A of active earth-satellite communication systems. Rec. 406 Maximum effective radiated powers of line-of- sight radio-relay system transmitters (non- stationary satellites) — 585 — Doc. Origin Title Reference IV/62 United Kingdom Feasibility of frequency sharing between com Q. 235 ax/60) munication-satellite systems and terrestrial S.P. 235A radio services. Interference from communica tion-satellite systems to radio-relay systems car rying 625-line television signals IV/63 United Kingdom Site shielding-factor to be used in calculation of Draft Q. coordination distance IV/64 Federal Republic Feasibility of frequency sharing between com S.P. 235A, § 1 of Germany \ munication-satellite systems and terrestrial radio services. Criteria affecting the selection of sites for earth stations in the communication- satellite system IV/65 United Kingdom Feasibility of frequency sharing between com Q. 235 ax/7i) munication-satellite and terrestrial radio ser S.P. 235A vices. Interference between stationary commu Rees. 358, 406 nication-satellite systems and line-of-sight radio-relay systems IV/6 6 United Kingdom Advantages and disadvantages of compandors Q. 235 in communication-satellite systems S.P. 235D IV/67 Chairman, Interim report by the Chairman, Study Group and Rev. 1 Study Group VI IV. Space systems and radioastronomy IV/68 United Kingdom Interference within and between communica S.P. 235C tion-satellite systems IV/69 Federal Republic Technical characteristics of communication- Q. 235, § 5 (IX/76) of Germany satellite systems Q. 192(IX) IV/70 U.R.S.I. Factors affecting the selection of frequencies Q. 239 for telecommunications with spacecraft re-enter Rep. 222 ing the earth’s atmosphere IV/71 Australia The OH-lines in the radio spectrum Q. 244 IV/72 Australia Proposed modification to Report 224. Radio- Rep. 224 astronomy IV/73 Australia Proposed modification to Report 225. The Rep. 225 possibility of frequency sharing between radio- astronomy and other services IV/74 C.C.I.R. Secretariat Submission of Doc. VI/8 to Study Group IV. S.P. 205(VI) Effects of radio noise in space on communica tions with spacecraft IV/75 Australia Areas in the radio-frequency spectrum of inter Q. 244 est to radioastronomy IV/76 France Terms and definitions relating to space radio Rep. 204 (XIV/3) communication and Add. 1 IV/77 France Antennae for space systems Q. 234 IV/78 Federal Republic Feasibility of frequency sharing between com S.P. 235A, § 5 of Germany munication-satellite systems and terrestrial radio services IV/79 Canada Co-channel interference caused by scattering Q. 244 from rain IV/80 Canada Protection to the radioastronomy service Q. 244 26 — 586 — D oc. Origin Title Reference IV/81 Canada Sharing of frequency bands between communi- Q. 235 cation-satellite systems and tropospheric-scatter terrestrial radio-relay systems IV/82 United States Technical feasibility of direct broadcasting from Q. 241 (X /38) of America earth satellites. Sharing considerations for band 8 frequency-modulation sound broad casting IV/83 United States Technical feasibility of direct broadcasting from Q. 241 (XI/37) of America earth satellites. Sharing considerations' for bands 8 and 9 television broadcasting IV/84 United States Communication-satellite systems and line-of- Rec. 358 (IX/81) of America sight radio-relay systems sharing the same fre quency bands IV/85 United States Communication-satellite systems — Frequency Rep. 209 (IX/82) of America sharing between communication-satellite sys tems and terrestrial services IV/86 United States Determination of power flux-density required Q. 235 (IX/83) of America by communication-satellite systems IV/87 Japan Interference at communication satellite caused S.P. 235B by terrestrial line-of-sight radio-relay system transmitter IV/88 Study Group IV Summary record of the first meeting IV/89 Study Groups Summary record — Joint opening session (VIII/27) IV and VIII and Corr. IV/90 Study Group IV Summary record of the second meeting IV/91 United States Corrigenda to Recommendations 356, 357, 358, Q. 235 of America 359 and 360 relating to communication-satellite S.P. 235A systems and radio-relay systems sharing the Rees. 356, 357, same frequency bands 358, 359 and 360 IV/92 United States Power limits of earth station in the communi- Rep. 209 of America cation-satellite service IV/93 Japan Communication-satellite systems. Feasibility of S.P. 235A frequency sharing between communication- satellite systems and terrestrial radio services IV/94 Working Party Communication-satellite systems Rep. 214 IV-A-2 IV/95 Sub-Group IV-B Active communication-satellite systems. Feasi Rec. 355 bility of sharing frequency bands with terres trial radio services IV/96 Sub-Group IV-C Interference and other special considerations Draft Rep. for telecommunication links for the manned and unmanned space-research service IV/97 Sub-Group IV-A Results of Working Group on noise IV/98 Sub-Group IV-C Frequency requirements for radionavigation- satellite systems — 587 — Doc. Origin Title Reference IV/99 Sub-Group IV-C Use of satellites for terrestrial navigation Q. 242 IV/100 C.C.I.R. Secretariat List of documents issued (IV/51 to IV/100) IV/101 Sub-Group IV-C Draft Report — Feasibility of frequency shar Q. 242 and ing between the radionavigation-satellite ser Rev. 1, 2 vice and terrestrial services IV/102 Sub-Group IV-C Meteorological satellites Draft Rep. IV/103 Sub-Group IV-C Radio spectrum requirements for meteorologi Draft Rep. cal satellites IV/104 Sub-Group IV-A Communication-satellite systems — The effects Proposed mod. and Add. 1 of Doppler frequency-shifts, transmission time- to Rep. 214 Rev. 1, delays and switching discontinuities Corr. 1 IV/105 Sub-Group IV-A Draft revision of Recommendations 353, 356 and and 357 Rev. 1, 2 IV/106 Sub-Group IV-A Draft revision of Report 206 IV/107 Sub-Group IV-A Draft revision of Report 215 — Feasibility of Q. 241 direct sound and television broadcasting from satellites IV/108 Sub-Group IV-A Feasibility of direct sound and television broad Draft rev. of casting from satellites Q. 241 IV/109 Sub-Group IV-A Draft Report — Radiation diagrams of anten Q. 234 and nae at communication-satellite earth station, Corr. 1, 2 for use in interference studies IV/110 Sub-Group IV-A Draft revision of Report 212 1V/111 Sub-Group IV-E Protection of frequencies used for radioastro Revision of nomical measurements Rec. 314 IV/112 Sub-Group IV-B Draft Report — Feasibility of frequency shar S.P. 235A, § 1 ing between communication-satellite systems and terrestrial radio services. Site selection criteria for earth stations in the communica tion-satellite service IV/113 Sub-Group IV-B Sharing frequency bands between communica Draft Rep. and Corr. tion-satellite systems and tropospheric-scatter terrestrial radio-relay systems IV/114 Sub-Group IV-B Draft Report — Comments on theoretical and S.P. 235A experimental values of receiver transfer charac teristics IV/115 Sub-Group IV-D Draft revision of Report 205 and Rev. 1 IV /116 Sub-Group IV-C Maintenance telemetering, tracking and tele Draft Rep. command for developmental and operational satellites. Frequency sharing between earth- satellite telemetering or telecommand links and terrestrial services 26 Doc. Origin Title Reference IV/117 Sub-Group IV-B Comments on E.A.R.C. procedure for calculat Draft Rep. Rev. 1 and ing coordination distance between earth sta (Annex to Corr. 1 tions and terrestrial stations sharing the same Rec. No. 1A) frequency band in the range 1-10 GHz, Rec ommendation No. 1A, Annex IV/118 Sub-Group IV-B Adequacy of using the spectral density of Draft Rep. power or power flux as an interference criterion IV/119 Sub-Group IV-B Line-of-sight radio-relay systems sharing the Draft rev. same frequency bands as the satellite receivers of Rec. 406 of active earth-satellite communication systems. Maximum radiated powers of line-of-sight radio-relay system transmitters IV/120 Sub-Group IV-B Proposed revision of Study Programme 23 5A(IV) and Rev. 1 IV/121 Sub-Group IV-B Proposed revision of Study Programme 235B(IV) IV/122 Sub-Group IV-D Site shielding-factor to be used in calculating Draft Q. and Rev. 1 coordination distances (Rev. of Doc. IV/63) IV/123 Sub-Group IV-D Frequency utilization above the ionosphere and Draft Q. on the far side of the Moon IV/124 Study Group IV Summary record of the third meeting IV/125 Sub-Group IV-D Propagation factors affecting coordination dis Draft Q. and tance between terrestrial radio-relay systems Rev. 1, 2 and the earth stations of space systems IV/126 Sub-Group IV-C Study Programme — Space research and main Q. 236 and Rev. 1 tenance telecommunication systems IV/127 Sub-Group IV-B Draft Report — Power flux-density at the sur Q. 235 face of the earth from communication satellites IV/128 Working Party Report by the Chairman IV-B-1 IV/129 Study Group IV Contributions to the noise temperature of an Draft Q. earth-station receiving antenna IV/130 Sub-Group IV-A Proposed revision of Study Programme 23 5E 1V/131 Sub-Group IV-A Earth-station antennae for the communication Draft Rep. and satellite service Corr. 1, 2 IV/132 Sub-Group IV-A Proposed revision of Report 211 1V/133 Sub-Group IV-B Draft letter 1V/134 Sub-Group IV-A Draft Recommendation — Frequency selection Q. 235 and carrier energy-dispersal for communica S.P. 235A tion-satellite systems — 589 — Doc. Origin Title Reference IV/135 Sub-Group IV-A Frequency sharing between communication- S.P. 235F and Corr. 1 satellite systems and terrestrial radio-relay sys tems. Energy dispersal in communication- satellite systems IV/136 Study Group IV Summary record of the fourth meeting IV/137 Sub-Group IV-E Report — Radioastronomy. Characteristics, Q. 244 and Corr. 1 and factors affecting frequency sharing with other services , IV/138 Sub-Group IV-B Draft Report — Corrigenda to Recommenda tions 358 and 359 IV/139 Sub-Group IV-E Draft New Report — The OH-lines in radio- Q. 244 astronomy IV/140 Sub-Group IV-A Draft revision of Report 2 1 1 — Bandwidth Q. 235 requirements for communication-satellite sys S.P. 235D tems employing angle-modulation IV/141 Sub-Group IV-A Draft revision of Report 213 — Factors affect S.P. 235E ing multi-station access in communication- satellite systems IV/142 Sub-Group IV-D Scattering from precipitation as a factor in the Draft new Q. siting of earth stations IV/143 Sub-Group IV-C Report of the Chairman IV /144 Sub-Group IV-B Interpretation of the term “Effective radiated power” IV/145 Working Party Report by the Chairman IV-B-2 IV/146 Sub-Group IV-D Factors affecting the selection of frequencies for Proposed rev. telecommunications with spacecraft re-entering of Rep. 222 the earth’s atmosphere IV/147 Sub-Group IV-D Draft letter to the Chairman, Study Group VI from the Chairman, Study Group IV IV/148 Sub-Group IV-D Draft letter to the Chairman, Study Group V from the Chairman, Study Group IV IV/149 Study Group IV Communication of Doc. VIII/44 IV/150 C.C.I.R. Secretariat List of documents issued (IV/101 to IV/150) IV /151 Sub-Group IV-A List of documents assigned to Sub-Group IV-A and action taken upon them IV/152 and Sub-Group IV-B Draft Annex I — Angles of separation occurring R ep .210 Corr. 1, 2 between satellites IV/153, Sub-Group IV-B Draft Annex II — Method of determination of Rep. 210 Rev. 1 the level of interference and Corr. 1 — 590 — Doc. Origin Title Reference IV/154 Sub-Group IX-B Proposed modification to Doc. IV/105 (Rev. 2) Rec. 357 ax/103) IV/155 “Terminology” Report and Rev. 1 Working Party IV/156 Sub-Group IV-E Report by the Chairman IV /157 Sub-Group IV-B Frequency sharing between communication- Rep. 209 and Rev. 1 satellite systems and terrestrial services IV/158 Sub-Group IV-B Report by the Chairman of Working Party IV-B-4 IV/159 Sub-Group IV-B Retention of Recommendation 355 Rec. 355 IV/160 Study Group IV Summary record of the fifth meeting IV/161 Sub-Group IV-D List of documents assigned to Sub-Group IV-D and action taken upon them IV/162 Study Group IV Summary record of the sixth meeting IV/163 C.C.I.R. Secretariat Status of texts concerning Study Group IV IV/164 Study Group IV Summary record of the seventh meeting IV/165 Sub-Group IV-B Report by the Chairman IV /166 C.C.I.R. Secretariat List of participants — Interim Meetings (VII/56) (Monte Carlo, 1965) (VIII/67) (IX/159) IV/167 C.C.I.R. Secretariat List of documents issued (IV/151 to IV /167) IV/168 C.C.I.R. Secretariat Comments by United Kingdom on Questions (V/86) 285(1V), 286(IV) IV/169 United States Proposed modification to Draft Report L.3.a Draft Rep. of America (IV) — Feasibility of direct sound and televi sion broadcasting from satellites IV/170 United States Factors affecting freedom of access in commu S.P. 235E of America nication-satellite systems. Random multiple- access systems : important boundaries of earth stations IV/171 United States Proposed modification to Draft Report L.2.n Q. 234 of America , (IV) — Radiation diagrams of antennae at communication-satellite earth stations, for use in interference studies IV/172 United States Proposed modification to Recommendation 355 Q. 235 (IX /179) of America — Active communication-satellite systems. S.P. 235A Feasibility of sharing bands with terrestrial radio services IV/173 United States Proposed modification to Draft Recommenda Q. 235 (IX/180) of America tion L.2.b(IV) — Communication-satellite sys S.P. 235A tems sharing the same frequency bands as line- • of-sight radio-relay systems — 591 — D oc. Origin Title Reference IV /174 United States Proposed modification to Draft Recommenda Q. 235 (IX/181) of America tion L.2.c(IV) — Communication-satellite sys S.P. 235A tems sharing frequency bands with line-of-sight radio-relay systems IV/175 United States Proposed modification to Recommendation 360 Q. 235 (IX/182) of America — Criteria for selection of preferred reference S.P. 235A frequencies for communication-satellite sys tems sharing frequency bands with line-of-sight radio-relay systems IV/176 United States Proposed modification to Recommendation Draft Rec. (IX/183) of America F.5.c(IX) — Lirie-of-sight radio-relay systems sharing the same frequency bands as the satellite receivers of active earth-satellite communica tion systems IV/177 United States Proposed modification to Draft Recommenda Q. 235 (IX/184) of America tion L.2.d(IV) — Communication-satellite sys S.P. 235A tems and line-of-sight radio-relay systems shar ing the same frequency bands. Maximum allowable values of power flux-density at the surface of the earth produced by communica tion satellites IV/178 United States Proposed modifications to Draft Report L.2.k S.P. 235H of America (IV) — Factors affecting multi-station access in communication-satellite systems IV/179 United Kingdom Frequency sharing between communication- S.P. 235J satellite systems and terrestrial radio-relay sys tems. Energy dispersal in communication- satellite systems IV/180 United Kingdom Proposed modifications to Draft Report L.2.f Draft Rep. (IV) — Technical characteristics of communi Q. 235 cation-satellite systems IV/181 United Kingdom Proposed modifications to Report 208 — Tech Rep. 208 nical characteristics of communication-satel Q. 235 lite systems IV/182 United Kingdom Antennae for space systems — Summary of DraftfRep. antenna characteristics at Goonhilly Earth Sta tion, U.K. IV/183 United Kingdom Proposed modifications to Report L.8 .b(IV) — Draft Rep. Radioastronomy. Characteristics and factors affecting frequency sharing with other services IV/184 United Kingdom Proposed modifications to Recommendation 365 Rec. 365 — Telecommunication links for deep-space research. Frequency, bandwidths and inter ference criteria IV/185 United Kingdom Proposed modifications to Recommendation 364 Rec. 364 — Telecommunication links for near-earth re search satellites. Frequencies bandwidths and interference criteria IV/186 United Kingdom Proposed modifications to Recommendation 366 Rec .[3 66 — Telecommunication links for manned re search spacecraft — 592 — D oc. Origin Title Reference IV/187 United Kingdom Proposed modifications to Study Programme S.P. 236A 236A(IV) — Space research and maintenance telemetering, tracking and telecommand sys tems. Possibilities of sharing and protection criteria IV/188 United Kingdom Proposed modifications to Draft Report L .l.a Draft Rep. (IV) — Factors affecting the selection of fre quencies for telecommunications with and between spacecraft IV/189 United Kingdom Proposed modifications to Draft Report L .8.C Draft Rep. (IV) — The OH-lines in radioastronomy IV /190 United Kingdom Feasibility of frequency sharing between com Q. 235 (IX/186) munication-satellite systems and terrestrial S.P. 235A radio services. Intersection of the beams at the antennae of radio-relay system stations in the U.K. with the stationary satellite orbit IV/191 United Kingdom Feasibility of frequency sharing between com S.P. 235F (IX/187) munication-satellite systems and terrestrial radio services. Determination of the power per 4 kHz which may be radiated in the hori zontal plane by communication-satellite earth stations IV/192 United Kingdom Revision of Draft Report L.2.o(IV), and Draft Draft Rep. (IX/188) Recommendation L.2.d(IV) — Power flux den Draft Rec. sity at the surface of the earth from communi cation-satellites IV /193 United States Proposed modifications to Report L.8 .b(IV) — Draft Rep. of America Radioastronomy. Characteristics and factors affecting frequency sharing with other services IV /194 United States Proposed modifications to Report 226 — Fac Q. 245 of America tors affecting the possibility of frequency shar Rep. 226 ing between radar astronomy and other services IV/195 United States Line frequencies of interest to radioastronomy Q. 244 of America lying in or near bands identified with radio- astronomy by the E.A.R.C., Geneva, 1963 IV/196 United States Proposed modifications to Report 223 — Line Q. 244 of America frequencies or bands of interest to radioastron Rep. 223 omy and related sciences in the 30 to 300 GHz range arising from natural phenomena IV/197 United States Communication-satellite systems. General con Rep. 206 of America siderations relating to the choice of orbit satellite and type of system IV/198 United States Communication-satellite systems. The effects Draft Rep. of America of Doppler frequency-shifts and switching discontinuities IV/199 United States Communication-satellite systems. The effects Draft Rep. o f America of time delay IV/200 C.C.I.R. Secretariat List of documents issued (IV/168 to IV/200) — 593 — Origin Title Reference IV/201 United States Proposed modifications to Draft Report L.2.g Q. 235, 285 (IX/189) of America (IV) — Frequency sharing between communi S.P. 235B (V /115) cation-satellite systems and terrestrial services and 311E(V) Draft Rep. IV/202 United States Proposed modifications to Draft Report L.7.b Draft Rep. of America (IV) — Factors affecting the selection of frequen Q. 239 cies for telecommunications with spacecraft re entering the earth’s atmosphere IV/203 United States Draft Recommendation—Telecommunication Rees. 364, 365, of America links for the manned and unmanned space 366 research service. Frequencies, bandwidths and interference criteria IV/204 United States Proposed modifications to Draft Report L.7.a Draft Rep. of America (IV) — Interference and other special consider S.P. 235B ations for telecommunication links for the man ned and unmanned spacecraft in the space research service IV/205 United States Technical feasibility of direct broadcasting from Q. 241 (X/125) of America earth satellites. Sharing considerations for Q. 306(X) AM broadcasting in band 7 IV/206 United States A study o f possible zonal television signal Q. 241 (XI/134) of America standards for television broadcasting from Q. 306(XI) satellites IV/207 United States Draft Study Programme — World-wide stan- Draft S.P. (XI/135) of America dard for television broadcasting from satellites . IV/208 United States Draft Study Programme — Composite 625-line Draft S.P. (X I/136) of America signal for television broadcasting from satellites IV/209 United Kingdom Proposed modification to Draft Recommen- Draft Rec. (IX/191) dation F.5.c(IX) — Interference from terres- Q. 235 trial line-of-sight radio-relay systems to com- S.P. 235A munication-satellite systems IV/210 United Kingdom Proposed revision of Draft Report L.2.h(IV)— Draft Rep. Frequency sharing within and between commu- Q. 235 nication-satellite systems S.P. 235C IV/211 United Kingdom Proposed amendments to Draft Report L.4.b Draft Rep. (IV) — Use of satellites for terrestrial naviga- Q. 242 tion IV/212 United Kingdom Proposed amendments to Draft Recommenda Draft Rec. tion L.4.a(IV) — Frequency requirements of Q. 242 radionavigation-satellite systems IV/213 United Kingdom Proposed amendments to Draft Reports L.5.a Draft Rep. (IV) and L.5.b(IV) — Radiocommunications Q. 243 for meteorological-satellite systems S.P. 243A IV/214 Australia Proposed modification to Draft Report L.8 .b Draft Rep. (IV) — Radioastronomy : IV/215 Australia Proposed modification to Draft Report L.8 .c Draft Rep. (IV) — The OH-lines in radioastronomy IV/216 Federal Republic Antennae for space systems Q. 234 of Germany — 594 — Doc. Origin Title Reference IV/217 Federal Republic Technical characteristics of communication- Q- 235, § 4 (IX/211) of Germany satellite systems IV/218 Federal Republic Frequency sharing between communication- Q. 235 (IX/212) of Germany satellite systems and terrestrial services Draft Rep. IV/219 Japan System for the transmission of television signals S.P. 235D (CMTT/59) using a positive synchronizing signal and non Q. 121(CMTT) linear pre-emphasis IV/2 2 0 Japan Frequency-sharing between communication- S.P. 235F satellite systems and terrestrial radio services. and 235G Reduction of mutual interference between a communication-satellite system and a terres trial radio-relay system by using pulse-code modulation on the terrestrial radio-relay system 1V/221 Japan Site shielding factor to be used in calculating Q. 287 coordination distances. Method of calcula tion of the transmission loss behind multiple mountains IV/2 2 2 Federal Republic Noise temperature of an earth-station receiving Q. 284 of Germany antenna IV/223 Federal Republic Noise temperature of an earth-station receiving Q. 284, § 1.6 of Germany antenna IV/224 France Proposed revision of Question 288(IV) — Fre Q. 288 (V/104) quency utilization above the ionosphere and on (VI/170) the far side of the Moon IV/225 France Propagation data necessary for radio systems Q. 285, 286, 28 (V/105) and satellite telecommunication systems and 311(V) IV/226 Canada Factors affecting freedom of access in commu S.P. 235H nication-satellite systems. Effects of varia tions in channel transmission delay on the choice of time-sharing or frequency-sharing of the satellite repeater IV/227 Canada Effects on system performance due to water on Q. 284 and Corr. 1 radomes and 311(V) IV/228 Canada Frequency sharing between communication- Q. 235 (IX/214) satellite systems and terrestrial radio services. S.P. 235F Visibility of antennae of Canadian radio-relay systems in the 4 and 6 GHz bands to the sta tionary orbit and the longitudinal distribution of the intersecting points IV/229 Canada Frequency sharing between communication- Q. 235 (IX/215) satellite systems and terrestrial radio services. S.P. 235F Power flux-density at the surface of the earth Draft Rec. from communication satellites 1V/230 Canada Frequency sharing between communication- Q. 235 (IX/216) satellite systems and terrestrial radio services. S.P. 235F and Corr. 1 Calculation of coordination and separation Draft Rep. distances — 595 — D oc. Origin Title Reference IV/231 Canada Line-of-sight radio-relay systems sharing the Draft Rec. (IX/217) same frequency bands as the satellite receivers of active earth-satellite communication systems. Maximum effective radiated powers of line- of-sight radio-relay system transmitters (6 GHz band) IV/232 Canada Proposed modifications to Draft Recommen- Draft Rec. a x / 218) dation L.2.b(IV) — Maximum allowable values of interference in a telephone channel of a com munication-satellite system, and to Draft Rec ommendation L.2.c(IV) — Maximum allow able values of interference in a telephone chan nel of a radio-relay system IV/233 Canada Frequency sharing between communication- Q. 235 ax/219) satellite systems and terrestrial radio-relay sys- Draft Rep. terns. Principles of sharing IV/234 Australia Exposure of radio-relay systems in Australia Q. 235 a x / 220) Draft Rec. IY/235 Federal Republic Active communication-satellite systems. A Q. 235, § 6 of Germany comparative study of possible methods of mod- S.P. 235D, § 2 ulation Draft Rep. IV/236 Denmark, Norway, Interference measurements. Some test results S.P. 235F Sweden from the Scandinavian experimental earth sta tion, Rao IV/237 United States Communication-satellite systems sharing the Q. 235 ax/229) of America same frequency bands as line-of-sight radio- S.P. 235F relay systems. Detailed analysis of inter- Draft Rec. ference relations between stationary communi cation satellites and a long east-west radio-relay system IV/238 I.U.C.A.F. Proposed modifications to Draft Recommen- Draft Rec. dation L.8 .a(IV) — Protection of frequencies used for radioastronomical measurements IV/239 France Draft revision of Report 204 — Terms and Rep. 204 (XIV / 6) definitions relating to space radiocommunica tion IV/240 France Proposed revision of the Annex to Draft Report Draft Rep. L.2.i(IV) — Active communication-satellite systems. A comparative study of possible methods of modulation IV/241 France Technical characteristics of communication- Draft Rep. satellite systems. Pre-emphasis characteris tics of frequency-modulation systems for fre quency-division multiplex telephone transmis sion IV/242 Chairman, Report by the Chairman, Study Group IV — Study Group IV Space systems and radioastronomy IV/243 Italy A case of high shielding from a single ground Q. 287 relief — 596 — D oc. Origin Reference IV/244 I.F.R.B. Sharing of the frequency bands between tele Q. 237 command, telemetry, tracking, or data trans S.P. 236A mission of the space services and terrestrial services IV/245 I.F.R.B. Propagation factors affecting the calculating of Q. 285 coordination distances IV/246 I.F.R.B. Site-shielding factor in calculations of coordina Q. 287 tion distance IV/247 C.C.I.R. Secretariat Report by Joint Special Study Group C Report by (IX/240) (C.C.I.T.T./C.C.I.R.) Joint Special S.G. C IV/248 Telespazio Deterioration due to rain and wind of the ratio Rep. 207 gain thermal noise temperature of an earth station IV/249 C.C.I.R. Secretariat Note by the Director a.i., C.C.I.R. Q/240 IV/250 C.C.I.R. Secretariat List of documents issued (IV/201 to IV/250) IV/251 France and U.S.S.R. Experimental transmission between Moscow Q. 235 (XI/166) and Paris of SECAM III colour television sys S.P. 117 (CMTT/68) tem signals via the satellite MOLNYA I Q. 121 (CMTT) IV/252 Working Group IV-A Proposed modifications to Report 208 — Tech nical characteristics of communication-satellite systems , IV/253 Japan Number of antennae of N.T.T.P.C. radio-relay Q. 235 systems operating in the 4 and 6 GHz bands, S.P. 235F beams of which intersect the orbits of stationary satellites IV/254 Canada Antennae for space systems — Summary of the Q. 234 antenna characteristics at the Mill Village earth station, Nova Scotia, Canada IV/255 Joint IV/IX Summary record of the first meeting (IX/247) Study Group IV/256 Study Group IV Summary record of the first meeting IV/257 Working Group IV-A Proposed modification to Draft Report L.3.a (IV) — Feasibility of direct sound and televi C sion broadcasting from satellites IV/257 Joint Working Proposed modification to Draft Report L.3.a Q. 283 Rev. 1 Group IV/X/XI (IV) — Feasibility of direct sound and televi- Q. 306(X/XI) (X/217) sion broadcasting from satellites Rec. No. 5A (XI/188) (E.A.R.C., Add. 1 Geneva, 1963) and Corr. 1 IV/258 Working Group IV-A Earth-station antennae for the communication Draft Rep. satellite service. Summary of antenna char acteristics at Goonhilly Earth Station, United Kingdom IV/259 Working Group IV-A Active communication-satellite systems for fre- Q. 235 quency-division multiplex telephony. Allow able noise power in the basic hypothetical reference circuit — 597 — D oc. Origin Title Reference IV/260 United States Active communication-satellite experiments. Q. 235 of America Results of tests and demonstrations IV/261 Working Group IV-A Proposed modifications to Draft Report L.2.f and Rev. 1 (IV) — Technical characteristics of communi cation-satellite systems. General considera tions relating to the choice of orbit, satellite and type of system IV/262 Study Group IV Draft Report — Terms and definitions relating (XIV/12) to space radiocommunications and Corr. 1, 2 IV/263 Study Group IV Draft Recommendation — Nomenclature con (IX/259) cerning radiated power (XIV/13) and Corr. 1 IV/264 Study Group IV Proposed modification to Draft Report L.8 .C Q. 244 (IV) — The OH-lines in radioastronomy IV/265 Study Group IV Proposed modifications to Draft Recommen dation L.g.a(IV) — Protection of frequencies used for radioastronomical measurements IV/266 Study Group IV Proposed modifications to Report 223 — Line frequencies or bands of interest to radio- astronomy and related sciences, in the 30 to 300 GHz range arising from natural phenomena IV/267 Working Group IV-A Draft Report — Communication-satellite sys tems. The effects of time delay IV/268 Working Group IV-A Proposed modification to Draft Report L.2.n (IV) — Radiation diagrams of antennae at communication-satellite earth stations, for use in interference studies IV/269 Working Group IV-A Earth-station antennae for the communication satellite service. Summary of the antenna characteristics at the Mill Village earth station, Nova Scotia, Canada IV/270 Working Group IV-A Earth-station antennae for the communication satellite service. Summary of antenna char acteristics at Raisting earth station (Federal Republic of Germany) IV/271 Working Group IV-A Draft Report — Frequency sharing between communication-satellite systems and terrestrial radio-relay systems. Energy dispersal in com munication-satellite systems with frequency- modulation of the radio-frequency carrier IV/272 Study Group IV Draft Report L.8 .b(IV) — Radioastronomy. Q. 244 Characteristics and factors affecting frequency sharing with other services IV/273 Study Group IV Proposed continuation of Questions 244(IV) and 245(IV) IV/274 Study Group IV Proposed modifications to Report 226 — Fac tors affecting the possibility of frequency shar ing between radar astronomy and other services — 598 — Doc. Origin Title Reference IV/275 Ad Hoc Joint IV/IX Summary record of the second meeting (IX/271) Study Group IV/276 Terminology Draft letter from the Chairman of Study (IX/274) Working Group of Group IV to the Chairman of Study Group XIV (XIV/15) Study Group IV and Corr. 1, 2 IV/277 Working Group IV-A Draft revision of Draft Report L.2.j(IV) — Q. 235 Communication-satellite systems for frequency- division multiplex telephony and monochrome television. Use of pre-emphasis by frequency modulation systems IV/278 Working Group IV-C Proposed amendments to Draft Recommen Q. 242 dation L.4.a(IV) — Frequency requirements of radionavigation-satellite systems IV/279 Working Group IV-C Proposed amendments to Draft Report L.4.b Q. 242 (IV) — Use of satellites for terrestrial naviga tion IV/280 Working Group IV-C Proposed modifications to Study Programme 236A(IV) — Space research and maintenance telemetering, tracking and telecommand sys tems. Possibilities of sharing and protection criteria IV/281 Working Group IV-C Proposed modifications to Recommendation 366 — Telecommunication links for manned research spacecraft IV/282 Working Group IV-C Proposed modifications to Recommendation 365 — Telecommunication links for deep-space research. Frequency, bandwidths and inter ference criteria IV/283 Working Group IV-C Proposed modifications to Recommendation 364 — Telecommunication links for near-earth research satellites. Frequencies, bandwidths and interference criteria IV/284 Joint Study Modifications to Draft Recommendation (IX/281) Group IV/IX L.2.c(IV) — Maximum allowable values of interference in a telephone channel of a radio relay system IV/285 Joint Study Modifications to Draft Recommendation (IX/282) Group IV/IX L.2.b(IV) — Maximum allowable values of interference in a telephone channel of a commu nication-satellite system IV/286 Joint Study Draft Recommendation — Frequency sharing Q. 235 (IX/283) Group IV/IX between active communication-satellite systems and terrestrial radio services in the same fre quency bands IV/287 Working Group IV-A Proposed revision of Draft Report L.2.k(IV) — Factors affecting multi-station access in com munication-satellite systems 00 Joint Study Draft Recommendation — Communication- Q. 235 00 Group IV/IX satellite systems and line-of-sight radio-relay systems sharing the same frequency bands. Maximum allowable values of power flux-den sity at the surface of the earth produced by communication satellites — 599 — D oc. Origin Title Reference IV/289 Working Group IV-A Proposed amendment to Draft Report L.2.n (IV) IV/290 Working Group IV-A Communication-satellite systems — The effects of Doppler frequency-shifts and switching dis continuities IV/291 Working Group IV-C Proposed amendments to Draft Reports L.5.a (IV) and L.5.b(IV) — Radiocommunications for meteorological-satellite systems IV/292 Working Group IV-A Draft Report L.2.i(IV) — Active communica Q. 235 and and Rev. 1 tion-satellite systems. A comparative study S.P. 235D of possible methods of modulation 1V/293 Ad hoc Joint First report by the Ad hoc Joint Study Group (X/190) Study Group IV/X/XI IV/X/XI — Satellite broadcasting (XI/186) IV/294 Joint Study Draft Report — Feasibility of frequency shar- S.P. 235F (IX/286) Group IV/IX ing between communication-satellite systems and terrestrial radio services. Maximum power in any 4 kHz band which may need to be radiated in the horizontal plane by active communication-satellite earth stations IV/295 Joint Study Draft revision of Question 306(X/XI) — Direct (X/209) Group IV/X/XI broadcasting service from satellites. Sound (XI/187) and television IV/296 Joint Working Draft Report — Techniques of calculating (IX/290) Group IV/IX-C interference noise in communication-satellite and Corr. 1 receivers and terrestrial radio-relay receivers IV/297 Joint Working Draft Recommendation — Line-of-sight radio- (IX/291) Group IV/IX-A relay systems sharing the same frequency bands and Rev. 1 as the space station receivers of active satellite- communication systems. Maximum direc tional radiated power of line-of-sight radio relay system transmitters IV/298 Joint Study Draft Report — Power flux-density at the sur Q. 235 (IX/293) Group IV/IX face of the earth from communication satellites IV/299 Joint Study Proposed modification to Draft Recommen- (IX/295) Group IV/IX dation L.2.b(IV) — Communication-satellite systems sharing the same frequency bands as line-of-sight radio-relay systems IV/300 C.C.I.R. Secretariat List of documents issued (IV/251 to IV/300) IV/301 Joint Study Proposed modification to Draft Recommen- (IX/296) Group IV/IX dation L.2.c(IV) — Communication-satellite systems and line-of-sight radio-relay systems, sharing the same frequency bands IV/302 Joint Study Proposed modification to Recommendation 360 (EX/297) Group IV/IX — Communication-satellite systems and line- and Corr. 1 of-sight radio-relay systems sharing the same frequency bands. Criteria for the selection of preferred reference frequencies for communica tion-satellite systems — 600 — D oc. Origin Title Reference IV/303 Joint Working Draft Report — Determination of coordination » (IX/307) Group IV/IX-C distance and Rev. IV/304 Working Group IV-D Proposed modifications to Draft Report L .l.a (IV) — Factors affecting the selection of fre quencies for telecommunications with and between spacecraft IV/305 Working Group IV-D Proposed modifications to Draft Report L.7.b (IV) — Factors affecting the selection of fre quencies for telecommunications with space craft re-entering the earth’s atmosphere IV/306 Working Group IV-D Proposed amendment to Question 288 — Pro posed new question for Study Group VI — Proposed new question for Study Group V IV/307 Working Group IV-A Proposed revision of Report 207 — Active com Q. 235 munication-satellite experiments. Results of tests and demonstrations IV/308 Working Group IV-A Draft amendment to Draft Recommendation L.2.a(IV) IV/309 Working Group IV-A Draft revision of Study Programme 235J(IV) IV/310 Study Group IV Report of the Terminology Working Group (XIV/17) IV / 311 Joint Working Draft amendment to Recommendation L.2.e (IX/301) Group IV/IX-C (IV) IV/312 Study Group IV Proposed revision of Draft Report L.2.h(IV) Q. 235 and Corr. I — Frequency sharing within and between com S.P. 235C munication-satellite systems IV/313 Joint Working Draft modifications to Report L.2.g(IV) — Q. 235 (IX/302) Group IV/IX/D Frequency sharing between communication- satellite systems and terrestrial services IV/314 Joint Working Draft Report — Exposures of radio-relay Q. 235 (IX/303) Group IV/IX antennae to communication satellites IV/315 Working Group IV-C Proposed modifications to Draft Report L.7.a S.P. 235B (IV) — Interference and other special consi derations for telecommunication links for the manned and unmanned spacecraft in the space research service IV/316 Joint Working Summary record of the third meeting (IX/304) Group IV/IX IV/317- Working Group IV-D Draft Question — Propagation factors affecting sharing of the radio-frequency spectrum and coordination between space and terrestrial radio-relay stations — 601 — Doc. Origin Title Reference- IV/318 Working Group IV-D Draft Report — Performance of earth-station Q. 234, 284 receiving antennae. Effects of rain on radomes and of solar and cosmic noise IV/319 Joint Study Draft Question — Feasibility of direct sound (X/210) Group IV/X/XI and television broadcasting from satellites (XI/194) IV/320 Joint Working Draft Study Programme — World-wide stand (X/211) Group IV/X/XI ard for television broadcasting from satellites (XI/195) IV/321 Joint Working Draft Study Programme — Composite 625-line (X/212) Group IV/X/XI signal for television broadcasting from satellites (XI/196) IV/322 Sub-Group IV-D Proposed additions to Doc. IV/303 IV/323 Sub-Group IV-D Proposed new Report — Estimating inter ference probabilities. Propagation considera tions IV/324 Study Group IV Summary record of the second meeting IV/325 Working Group IV-A List of documents considered and put forward by Working Party IV-A IV/326 Joint Study Modifications to Draft Recommendation L.2.c (IX/305) Group IV/IX (IV) — Communication satellite systems and line-of-sight radio-relay systems sharing the same frequency bands. Maximum allowable values of interference in a telephone channel of a radio-relay system IV/327 Joint Study Proposed modification to Draft Recommenda (IX/306) Group IV/IX tion L.2.b(IV) — Communication satellite sys tems and line-of-sight radio-relay systems sharing the same frequency bands. Maximum allowable values of interference in a telephone channel of a communication-satellite system IV/328 Ad hoc Joint IV/IX Summary record of the fourth meeting (IX/308) Study Group IV/329 Study Group IV Summary record of the third meeting IV/330 C.C.I.R. Secretariat Submission of Doc. V/153 — Draft Report — S.P. 311E(V) Influence of scattering from precipitation on the siting of earth stations IV/331 Study Group IV Status of texts and Rev. 1 IV/332 Joint Study Second Report — Broadcasting from satellites (X/219) Group IV/X/XI (XI/215) IV/333 Joint Study Documents submitted by Joint Study Group (IX/309) Group IV/IX IV/IX IV/334 Study Group IV Summary record of the fourth meeting IV/335 Study Group IV Summary record of the fifth meeting IV/336 C.C.I.R. Secretariat List of documents issued (IV/301 to IV/336) LIST OF DOCUMENTS OF THE Xlth PLENARY ASSEMBLY ESTABLISHED BY STUDY GROUP IV Title Final text IV/1001 Terms and definitions relating to space radiocommunications Rep. 204-1 IV /1002 Radioastronomy — Characteristics and factors affecting frequency Rep. 224-1 sharing with other services IV/1003. Factors affecting the possibility of frequency sharing between radar Rep. 226-1 astronomy and other services IV/1004 Frequency sharing between communication-satellite systems and Rep. 384 terrestrial radio-relay systems. Energy dispersal in communication- satellite systems with frequency-modulation of the RF-carrier IV/1005 The OH-lines in radioastronomy Rep. 397 IV/1006 Active communication-satellite systems for frequency-division mul Rep. 208-1 tiplex telephony and monochrome television IV/1007 Line frequencies or bands of interest to radioastronomy and related Rep. 223-1 sciences, in the 30 to 300 GHz range arising from natural phenomena IV/1008 Technical characteristics of communication-satellite systems — Rep. 206-1 and Corr. 1 General considerations relating to the choice of orbit, satellite and type of system IV/1009 Earth-station antennae for the communication-satellite service Rep. 390 IV/1010 Radiation diagrams of antennae at communication-satellite earth Rep. 391 stations, for use in interference studies IV/1011 Active communication-satellite systems for frequency-division mul Rep. 212-1 tiplex telephony and monochrome television. Use of pre-emphasis by frequency-modulation systems 1V/1012 Active communication-satellite systems for frequency-division mul Rec. 353-1 and Rev. 1 tiplex telephony. Allowable noise power in the |basic hypothetical reference circuit IV/1013 Protection of frequencies used for radioastronomical measurements Rec. 314-1 IV/1014 Communication-satellite systems and line-of-sight radio-relay sys Rec. 360-1 tems sharing the same frequency bands. Criteria for the selection of preferred reference frequencies for communication-satellite sys tems IV/1015 Use of satellites for terrestrial navigation Rep. 216-1 IV/1016 Interference and other special considerations for telecommunication Rep. 219-1 links for the manned and unmanned spacecraft in the space research service IV/1017 Maintenance telemetering, tracking and telecommand for develop Rep. 396 mental and operational satellites. Frequency sharing between earth-satellite telemetering or telecommand links and terrestrial ser vices — 603 — Title Final text IV/1018 Feasibility of frequency sharing between the radionavigation-satellite Rep. 394 service and terrestrial services IV/1019 Feasibility of frequency sharing between communication-satellite Rep. 385 systems and terrestrial radio services. Site selection criteria for earth stations in the communication-satellite service IV /1020 Feasibility of direct sound and television broadcasting from satellites Rep. 215-1 IV/1021 Communication-satellite systems. The effects of Doppler fre Rep. 214-1 quency-shifts and switching discontinuities IV/1022 Determination of coordination distance Rep. 382 IV /1023 Communication-satellite systems. The effects of transmission Rep. 383 delay IV/1024 Performance of earth-station receiving antennae. Effects of rain on Rep. 392 radomes and of solar and cosmic noise IV /1025 Frequency sharing between communication-satellite systems and Rep. 209-1 terrestrial services IV /1026 Feasibility of frequency sharing between communication-satellite Rep. 386 systems and terrestrial radio services. Maximum power in any 4 kHz band which may need to be radiated in the horizontal plane by active communication-satellite earth stations IV/1027 Estimating interference probabilities between space systems and ter Rep. 389 restrial radio-relay services IV/1028 Techniques of calculating interference noise in communication-satel Rep. 388 lite receivers and terrestrial radio-relay receivers IV/1029 Communication-satellite systems and line-of-sight radio-relay sys Rec. 356-1 tems sharing the same frequency bands. Maximum allowable values of interference in a telephone channel of a communication- satellite system IV /1030 Communication-satellite systems and line-of-sight radio-relay sys Rec. 357-1 tems sharing the same frequency bands. Maximum allowable val ues interference in a telephone channel of a radio-relay system IV/1031 Frequency selection and carrier energy dispersal for communication- Rec. 446 satellite systems IV/1032 Space research and maintenance telemetering, tracking and telecom S.P. 3A/IV mand systems. Possibilities of sharing and protection criteria IV/1033 Definitions concerning radiated power Rec. 445 IV/1034 Communication-satellite systems and line-of-sight radio-relay sys Rec. 358-1 tems sharing the same frequency bands. Maximum allowable values of power flux-density at the surface of the earth produced by communication satellites IV/103 5 Propagation factors affecting sharing of the radio-frequency spectrum Q. 14/IV and coordination between space and terrestrial radio-relay systems IV/1036 Frequency requirements of radionavigation-satellite systems Rec. 361-1 — 604 — Title Final text IV/1037 Telecommunication links for deep-space research. Frequencies, Rec. 365-1 bandwidths and interference criteria IV/1038 Telecommunication links for manned research spacecraft Rec. 366-1 IV/1039 Telecommunication links for near-earth research satellites. Fre Rec. 364-1 quencies, bandwidths and interference criteria IV/1040 Frequency utilization above the ionosphere and on the far side of Q. 15/IV the Moon IV/1041 Communication-satellite systems and terrestrial radio systems shar Rec. 359-1 ing the same frequency bands. Determination of the coordination distance IV/1042 Frequency sharing between active communication-satellite systems Rec. 355-1 and terrestrial radio services in the same frequency bands IV/1043 Shielding effects due to the ionosphere Q. 16/IV IV/1044 Shielding effects due to the Moon Q. 17/IV IV/1045 Power flux-density at the surface of the earth from communication Rep. 387 satellites IV/1046 Factors affecting the selection of frequencies for telecommunications Rep. 222-1 with spacecraft re-entering the earth’s atmosphere IV/1047 Radiocommunications for meteorological-satellite systems Rep. 395 IV/1048 Active communication-satellite experiments. Results of tests and Rep. 207-1 demonstrations IV/1049 Frequency sharing within and between communication-satellite sys Rep. 210-1 tems IV/1050 List of documents issued (IV/1001 to IV/1056) IV/1051 Factors affecting the selection of frequencies for telecommunications Rep. 205-1 with and between spacecraft IV/1052 Emergy dispersal in communication-satellite systems S.P. 2F/IV IV/1053 Feasibility of direct sound and television broadcasting from satellites Q. 12/IV IV/1054 Exposures of radio-relay antennae to communication satellites Rep. 393 1V/1055 Active communication-satellite systems — A comparative study of Rep. 211-1 possible methods of modulation IV/1056 Factors affecting multiple access in communication-satellite systems Rep. 213-1 PRINTED IN SWITZERLAND