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© International Telecommunication Union INTERNATIONAL RADIO CONSULTATIVE COMMITTEE
C.C.I.R.
XHIth PLENARY ASSEMBLY
GENEVA, 1974
VOLUME VI
IONOSPHERIC PROPAGATION (STUDY GROUP 6)
Published by the INTERNATIONAL TELECOMMUNICATION UNION GENEVA, 1975 INTERNATIONAL RADIO CONSULTATIVE COMMITTEE
C.C.I.R.
XHIth PLENARY ASSEMBLY
GENEVA, 1974
VOLUME VI
IONOSPHERIC PROPAGATION (STUDY GROUP 6)
Published by the INTERNATIONAL TELECOMMUNICATION UNION GENEVA, 1975
ISBN 92-61-00071-1 PAGE INTENTIONALLY LEFT BLANK
PAGE LAISSEE EN BLANC INTENTIONNELLEMENT RECOMMENDATIONS AND REPORTS
6A Ionospheric and solar indices
6B Prediction of MUF
6C Field strengths between about 1*5 and 40 MHz
6D Atmospheric and man-made radio noise
6E Ionospheric mapping
IONOSPHERIC 6F Problems of interference PROPAGATION
6G Field strengths below about 1-5 MHz
6H Fading of signals received via the ionosphere
6S Topics related to space systems
QUESTIONS AND STUDY PROGRAMMES, DECISIONS, RESOLUTIONS AND OPINIONS (Study Group 6) PAGE INTENTIONALLY LEFT BLANK
PAGE LAISSEE EN BLANC INTENTIONNELLEMENT DISTRIBUTION OF TEXTS OF THE XHIth PLENARY ASSEMBLY OF THE C.C.I.R. IN VOLUMES I TO XIII
Volumes I to XIII, XHIth Plenary Assembly, contain all the valid texts of the C.C.I.R. and succeed those of the Xllth Plenary Assembly, New Delhi, 1970.
1. Recommendations, Reports, Decisions, Resolutions, Opinions
1.1 Numbering o f these texts Recommendations, Reports, Resolutions and Opinions are numbered according to the system in force since the Xth Plenary Assembly. In conformity with the decisions of the Xlth Plenary Assembly, when one of these texts is modified, it retains its number to which is added a dash and a figure indicating how many revisions have been made. For example: Recommendation 253 indicates the original text is still current; Recommendation 253-1 indicates that the current text has been once modified from the original, Recommendation 253-2 indicates that there have been two successive modifications of the original text, and so on. The XHIth Plenary Assembly adopted a new category of texts known as Decisions, by which Study Groups take action, generally of an organizational nature, relative to matters within their own terms of reference, particularly the formation of (Joint) Interim Working Parties (see Resolution 24-3, Volume XIII). Although the Plenary Assembly did adopt in the form of Resolutions a number of texts which fell into the category of Decisions after amendment of Resolution 24-2, these texts are published in the Volumes of the XHIth Plenary Assembly as Decisions, for practical reasons. When one of these texts is so published, a reference to the Resolution on which the text is based, is given in parenthesis below the title. The tables which follow show only the original numbering of the current texts, without any indi cation of successive modifications that may have occurred. For further information about this numbering scheme, please refer to Volume XIII.
1.2 Recommendations
Number Volume Number Volume Number Volume
45 VIII 310, 311 V 436 III 48, 49 X 313 VI 439 VIII 77 VIII 314 II 441 VIII 80 X 325-334 I ' 442, 443 I 100 I 335, 336 III 444 IX 106 in 337 I 445 I 139, 140 X 338, 339 III 446 IV 162 m 341 I 447-450 X 166 XII (CMV) 342-349 III 451 XII (CMTT) 182 I 352-354 IV '452, 453 V 205 X 355-359 IX 454-456 III 214-216 X 361 VIII 457-460 VII 218, 219 VIII 362-367 II 461 XII (CMV) 224 VIII 368-370 V 462, 463 IX 237 I 371-373 VI 464-466 IV 239 I 374-376 VII 467, 468 X 240 III 377-379 I 469-472 XI 246 III 380-393 IX 473-474 XII (CMTT) 257, 258 VIII 395-406 IX 475-478 VIII 262 X 407-416 X 479 II 265, 266 XI 417-418 XI 480 III 268 IX 419 X, XI 481-484 IV 270 IX 421 XII (CMTT) 485, 486 VII 275, 276 IX 422, 423 VIII 487-496 VIII 283 IX 427-429 VIII 497 IX 289, 290 IX 430, 431 XII (CMV) 498, 499 X 302 IX 433 I 500, 501 XI 305, 306 IX 434, 435 VI 502-505 XII (CMTT) L3 Reports
Number Volume Number Volume Number Volume
19 III 275-282 I ,424-426 V 32 X 283-289 IX 428 V 42 III 292, 293 X 429-432 VI 79 X 294 XI 433-437 III 93 VIII 298-305 X 438, 439 VII 106, 107 III 306, 307 XI 440 0 109 III 311, 312 XI 441 XII (CMV) 111 III 313 XII (CMTT) 443-446 IX 112 I 315 XI 448, 449 IX 122 XI 318, 319 VIII 451 IV 130 IX 321 XII (CMV) 453-455 IV 137 IX 322 0 456 II 176-189 I 324-330 I 457-461 X 190 X 335 XII (CMV) 463-465 X 192, 193 I 336 V 467, 468 X 195 III 338 V 469-473 XI 196 I 340 0 476-485 XI 197, 198 III 341-344 VI 486-488 XII (CMTT) 200, 201 III 345-357 III 490, 491 XII (CMTT) 202 I 358, 359 VIII 493 XII (CMTT) 203 III 361 VIII 495-498 XII (CMTT) 204-208 IV 362-364 VII 499-502 VIII 209 IX 366 VII 504-513 VIII 211-214 IV 367-373 I 515 VIII 215 IX 374-382 IX 516 X 216 VIII 383-385 IV 518 VII 222-224 II 386-388 IX 519-534 I 226 II 390, 391 IV 535-548 II 227-229 V 393 IX 549-551 III 233-236 V 394 VIII 552-561 IV 238, 239 V 395, 396 II 562-570 V 241 V 398-401 X 571-575 VI 245-251 VI 403 X 576-580 VII 252 0 404, 405 XI 581-603 VIII 253-256 VI 409 XI 604-615 rx 258-266 VI 410-412 XII (CMTT) 619-622 X 267 VII 413-415 0 623 X,XII (c m t t ) 269-271 VII 417-420 I 624-634 XI 272, 273 I 422, 423 I 635-649 XII (CMTT) 650 XII (CMV)
0) Published separately.
1.4 Decisions
Number Volume Number Volume Number Volume
1 I 6-11 VI 17 XI 2 IV 12-14 VII 18 XII (CMTT) 3-5 V 15 VIII 19,20 XII (CMV) 16 IX
1.5 Resolutions
Number Volume Number Volume Number Volume
4 VI 22, 23 XII (CMV) 43,44 I 14 VII 24 x m 48 VI 15, 16 I 26, 27 XIII 59 X 20 VIII 33 x i n 60, 61 XIII 36, 39 XIII 1.6 Opinions
Number Volume Number Volume Number Volume
2 I 29, 30 I 45, 46 VI 11 I 32 I 47,48 VII 13, 14 IX 34, 35 I 49 VIII 15, 16 X 36 VII 50 IX 22, 23 VI 38-40 XI 51, 52 X 24 VIII 41 XII (CMTT) 53, 54 XI 26-28 VII 42, 43 VIII 55 XII (CMTT) . 44 I
2. Questions and Study Programmes
2.1 Text numbering
2.1.1 Questions Questions are numbered in a different series for each Study Group; where applicable a dash and a figure added after the number of the Question indicate successive modifications. The number of a Question is completed by an Arabic figure indicating the relevant Study Group. For example: — Question 1/10 would indicate a Question of Study Group 10 with its text in the original state; — Question 1-1/10 would indicate a Question of Study Group 10, whose text has been once modified from the original; Question 1-2/10 would be a Question of Study Group 10, whose text has had two successive modifications.
2.1.2 Study Programmes Study Programmes are numbered to indicate the Question from which they are derived if any, the number being completed by a capital letter which is used to distinguish several Study Programmes which derive from the same Question. The part of the Study Programme number which indicates the Question from which it is derived makes no mention of any possible revision of that Question, but refers to the current text of the Question as printed in this Volume. Examples: — Study Programme 1A/10, which would indicate that the current text is the original version of the text of the first Study Programme deriving from Question 1/10; — Study Programme lC/10, which would indicate that the current text is the original version of the text of the third Study Programme deriving from Question 1/10; — Study Programme 1 A-1/10 would indicate that the current text has been once modified from the original, and that it is the first Study Programme of those deriving from Question 1/10. It should be noted that a Study Programme may be adopted without it having been derived from a Question; in such a case it is simply given a sequential number analogous to those of other Study Programmes of the Study Group, except that on reference to the list of relevant Questions it will be found that no Question exists corresponding to that number.
2.2 Arrangement o f Questions and Study Programmes The plan shown on page 8 indicates the Volume in which the texts of each Study Group are to be found, and so reference to this information will enable the text of any desired Question or Study Programme to be located. PLAN OF VOLUMES I TO XIII Xinth PLENARY ASSEMBLY OF THE C.C.I.R.
(Geneva, 1974)
V o l u m e I Spectrum utilization and monitoring (Study Group 1).
V o l u m e II Space research and radioastronomy (Study Group 2).
V o l u m e III Fixed service at frequencies below about 30 MHz (Study Group 3).
V o l u m e IV Fixed service using communication satellites (Study Group 4).
V o l u m e V Propagation in non-ionized media (Study Group 5).
V o l u m e VI Ionospheric propagation (Study Group 6).
V olume VII Standard frequency and time-signal services (Study Group 7).
V olume VIII Mobile services (Study Group 8).
V o l u m e IX Fixed service using radio-relay systems (Study Group 9). Coordination and frequency sharing between systems in the fixed satellite service and terrestrial radio-relay systems (subjects common to Study Groups 4 and 9).
V o l u m e X Broadcasting service (sound) including audio-recording and satellite applications (Study Group 10).
V o l u m e XI Broadcasting service (television) including video-recording and satellite applications (Study Group 11).
V o l u m e XII Transmission of sound broadcasting and television signals over long distances (CMTT). Vocabulary (CMV).
V o l u m e XIII Information concerning the XlHth Plenary Assembly. Structure of the C.C.I.R. Complete list of C.C.I.R. texts.
Note. — To facilitate reference, page numbering is identical in all three versions of each Volume, that is, in English, French and Spanish. VOLUME VI *
INDEX OF TEXTS**
IONOSPHERIC PROPAGATION (Study Group 6)
Page Introduction by the Chairman, Study Group 6 ...... 13 s e c t io n 6A: Ionospheric and solar indices...... 17 s e c t io n 6B: Prediction of M U F ...... 31 s e c t io n 6C: Field strengths between about 1*5 and 40 M H z ...... 49 s e c t io n 6D: Atmospheric and man-made radio noise ...... 63 s e c t io n 6E: Ionospheric m apping ...... 71 s e c t io n 6F: Problems of interference...... 75 s e c t io n 6G: Field strengths below about 1-5 M H z ...... 107 s e c t io n 6H: Fading of signals received via the ionosphere...... 207 s e c t io n 6S: Topics related to space systems...... 219
RECOMMENDATIONS Section Rec. 313-2 Exchange of information for the preparation of short-term forecasts and the transmission of ionospheric disturbance warnings...... 6A 17 Rec. 371-2 Choice of basic indices for ionospheric propagation ...... 6A 18 Rec. 372-1 Use of data on radio noise ...... 6D 63 Rec. 373-3 Definitions of maximum transmission frequencies 6B 31 Rec. 434-2 C.C.I.R. Atlas of Ionospheric Characteristics . 6E 71 Rec. 435-2 Prediction of sky-wave field strength between 150 and 1600 k H z ...... 6G 107
REPORTS Report 245-3 Long-term prediction of solar indices ...... : ...... 6A 19 Report 246-3 Choice of basic indices for ionospheric propagation 6A 20 Report 247-3 Identification of precursors indicative of short-term variations and evaluation of the reliability of short-term forecasts of ionospheric propagation conditions 6A 22 Report 248-3 Availability and exchange of basic data for radio propagation forecasts . . 6A 26 Report 249-3 Ionospheric sounding at oblique incidence 6B 32
* Although the working documents mentioned in this Volume bear the reference “ Period 1970-1973 ” for documents published during 1971 and 1972 and “ Period 1970-1974 ” for those published during 1973 and 1974, they are, of course, all documents of the period 1970-1974, between the Plenary Assembly of New Delhi and that of Geneva. For this reason, all references to these documents in this Volume take the form “ Period 1970-1974 ”. ** In this Volume, Recommendations and Reports dealing with the same subject are collected together. These texts are numbered in such a manner that they cannot be presented in numerical order and at the same time, in numerical sequence of pages. Consequently, this index, in numerical order of texts, does not follow the numerical sequence of pages. — 10 —
Section Page Report 250-3 Long-distance ionospheric propagation without intermediate ground reflection 6B 35 Report 251-1 Intermittent communication by meteor-burst propagation ...... 6F 75 Report 252-2 C.C.I.R. interim method for estimating sky-wave field strength and transmis sion loss at frequencies between the approximate limits of 2 and 30 MHz (Text published separately)...... 6C 49 Report 253-2 Methods of systematic measurement of sky-wave field strength and transmis sion loss at frequencies above 1-5 M H z ...... 6C 49 Report 254-3 Measurement of atmospheric radio noise from lightning...... 6D 64 Report 255-3 Basic prediction information for ionospheric propagation ...... 6B 36 Report 256-2 Maximum transmission frequencies...... 6B 40 Report 258-2 Man-made radio n o ise ...... 6D 68 Report 259-3 VHF propagation by regular layers, sporadic E or other anomalous ionization 6F 76 Report 260-2 Ionospheric-scatter propagation ...... 83 6F Report 261-3 Back-scattering...... 6B 41 Report 262-3 VLF propagation in and through the ionosphere...... 6S 219 Report 263-3 Ionospheric effects upon Earth-space propagation...... 6S 223 Report 264-3 Sky-wave propagation curves between 300 km and 3500 km at frequencies between 150 kHz and 1600 kHz in the European Broadcasting Area. . . 6G 108 Report 265-3 Sky-wave propagation at frequencies below 150 kHz with particular emphasis on ionospheric effects...... 6G 123 Report 266-3 Ionospheric propagation characteristics pertinent to radiocommunication systems design (Fading) ...... 6H 207 Report 322-1 World distribution and characteristics of atmospheric radio noise (Text published separately)...... 70 6D Report 340-2 C.C.I.R. Atlas of Ionospheric Characteristics (Text published separately) . 6E 72 Report 341-2 HF propagation by ducting above the maximum of the F region...... 6F 92 Report 342-2 Radio noise within and above the ionosphere ...... 6S 237 Report 343-2 Special problems of HF radiocommunication associated with the equatorial ionosphere...... 6F94 Report 344-2 Prediction of sporadic E ...... 6F 102 Report 429T1 Ground and ionospheric sid e -sca tte r...... 6B 46 Report 430-1 Improvement in the world-wide ionospheric observing programme for numerical mapping purposes...... 6E 72 Report 431-1 Analysis of sky-wave propagation measurements for the frequency range 150 kHz to 1600 k H z ...... 6G 145 Report 432 The accuracy of predictions of sky-wave field strength in bands 5 (LF) and 6 ( M F ) ...... 6G 171 Report 571 Comparisons between observed and predicted sky-wave field strength and transmission loss at frequencies between 2 and 30 M H z ...... 6C 56 Report 572 Estimation of sky-wave field strength and transmission loss at frequencies between 2 and 30 MHz (A proposed plan for revision of the C.C.I.R. interim m ethod)...... 58 6C Report 573 The characteristics of sporadic E ...... 6F 103 Report 574 Ionospheric cross-modulation ...... 6G 178 Report 575 Methods for predicting sky-wave field strengths at frequencies between 150 kHz and 1600 k H z ...... 6G 186 — 11 —
Page
QUESTIONS AND STUDY PROGRAMMES
Question 2-2/6 Geographic distribution and programme of regular ionospheric observations . . 247 Study Programme 2A-2/6 Improvement in the world-wide ionospheric observing programme for numerical mapping purposes...... 247 Question 3-2/6 Side-scatter resulting from ground or ionospheric irregularities...... 248 Question 4-1/6 Propagation by way of sporadic E and other anomalous io n izatio n ...... 248 Study Programme 4A-2/6 Prediction of sporadic E ...... 249 Study Programme 4B-2/6 Propagation by way of sporadic E and other anomalous ionization . 249 Question 5-2/6 Propagation between stations below the ionosphere by ducting above the ionization maximum of the F region ...... 250 Question 6-2/6 Special problems of HF radiocommunications associated with the equatorial ionosphere...... 251 Study Programme 6A/6 Special problems of HF radiocommunications associated with the equatorial ionosphere...... 251 Question 7-1/6 Radio n o is e ...... 252 Study Programme 7A/6 Radio noise within and above the ionosphere...... 252 Study Programme 7B/6 Measurement of atmospheric radio noise caused by lightning . . . 253 Study Programme 7C/6 Mari-made radio n o is e ...... 254 Question 8/6 Choice and prediction of long-term basic indices for ionospheric propagation . . . 255 Study Programme 8A-1/6 Long-term predictions of solar in d ic e s ...... 255 Study Programme 9A-1/6 Basic prediction information for ionospheric propagation...... 256 Question 10/6 Short-term predictions of operational parameters for ionospheric radiocommuni cations ...... 256 Study Programme 10A-1/6 Identification of indicators and precursors for short-term variations and disturbances of the ionosphere and radio circuits . . . . ' ...... 257 Question 11-1/6 Sky-wave field strength and transmission loss at frequencies above 1 -5 MHz . . . 257 Study Programme 11A-2/6 Estimation of sky-wave field strength and transmission loss at frequencies above 1-5 M H z ...... 258 Study Programme 12A-1/6 ionospheric sounding at oblique incidence ...... 259 Study Programme 14A-1/6 Back-scattering...... 259 Question 16-1/6 Ionospheric propagation characteristics pertinent to radiocommunication systems design...... 260 Study Programme 16A-2/6 Ionospheric propagation characteristics pertinent to radiocommuni cation systems d e sig n ...... 261 Question 17-1/6 Propagation at frequencies below 1600 kHz with particular emphasis on ionospheric effects...... 262 Study Programme 17A-2/6 Sky-wave propagation at frequencies between 150 and 1600 kHz . . 262 Study Programme 17B-1/6 Propagation at frequencies below 150 kHz with particular emphasis on ionospheric effects...... 263 Question 18-1/6 Ionospheric factors influencing communication and navigation systems involving spacecraft...... 263 Study Programme 18A-2/6 Ionospheric influences on space communications at frequencies below about 1-5 M Hz...... 264 Study Programme 18B-1/6 Ionospheric influences on space communications at frequencies above about 1-5 M Hz...... 264 — 12 —
Page Question 23-1/6 Ionospheric cross-m odulation...... 265 Study Programme 23A/6 Ionospheric cross-modulation...... 265
DECISIONS Decision 6 Sky-wave field strength and transmission loss at frequencies above 1*5 MHz . . . 266 Decision 7 Basic long-term ionospheric predictions...... 267 Decision 8 Sky-wave propagation at frequencies between 150 and 1600 k H z ...... 267 Decision 9 Propagation at frequencies below 150 kHz with particular emphasis on ionospheric effects...... 268 Decision 10 Short-term predictions of operational parameters for ionospheric radiocommuni cations ...... 269 Decision 11 VHF propagation by sporadic E ...... 270
RESOLUTIONS Resolution 4-2 Dissemination of basic indices for ionospheric propagation...... 270 Resolution 48-1 Development of short-term indices for ionospheric propagation...... 271
OPINIONS Opinion 22-2 Routine ionospheric sounding ...... 272 Opinion 23-2 Observations needed to provide basic indices for ionospheric propagation .... 272 Opinion 45 Evaluation of the C.C.I.R. interim method for estimating sky-wave field strength and transmission loss at frequencies between 2 and 30 M H z ...... 273 Opinion 46 Establishment of a central card index of results of field-strength measurements on medium and low-frequency waves...... 274 — 13 —
IONOSPHERIC PROPAGATION
STUDY GROUP 6
Terms of reference: To study the propagation of radio waves (including noise) through the ionosphere, with the object of improving radiocommunication.
Chairman: D. K. B a il e y (United States o f America) Vice-Chairman: Miss G. P il l e t (France)
INTRODUCTION BY THE CHAIRMAN OF STUDY GROUP 6
1. The tasks of the Study Group Study Group 6 is concerned with the acquisition of knowledge of the propagation of radio waves through the ionosphere and its application to telecommunication problems. Since the Study Group does not deal directly with telecommunication systems, its main efforts are to codify experience and knowledge of the properties of the ionosphere and of background radio noise in forms as widely applicable as possible, with respect to geographical position, season, time of day, and solar-terrestrial relations. Moreover, the Study Group takes active account of special features of ionospherically propagated radio waves. Finally, it proposes actions intended to stimulate the collection of essential ionospheric and solar-terrestrial data, so that the basis for the required predictions associated with ionospheric propagation can be maintained and improved.
2. Problems already settled The subjects indicated under this heading are not, in fact, finally solved, but codified Reports have been prepared with practical usage in mind. The information contained in these Reports represents the most reliable data upon which the C.C.I.R. can agree at present, but revision will clearly be required from time to time.
2.1 C.C.I.R. Atlas of ionospheric characteristics This Atlas enables the prediction of EJF’s (standard MUF’s) and is contained in Report 340-2, published separately. It consists of the 1966 edition of Report 340 and its Addenda and Supplement, approved in 1970 and 1974. The original edition represented the culmination of several years of work by an International (now called Interim) Working Party of the Study Group, under the chairmanship of Dr. W. G. Baker (Australia). The Supplement concerns predictions of the Es and FI layers and the corre lation of different solar and ionospheric indices, and was prepared by Interim Working Party' 6/3, under the chairmanship of Mrs. Margo Leftin (U.S.A.). A continually updated version of the Atlas is also available in the form of punched cards, suited to users with extensive requirements for the information contained therein, and having access to modern computers. The charts may, however, be used by anyone and are especially apt for the needs of developing countries. Recommendation 434-2 endorses the use of the Atlas.
2.2 Atmospheric radio noise Report 322, adopted by correspondence after the Xth Plenary Assembly, and published separately, makes it possible to estimate the limitations imposed on radiocommunications by naturally occurring atmospheric radio noise. These limitations are the most severe in many tropical regions and are therefore — 14 —
of particular concern to a number of developing countries. In Report 322-1, some indication is included of likely levels of man-made noise in order to clarify the conditions under which it is necessary to take account of atmospheric radio noise. There is much agreement that a revision of the Report is required and is technically feasible. It is hoped that interested Administrations will support work to this end. Meanwhile, as no active work is now in progress, Resolution 8-2 which established Interim Working Party 6/2 has been deleted. The revision should take detailed account of such matters as:
— more recent observational material including that on the characteristics and distribution of noise sources; — directional characteristics of receiving antennae; — the improved knowledge of ionospheric propagation including variation with solar cycle (see Reports 340-2, 252-2, new Report 575 and Report 265-3).
Recommendation 372-1 continues the endorsement of Report 322-1 for provisional use.
2.3 Sky-wave field strength at frequencies between 2 and 30 MHz Report 252-2 contains the C.C.I.R. interim method for estimating sky-wave field strength and transmission loss at frequencies between the approximate limits of 2 and 30 MHz. A description and listing of the programme, written in FORTRAN language, complete the Report, which is the result of the work of Interim Working Party 6/1, under the chairmanship of Mr. G. Haydon (U.S.A.). Opinion 45 (replaces Resolution 49) calls for the evaluation of this interim method, and a plan for proposed revision is contained in Report 572.
2.4 Predictions of ionospheric field strength and propagation loss for the frequency range between 150 and 1500 kHz Interim Working Party 6/4, under the chairmanship of Dr. P. Knight (United Kingdom), continuing the work begun under the previous Chairman, Mr. G. Millington (United Kingdom), has prepared Report 575. This new Report, which was completed in time for the Regional Administrative Radio Conference for LF/MF Broadcasting (October, 1974), replaces the material applicable solely to the European Broadcasting Area (Report 264-3) and the Cairo Curves of 1938, has world-wide applicability. Recommendation 435-1 which endorses, for provisional use, the material applicable only to the European Broadcasting Area as given in Report 264-3, has been revised (Recommendation 435-2) to endorse similarly the much more general results given in Report 575. It is planned to combine certain material in Reports 264-3 and 431-1 in the next working period. Experience with use of the material in the new Report 575 will doubtless give rise to suggestions for minor improvements.
2.5 Propagation at frequencies below about 150 kHz Sky-wave propagation at frequencies below about 150 kHz, with particular emphasis on ionospheric effects, is the subject of Report 265-3. It is hoped that this Report can be reviewed and usefully modified in the next working period.
2.6 Solar and ionospheric indices for use in connection with ionospheric propagation predictions There is now general agreement within the C.C.I.R., as embodied in Recommendation 371-2, that three indices, of which two, R12 and
3. Radiocommunication involving space vehicles To the extent that the ionosphere, and the ionization in the space beyond the ionosphere, affect radiocommunication at the frequencies employed, there is a growing role for the Study Group. Its recent work in this area is represented by Reports 262-3 and 263-3. Question 18-1/6 outlines some general goals.
4.- Problems under further study It will be evident that the problems described in § 2 as being “ already settled ” are not really solved to such an extent that no further—and possibly significant—improvements might still be made. To this list should be added the subject of § 3, and the study of short-term predictions of operational characteristics for ionospheric radiocommunications, which is being considered by Interim Working Party 6/7 under the chairmanship of Mr. C. G. McCue (Australia).
5. Problems of interest to developing countries Certain subjects under study, in addition to those already discussed, are of potential interest to developing countries.
5.1 Propagation by way of sporadic E and other localized D- and E-region ionization Question 4-1/6 has given rise to Study Programmes 4A-2/6 and 4B-2/6 on the subject of sporadic-E propagation. The outcome of such studies should be carefully watched by developing countries situated in low magnetic latitudes, where certain regular features of the sporadic-E region may offer useful possibilities for radiocommunication at both HF and VHF. Reports 259-3 and 260-2 should be borne in mind in connection with this problem.
5.2 Special problems o f HF radiocommunication associated with the equatorial ionosphere Question 6-2/6 and Report 343-2, with the above title, raise additional specific problems associated with the equatorial ionosphere. Further developments on this topic should be kept under observation by developing countries.
5.3 Improvement in the world-wide ionospheric observing programme for numerical mapping purposes The problems raised by Study Programme 2A-2/6, § 1, have special relevance to the territories of some developing countries, and deserve consideration by the latter. Simultaneously, note should be taken of Opinion 22-2, which deals with routine ionospheric sounding.
6. Concluding comments The advent of artificial earth satellites and space vehicles of many kinds in the last decade and a half has given rise to new technologies of vast importance—in the first instance to the more highly developed countries—for telecommunication. As a consequence, interest in the further development of more traditional uses of the ionosphere has slackened in these countries in most subject areas discussed above except for the economically and politically important aspect of broadcasting. This change of interest has come just at the time when many of the developing countries, nearly all of which lie in equatorial, sub-tropical and low temperate latitudes, can derive in highly practical ways much benefit from exploiting the older methods of ionospheric telecommunication. It would seem therefore that, in the immediate future, particular attention should be paid to the outstanding problems of ionospheric telecommunication in low and equatorial latitudes, as described above. PAGE INTENTIONALLY LEFT BLANK
PAGE LAISSEE EN BLANC INTENTIONNELLEMENT — 17 — Rec. 313-2
SECTION 6A: IONOSPHERIC AND SOLAR INDICES
RECOMMENDATIONS AND REPORTS
Recommendations
RECOMMENDATION 313-2
EXCHANGE OF INFORMATION FOR THE PREPARATION OF SHORT TERM FORECASTS AND THE TRANSMISSION OF IONOSPHERIC DISTURBANCE WARNINGS
(1951 - 1959 - 1966 - 1974)
The C.C.I.R.,
CONSIDERING
that some radiocommunication services would benefit from having the earliest possible warnings of the probable onset of disturbances to ionospheric propagation;
UNANIMOUSLY RECOMMENDS
1. that each country participating in radio propagation research should designate an official agency for the reception, coordination and exchange of information required for the preparation of short-term forecasts, and for liaison with corresponding agencies in other countries; 2. that such information should be forwarded to the above agencies by the most direct means of telecom munication ; 3. that data needed for short-term forecasting within 48 hours should be disseminated in accordance with the International Ursigram and World Days Service (IUWDS) decisions by suitable available communi-] cation channels, while other data should be disseminated by ordinary post or airmail, or, if requested, by radio or other rapid means of communication, and that short regular transmissions giving short term warnings of ionospheric disturbances should be effected by long-range radio stations; 4. that codes to be used for the above communication and dissemination should be fully standardized in accordance with IUWDS decisions and actions; 5. that Administrations and operating agencies using the above services should be invited to compare the forecasts with the actual behaviour of radio circuits, to evaluate the accuracy of the forecasts, and to provide records and make any suggestions which might assist the studies undertaken to improve the methods used; 6. that it is desirable that a common method, based on the work on Question 8/6 be adopted to describe ionospheric perturbations and variations, for correlation with forecasts and the behaviour of operating radio services; 7. that, where Administrations have provided facilities for the rapid interchange of information in connec tion with the IUWDS, these facilities should be maintained, and, if necessary, extended in the future. Rec. 371-2 — 18 —
RECOMMENDATION 371-2
CHOICE OF BASIC INDICES FOR IONOSPHERIC PROPAGATION
(1963 - 1970 - 1974)
The C.C.I.R.,
CONSIDERING
(a) that continuous observations of sunspots have been made for a longer period of time than for any other index of solar activity; (b) that the 12-month running mean sunspot number, R12, is the only index which has, up to now, been sufficiently studied to allow predictions to be made by objective methods;
UNANIMOUSLY RECOMMENDS
1. that the 12-month running mean sunspot number R12, be adopted as the index to be used for all ionospheric predictions for dates more than twelve months ahead of the date of the last observed value; 2. that the 12-month running mean sunspot number R12, or the ionospheric index IF2, or the index O (as defined in Report 246-3) should be adopted as the index to be used for predicting monthly median values of foF2 and M(3000)F2 for dates certainly up to six, and perhaps up to twelve months ahead of the date of the last observed values. Caution should be shown in the use of IF2 at high magnetic latitudes, where the resulting ionospheric predictions may not be sufficiently accurate; 3. that $ should be adopted as the index to be used for predicting monthly median values of foE and foFl for dates certainly up to six, and perhaps up to twelve months ahead of the date of the last observed value. — 19 — Rep. 245-3
6A: Reports
REPORT 245-3 *
LONG-TERM PREDICTION OF SOLAR INDICES
(Study Programme 8A-1/6)
(1963 - 1966 - 1970 - 1974)
There is as yet no method whereby it is possible to predict accurately the next sunspot cycle, or, more generally, a cycle which has not yet begun. Sunspot-cycle parameters that have been calculated by using harmonic analysis or empirical and statistical laws observed over some earlier and even recent cycles, have not proved useful in predicting a new cycle. After a sunspot minimum has been observed, future development of the cycle can be extrapolated to a certain extent, although the deviations have been observed to be rather extreme. In the United States of America, 12-month running averages of relative sunspot numbers, R12, are predicted by computer using an improvement of the McNish-Lincoln objective method by Stewart and Ostrow [1970]. First a mean cycle is computed by taking the mean of all past values of the sunspot numbers ordered from the sunspot minimum of each cycle through eleven years thereafter. For prediction of a value in the current cycle the first approximation is the value of the mean cycle at the stated time after minimum. This estimate is improved by adding a correction proportional to the departure of the last observed value for the current cycle from the mean cycle. The correction factor for each month is determined by the method of least squares, based on sunspot data for 1834 to 1964. With the current computer programmes a new prediction for each month of the remainder of the cycle can be made as soon as a new observed value becomes available. The statistical uncertainty of the prediction increases rapidly with time after the last observed value, being fairly small for the first few months but becoming fairly large for predictions 12 months or more in advance. As soon as a minimum is identified, new correction factors can be computed by including the observed values for the preceding cycle, for appli cation to the new cycle. A study undertaken by the Secretariat of the C.C.I.R. in 1956 of several different kinds of prediction methods concluded that it was not possible to improve on an error of approximately 10 in the prediction of the solar index, R12. Only the statistical methods (comparison of cycles or autocorrelation) are considered useful, the methods based on harmonic analysis having never produced satisfactory results. The moving average 10-7 cm solar radio noise flux would be preferable to sunspot numbers for predicting long-term solar activity. However, the time series of solar flux observations is too limited for present statistical analysis prediction techniques. This parameter can be predicted from R12 with reasonable accuracy [Stewart and Leftin, 1972]. A method for predicting the ionospheric index Q>F2 with the aid of the I.T.U. computer was proposed by Joachim et al. [1972] and Joachim and Kralik [1970]. It is based on the Fourier analysis of the time series of the expansion of * Adopted unanimously. Rep. 245-3, 246-3 — 20 — R eferences Jo a c h im , M. [1973] Zur “ Stabilitat ” des ionospharischen Index Jo a c h im , M., G rom ov, A. and G u il l o t ,P . [1972] Previsions des indices et ®p2 de la propagation ionospherique. C. R. Acad. Sci. (Paris), B, 275, 473-476. Jo a c h im , M. and K r Ali'k , F. [1970] Predicting ionospheric index OF2 during a complete cycle. Telecommunication Journal, 37, 587-591. S te w a r t , F.G. and L e ft in , M. [1972] Relationship between Ottawa 10-7 cm solar radio noise flux and Zurich sunspot number. Telecommunication Journal, 39, 159-169. S te w a r t , F.G. and O st r o w , S.M. [1970] Improved version of the McNish-Lincoln method for prediction of solar activity. Telecommunication Journal, 37, 228-232. V it in s k i, J.I. [1973] Cycles and predictions of solar activity. (In Russian.) Akademiia Nauk, Leningrad, U.S.S.R. REPORT 246-3 * CHOICE OF BASIC INDICES FOR IONOSPHERIC PROPAGATION (Recommendation 371-2) (1953 - 1956 - 1959 - 1963 - 1966 - 1970 - 1974) 1. Introduction The concept of basic indices for ionospheric propagation expresses the belief that there are certain measurable quantities concerned with solar radiation which can be used for the description and the prediction of the important characteristics of the ionosphere, such as the critical frequencies of the various layers and the MUF factor, M(3000)F2. The month-to-month changes of solar activity in general contain three components: — a fairly regular component with a period of about 11 years, which represents a well known cycle of solar activity; — a component that has a quasi-period of about a year or a little less, which generally appears near the peak of the solar cycle and which lasts for only a year or two; and — fluctuations which appear to be erratic and which may represent rapid changes in solar activity that cannot be resolved when a monthly mean index is used as an index. 2. Sunspot numbers For studies of the main component of the solar cycle the 12-month running mean sunspot number R12 is used because the resultant smoothing considerably reduces complicated rapidly-varying components but does not obscure the slowly-varying component. The definition of R12 is ”+5 1 * 12 ~ 12 ^ R k + ~ 2 (^ + 6 + ^h - 6) n — 5 in which Rk is the mean value of R for a single month k, and R12 is the smoothed index for the month represented by k = n. The two main disadvantages in the use of R12 are: — the most recent available value is necessarily centred on a month, at least 6 months earlier than the present time; — it cannot be used to predict the shorter-term variation in solar activity. * Adopted unanimously. — 21 — Rep. 246-3 Nonetheless, R12 appears to be the most useful parameter for long term studies and predictions concerning the F2 layer. It appears to be desirable and possible to make shorter-term predictions for the E layer. A shorter-term index also may be applicable for the F2 layer. It must be remembered however that by far the longest series of existing data are the sunspot numbers themselves. 3. Index 0 Consistent and reasonably long series of observations at about 10 cm have been made by Canadian, Japanese and other laboratories. The Ottawa observations of solar radio noise flux, 0 , expressed in units of 10~22 W/m2/Hz, ought to be regarded as the reference index data. Monthly mean values of O are more closely correlated with E-layer critical frequency than are noise flux values at other wave lengths [Kundu, I960]. A simple relation has also been reported between the monthly average of planetary F2 layer ioniza tion and 0 [Chaman Lai, 1967]. 4. Ionospheric index IF2 An index IF2 has been developed, based on measurement of foF2 at noon at long established ionospheric observatories. The index is available for the period since 1938 (see the Telecommunication Journal). Until 1970 it was prepared from measurements at 10 observatories. Since then measurements from the 13 observatories listed in Opinion 23-2 have been, used. It would be possible to replace any or all of the 13 stations presently used in calculating IF2 with other suitable stations which have been in continuous operation at least since 1963. Minnis [1964] showed that IF2 was more closely correlated with foF2 than was any other index available at that time. 5. Index 0 F2 A modification to the IF2 index, designated 0 F2 has been proposed based on a curvilinear correlation between 0 and foF2 for the same stations as those used for the calculation of IF2 [Joachim, 1968]. 6. Index T A further index, T, developed in Australia [I.P.S.D., 1968] is based on the mean of the 24 hourly median values of foF2 measured at 30 stations. Apart from the use of data from all hours, the index is similar to that for IF2. 7. Correlation between the various indices It should be noted that the correlation between these indices and the actual ionospheric characteristics does not necessarily imply a causal relationship, but rather an indication of associated phenomena. The regression between the 10 cm flux, 0 , and R12 is practically linear for the higher sunspot numbers whereas the IF2 values show a hysteresis effect and a curvilinear regression with R12 or 0 . A study of the correlation of R12 and 0 F2 has shown no significant hysteresis effect in this case [Joachim and Krupin, 1969]. At high latitudes, where corpuscular radiation strongly influences ionospheric variations, a special four-term index may be necessary [Verdile and Mesterman, 1973]. 8. Conclusion The indices appropriate in different circumstances are given in Recommendation 371-2. Measured and predicted values of these indices, together with values for 0 F2, are published in the monthly circulars of the C.C.I.R. as well as in the Telecommunication Journal. R eferences C h a m a n L a l [1967] The seasonal and secular characteristics of the averaged diurnal value of the F2 layer critical frequency (E foF2) at conjugate magnetic latitudes. Telecommunication Journal, 34, 389-393, Rep. 246-3, 247-3 — 22 — I.P.S.D. [1968] The development of the ionospheric index T. Report IPS-R11. Jo a c h im , M. [1968] ®F2, the ionospheric index defined by computer. Telecommunication Journal, 35, 678-679. Jo a c h im , M. and K r u p in , Y. [1969] Correlation entre les indices i?12 et Kundu, M.R.J. [1960] Solar radio emission on centimeter waves and ionization of the E layer of the ionosphere. J. Geophys. Res., 65, 3903. ‘ M in n is , C.M. [1964] Ionospheric indices in Advances in Radio Research, Vol. II, Ed. J.A. Saxton. Academic Press, London and New York. V er d ile, J.L. and M esterm an, I. [1973] Dindmica de la capa F de la ionosfera en altas latitudes (Dynamics of the ionospheric F-layer at high latitudes). L.I.A.R.A. Report C-23. Avda. Libertador 327, Vicente Lopez (B.A.) Argentina. REPORT 247-3 * IDENTIFICATION OF PRECURSORS INDICATIVE OF SHORT-TERM VARIATIONS AND EVALUATION OF THE RELIABILITY OF SHORT-TERM FORECASTS OF IONOSPHERIC PROPAGATION CONDITIONS (Study Programme 10A-1/6) (1959 - 1963 - 1966 - 1970 - 1974) 1. Identification of precursors Short-term variations of ionospheric propagation conditions are of different kinds: — sudden ionospheric disturbances (SIDs), which occur at the moment of a solar flare, manifest themselves in the sunlit zone and last from about ten minutes to an hour; — polar cap absorptions (PCAs), which begin from about 20 minutes to several hours after certain flares, manifest themselves in the polar caps and extend at times to the sub-auroral zone, and may last several days; — ionospheric storms, which are associated with geomagnetic storms and last for one or more days. Most of the work on ionospheric storms that disturb radio propagation conditions for a long time relates to geomagnetic storms. Around the minimum epoch of the solar activity cycle these geo magnetic storms are apt to recur every 27 days. During the period of maximum solar activity many of the geomagnetic storms are linked with certain types of solar flares. The problem is to identify these flares and to forecast them, using the available data on solar activity. In this Report, types of solar and geophysical activity and their application to ionospheric variations will be discussed as well as the services which provide solar and ionospheric alerts and forecasts. Alerts are used to identify ionospheric blackouts as SIDs, PCAs or onsets of geomagnetic storms. Forecasts of the occurrence of SIDs and PCAs can be made with some uncertainty a few days in advance, but the times of actual onset cannot as yet be forecast. Forecasts can provide advance notification of the pro bability of occurrence and time of onset of geomagnetic storms which in turn affect ionospheric communications, particularly in high latitudes. 2. Solar flares Ha flares are very frequent phenomena in the years around sunspot maximum, but only a few percent of the flares produce the ionospheric perturbations described in § 1. In a schematic way one can describe the total flare phenomenon as follows: * Adopted unanimously. — 23 — Rep. 247-3 — Ha flares are accompanied by particle accelerations. These cause a PCA when particles attain energies of the order of 10 to 20 MeV and they are propagated in the direction of the earth; — certain phases of these accelerations are accompanied by a shock wave which travels through the interplanetary medium and causes geomagnetic storms, auroras and ionospheric disturbances; — depending upon their energy and the structure of the solar corona, these particles emit radio waves, UV radiation and X-rays. The X-rays cause SIDs which affect ionospheric paths located in the sunlit zone of the earth. 3. Use of solar observations There are many types of solar observations which are used for the prediction of Ha flares and their effects on communications. 3.1 These observations can be used to indicate a few hours to a few days in advance the regions on the solar disk which produce Ha flares and their effects. The basic data for the predictions are solar spectro- heliograms (Ha, calcium), solar magnetograms, radio maps in centimetric wavelengths and daily reports of the observed Ha flares [Simon et al., 1969]. 3.2 Some of the data on Ha flares are such that one can predict the onset of a PCA a few tens of minutes in advance. These data are the position of the flare on the solar disk relative to the position of the sunspots, centimetric bursts, spectral radio bursts and hard X-ray bursts which are observed by satellite [Bailey, 1964; Croom, 1971; Hakura, 1967; Lange-Hesse and Rinnert, 1970; Svestka, 1972; Yoshida and Akasofu, 1965]. 3.3 Centimetric and metric radio bursts, the optical importance of the Ha flare and to a lesser degree its position on the solar disk are such that one can predict a few days in advance the onset of a geomagnetic storm to within the order of half a day [Bell, 1963; Caroubalos, 1964; Castelli and Aarons, 1970; Pick, 1961; Simon, 1973]. 4. The interplanetary medium It is divided into “ streams” of solar wind and into north and south sectors of the magnetic field. After certain Ha flares, shock waves and clouds of solar particles are propagated through the interplan etary medium. Data furnished by satellites and space probes are proving useful in the prediction and the confirmation of ionospheric conditions. — The measurement of proton flux with energy > 1 0 MeV by geostationary or other satellites can confirm PCA. — The beginning of certain geomagnetic storms is associated with the arrival of a shock wave. Depending on the location of the space probe this shock wave could be detected before the storm (the velocity of the shock wave is between 800 and 1000 km/s). — The role of the solar wind and of the direction of the interplanetary magnetic field relative to the geomagnetic storms which recur at 27-day intervals is not yet fully understood. — The sector crossings of the interplanetary magnetic field are statistically related to an increase in geomagnetic activity. However, the appearance Of the south component of the interplanetary magnetic field is probably related to the beginning of the geomagnetic activity [Akasofu et al., 1973; Foster et al., 1971]. Satellite or space probe surveillance of the interplanetary magnetic field can therefore be used to predict geomagnetic storms or confirm their beginnings. 5. Solar and geophysical activity forecast services Solar and geophysical activity forecasting is a complex operation which requires the rapid assembly of numerous data and immediate distributions of the forecasts or alerts to communicators. This is carried out in regional warning centres of varying sizes working in collaboration with each other within Rep. 247-3 — 24 — the International Ursigram and World Days Service (I.U.W.D.S.). Forecasting methods have developed rapidly in the last few years. The basic data have been described in earlier sections of this Report. In order to determine their accuracy solar forecasts are currently being evaluated [Simon and McIntosh, 1972]. At present, solar phenomena forecasts are issued daily, covering a period from one to three days. Alerts identifying SIDs, PCAs and the onset of geomagnetic storms are also issued by the forecast centres of the I.U.W.D.S. Advance predictions of geomagnetic storms are made by several of the centres. 6. Ionospheric prediction services Ionospheric and magnetic disturbances and short-term radio propagation prediction services are carried on by the U.S.S.R., predicting conditions for whole days or for half days in advance. They estimate the predicted deviations of foF2, the level of sporadic-E ionization, the degree of absorption, and the presence of spread echoes [Zevakina et al., 1967]. The forecast methods have been extended by considering changes of N(h) profiles during disturbances and the statistical deviations of ionospheric parameters from their normal values [Zevakina and Liakhova, 1971]. Computer-oriented programmes using observed and predicted solar and geophysical parameters are now used by the United States of America to produce synoptic maps of deviations of foF2 from the monthly median, and to forecast the time, location, probability and magnitude of ionospheric variations caused by solar flares, solar protons and magnetic storms. 7. Usefulness and reliability of short-term forecasts Verification techniques have involved consideration of both the number of storm days for which correct forecasts were issued and the number of quiet days for which storm forecasts or false alarms were issued. Radio propagation quality figures for specific geographic areas have been prepared by the United States of America and the Federal Republic of Germany for use in forecasts and their verification. In the U.S.S.R. statistical studies of the changes in foF2 have indicated “ forbidden ” time intervals for storm commencements at their ionosphere stations [U.S.S.R., 1957]. Relatively stable communications have been predicted in the U.S.S.R. in the auroral zone utilizing real-time monitoring since it has been observed that strong fluctuations in foF2 do not take place over periods of 15-30 minutes when F2-layer reflections exist. In the United States of America the deviations of the daily foF2 from the computed monthly median have been analysed for all stations for selected months in 1958. Systematic world-wide patterns are observed in the deviations, particularly during ionospheric disturbances [Jones et al., 1973]. 8. Relationships between characteristics of disturbances and performance of radio circuits In Canada, studies by Jelly [1970] have indicated that there exist longitudinal and latitudinal variations of absorption associated with geomagnetic substorms around the northern auroral zone. The absorption at a particular L shell [Mcllwain, 1961] of about 5-2 was reported to extend from the midnight region through morning hours around to the afternoon region with maxima at night and in the morning hours. The study indicates that absorption at a location can be predicted from the measurements taken at another location on the same L shell. A latitudinal boundary between abrupt and smooth absorption has been observed during geomag netic substorms at night. This boundary is located at an invariant latitude of 65° ± 1-5°. The absorption first appeared in the vicinity of this boundary for most of the substorms studied. The location of the boundary was found to have a definite dependence on magnetic activity as defined by Kp. In India, studies by Aggarwal and Reddy [1973] indicate that geomagnetic storms at low latitudes produce very little change in the F2-layer propagation parameters such as the MUF. However, it appears that large solar flares, which are not necessarily accompanied by geomagnetic storms, may produce great depressions in the MUF values two to three days after the optical flares. Thus certain types of solar flares may be used as precursors to predict the depressed MUF values that are likely to follow in low latitudes. — 25 — Rep. 247-3 R e f e r e n c e s A g g a r w a l , S. and R e d d y , B.M. [1973] The effects of magnetic and solar activity on H.F. communication via F2-Region, presented at the Symposium on Aeronomy and Radio Wave Propagation held at NPL, India. A k a so fu , S.I., P e r r ea u l t , P .D ., Y a su h a r a , F. and M e n g , C .I. [1973] Auroral substorms and the interplanetary magnetic field. J. Geophys. Res., 78, 7490-7508. B a iley , D.K . [1964] Polar-cap absorption. Planet. Space Sci., 12, 495-541. B e ll, B. [1963] Type IV solar radio bursts, geomagnetic storms, and polar-cap absorption (PCA) events. Smithsonian Contributions to Astrophysics, 8, 119-131. C a r o u b a lo s, C. [1964] Contribution a l’etude de l’activite solaire en relation avec ses effets geophysiques (Contribution to the study of solar activity in relation to geophysical effects). Ann. Astrophys., 27, 333. C aste lli, J.P. and A a r o n s, J. [1970] Radio burst spectra and the short term prediction of solar proton events. Ionospheric forecasting, Ed. V. Agy, AGARD Conference Proceedings No. 49. C room , D.L. [1971] Forecasting the intensity of solar proton events from the time characteristics of solar microwave bursts. Solar Physics, 19, 171-185. F oster, J.C., F a ir fie l d , D .H ., O g il v ie, K.W. and R osenberg , T.J. [1971] Relationship of interplanetary parameters and occur rence of magnetospheric substorms. J. Geophys. Res., 16, 6971-6975. H a k u r a , Y. [1967] Entry of solar cosmic rays into the polar cap atmosphere. J. Geophys. Res., 72, 1461-1472. Jel ly , D.H. [1970] On the morphology of auroral absorption during substorms. Can. J. Phys., 3, 48. Jo n es, W.B., G allet, R.M ., L e f t in , M. and S t e w a r t , F.G. [1973] Analysis and representation of the daily departures of the foF2 from the monthly median. Office of Telecommunications, Report 73-12, U.S. Department of Commerce. L a n g e -H esse, G. and R in n e r t , K. [1970] The reliability of transpolar VLF measurements as a method to forecast H F propagation disturbances. Ionospheric Forecasting. Ed. V. Agy, AGARD Conference Proceedings No. 49. M cI l w a in , C.E. [1961] Coordinates for mapping the distribution of magnetically trapped particles. J. Geophys. Res., 66, 3681. P ic k , M. [1961] Evolution des emissions radioelectriques solaires de type IVet leur relation avec d’autres phenomenes solaires et geophysiques. (Evolution of solar radio emissions of Type-IV and their relation to other solar and geophysical phenomena.) Ann. Astrophys., 24, 183. S im o n , P. [1973] Some comments on the forecasting of solar activity. Synoptic codes for solar and geophysical data, Third Revised Edition, IUWDS Secretariat, NOAA, Boulder, Colorado. S im o n , P., M artres, M.J. and L e g r a n d , J.P. [1969] Flare forecasting. Symposium on Solar Flares and Space Research, Ed. North Holland. S im o n , P. and M cI n t o sh , P.S. [1972] Survey of current solar forecast centers. Solar Activity Observations and Predictions, 343-359, Eds. P.S. McIntosh and M. Dryer, M.I.T. Press, Cambridge. S v estk a, Z. [1972] Several solar aspects of flare associated particle events. Solar Activity Observations and Predictions, 141-162, Eds. P.S. McIntosh and M. Dryer, M.I.T. Press, Cambridge. U.S.S.R. [1957] Annals of the corpuscular conference of the Academy of Sciences of the U.S.S.R., Moscow (in Russian). Y o sh id a , S. a n d A k a so fu , S.I. [1965] A study of the propagation of solar particles in interplanetary space. The center-limb effect of the magnitude of cosmic-ray and geomagnetic storms. Planet, and Space Science, 13, 435-448. Z ev a k in a , R.A., L a v r o v a , Ye.V. and L ia k h o v a , L.N. [1967] Principles for predicting ionospheric and magnetic disturbances, and short-range radio propagation prediction services. (In Russian.) Published by “ Nauka ”, Moscow. Z ev a k in a , R.A. and L ia k h o v a , L.N. [1971] Ionospheric disturbances and their effects on radio propagation. (In Russian.) Published by “ Nauka ”, Moscow. Rep. 248-3 — 26 — REPORT 248-3 * AVAILABILITY AND EXCHANGE OF BASIC DATA FOR RADIO PROPAGATION FORECASTS (Recommendation 313-2) (1951 - 1953 - 1956 - 1959 - 1963 - 1966 - 1970 - 1974) 1. Introduction Propagation of radio signals in the range 3 to 30 MHz is practicable over any but the shortest distances, mainly because of the possibility of obtaining ionospheric and ground reflections which result in small values of attenuation. Satisfactory communications for a given circuit can generally be obtained if the operating frequency lies between a lower (LUF) and an upper (operational MUF (MUF)) frequency limit. These are determined by ionospheric characteristics. The operational range of frequencies has been found to be even more restricted with some forms of high capacity communication systems. Since only a limited range of frequencies can be used, it is desirable to have, as far in advance as possible, information on the probable values of these upper and lower limits, as well as short-term forecasts and disturbance warnings. Collectively, these predictions (long-term) and forecasts (short term) and disturbance warnings provide information for planning and operating personnel, that can be utilized in making the most economical use of the limited resources of equipment and frequency spectrum. The long and medium-term predictions are indicative of representative ionospheric conditions, so that it is extremely useful to operating personnel to be warned of impending ionospheric disturbances in order that traffic can be re-routed, instructions can be issued in advance to cover temporary adjust ments in the normal operating frequency, and the performance of other systems affected by the ionosphere can be assessed. 2. Available data for radio-propagation forecasts 2.1 Long-term predictions Organizations in several countries now prepare predictions of ionospheric conditions from one month up to twelve months in advance (see Annex); for general planning purposes, predictions for a complete solar cycle are also made by some organizations. These predictions are for representative ionospheric conditions. The information is usually issued in the form of charts which are applicable to any part of the world and are available for interchange between the organizations undertaking this service. 2.2 Forecasts of disturbances Organizations in several countries now prepare forecasts of ionospheric disturbances from a few hours to twenty-seven days in advance (see Annex). These forecasts are supplemental to the long-term predictions, since the occurrence of ionospheric disturbances, which cannot be forecast for long periods in advance, may modify considerably the frequency range within which satisfactory operation can be maintained on a particular circuit. Operating organizations have shown interest in these short-term forecasts to such an extent that they are now being regularly transmitted by radio at scheduled times (see Annex). 2.3 Working documents for long-term predictions The C.C.I.R. Atlas of ionospheric characteristics, Report 340-2, is the source of standard MUF (EJF) and FOT for use with predicted 12-month running mean sunspot numbers R 12 in making long term predictions for any part of the world. * Adopted unanimously. — 27 — Rep. 248-3 3. Exchange of basic data used in short-term forecasts 3.1 For many years, scientific information of direct interest to those concerned with ionospheric forecasts and disturbances has been broadcast by certain countries, in programmes known as Ursigrams. Since 1962, through the International Ursigram and World Days Service (I.U.W.D.S.) (a Permanent Service of U.R.S.I. in association with I.A.U. and I.U.G.G. adhering to the Federation of Astronomical and Geophysical Services), these data are collected, coordinated and exchanged by rapid means through suitable interchange synoptic codes. These programmes provide a means of exchange of summary information required within 48 hours, after its collection, for the preparation of short-term forecasts and similar urgent purposes. These exchanges are made through regional networks, composed of observatories, laboratories, communication agencies and regional centres. The regional centres in turn exchange, once a day, summaries of information on solar flares, sudden ionospheric disturbances, solar corona and radio emission, sunspots, ionospheric and magnetic activity, as well as forecasts. The regional warning centres (RWC) in Australia, France, Federal Republic of Germany, Japan, and U.S.S.R., plus associate regional centres in the Czechoslovak Socialist Republic, India, Sweden and U.S.S.R. (Irkutsk), collect data in their regions and forward them by telegraph to the I.U.W.D.S. World Warning Agency (at Boulder, Colorado, U.S.A.), which has also collected data from its region. The I.U.W.D.S. World Warning Agency makes the final decisions, having advice available from the other centres, whether or not to declare a world-wide GEOPHYSICAL ALERT (issued shortly after an exceptional solar or geophysical event has occurred or started)—periods during which many geophysical stations carry out special observing programmes. These decisions are distributed through the world to scientific stations participating in the programme by various rapid means, in particular over the meteorological teleprinter networks coordinated by the W.M.O. 3.2 Types of data exchanged among the various regional centres are those concerning solar flares, solar corona, solar radio emission, cosmic rays, critical frequencies of the ionosphere, ionospheric disturbances, terrestrial magnetism and radio-propagation quality. Data are collected and transmitted in simple synoptic codes. Code booklets are available from Dr. P. Simon, Chairman, I.U.W.D.S. Steering Committee, Observatoire de Paris, 92190 Meudon (France) or Mr. R. B. Doeker, Secretary for Ursigrams, I.U.W.D.S. Steering Committee, NOAA, Boulder, Colorado, 80302, U.S.A. The regional centres from which details may be obtained concerning data and schedules of broadcasts and reports are listed in the Part E of the I.U.W.D.S. Synoptic Codes for Solar and Geophysical Data, Third Revised Edition, 1973. 3.3 The Annex lists the centralizing agencies, which have been designated for the reception, coordination, liaison and exchange of information relating to radio propagation. ANNEX LIST OF ORGANIZATIONS CONCERNED WITH THE EXCHANGE OF DATA AND THE ISSUING OF FORECASTS OF PROPAGATION CONDITIONS A: an agency for the general exchange of information on propagation. RC: a regional centre of the I.U.W.D.S. for the rapid exchanges of data required for short-term forecasts of disturbances. L: the organization issues long-term predictions. The-periods ahead for which predictions are made are shown (in months). S: the organization issues short-term forecasts of disturbances, WDC: designated as a world data centre beginning with the IGY. Rep. 248-3 — 28 — Country Organization Address A RC L S Germany F.T.Z. Fernmeldetechnisches Zentralamt FI 33 (Federal (Arbeitsgemeinschaft Ionosphare), Republic of) D 6100 Darmstadt, am Kavalleriesand, 3 Tel. Add.: Ionosphare, Darmstadt Telex 419209 X X 3 X Argentina L.I.A.R.A. L.I.A.R.A. Av. Libertador No. 327, Vicente Lopez, (B.A.) X 6 Australia Officer-in-charge, International Section, P.M.G.’s Department, Treasury Gardens, Melbourne, C.2 Tel. address: Gentel, Melbourne X I.P.S. Ionospheric Prediction Service, P.O. Box 702, Darlinghurst 2010 Tel. address: IPSO, Sydney X 3 X Belgium Chef du Service du Rayonnement, Institut Royal meteorologique, 3, Avenue Circulaire, Uccle, Brussels X Brazil C.T.A. Centro technico de Aeronautica, S. Jose dos Campos, Sao Paulo X 1 I.P.M. Instituto de Pesguisos de Marinha, Ministerio de Marinha, Rio de Janeiro 1 Canada D.O.C. Telecommunications Regulation Branch Berger Building 100 Metcalfe Street Ottawa, Ontario K1A OC8 X Communications Research Centre, Shirley Bay P.O. Box 490, Station ’A ’ Ottawa, Ontario K IN 8T5 6 Spain Departamento de Servicios Tecnicos de Telecomunicacion, Direction General de Correos y Telecomunicacion, Madrid X United States N.O.A.A. Space Environment Services Center Environmental Boulder, Colorado, 80302 Research Laboratories X X« N.O.A.A. World Data Center A for Solar- Environmental Terrestrial Physics Data Service Boulder, Colorado 80302 see N ote 0 Solar and geophysical information radiated from WWV and WWVH. Note.—World Data Center A for Solar-Terrestrial Physics receives and distributes ionospheric data from a few geographical areas not directly represented through membership in the I.T.U. — 29 — Rep. 248-3 Country Organization Address A RC L S United States Office of Institute for Telecommunication Telecommuni Sciences cations Boulder, Colorado 80302 X K 1) x ( 2) France C.N.E.T. Service des Ursigrammes Observatoire de Paris F-92190 Meudon X X X 0 Division des Previsions Ionospheriques 3 Avenue de la Republique F-92131 Issy-les-Moulineaux X 6 X ( 3) India Council of The Secretary, Scientific and Radio Research Committee, Industrial National Physical Laboratories, Research Hillside Road, New Delhi, 12 X X 6 India Kodaikanal Meteorological Observatory X Dept. All India Research Department, All India Radio Radio Indraprastha Estate, New Delhi-1 X Italy Istituto Nazionale di Geofisica, Citta Universitaria, Roma Tel. address: Geofisica, Roma (All messages should begin with the word “ Ionosphere ”) X Japan R.R.L. Radio Research Laboratories, Ministry of Posts and Telecommunica tions, 2-1, Nukui-kita-machi, 4-chome, Koganeishi, Tokyo, 184 X X 3 X ( 5) Mexico S.C.T. Direction general de Telecomunicaciones, Estacion de sondeo ionosferico, Xola y Universidad, Mexico, (12) D.F. X Netherlands P.T.T. Afdeling “ Ionosfeer en Radioastronomie ” St. Paulus St. 4, Leidschendam X German R.F.Z. Rundfunk und Fernsehtechnisches Democratic Zentralamt, Berlin-Adlershof, Agastrasse Republic Telex: 0158720 X 3 H.H.I. Heinrich-Hertz-Institut fur solar- terrestrische Physik Juliusruh/Rugen Telex: 318422 X X 0 Predictions for one month ahead to end of the present solar cycle. (2) Warnings radiated from WWV. (3) Weekly Telex messages. (4) Warnings radiated from Pontoise. (5) Warnings radiated from JJY. Rep. 248-3 — 30 — Country Organization Address A RC L S United Appleton Director, Appleton Laboratory, Kingdom Laboratory Ditton Park, Slough, SL3 9JX Telex: 848369 Appleton SL X 6 M.R.P.S. Marconi Radio Propagation Services Baddow Research Laboratories Great Baddow, Essex, CM2 8HN 6 X Republic of C.S.I.R. Telecommunication Research Laboratory, South Africa Department of Electrical Engineering, University of Witwatersrand, Johannesburg X 1 Sweden Central Administration of Swedish Telecommunications, Development Department, S-123 86 Farsta Tel. Add.: Radiogen, Stockholm X X 3 X Switzerland Division Radio et Television Direction Generale des P.T.T. 21, Viktoriastrasse CH-3000 Bern 33 X Czechoslovak Geophysical Institute, Academy of Sciences, S.R. Bocni 2, 14100 Praha 4, Sporilov X U.S.S.R. IZMIRAN Scientific Research Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Moskovskaya Obi., P/O Akademgorodok X X 3 X 0 SIBIZMIR Siberian Institute of Terrestrial Magnetism, Ionosphere and Radio Propagation, Irkutsk 3, Box 65 X X X 0 Warnings radiated from RDZ and RND. — 31 — Rec. 373-3 SECTION 6B: PREDICTION OF MUF RECOMMENDATIONS AND REPORTS Recommendations RECOMMENDATION 373-3 DEFINITIONS OF MAXIMUM TRANSMISSION FREQUENCIES (1959 - 1963 - 1966 - 1970 - 1974) The C.C.I.R., CONSIDERING that prediction services, scientists and operators have different requirements for definitions of the maximum transmission frequency; UNANIMOUSLY RECOMMENDS 1. that the term operational MUF, or simply MUF, should denote the highest frequency that permits acceptable operation between given points at a given time, and under specified working conditions; 2. that the term classical MUF or junction frequency (JF) should denote the highest frequency that can be propagated in a particular mode between specified terminals by ionospheric refraction alone; on an oblique ionogram it is the frequency at which the high- and low-angle rays merge into a single ray; 3. that the term standard MUF or estimated junction frequency (EJF) should denote an approximation to the classical MUF or junction frequency obtained by application of the conventional transmission curve to vertical-incidence ionograms, together with the use of a distance factor, or, optionally, by some other suitable theoretical method; 4. that the term maximum observed frequency (MOF), should denote the highest frequency at which signals are observable on an oblique-incidence ionogram. Rep. 249-3 — 32 — 6B: Reports REPORT 249-3 * IONOSPHERIC SOUNDING AT OBLIQUE INCIDENCE (Study Programme 12A-1/6) (1959 - 1963 - 1966 - 1970 - 1974) 1. Introduction This Report concerns chiefly experiments in which sounding equipment is located at both ends of the propagation path. Equipment for operational use in this way is now offered by several manu facturers. Single frequency pulse transmissions reveal more information about ionospheric modes than CW transmissions; however, the use of sweep-frequency or step-frequency transmissions provides much more information and easier interpretation of data. Administrations are encouraged to develop special sounding techniques to minimize the possibilities of harmful interference to other users of the spectrum. One useful technique is that of FM-CW, or “ Chirp ” sounding, which has low peak power requirements of a few watts and results in low interference to other users [Fenwick and Barry, 1965], 2. Results of sounding at HF and VHF 2.1 The existing theories of propagation seem to be adequate for calculating standard MUF (EJF) for distances up to several hundred kilometres. Oblique incidence pulse tests in Canada [Hatton, 1961], France [Delobeau et al., 1955], Federal Republic of Germany [Moller, 1960], Japan [Aono, 1962], the United States of America [Agy and Davies, 1959; Tveten, 1961] and in Africa [Davies and Barghausen, 1966] indicated that the classical MUF (JF) is observed to be 3 to 10% higher than the standard MUF (EJF) for paths up to about 4000 km. It should be noted, however, that at low latitudes anomalously high operational MUFs associated with F scatter are encountered at night [Davies and Barghausen, 1966; Neilson, 1966]. 2.2 For high-latitude paths, the classical MUF (JF) for transmission distances at the 2000-3400 km range is normally controlled by the FI layer in the summer [Petrie and Stevens, 1965]. For long paths, the one hop F2 (Pedersen ray) is the principal mode [Hagg and Rolfe, 1963] under certain conditions over a wide range of latitudes. The MOF may however be controlled by an Es mode on some occasions. The reflection coefficient of Es and the ionosonde equipment characteristics must be considered in estimating the usefulness of Es as a reflector at oblique incidence [Stevens, 1968a]. 2.3 Magneto-ionic splitting of x and o components at the MOF was calculated by Kopka and Moller [1968] using ray tracing. Results are given for a 2000 km path as a function of both magnetic latitude and magnetic azimuth. In general, the magneto-ionic splitting is less for an east-west path than a north- south path. Aono [1962] and Davies and Barghausen [1966] found a separation of about f L on north- south paths. Agy and Davies [1959] found that there was a separation of about 0-2 MHz on a 2400 km east-west path and that the separation decreased with an increase in distance. 2.4 A study of pulse amplitudes at frequencies near the classical MUF (JF) for a 2370 km east-west path in the United States of America indicates that “ classical MUF (JF) focusing ” amounts, on the average, to no more than about 6 dB [Davies, 1965]. 2.5 Reciprocity measurements using oblique pulse methods have been made to record the signal strength and fading characteristics; high correlation between fading pulsed signals received simultaneously * Adopted unanimously. — 33 — Rep. 249-3 at each end of a path is observed when the signal contains only one magneto-ionic component. When both magneto-ionic waves were present of comparable strength, and using horizontal antennae for transmitting and receiving, the fading correlation was low [Jull and Pettersen, 1964]. Theoretical studies have explained these observations and have indicated a corrective procedure [Budden and Jull, 1964], 2.6 Chirp sweep-frequency oblique-sounding polarization measurements of F-layer reflections over a 1300 km north-south path and a 1900 km east-west path have shown average polarization rotations of 1 turn/MHz and 0-25 turn/MHz respectively. Higher rates were observed during night than for day. Such polarization rotation rates indicate that when linearly polarized antennae are used in temperate zones signal bandwidths exceeding about 100 kHz on north-south paths, or 400 kHz on east-west paths will result in signal envelope distortion [Epstein, 1969]. 3. Angle of arrival measurements There can be notable departures between the calculated and observed results if calculations are made using simple theoretical models. This agreement can be improved by the inclusion of electron density gradients in ray tracing [Rottger, 1967]. 4. Application to practical HF communication links Operational applications of HF oblique incidence sounders are treated by Study Group 3 [C.C.I.R., 1974]. Results from oblique incidence sounding studies have shown that such equipments may be used in parallel with communications systems to determine optimum, short-term operating frequencies. A number of studies have now been carried out which are designed to determine the improvements that can be achieved from the use of oblique sounding information [Probst, 1968 and Stevens, 1968b]. Improvement in radiocommunications during disturbed propagation conditions has been achieved due to effective selection and use of available operating frequencies when predictions or prearranged operating frequency schedules are inaccurate [Jull et al., 1962]. It has been found that oblique sounding information can be a valuable aid in assessing the performance of the communication system. With propagation removed as an unknown variable, any defects in, or limitations of, the communication system performance or operation can be more readily discovered. However, experiments involving ionospheric sounding at oblique incidence disclose a number of problems when the sounding equipment is used in parallel with the communications equipment or when the communications equipment is used to perform the sounding. The following must be considered: — any differences in operational sensitivity between communication and sounding equipment parameters; — inaccuracies which result from sounding over an ionospheric path separated from the communications path (particularly when the sounding data are used for quantitative prediction of signal levels). Studies [Jull, 1968] suggest that for a separation of 32 km, the r.m.s. differences in signal levels amounted to 5 dB when the sounding information was averaged over eight minutes; — that ionospheric non-reciprocity results in unequal levels of signals travelling in opposite direction between linear antennae. The path non-reciprocity which arises when polarization fading was present can be appreciably reduced by averaging signal levels for eight minutes [Jull, 1968]; — any differences of performance of the sounder and communication equipment in the presence of interference; — the difficulty in determining the sounding repetition rate required to ensure the validity of the information when ionospheric conditions begin to vary, as during disturbed periods; — the difficulty in determining a sounding signal that is representative of the modulation of the communications system. Rep. 249-3 — 34 — R eferences A g y , Y. and D a v ies, K. [1959] Ionospheric investigations using the sweep-frequency pulse technique at oblique incidence. NBS Journ. o f Research, 63D, 151-174. A o n o , Y. [1962] Study of radio wave propagation in sweep frequency pulse transmission tests in Japan. J. Radio Res. Labs. (Japan), 9, 127. B u d d e n , K.G. and Ju l l , G.W. [1964] Reciprocity and non-reciprocity with magneto-ionic rays. Can. Journ. Phys., 42, 113. C.C.I.R. [1974] Report 357-1. Documents of the XHIth Plenary Assembly, Geneva. D a v ies, K. [1965] Ionospheric radio propagation. NBS Monograph 80, 181, U.S. Government Printing Office, Washington D.C. D avies, K. and B a r g h a u se n , A.F. [1966] The effect of spread F on the propagation of radio waves near the equator. Spread F and its effects upon radiowave propagation. Agardograph 95, Ed. P. Newman, Technivision, 437-466. D elo beau, F., E y f r ig , R. and R a w e r , K. [1955] Resultats experimentaux de transmission ionospherique d’impulsions a incidence oblique (Experimental results of ionospheric pulse transmissions at oblique incidence). Ann. des telecomm., 10, 55-64. E p st e in , M.R. [1969] Polarization of ionospherically propagated HF radio waves with applications to radiocommunications. Radio Science, 4, 53-67. F e n w ic k , R.B. and B a r r y , G.H. [1965] Step by step to a linear frequency sweep. Electronics, 38, 66-70. H a g g , E.L. and R olfe, W. [1963] A study of transatlantic radio propagation modes at 41 -5 MHz. Can. Journ. Phys., 41, 220. H a t t o n , W.L. [1961] Oblique-sounding and HF radiocommunication. IRE Trans., PGCS-9, 275. J u l l , G.W. [1968] HF spatial and temporal propagation characteristics and sounding assisted communications. Ionospheric Radio Communications, Ed. K. Folkestad, Plenum Press, New York, 225-241. Ju l l , G.W., D o yle, D.J., Ir v in e , G.W. and M u r r a y , J.P. [1962] Frequency sounding; techniques for HF communications over auroral zone paths. Proc. IRE, 50, 1676-1682. Ju l l , G.W. and P ettersen, G.W.E. [1964] Origin of non-reciprocity on high frequency ionospheric paths. Nature, 201, 483-484. K o p k a , H. and M o ll er , H.G. [1968] MUF calculations including the effect of the earth’s magnetic field. Radio Science, 3, 53-56. M oller, H.G. [1960] Ergebnisse der Impulstibertragung mit veranderlicher Frequenz auf der Strecke Sodankyla-Lindau (Results of variable frequency pulse transmissions over the path Sodankyla-Lindau). Kleinheubacher Berichte, 7, 115-123. N eilso n , D. [1966] Oblique sounding of a transequatorial path. Spread F and its effects upon radiowave propagation and communication. Agardograph 95, Ed. P. Newman, Technivision, 467-490. P etrie, L.E. and S tevens, E.E. [1965] An FI layer MUF prediction system for northern latitudes. IEEE Trans. Ant. Prop., AP-13, 542-546. P robst [1968] The CURTS concept and current state of development. Ionospheric Radio Communications. Ed. K. Folkestad, Plenum Press, New York, 370-379. R o ttg er, J. [1967] Messung des Erhebungswinkels von Kurzwellen bei schragem Einfall in die Ionosphare (Measurement of the angle of HF radiation at oblique incidence in the ionosphere). Thesis at the Institute of Geophysics and Meteorology of the Technische Hochschule Braunschweig. S tevens, E.E. [1968a] The significance of sporadic E propagation in determining the MUF. Ionospheric Radio Communications. Ed. K. Folkestad, Plenum Press, New York, 289-293. S tevens, E.E. [1968b] The CHEC sounding system. Ionospheric Radio Communications. Ed. K. Folkestad, Plenum Press, New York, 359-369. T vet en , L.H. [1961] Long-distance one-hop FI propagation through the auroral zone. J. Geophys. Res., 66, 1683-1684. — 35 — Rep. 250-3 REPORT 250-3 * LONG-DISTANCE IONOSPHERIC PROPAGATION WITHOUT INTERMEDIATE GROUND REFLECTION (Questions 5-2/6 and 6-2/6) (1959 - 1963 - 1966 - 1970 - 1974) 1. It is now well established that there are modes of propagation controlled by the regular ionospheric regions, for which HF and VHF radio waves can travel to great distances in or below the ionosphere, over low absorption paths without intermediate ground reflection. The distances in question extend from the classical geometrical limit for a single hop out to more than 10 000 km. Modes of propagation which are now recognized include: normal one-hop propagation including both direct and Pedersen-rays; ionospheric ducting between E and F regions; and those resulting from horizontal ionization gradients, such as the equatorial F-region ionization trough. 2. Both direct and Pedersen-ray propagation have been observed well beyond the classical limit of a single hop (4000 km) usually when reflection heights exceed the average value normally assumed for calculations, or because of horizontal ionization gradients. The one-hop mode provides explanations for communi cation often observed at distances above the classical geometrical limit of single-hop propagation [Muldrew and Maliphant, 1962]. 3. Long-range transequatorial propagation has often been explained by a modification of normal one-hop propagation by a mechanism in which a ray is refracted through an ionization gradient to emerge from the ionosphere, travels in a straight line above the earth’s surface, and re-enters the ionosphere to undergo further refraction back to the ground [Stein, 1958; Kerblai and Kovalevskaya, 1967]. Both layer tilt and scattering processes may affect this type of long distance propagation [Tao et al., 1970]. 4. The proposed mechanism of a series of internal ionospheric reflections or refractions is attractive for explaining round-the-world echoes which, under certain conditions, show remarkably low dispersion and attenuation between successive echoes [Fenwick and Villard, 1963]. 5. Satellite soundings have shown the existence of significant sheets of ionization irregularities above the height of maximum electron density of the F region. These sheets probably extend below the height of maximum electron density and may be responsible for long distance propagation of frequencies above the standard MUF (Hagg and Rolfe, 1963]. At low latitudes they are probably responsible for transequatorial scatter propagation [Davies and Barghausen, 1966]. 6. Theoretical studies have been made of the duct mode of propagation [Chvojkova, 1965]. R eferences C h v o j k o v a , E. [1965] Analytic formulae for radio path in spherically stratified ionosphere. Radio Science, 69D, 453-457. D a vies, K. and B a r g h a u se n , A.F. [1966] The effect of spread F on the propagation of radiowaves near the equator. Agardo graph 95, 437-466. F e n w ic k , R.B. and V il l a r d , Jr. O.G. [1963] A test of the importance of ionosphere—ionosphere reflections in long distance and around-the-world high-frequency propagation. J. Geophys. Res., 68, 5659-5666. H a g g , E .L . and R olfe, W . [1963] Study of transatlantic radio propagation modes at 41 -5 Mc/s. Can. J. Phys., 41, 220. K er b la i, T. and K ovalevskaya , E. [1967] Calculation of the MUF when there are horizontal irregularities in the ionosphere (in Russian). Geomagnetism i Aeronomiia, VII, 1, 123-127. M u l d r e w , D.B. and M a l ip h a n t , R.G. [1962] Long-distance one-hop ionospheric radio propagation. J. Geophys. Res., 67, 1805-1815. * Adopted unanimously. Rep. 250-3, 255-3 — 36 — S t e in , S. [1958] The role of ionospheric layer-tilts in long-range high frequency radio propagation. J. Geophys. Res., 63, 217-241. T a o , K., O c h i, F., Y a m a o k a , M., W a t a n a b e , S., W a t a n a b e , C. and T a n o h a t a , K. [1970] Experimental results of YHF trans equatorial propagation. J. Radio Res. Labs. (Japan), 17, 83-101. REPORT 255-3 * BASIC PREDICTION INFORMATION FOR IONOSPHERIC PROPAGATION (Study Programme 9A-1/6) (1959 - 1963 - 1966 - 1970 - 1974) 1. Prediction of the standard MUF (EJF) When considering the prediction of ionospheric propagation conditions, it is essential to consider separately the translation of vertical-incidence ionospheric data into oblique-incidence characteristics, and the prediction of vertical incidence data from values of the ionospheric indices, which are themselves the subject of still other predictions. Many Administrations use prediction methods involving charts of ionospheric characteristics based on predicted vertical incidence data. These predicted characteristics have usually been presented in graphical form, showing the world-wide distribution of the ionospheric characteristics. The representation is now, however, frequently given in the form of mathematical expressions and coefficients which may be readily manipulated by electronic computers. These representations necessarily involve extrapolation into areas of the world where ionospheric data are few or lacking. They are also dependent upon correlation with a solar activity index for prediction purposes. Moreover, this solar activity index is itself a predicted one. Obviously there are several potential sources of error inherent in the above process of prediction of radiocommunication frequencies. These sources of error include the following: — the lack of data in certain geographic areas (see Report 430-1); — the smoothing of basic data inherent in the mathematical representation; — uncertain and variable relationships between the ionospheric characteristics and the solar activity index both in the rising and descending phases of the solar cycle, and also from cycle to cycle; — unsatisfactory and inadequate methods of predicting the solar activity index or indices (Report 246-3); — the difference between standard MUF (EJF) and the operational MUF (see § 2); — the inadequacy of usable methods for taking account of the statistical variations of ionospheric parameters, including the standard MUF (EJF), around the monthly median. The statistical distributions around the monthly median are often quite broad [King and Slater, 1973] and the spread of the distribution is not necessarily proportional to the value of the monthly median; — difficulty in the translation of vertical ionosonde data to propagation predictions for operational communication circuits. The differences can be particularly large when steep horizontal gradients and inhomogeneities in ionization occur in the transmission path. Examples of this are given [Moller, 1960; Williams, 1962; Barghausen and Finney, 1967], where exceptionally high values of the ratio of the standard MUF (EJF) to the classical MUF (JF) are attributed to non-symmetrical paths. * Adopted unanimously. — 37 — Rep. 255-3 2. The ratio of MUF to the standard MUF (EJF) 2.1 In general the ratio of operational MUF to standard MUF (EJF) seems to be larger for night-time than for daytime conditions, while at middle latitudes this ratio seems to be smaller in summer for both day and night than for winter nights. The largest values of this ratio for middle and high latitudes usually occur in winter when the MUF (EJF) is low. The ratio tends to increase for circuits passing near the auroral zones. It is possible to describe this observed variation, as a first approximation, by an empirical formula based on the dynamics of the ionosphere as a function of the variation of the standard MUF (EJF) [Beckmann, 1967; Beckmann and Ochs, 1969; Beckmann, 1970]. The effect of the irregularities of the ionosphere becomes more apparent as the height of the F2 layer increases and its critical frequency decreases. The differences between the operational MUF and the standard MUF (EJF) can be explained by various ionospheric phenomena, such as scattering in the E and F regions, off-great-circle propagation, and propagation by unusual modes when ionization irregularities exist in the E and F regions. Also, spread F may be an important factor in the increasing ratio. It is also possible that some of this effect may result from inadequacies in the theoretical formulae and ray-tracing methods used. As might be expected, when comparisons are made between observed and predicted values of the MUF the differences are greater than in the case when actual ionospheric vertical incidence observations are used to calculate these values. Errors in the prediction of the standard MUF (EJF) or of the operational MUF may arise from ionospheric characteristics that are not well predicted; thus an important source of error may well be the prediction of the critical frequencies and heights of the layers rather than the conversion to the standard MUF (EJF). 2.2 Spread F may be an important factor in causing the above-mentioned differences between the operational MUF (MUF) and the standard MUF (EJF). Numerical maps of spread F occurrence have been developed in the United States of America for years of high and low solar activity (see Report 340-2). The dispersion of the standard MUF (EJF) about the monthly median exhibits variations that are similar to the variations of the probability of frequency spreading. Both the MUF-dispersion and the spread-F probability show the following variations: — a diurnal maximum in night-time hours; — a seasonal maximum in winter or equinox at high latitudes; — a positive correlation with solar activity at high latitudes and a negative correlation at medium and low latitudes. The U.R.S.I. Ionospheric Network Advisory Group (I.N.A.G.) has issued rules for the deter mination of a vertical incidence spread F parameter fxl. These rules, and illustrations of typical ionograms, appear in the second edition of the U.R.S.I. Handbook of Ionogram Interpretation and Reduction [Piggott and Rawer, 1972], Some Administrations are investigating the application of this parameter to predictions. 3. The effect of the Fl-layer on radiocommunications The fact that the determination of the Fl-layer standard MUF by an extension of E-layer nomograms was but a temporary expedient has been overlooked for a long time. The ionospheric reflection point for oblique propagation often lies in the region represented by the Fl-layer trace on the ionogram. An Fl-layer prediction system has been developed for transmission distances in the range 2000-3400 km (C.C.I.R. Report 340-2). The geomagnetic influence on foFl has been found and reduced to an empirical formula as a function of the geomagnetic latitude [Du Charme et al., 1973]. A comparison of this prediction method with monthly median values of foFl observed during 1967-1969 at 28 vertical incidence sounding stations indicated that the r.m.s. deviation for most stations is about 0-2 MHz or less; the error in the equatorial regions being about 1-5 times greater than that at high latitudes. Another method for the prediction of foFl and of the maximum solar zenith angle of foFl occurrence [Rosich and Jones, 1973] has been developed in the United States of America. This model predicts foFl values to a similar accuracy to that above, but also gives a better prediction of the occurrence of the Fl-layer. Rep. 255-3 — 38 — 4. The effect of E-region ionization Little difficulty is encountered in introducing the effect of the regular E layer into prediction systems. The world-wide occurrence of sporadic E has been predicted on a statistical basis [Leftin et al., 1968; Leftin and Ostrow, 1969]. However, for radiocommunication purposes its effects are not well understood. A system of prediction has been developed which takes into account the probability of Es reflection at a given frequency, the frequency of the wave reflected with a given probability and the frequency at which the Es ionization acts as a screen against reflection from higher layers [Kerblai, 1960; Kerblai, 1964]. Analysis of oblique, sounding observations over several years indicates that sporadic E was useful for propagation on a transatlantic path (Ottawa-The Hague) less than 1 % of the time and on a trans-auroral path (Winnipeg-Resolute) only 7% of the time [Hagg and Rolfe, 1963]. Measurements made on a trans-equatorial path between Lindau and Tsumeb suggest that during the summer daytime the maximum observed frequency (MOF) was determined by Es [Mewes and Moller, 1974]. A prediction method which incorporates the tropical Es-layer has been described [Kift, I960]. 5. Propagation modes and other factors Methods of standard MUF (EJF) calculation which take modes of propagation into account can often give results which are more accurate than those which do not. In using such methods, ground scattering and reflection, and horizontal ionization gradients, must be carefully considered. The availability of high speed digital computers has stimulated the development of a wide variety of methods for ray tracing and has also made possible the incorporation of more realistic models for the ionosphere than the two or three-parabolic representations. In order to increase the accuracy of radio path parameters, predictions of the geometric parameters of N(h) profiles of both the quiet and disturbed ionosphere, as well as predictions of the virtual reflection heights of the ionosphere, and of the standard deviation of these heights, are made in the U.S.S.R. [C.C.I.R., 1966-1969a; C.C.I.R., 1970-1974a; C.C.I.R., 1970-1974b]. An analytic method of computing ray paths has been given by Chvojkova [1969]. Ray trajectories in an ionosphere having a horizontal ionization gradient, as observed by Bibl [1964] [Kerblai and Volochinova, 1964], have been computed [Kerbla'i and Kovalevskai'a, 1967; Kovalevskaia and Kornitzkaia, 1969]. It has been shown by ray tracing that, in most cases, inhomogeneities of appropriate scale produce a standard MUF (EJF) increase of several MHz and differences between the angles of departure and arrival of up to 7°; while the predicted increase in spatial attenuation caused by the inhomogeneities, especially in the high angle ray with increasing frequency, was found to be in good agreement with observed results. A prediction method is available in the U.S.S.R. which takes account qf the electron density gradient in an arbitrary direction; predictions may thus be made of the hop length, the angles of radiation and arrival, in both azimuth and elevation and of the MUF in a horizontally inhomogeneous ionosphere [Kovalevskaia and Kerblia, 1971]. In considering the operational MUF, it must be remembered that its relationship to the standard MUF (EJF) depends upon the type of service and the effective radiated power, as well as on the required minimum field strength. 6. Analysis of foF2 variations India reported an analysis of foF2 variations as a function of time and day and solar zenith angle. Examples are given for 12 stations [Chaman Lai, 1966]. R eferences B a r g h a u s e n , A.F. and F i n n e y , J.W. [1967] Computer ray tracing techniques to determine the effect of ionospheric tilts on M UFs. AWS Tech. Rept. 196. B e c k m a n n , B . [1967] Notes on the relationship between the receiving-end field strength and the limits of the transmission fre quency range MUF, LUF. NTZ-Comm . /., 6, 37-47. — 39 — Rep. 255-3 B e c k m a n n , B. [1970] FTZ-report A445-TBR8. B e c k m a n n , B . and O c h s, A. [1969] Vorhersage der ionospharischen Kurzwellenausbreitung (Prediction for the ionospheric short-wave propagation). Jahrbuch des elektrischen Femmeldewesens, 281-365. Bibl, K. [1964] The detection of an important anomaly in the F-region ionization above Central Europe. Ann. de geophys. 20, 447-453. * C.C.I.R. [1966-1969a] Doc. VI/78, U.S.S.R. C.C.I.R. [1970-1974a] Doc. 6/90, U.S.S.R. C.C.I.R. [1970-1974b] Doc. 6/91, U.S.S.R. C h a m a n L al [1966] The development of a formula for the seasonal and secular characteristics of F2-layer critical frequency (SfoF2). Telecommunication Journal, 33, 257-262. C h vo jk o v a , E. [1969] Formulae for ray path in ionized layers with application to oblique ionogram and duct modes. Radio Science, 4, 23-27. D u C h arm e, E.D., P etrie, L.E. and E y f r ig , R. [1973] A method for predicting the Fl-layer critical frequency based on the Zurich smoothed sunspot number. Radio Science, 8, (New Series), 837-839. H a g g , E.L. and R olfe, W. [1963] A study of transatlantic radio propagation modes at 41 *5 Mc/s. Can. Journ. Phys., 41, 220-233. K erblai, T.S. [1960] Relation between critical Es frequencies and characteristics of apparatus (in Russian). Ionospheric Studies, 5, 50-63. K erblai, T.S. [1964] Instruction on calculation of HF communications reflected from the Es-layer (in Russian). Science, Moscow. K erblai, T.S. and K ovalevskaia , E. [1964] Estimation of the skip distance in the horizontal inhomogeneous ionosphere (in Russian). Geomagnetism i Aeronomiia, 5, 962. K erblai, T.S. and K ovalevskaia, E. [1967] Calculation of MUF in the presence of a horizontally heterogeneous ionosphere (in Russian). Geomagnetism i Aeronomiia, 7, 123-127. K erblai, T.S. and V o l o c h in o v a , E. [1964] Estimation of the tilts of the isosurfaces of NF2 on the basis of Nh profiles (in Russian). Geomagnetism i Aeronomiia, 1, 61. K ift , F. [1960] The propagation of high frequency radio waves to long distances. Proc. I.E.E., 107B, 127-140. K in g , J.W. and S la t er , A.J. [1973] Errors in predicted values of foF2 and hmF2 compared with the observed day-to-day variability. I.T.U. Telecommunication Journal, 40, 766-770. K ovalevska'i'a , E. and K erbla'i, T.S. [1971] Calculation of the length of hop, MUF and angles of arrival of radio waves, taking account of the horizontal irregularity of the ionosphere (in Russian). “ Nauka ” Publishing House, Moscow. K ovalevskaia, E. and K o r n it z k a ia , A.A. [1969] Influence of the horizontal gradients of electron density distribution on the value of MUF, hop length and angles of arrival (in Russian). Geomagnetism i Aeronomiia, 9, 290-293. L ef t in , M., O st r o w , S.M. and P r esto n, C. [1968] Numerical maps of foEs for solar cycle maximum and minimum. ESSA Techn. Rep., ERL 73-ITS-63, Boulder, Colo. L eftin , M. and O st r o w , S.M. [1969] Numerical maps of fbEs for solar cycle minimum. ESSA Techn. Rep., ERL 124-ITS-87, Boulder, Colo. M ew es, E.R. and M oller, H.G. [1974] Der Einfluss der sporadischen E-Schicht auf eine transaquatoriale Kurzwellen-Obertragung (The influence of sporadic E on transequatorial HF propagation). Kleinheubacher Berichte, 17, 443-447. M oller, H.G . [1960] Ergebnisse der Impulsiibertragung mit veranderlicher Frequenz auf der Strecke Sodankyla -Lindau (Results of variable frequency pulse transmissions over the path Sodankyla-Lindau). Kleinheubacher Berichte, 7, 115-123. P ig g o t t , W.R. and R a w e r , K. [1972] U.R.S.I. Handbook of ionogram interpretation and reduction. World Data Center A for Solar-Terrestrial Physics, Report UAG-23, NOAA, Boulder, Colo. R o sic h , R.K. and Jo nes, W.B. [1973] The numerical representation of the critical frequency of the FI region of the ionosphere. Office of Telecommunications, Report 73-22, U.S. Dept, of Commerce. W illiam s, H.P. [1962] Increase in MUF due to horizontal gradients. Nature, 196, 256. B ibliography M esterm an, I., T r e n t in i, F. and F e r n a n d e z Sa r m ien t o , N. [1972] Planificacion de redes antarticas para telecomunicaciones en ondas decametricas (Planning of Antarctic HF Telecommunication nets). LIARA-C18. (Also available in English.) Rep. 256-2 — 40 — REPORT 256-2 * MAXIMUM TRANSMISSION FREQUENCIES (Recommendation 373-3) (1959 - 1963 - 1966 - 1974) 1. The operational MUF (or simply the MUF) is the highest frequency that permits acceptable operation between given points at a given time and under specified working conditions. When referring to the operational MUF, all pertinent details of the actual system should be given, including antennae, power output, class of emission, information rate and the required signal-to-noise ratio. The operational MUF refers to propagation in the actual ionosphere, in which irregularities in ionization, scattering, and partial reflection at steep gradients of ionization density, frequently occur. It commonly refers to propagation by whatever mechanism, layer or mode, gives the highest permissible frequency. 2. The classical MUF or the junction frequency (JF) is the highest frequency that can propagate by a particular mode between specified terminals by ionospheric refraction. The classical MUF is independent of power, and is calculable in principle by ray optics in the presence of the Earth’s magnetic field. It is also measurable by oblique-incidence pulse transmission, and refers to the frequency at which the high angle and low angle traces are seen to merge on the ionogram. However, no account is taken of the nose extension observed in some ionograms. In cases where scatter, interference, or most of all poor definition prevents a clear junction from being recognized, extrapolation of the low-and high-angle traces may be used with caution to approximate the junction frequency. Effects due to partial reflections and to scattering are not considered in the determination of the junction frequency. However, large-scale irregularities or “ layer tilts ” and the anisotropy caused by magneto-ionic double refraction, are included. This means that the ray path may deviate somewhat from a great circle. 3. The standard MUF or estimated junction frequency (EJF) is an analytical approximation 'to the classical MUF or junction frequency (JF). It is found by combining data obtained from vertical soundings with a simplified analytical solution of the oblique-incidence refraction problem. The basic ionospheric parameters generally used for the F2 layer are: — foF2 and M(3000)F2 for the conventional transmission curve method; — foF2, hc, and qc for the parabolic layer method (hc and qc were formerly known as hm and ym/2, respectively). 4. The maximum observed frequency (MOF) is the highest frequency that can be detected on an oblique- incidence ionogram. With the advent of “ radiocommunications sounding ” used in conjunction with operational systems, such a term is required. In general, MOF is defined on a per mode basis, that is, defined in conjunction with other parameters that identify the mode. When stated without qualifi cation, MOF should imply the highest of all propagation frequencies for a given path. The MOF is actually the operational MUF of the oblique-incidence sounder, but not, in general, that of any associated telecommunication system. Note. — The standard MUF for a path length of 3000 km is found directly by application of the standard 3000 km transmission curve. For other path lengths it is usually found by applying distance factors, a rapid but sometimes poor approximation. Better values can be obtained by using transmission curves for these path lengths or by using the parabolic model, provided the proper allowance is made for underlying ionization. Both of these methods neglect the effects of horizontal gradients and layer tilts, and use approximations for the effects of ionosphere curvature and the geomagnetic field. * Adopted unanimously. — 41 — Rep. 261-3 REPORT 261-3* BACK-SCATTERING (Study Programme 14A-1/6) (1963 - 1966 - 1970 - 1974) 1. Introduction A radio wave propagated via the ionosphere is partially scattered by the irregularities of the ground or sea, and even to some extent by those of the ionosphere itself. Scattering coefficients of the Earth’s surface may be quite variable depending upon the nature of the surface and the elevation angle [Steel, 1964]. Some of the scattered energy is returned over the same propagation path to the vicinity of the transmitter [Villard and Peterson, 1952a; Shearman, 1955; Silberstein, 1954; Peterson, 1957; Kono, 1950; Dieminger, 1951] and, by measuring the time delay of the echo, the slant range to the scattering source may be inferred. By employing a directional antenna rotating slowly around the vertical, the azimuth to the echoing region can be estimated. If information about ionospheric height and tilt is available, the measured slant range of the back- scatter echo may be converted to ground range [Kosikov, 1959; Bibl, 1960a]. Thus, with a single station, the condition of the ionosphere can be determined in all directions from the station, generally to distances of the order of several thousand kilometres [Peterson et al., 1959]. Relatively large pulse widths (1 ms) are typically employed to capitalize on the “ time focusing ” which occurs just beyond the edge of the skip zone [Peterson, 1951; Beckmann and Vogt, I960]. This method is often called simply “ back- scatter sounding ”. An alternative method of measurement is to vary the frequency at a given azimuth, rather than varying the azimuth. This has been called “ sweep-frequency back-scatter sounding ”, and gives more definite information about the propagation modes present at a given azimuthal direction. 2. Identification of sources of back-scatter To establish the bearing of a back-scatter echo, the combined azimuthal directivity of the transmitting and receiving antennae must be sufficient to discriminate against off-bearing echoes. However, because of ionospheric tilts, the possible errors in azimuthal information become progressively greater with the number of hops involved. To estimate the range of back-scatter sources from oscilloscope records it is necessary to postulate the active modes. Their identification is assisted by a knowledge of the combined vertical directivity of the antennae, but unless vertical-incidence data are available, caution is needed in drawing conclusions. In one experiment, however, when the ionospheric height data were not available, the ground range was inferred using an antenna with a vertical pattern split into two distinct lobes: by showing an overlapping of the echoes from these two lobes the phenomenon of focusing has been demonstrated [Ranzi, 1961]. Information concerning elevation angle versus back-scatter delay, such as can be obtained with a vertical angle scanning antenna, can be processed to provide ionospheric model parameters, including the longitudinal component of tilt in the direction of interest. Such information and measurement of the elevation angle of arrival of a specific back-scatter source allow a quite accurate determination of the range to the source. The focusing effect is characteristic of F-layer echoes near the skip' distance, and the probability of occurrence of this effect appears to be greatest in the evening in winter and at night in summer. The absence of focusing can be used to identify Es modes [Bibl, 1960b]. Elevation angle measurements of back- scatter by Es modes also identify them clearly and unambiguously [Tveten and Hunsucker, 1969]. Another method of identifying Es modes is the use of the azimuth-sweeping back-scatter sounder: motion of isolated Es patches can be easily traced and a general westward drift has been noted around 40° north latitude [Villard and Peterson, 1952b; Clark, 1956; Harwood, 1961; Egan and Peterson, 1962; Bibl, 1961; Shearman and Harwood, I960]. * Adopted unanimously. Rep. 261-3 42 - This type of sounder occasionally observes slight changes in range of the skip-zone echo, these changes in range being a function of the azimuth. These are consistent with a large-scale single wave of altered F-region ionization, the wave sweeping over the observing station at speeds of the order of 1000 km/hr [Tveten, 1961]. In addition, when sufficiently short pulses are used for back-scatter sounding, irregularities in signal amplitude which may change range with time are seen. These signal enhancements appear to be caused by moving irregularities in the ionosphere which cause signal focusing [Tveten, 1961; Shearman, 1963] or by certain special conditions [Dueno, 1963]. A lack of correlation was often observed between the transmission loss in a given radiocommuni cation link and the total loss in the back-scatter soundings performed along the same path as the radio link. This is due to the fact that the intensity of the back-scatter echoes strongly depends upon the presence of focusing F2-region irregularities, which have no detectable influence on the transmission loss in a radiocommunication link. It is also observed that this focusing effect is stronger when the back- scatter sounding in temperate latitudes is performed in the direction of the Earth’s magnetic field [C.C.I.R., 1963-1966]. The amplitude of the back-scatter signals shows a negative correlation with the occurrence of spread-F [C.C.I.R., 1966-1969]. 3. Practical applications Back-scatter tests have been carried out at fixed frequencies to determine the usefulness of this technique in the operation of the HF broadcasting service. Service areas deduced from these tests were found to be in better agreement with listener reports than predictions based on vertical-incidence soundings. The echo patterns showed clearly the difference in coverage over the service area on changing the transmitting antenna. Some similar tests have shown that it is possible to estimate, with an accuracy of a few minutes, the fade-out time of signals on a telegraph circuit of average length [Beckmann and Vogt, 1960; Shearman, 1961; Beckmann and Vogt, 1955]. Back-scattering technique is used for deter mination of MUF and field strength within the service area [Tchernov, 1971], Back-scatter observations have shown that the skip zone of an Indian broadcast transmission was smaller than had been anticipated from consideration of the F layer alone and this was found to be due to Es propagation [Mitra and Iyengar, 1954]. Back-scatter techniques have been used to observe the effects of solar flares [Barry and Widess, 1962]. 4. Unusual phenomena Back-scatter experiments have shown long-range echoes out to 10 000 km which are stronger than multi-hop echoes present at the same time at shorter range. They sometimes persist after the multi-hop echoes have faded out. Such long-range echoes, without intermediate ground reflections, are observed primarily during dawn, twilight, or trans-equatorial propagation conditions (see Report 250-3). They are generally attributed to layer tilts and have been the subject of considerable study [Stein, 1958; Southworth, I960]. Back-scatter observations have been reported suggesting the existence of an interesting mode of propagation in which an HF signal at a frequency, always somewhat above the penetration frequency of the ionosphere in the direction of the magnetic field, is apparently guided along the field lines [Gallet and Utlaut, 1961]. Other work at 40-3 MHz has shown echoes with delay up to 50 times greater than those for E reflections [Maynard, 1961]; they involve off-path auroral echoes as well as one- and two-hop F2 ground- scatter reflections from as far as 6000 km. During winter daytime, F2 reflections were nearly always involved. Observations have been reported of anomalous back-scatter returns observed at the time a satellite passes through an area accessible to the sounder or through the region in the opposite hemisphere magnetically conjugate [Tiuri and Kraus, 1963]. 5. Direct back-scatter from ionospheric irregularities 5.1 Field-aligned ionization Irregularities and gradients have been observed experimentally in the ionosphere, which are capable of giving direct back-scatter echoes at frequencies well above the frequencies normally reflected from — 43 — Rep. 261-3 the ionosphere by over-dense ionization. The process is an inefficient one. Most of the energy passes through the scattering region, but a substantial echo is nevertheless observable, because of the large volume of scatterers observed instantaneously by the radar. Those echoes have the unique property that they are strongest when observed from an orientation at right angles to the direction of the Earth’s magnetic field. An empirical description of the scattering process has been given by Booker [1956]. Possibilities of communication by those modes have only partially been explored. 5.2 Auroral ionization Near the auroral zone, the aurora has sufficiently intense scattering properties to scatter HF, VHF, and even UHF radar waves. A survey of work prior to 1955 is contained in [Little et al, 1956; Presnell et .al., 1959; Fricker et al., 1957; Bates, 1961]. These echoes are closely related to the visible aurora, and have a diurnal variation which is maximum near midnight. The seasonal variation has a maximum at the equinoxes. In addition to a dependence upon the geometry of the magnetic field, the auroral echoes appear to be confined to heights of the order of 100 to 125 km, in contrast to the much greater heights, at which visible light from the auroral rays can be detected. When the E region is sunlit, another type of auroral echo appears [Presnell et al., 1959]. These echoes appear to come from height regions about 10 km in thickness [Unwin, 1958] but of great lateral extent. Systematic studies of auroral back-scattering have now been carried out on scaled systems operating in one experiment at four frequencies between 42 and 104 MHz inclusive [Lyon and Forsyth, 1962; Lyon, 1962], and in another at two frequencies near 500 and 1000 MHz [Blevis et al., 1963]. These studies indicate that critical reflection may occur at frequencies as high as 100 MHz during auroral disturbances supporting the use of a theoretical model which includes both weak scattering and critical reflection [Moorcroft, 1961], to explain echoes at these frequencies. At higher frequencies, the evidence suggests that auroral back-scatter echoes arise only by weak-scattering from non-isotropic, field-aligned irregularities, although present theories appear somewhat inadequate in explaining all of the results. Multiple scattering may also be significant. Direct back-scatter from irregularities in the F-region at heights of 250-400 km apparently unrelated to visual aurora have been obtained [Bates, 1959], with winter daytime echoes appearing more “ patchy ” than those seen at night [Bates, I960]. Even at latitudes far from the auroral zone, HF ground back-scatter equipment has observed distant echoes from auroral ionization, with the aid of ionospheric bending and possibly an additional reflection from the ground [Peterson and Leadabrand, 1954]. 5.3 Field-aligned ionization at middle latitudes HF radars, of the type used for ground back-scatter sounding, regularly observe field-aligned echoes that resemble auroral echoes in many ways but are not so intense and do not correlate with geomagnetic activity. An E-region type, associated with sporadic E, is distinctly separate from an F-region type associated with spread F [Peterson et al., 1955]. A 200 MHz observation, giving evidence for E-region field-aligned ionization, but possibly being related to meteoric ionization, has been observed over a middle-latitude oblique path [Heritage et al., 1959]. 5.4 Equatorial field-aligned ionization In the special case of the magnetic-dip equator, where the magnetic field is parallel to the Earth’s surface and to the ionosphere stratifications, F-region ionization, associated with spread-F conditions, as registered by a vertical-incidence sounder, actually consists of field-aligned horizontal irregularities which principally give echoes in the east-west plane through the vertical containing the station [Cohen and Bowles, 1961] and exhibit characteristic drift patterns [Clemesha, 1964]. Rep. 261-3 — 44 — 5.5 Artificial satellites Spread-F echoes have been observed by means of the “ Alouette ” topside-sounder satellite. Spread echoes occur largely at high latitudes and appear to arise from back-scatter from local irregularities in the turbulent ionosphere and also at times from clouds of ionization within the auroral zone [Petrie, 1963]. Equatorial spread F has also been observed [Muldrew, 1963; Lockwood and Petrie, 1963]. This spreading has been explained on the basis of north-south propagation along sheets of ionization aligned with the magnetic field, in which trapping by the field-aligned sheets occurs at levels near the normal height of F-layer reflection. R eferences B a r r y , G.H. and W id ess, P.R. [1962] The effect of a solar flare on the frequency of HF ground back-scatter. J. Geophys. Res., 67, 2707-2714. B ates, H.F. [1959] The height of irregularities in the arctic atmosphere. J. Geophys. Res., 64, 1257-1265. B ates, H.F. [1960] Direct HF back-scatter from the F-region. J. Geophys. Res., 65, 1993-2002. B ates, H.F. [1961] The slant-Es echo—A high-frequency auroral echo. J. Geophys. Res., 66, 447-454. B e c k m a n n , B. and V o g t , K. [1955] liber Beobachtungen der Riickstreuung (back-scatter) im Kurzwellengebiet an kommerziellen Telegraphie-Signalen (Observations on back-scattering of commercial telegraph signals in the HFband). FTZ, 8, 473-481. B e c k m a n n , B. and V o g t , K. [1960] Back-scatter Beobachtungen an Telegraphie-Signalen (Observations of back-scattering on telegraph signals). Electromagnetic Wave Propagation, 157-166. Ed. M. Desirant and J.L. Michiels, Academic Press. B ib l , K. [1960a] Aktive Hochfrequenzspektormeter fur die ionospharische Echolotung I: Direktregistrierung ionospharischer Charakteristiken (The active high frequency spectrometers for ionospheric sounding I. Direct recording of ionospheric characteristics). Arch, elektr. Ubertr., 14, 341-347. B ib l , K. [1960b] Experimental proof of focusing at the skip distance by back-scatter records. Proc. IRE, 48, 956-957. B ib l , K. [1961] Neuartige Back-scatter-Registrierungen und Ergebnisse (A new method of recording back-scatter and presenting the results). From the reports of the meeting on ionospheric physics and wave propagation. FTZ, Darmstadt, Kleinheu bacher Berichte, 8, 111-114. B l evis, B.C., D a y , J.W.B. and R oscoe, O.S. [1963] The occurrence and characteristics of radar auroral echoes at 488 and 944 Mc/s. Can. J. Phys., 41, 1359-1380. B ooker, H.G. [1956] Turbulence in the ionosphere with application to meteor-trails, radio-star scintillation, auroral radar echoes and other phenomena. J. Geophys. Res., 61, 673-705. C.C.I.R. [1963-1966] Doc. VI/56, Italy. C.C.I.R. [1966-1969] Doc. VI/62, Federal Republic of Germany. C l a r k , C. [1956] Motion of sporadic-E patches determined from high-frequency back-scatter records. Nature, 178, 48 6 -4 8 7 . C lem esha, B.R. [1964] An investigation of the irregularities in the F-region associated with equatorial type spread-F. J. Atmos. Terr. Phys., 26, 91-112. C o h e n , R. and B o w les, K.L. [1961] On the nature of equatorial spread-F. J. Geophys. Res., 66, 1081-1106. D ie m in g e r , W. [1951] The scattering of radio waves. Proc. Phys. Soc., 64-B, 142-158. D u e n o , B. [1963] Interpretation of some sweep-frequency back-scatter echoes. J. Geophys. Res., 68, 3603-3610. E g a n , R.D. and P eterso n, A.M. [1962] Back-scatter observations of sporadic-E. Ionospheric Sporadic E, 89-109. Ed. Smith, E.K. and Matsushita, S., Pergamon Press. F r ic k e r , S.J., I n g a l l s, R.P., S t o n e, M.L. and W a n g , S.C. [1957] U H F auroral observations. J. Geophys. Res., 62, 527-546. G allet, R.M. and U t l a u t , W.F. [1961] Evidence on the laminar nature of the exosphere obtained by means of guided high- frequency wave propagation. Phys. Rev. Letters, 6, 591-594. H a r w o o d , J. [1961] Some observations of the occurrence and movement of sporadic-E ionization. J. Atmos. Terr. Phys., 20, 243-262. H eritage, J.L., W eisbr o d, S. and F a y , W.J. [1959] Evidence for 200 Mc/s ionospheric forward-scatter mode associated with the earth’s magnetic field. J. Geophys. Res., 64, 1235-1241. K o n o , T. [1950] Experimental study on scattered echoes (IT). Reports of Ionospheric Research in Japan, 4, 189-199. K o sik o v, K.M. [1959] Oblique return sounding and problems of radio communication and broadcasting over long distances (in Russian). Elektrosviaz, 7, 699. L ittle, C.G., R a y t o n , W.M. and R oof, H.B. [1956] Review of ionospheric effects at VHF and UHF. Proc. IRE, 44, 992-1018. L o c k w o o d , G.E.K. and P etrie, L.E. [1963] Low latitude field strength aligned ionization observed by the Alouette topside sounder. Planet, and Space Science, 11, 327. — 45 — Rep. 261-3 L y o n , G.F. [1962] The effect of absorption and multiple scattering on radio-auroral observations. Can. J. Phys., 40, 739-748. L y o n , G.F. and F o r sy th , P.A. [1962] Radio-auroral reflection mechanisms. Can. J. Phys., 40, 749-760. M a y n a r d , L.A. [1961] Propagation of meteor-burst signals during the polar disturbance of November 12-16, 1960. Can. J. Phys., 39, 628. M it r a , S.N. and I y e n g a r , V.C. [1954] Ind. Journ. Phys., 28, 147. M oo rcro ft, D .R . [1961] M odels of auroral ionization. Can. J. Phys., 39, 677-715. M u l d r e w , D.B. [1963] Radio propagation along magnetic field-aligned sheets of ionization observed by the Alouette top-side sounder. J. Geophys. Res., 68, 5355-5370. P eterson, A.M. [1951] The mechanism of F-layer propagated back-scatter echoes. J. Geophys. Res., 56, 221. P eterson, A.M. [1957] Ionospheric back-scatter. Annals of the IGY III, Part IV, 361-381, Pergamon Press. P eterson, A.M. and L e a d a b r a n d , R.L. [1954] Long-range radio-echoes from auroral ionization. J. Geophys. Res., 59, 306-309. P eterson, A.M ., V il l a r d , O.G., L e a d a b r a n d , R .L . and G a ll a g h e r , P .B . [1955] Regularly-observable aspect-sensitive radio reflection from ionization aligned with the earth’s magnetic field and located within the ionospheric layers at middle latitude. J. Geophys. Res., 60, 497-512. P eterson, A.M ., E g a n , R.D. and P r a t t , D.S. [1959] The IGY three-frequency back-scatter sounder. Proc. IRE, 47, 300-314. P etrie, L .E . [1963] Top-side spread echoes. Can. J. Phys., 41, 194-195. P r essnell, R .I ., L e a d a b r a n d , R .L ., P eterso n, A.M ., D y c e , R .B ., S chlo bo hm , J.C. and B e r g , M.R. [1959] VHF and UHF radar observations of the aurora at College, Alaska. J. Geophys. Res., 64, 1179-1190. R a n z i, I. [1961] Back-scatter echo pattern and vertical radiation diagram. Centro radioelectrico sperimentale G. Marconi, Rome, N ote 8. Sh e a r m a n , E.D.R. [1955] A study of ionospheric propagation by means of ground back-scatter. Proc. I.E.E., Paper 1914R. S h e a r m a n , E.D.R. [1961] An investigation of the usefulness of back-scatter sounding in the operation of high frequency broad casting services. Proc. I.E.E., 108B, 361. S h e a r m a n , E.D.R. [1963] The amplitude and spectrum of ground back-scatter echoes. Proceedings of the International Conference on the Ionosphere 1962, 293-300. Ed. A.C. Stickland, Bartholomew Press. S h e a r m a n , E.D.R. and H a r w o o d , J. [1960] Sporadic-E observed by back-scatter techniques in the United Kingdom. J. Atmos. Terr. Phys., 18, 29. S il berstein, R. [1954] Sweep-frequency back-scatter—Some observations and deductions. Trans. IRE, AP-2, 56. S o u t h w o r t h , M.P. [1960] Night-time equatorial propagation at 50 MHz: First results from an IGY amateur observing programme. J. Geophys. Res., 65, 601-607. S teel, J.G. [1964] The effect of the ground back-scatter coefficient on observations of sporadic-E over sea, land and mountains. J. Atmos. Terr. Phys., 26, 323-324. S t e in , S. [1958] The role of ionospheric layer-tilts in long-range high-frequency radio propagation. J. Geophys. Res., 63, 217-241. T c h e r n o v , Y.A. [1971] Back-scatter sounding of the ionosphere (in Russian). Sviaz, Moscow. T iu r i, M.E. and K r a u s , J.D. [1963] Ionospheric disturbances associated with Echo I as studied with 19 MHz radar. J. Geophys. Res., 68, 5371-5386. T vet en , L.H. [1961] Ionospheric motions observed with high-frequency back-scatter sounders. NBS Journ. o f Research, 65-D, 115-127. T v eten, L.H. and H u n su c k e r , R.D. [1969] Remote sensing of the terrestrial environment with an HF radio high-resolution azimuth and elevation scan system. Proc. IEEE, 57, 487-493. U n w i n , R.S. [1958] The geometry of auroral ionization. J. Geophys. Res., 63, 501-506. V il l a r d , O.G. and P eterson, A.M. [1952a] Scatter-sounding: a technique for study of the ionosphere at a distance. Trans. IRE, PGAP-3, 186-201. V il l a r d , O.G. and P eterso n, A.M. [1952b] A method for studying sporadic-E clouds at a distance. Proc. IRE, 40, 992-994. Rep. 429-1 — 46 — REPORT 429-1 * GROUND AND IONOSPHERIC SIDE-SCATTER (Question 3-2/6) (1970 - 1974) HF and VHF radio waves propagated via the ionosphere have been observed to arrive from azimuths which differ considerably from the great circle direction. Many of the off-path signals exhibit the characteristics of a scatter signal with a typical fading period less than one second. The angle of arrival of these scatter signals is quite variable and the signal strength gradually decreases to a level 25 to 40 dB less than the great circle mode of propagation as the propagation path deviatesfrom the great circle path [Goddard, 1939; Feldman, 1939; Dieminger, 1951; Hagg and Rolfe, 1963; Edward and Jansky, 1941; Silberstein and Dickson, 1965]. In 1961 and 1962, measurements of azimuthal angles of arrival of transmissions from WWV, WSL, MSF and JJY were made at Moscow by means of a direction finder. As a result of measurements of WWV and WSL transmissions in autumn and winter from 1800 to 0500 UT, it has been established that the deviations from the great circle: — in 14% of the cases vary between ±5°; — in 33% of the cases vary between + 6° and + 25°; — in 53% of the cases vary between — 6° and — 100°. The side-scatter mode of propagation has also been studied using CW field strength data together with bearing and oblique incidence sounding data. The side-scatter mode of propagation is evident on the oblique ionograms but exact identification is only possible if the ionogram data are used in conjunction with CW field strength data. Non great-circle signals are predominantly the result of multi-hop modes of propagation in which side-scatter occurs from irregularities at the surface of the Earth. The lowest path loss occurs for the bearing of an intersection of the skip distances around the transmitter and receiver because of skip distance focusing. The directivity of the transmitting antenna influences the bearing of the maximum received signal strength [Miya and Ueno, I960]. The ground side-scatter mode of propagation accounts for the southerly deviation of B.B.C. television signals received in North America [Hagg and Rolfe, 1963], the southerly deviation of HF signals received in both directions between England and Japan [Wilkins, 1960; Kanaya and Yokoi, 1960a], the large deviations of radio signals from European and American stations observed in Japan [Miya and Kanaya, 1957; Miya, Ishikawa and Kanaya, 1957; Kanaya and Yokoi, 1960b and 1960c] and the wide range of azimuths over which the signals are received on short paths [Dieminger, 1951]. Several workers have suggested that non great-circle signals are produced by scatter from land areas; the signal strength of sea scatter is believed to be negligible in comparison with the signal strength of land scatter [Miya and Kanaya, 1955; Miya, Kobayashi and Wakai, 1951]. However, observations in North America at 41 MHz and 20 MHz indicate little difference in signal strength between scatter signals arriving from the land and from the sea [Hagg and Rolfe, 1963]. Ionospheric side-scatter from irregularities located in the auroral region have been reported by several workers. In some cases the non great-circle signals were attributed to direct scatter from ionospheric irregularities without an ordinary type of ionospheric reflection [Bates et al., 1966]. In other cases, they were attributed to multi-hop modes of propagation in which the strongest scatter signals were produced by irregularities in the ionospheric region where the radio waves are focused [Hagg and Rolfe, 1963]. On transmissions between Europe and southern Africa, azimuthal deviations of up to zk 50° were observed. By additional time delay measurements it was shown that these great-circle deviations are caused by F-layer inhomogeneities near the earth’s magnetic equator [Rottger, 1973a; Crochet, 1972] and in the auroral region [Rottger, 1973b]. Crochet assumes scatter from weak field aligned irregularities while Rottger assumes that the strongest non great-circle signals result from specular reflections at horizontal gradients of electron density. * Adopted unanimously. — 47 — Rep. 429-1 The ground and ionospheric side-scatter modes of propagation can lead to an extension of time beyond that which a given frequency can be used on the great-circle path and also leads to the use of frequencies higher than the standard MUF (EJF). Communication circuits may, however, be degraded by the interference caused by strong side-scatter signals. The direction from which the ground side-scatter signals are received can be predicted from skip distance data deduced from monthly predictions’ of standard MUF (EJF) [Hagg and Rolfe, 1963]. The non great-circle path MUF and also the direction from which ground side-scatter signals are received can be predicted by the two control-circles method using ionospheric prediction maps [Miya and Kanaya, 1955]. The ground scatter mode could maintain communication during disturbances; this mode often avoids propagating through regions of abnormally high absorption. Transmission experiments in the United States of America show that in the case of side-scatter both the average azimuth spread of direction of arrival and the fading spectrum width are greatly enhanced compared to great-circle propagation [Blair, 1971]. R eferences B ates, H .F ., A lbee, P.R. and H u n su c k e r , R.D. [1966] On the relationship of the aurora to non great-circle high frequency propagation. J. Geophys. Res., 71, 1413. B l a ir , J.C. [1971] The effectiveness of spectrum and azimuth spread measurements in detecting side scatter. Proceedings of the 4th Allerton Conference on DF, University of Illinois. C r o c h et, M. [1972] Propagation transequatoriale en dehors du grand cercle en ondes decametriques. Part I: Mise en evidence et morphologie. Ann. de geophys. 28 (1), 27-35. Part II: Application a l’etude par diffusion des irregularites. Ann. de geophys. 28(2), 381. D ie m in ger, W. [1951] The scattering of radio waves. Proc. Phys. Soc., 64-B, 142-158. E d w a r d , C.F. and Ja n s k y , K.G. [1941] Measurement of the delay and direction of arrival of echoes from nearby short wave transmitters. Proc. IRE, 29, 322. F e l d m a n , C.B. [1939] Deviations of short radio waves from the London-New York great circle path. Proc. IRE, 27, 625. G o d d a r d , D.R. [1939] Transatlantic reception of London television signals. Proc. IRE, 27, 692. H a g g , E.L. and R olfe, W. [1963] A study of transatlantic radio propagation modes at 41-5 Mc/s. Can. J. Phys., 41, 220. K a n a y a , S. and Y o k o i, H. [1960a] Simultaneous measurements of azimuthal arrival angles of signals in cooperation with United Kingdom—Determination of scattering area. KDD Technical Journal, 25, 38-43. K a n a y a , S. and Y o k o i, H. [1960b] The lateral deviation of radio waves propagated along the longer great circle path from Europe. Rep. Ion. and Space Res. in Japan, 14, 192-195. K a n a y a , S. and Y o k o i, H. [1960 c] The bearings of WWY signals, Washington, D.C., observed at Tokyo. Rep. Ion. and Space Res. in Japan, 14, 435-440. M iy a , K . and K a n a y a , S. [1955] Radio propagation prediction considering scattering wave on the Earth’s surface. Rep. Ion. Res. Japan, 9, 1. M iy a , K., K o b a y a sh i, T. and W a k a i, N. [1951] Field intensity of scattered wave in radio wave propagation. Rep. Ion. Res. Japan, 5, 55. M iy a , K . and K a n a y a , S. [1957] On the lateral deviation of radio waves coming from Europe. Rep. Ion. Res. Japan, 10,1-8. M iy a , K., I sh ik a w a , M. and K a n a y a S. [1957] On the bearing of ionospheric radio waves. Rep. Ion. Res. Japan, 11, 130-144. M iya, K. and U e n o , K. [1960] Influence of directivity of the transmitting antenna on the bearing of HF signals. Rep. Ion. and Space Res. Japan, 14, 187. R o t t g e r , J. [1973a] Wave-like structures of large-scale equatorial spread-F irregularities. J. Atmos. Terr. Phys. 35, 1195-1206. R ottg er, J. [1973b] Seitenscatter durch polares spread-F beobachtet auf der Strecke Lindau-Tsumeb (Side-scatter due to polar spread-F observed on the path Lindau-Tsumeb) Kleinheubacher Berichte, 16, 155-157. S ilberstein, R. and D ic k so n , F.H. [1965] Great circle and deviated path observations on CW signals using a simple technique. IEEE Trans. Ant. Prop., AP-13, 52. W il k in s , A.F. [1960] HF propagation—its present and future use for communications purposes. J. Brit. Inst. Radio Engrs. 20, 939. PAGE INTENTIONALLY LEFT BLANK PAGE LAISSEE EN BLANC INTENTIONNELLEMENT — 49 — Rep. 252-2, 253-2 SECTION 6C: FIELD STRENGTHS BETWEEN ABOUT 1-5 AND 40 MHz RECOMMENDATIONS AND REPORTS Recommendations There are no Recommendations in this section. Reports REPORT 252-2 * C.C.I.R. INTERIM METHOD FOR ESTIMATING SKY-WAYE FIELD STRENGTH AND TRANSMISSION LOSS AT FREQUENCIES BETWEEN THE APPROXIMATE LIMITS OF 2 AND 30 MHz (Study Programme 11 A-2/6) (1956 - 1959 - 1963 - 1966 - 1970) This Report, which was adopted unanimously, has been published separately. Work is continuing on the development of this method and revisions are being prepared by Interim Working Party 6/1 for the next Interim Meeting of C.C.I.R. Study Group 6 (see Report 572). REPORT 253-2 * METHODS OF SYSTEMATIC MEASUREMENT OF SKY-WAYE FIELD STRENGTH AND TRANSMISSION LOSS AT FREQUENCIES ABOVE 1-5 MHz (Study Programme 11 A-2/6) (1963 - 1966 - 1974) 1. Introduction A large number of field-strength measurements in the reference frequency band is required to provide the Interim Working Party 6/1 with experimental data to compare with the values of field strength derived from the various methods of prediction under consideration. Administrations or organizations which already have experimental data that are usable by the Interim Working Party are invited to forward their data to the Working Party through the Director, C.C.I.R., as soon as possible. * Adopted unanimously. Rep. 253-2 — 50 — 2. General considerations In order to obtain the desired experimental data it is necessary that: — suitable transmissions from stations in suitable locations and suitable receiving station locations be selected, and — data be recorded and compiled in a form suitable for comparison with calculated values of field strength, and preferably in a common format. The transmitter selected should be unambiguously identifiable and should be in use as continuously as possible, preferably 24 hours per day. The antenna characteristics of these stations, and also the frequency and power, must be known. Standard frequency transmissions have been used in the past as satisfying these requirements, but there now is serious mutual interference, due to frequency sharing among many of these transmissions. Nevertheless, this difficulty can be avoided by using a narrow-band receiver, e.g., having a bandwidth less than about 100 Hz, which is capable of discriminating the difference in the audio modulation frequency of each co-channel transmitter [Wakai et al., 1971]. The selection of transmission paths for these experiments should include path lengths both greater than 4000 km and shorter than 4000 km, with frequencies chosen accordingly. 3. Provision of transmissions Decision 6 envisages the cooperation of Administrations and organizations in the provision of transmissions. Such cooperation would obviously be subject to the limitations imposed by the normal working schedules of the transmitters in question, although the possibility of using such transmitters at special times on standard omnidirectional antennae could be explored. Organizations have been asked, in C.C.I.R. Circular AC/57, to give full details of those transmitters which they consider would be suitable, especially as regards their knowledge of the radiation polar- diagram in both the horizontal and vertical directions. As a guide, it is desirable, in long-distance propa gation, to have a vertical polar diagram approximating to that of a standard short vertical radiator for angles of elevation less than 20°. It is also necessary to rely upon the organization to choose transmitters, the antenna characteristics of which are known with sufficient accuracy, either by actual measurement or from design data and a knowledge of the suitability of the site. In addition, they are invited to consider the possibilities of making field-strength measurements of the distant transmitters used for their point-to-point telephone or telegraph service. Some of the advantages of this are: — no special transmissions are required, — reception is mainly free of interference, provided the bandwidth of the measuring set is not greater than that of the receiver normally in use for the reception of traffic, — full details of the transmissions are recorded in the logs of the commercial receiving station. 4. Receiving stations and recording techniques The need should be stressed for a coordinated programme of measurements and the presentation of the results in a uniform manner so that they may be comparable. It may be desirable that at certain key stations, prepared to make special measurements, e.g. of angles of arrival, there should be uniformity with regard to recording speeds, scales, etc. However, many organizations are already equipped to make field-strength measurements, using their own choice of time-constants, recording meters, calibration techniques, etc., and they would not necessarily be prepared to replace their existing facilities by a standard equipment. Since it will be their responsibility to analyse their own records, the use of standard equipment will not be of paramount importance. It is suggested, however, that the standard receiving antenna should be a small loop or a short vertical rod, for which the vertical polar diagram does not differ materially from that of an ideal Hertzian dipole over the range of angles involved in long-distance transmission. — 51 — Rep. 253-2 In any measuring equipment, the quantity measured is a voltage at some specified point, usually the input terminals of the receiver. The problem is then to relate this voltage to the value of the field strength existing at the antenna. Even with the standard antenna, the conversion of field strength refers only to the vertical component of the electric field and some assumption must be made with regard to the randomness of the polarization of the incident wave. Some organizations are equipped to make measurements using special antenna systems, such as rhombic arrays, designed for specific circuits to improve signal-to-noise ratios and to enable measurements to be made under conditions where a simple antenna would be unusable. It is difficult to interpret the results obtained on an extended antenna system, in the presence of a complex field built up of several waves incident at different angles but measurements made with such antennae may be acceptable for the purpose in hand, if they can be related consistently to those that would be obtained at the same time on a standard antenna. These relationships would have to be examined by the organizations concerned and applied by them in the reduction of their records to the standard form of hourly median values of field strength. Experimental data [Khmelnitsky, 1969] suggest that different methods of measurement may produce substantially different results when measuring the field strength of the same transmitter. For this reason it is highly desirable, when working out a method for measuring field strengths, that simultaneous measurements should be made with measuring sets using different calibrated antennae. The polar diagrams of the transmitting and receiving antennae must be known. In general, each measuring station is requested, in Circular AC/57, to prepare from the analysis of its own records the hourly median values and then to tabulate them with the monthly median values for each hour. A suggested form for presentation of the results is given in the Table in Annex I. The actual method of measurement in each case should be stated. It is realized that continuous recording may be impracticable and that measurements may only be made at certain times of day and on certain days of the week, but it is hoped that even so some useful information on monthly median values may be obtained for at least some hours of the day. Although the field-strength predictions are meant to refer to average propagation conditions, readings which correspond to obviously abnormal conditions, e.g. auroral effects or during times of sudden ionospheric disturbances and severe magnetic storms, should be included in the estimate of the monthly median values. Such readings in the table of hourly medians should be indicated by some suitable notation. Annex I gives details of a typical method of obtaining the required information from point-to-point telegraph and telephone transmissions. 5. Measurements of ionospheric absorption Provided that distance and operating frequency are suitably chosen, significant values of the ionospheric absorption can be obtained by means of relative field-strength measurements. Widespread use of this rather simple method could result in a considerable increase of basic data, needed for the evaluation of a more accurate method for the estimation of sky-wave field strength and transmission loss. Annex II intends to give some guidance for planning and executing such measurements. ANNEX I A METHOD FOR MAKING FIELD-STRENGTH MEASUREMENTS OF TRANSMISSIONS USED ON POINT-TO-POINT TELEGRAPH AND TELEPHONE SERVICES 1. As an example relating to point-to-point telegraph and telephone services, a brief description follows of the method in use by one organization. Rep. 253-2 — 52 — 2. Measurement of field strength Field-strength measurements are made at a measuring station using a communications receiver adapted for the purpose. A self-supporting vertical rod antenna, 4 m long, feeds a wideband amplifier, which in turn is connected to the communications receiver by a buried 75 O coaxial cable. The equipment is calibrated against a field-strength measuring set, when both sets are receiving signals from a portable transmitter several hundred metres away. The receiver is operated with charge and discharge time constants of 20 s. A pen recorder is used with a chart speed of one inch per hour. Calibration marks are shown on the charts at intervals, thus allowing the field strength to be read directly in dB (fxV/m). The measurements are made on multi-channel independent-sideband transmissions with suppressed carriers and the groups of individual channels may not be symmetrical about the carrier frequency. The receiver bandwidth is adjusted, so that it includes all the sidebands transmitted and the actual bandwidth used depends on the type of multi-channel signal being transmitted, e.g., a receiver bandwidth of 1*2 kHz might be used for a transmission consisting of two telegraph channels and a bandwidth of 8 kHz for a transmission consisting of two 3 kHz telephone channels, together with two telegraph channels. 3. Analysis of field-strength records In analysing the field-strength records, the hourly values are first extracted from the recorder charts and tabulated. From the tabulated values, the median, upper decile and lower decile values can be derived for each hour. It has been found that at this stage of the analysis, it is imperative to check the recorder chart against the receiving station logs to ensure that the recorded field strengths do, in fact, refer to the correct transmission and not to noise or interference. It may happen that the level of interference is not high enough to affect reception at the receiving station and, in such a case, there will probably be no reference to interference in the station logs. The interfering signal will only be detected by the presence of a residual signal after the wanted transmission has ceased. It has been found from the records obtained so far that this condition does not occur very often. When it does occur, the readings for that day for the particular frequency concerned must be discarded. To compare the recorded field strengths with the values obtained by calculation, using any of the existing methods, the recorded values are reduced to the value Fs, corresponding to the incident sky-wave field strength for a power of 1 kW radiated from a short vertical antenna. The value of Fs is obtained from the formula: FS = F „ - (P, — L) — Gt — H (1) where: Fs — sky-wave field strength, Fm = measured field strength, which is a resultant of the sky-wave dB (1 [xV/m) and ground-reflected waves, Pt = transmitter power dB (1 kW), Gt — transmitter antenna gain relative to the gain of a short vertical antenna * on the surface of a perfectly conducting earth, L = loss due to transmitter mismatch, antenna mismatch and feeder loss; a typical value for this is 3 dB, F[ = height-gain factor. A typical height-gain factor of — 1 dB allows for the fact that the measured value is the resultant of the direct and ground-reflected rays, whereas the calculated value normally refers to the incident sky wave. This factor assumes wave-arrival angles of 10° to 15° relative to the ground and reflection from ground of good conductivity (s = 10, a = 3 X 10~2 S/m), for a vertically polarized wave. The height-gain factor will vary with frequency and wave-arrival angle, but the variations can be neglected over the range of frequencies and angles likely to be encountered in the course of these measurements. * If an isotropic antenna is used as the reference, Gt would be increased by 4-8 dB. T able I MEASUREMENTS OF HF FIELD STRENGTH * — MEASURING STATION ...... CIRCUIT: Transmitter Receiver D istance...... (km) Power ...... (kW) Feeder and mismatch losses...... (dB) Antenna gain — ...... (dB rel. short vertical antenna) TRANSMITTER DETAILS: Frequency...... (MHz) Year Zone time: UT ± ...... h Month * The figures in this Table are the hourly median values of measured field strength. The field strength of the incident sky-wave for radiated power of 1 kW is obtained from equation (1) in § 3 o f Annex I. Rep. 253-2 — 54 — The height-gain factor would differ somewhat from the — 1 dB given above, if the ground conductivity were different. 4. Presentation of results A possible form of presentation is given in Table I, which contains the minimum data needed. Further useful information would include the count number and the notation used to describe the conditions attached to individual readings. ANNEX II MEASUREMENT OF THE IONOSPHERIC ABSORPTION BY MEANS OF FIELD-STRENGTH RECORDINGS 1. Introduction Systematic comparisons and several years of experience have shown that relative field-strength measurements are suitable for determining the ionospheric absorption around noon, provided that the parameters of the radio circuit meet certain requirements [Schwentek, 1965]. Rules for establishing such a circuit have been given in one of the I.Q.S.Y. Manuals [No. 4] but the calibration method described there is rather complicated. This Annex proposes a somewhat simplified method. In the terminology of the U.R.S.I. this method is called the A3 method, in contrast to the Al method (measurement of absorption using vertical incidence pulse transmissions) and the A2 method (measurement of absorption of extra-, terrestrial noise). These names have no relation whatsoever to the terms used for characterizing the class of emission. 2. Choice of frequency and distance Frequency and distance must be selected in such a way that the observed noon value of the field strength is essentially (to about 1 dB) given by one-hop E propagation, F propagation being prevented by blanketing and higher-order reflections being strongly absorbed. In mid-latitudes, frequencies of 2-3 MHz and distances of about 100-400 km give good results. In most countries suitable transmitters can be found in coast stations. 3. Choice of types of antenna Since the absorption is smaller for the ordinary than for the extraordinary component, the antennae should be designed so as to select the ordinary component. If the extraordinary component were used the measurement could be influenced by the stronger ordinary component, especially during high absorption conditions. Close attention must be given to the suppression of the ground wave, especially for the lower frequencies and shorter distances. For this reason, propagation paths over sea should not be used. In addition the antennae can be used to minimize the effects of modes other than the 1 X E mode. For instance, in mid-latitudes in the northern hemisphere the following antennae are recommended for a path running approximately in north-south direction: a horizontally polarized antenna at the northern terminal and a vertically polarized antenna at the southern terminal. With this combination the errors due to polarization effects can be expected to be less than 3 dB. The use of a polarimeter antenna at the receiving terminal would result in an even smaller error. 4. Receiving and recording equipment The requirements as regards the receiver and recorder are essentially the same as for other field- strength measurements (see, e.g., Annex I, § 2). If a double-sideband transmission is recorded, a — 55 — Rep. 253-2 narrow receiver bandwidth selecting only the carrier frequency may be used and would result in a better signal-to-noise ratio. 5. Calibration The simplicity of the method results mainly from the fact that only relative, not absolute, values of field strength are needed, nor do the antenna diagrams need to be known in detail. It is convenient to have an approximately logarithmic receiver response calibrated in terms of input voltage. Relative changes from day to day can be easily read out from these records. To obtain absolute values of absorption, however, some more information is needed. As absorption is negligible on magnetically quiet nights, night-time measurements can be used for calibration, provided that the reflection occurs essentially at the same height as in daytime, i.e. via a blanketing Es layer (this case is not too rare on the rather low frequencies used for these measurements). Under these conditions, the geometry of the path is the same, day and night, and the difference of the logarithmic field-strength values for night and day, Fn — Fd should directly give the absorption. However, in most latitudes the extraordinary component is strongly absorbed at noon but appears in the night. Therefore in this case, if the unwanted component is not suppressed by the antennae, the absorption is given by Fn — Fd — 3 dB. Likewise, if higher-order reflections are not negligible at night, Fn will be too great. The best calibration conditions are therefore encountered when a blanketing Es layer occurs around sunset. In this case, the higher-order modes are still absorbed and the calibration is easily obtained by extrapolation of the daily field-strength variation until sunset (a similar procedure could be used at sunrise; however, Es is less probable at sunrise). The cases where propagation takes place via the Es layer around sunset are easily identified and can be deduced from the field-strength records themselves. Of course, ionograms taken from a nearby ionosonde may facilitate the identification of Es propagation. It is recommended that records covering a period of about three months be used for the calibration, considering only those 10% of all days when the highest sunset values were obtained. The average of these few days should be taken for Fn. The accuracy can be expected to be better than about i 3 dB. This is reasonable since the daytime absorption is of the order of 30 dB. If possible, however, the more thorough procedure described in instruction Manual No. 4 [I.Q.S.Y.] ought to be employed. 6. Presentation of results Half-hourly or hourly values of daytime absorption loss, L, should be given in monthly tables. The characteristic parameters of the measuring circuit (frequency, distance, path orientation, type of antenna) should be clearly indicated on the sheets, so that a reduction to vertical incidence data is possible. R eferences I.Q.S.Y. Instruction Manual No. 4, Ionosphere. Issued by I.Q.S.Y. Secretariat, 6 Cornwall Terrace, London N.W .l. K h m el n it sk y , E.A. [1969] Osobennosti experimentalnogo opredelenia napryazhennosti polya v KV diapazone (Aspects of experimental determination of field strength in the short-wave range). Elektrosyvaz, 10. S c h w e n t e k , H. [1965] Ein Vergleich von Impuls- und Dauerstrichbeobachtungen auf derselben Obertragungsstrecke bei 2,5 MHz (Comparison of pulse and CW observations over the same circuit on 2-5 MHz). Forschungsbericht des Landes Nordrhein- Westfalen Nr. 1443, published by Westdeutsche Verlag, Koln and Opladen. W a k a i, N., Fujii, S. and M iy am oto, Y. [1971] Measurement of the field intensity of HF radio waves by means of a narrow band receiver. J. Radio Res. Labs., Japan, 18, 183-190. Rep. 571 — 56 — REPORT 571 * COMPARISONS BETWEEN OBSERVED AND PREDICTED SKY-WAVE FIELD STRENGTH AND TRANSMISSION LOSS AT FREQUENCIES BETWEEN 2 AND 30 MHz (Study Programme 11 A-2/6, Opinion 45) (1974) 1. Introduction This Report provides a brief summary of comparisons between observed and predicted sky-wave field strength and transmission loss at frequencies between 2 and 30 MHz. Such comparisons provide information on the reliability of various prediction methods and on improvements to be considered by Interim Working Party 6/1 for inclusion in Report 252-2. It is important that Administrations and organizations keep the C.C.I.R. informed on the current status of their comparisons. 2. Field strength at vertical incidence In the U.S.S.R. [C.C.I.R., 1970-1974a] a comparison has been made of calculated and observed values of vertical incidence absorption using four different prediction methods. The comparisons showed large differences among prediction methods and up to 25 dB at 2-4 and 4-0 MHz between calculated and observed values of absorption. 3. Field strength at oblique incidence Work in the United Kingdom [Bradley and Howard, 1973] describes signal-strength measurements made from August 1968 to July 1969 over a 3260 km temperate-latitude path (Cyprus to Slough), using pulse transmissions on 9-9, 17-5 and 23-0 MHz. Measured absorption for the 1F2 and 2F2 modes is compared with the values predicted by the methods of Piggott [1959], Lucas and Haydon [1966] and Barghausen et al. [1969]. Generally, the measured absorption exceeds that predicted, particularly at night. On average for all hours, the three sets of predictions give absorptions which are too low by 7-5 dB, 6-5 dB and 8-0 dB respectively. A further prediction method based on work by George and Bradley [1973] leads generally to somewhat larger predicted absorptions for the lower frequencies and for the 1F2 mode. Discrepancies are accordingly reduced to 5 dB. Observational data indicate a median excess system loss of 6 dB, while the corresponding value given by the interim C.C.I.R. field-strength prediction method (Report 252-2) for this is 9-5 dB. Absorption values obtained from measurements in the Federal Republic of Germany over a complete sunspot cycle resulted in a formula which describes the winter anomaly for middle northern latitudes and its dependence upon the solar activity. Measurements on board a ship in the eastern Atlantic area showed that the southern boundary of the winter anomaly lies between 30 and 40 degrees northern latitude [Elling et al., 1973; Rottger and Schwentek, 1974]. A comparison between the field intensities predicted by the C.C.I.R. interim method and those measured at three frequencies (5, 10 and 15 MHz) in four seasons for different epochs of solar activity, can be seen from a Table in Doc. 6/65 [C.C.I.R., 1970-1974b]. The predicted and observed values are generally in good agreement. However, it is suggested from the comparison that the following points have to be taken into account in the revision of Report 252-2: — at 5 MHz as an example of relatively short distance propagation, predicted values are higher (less than 5 dB) than those measured; * Adopted unanimously. — 57 — Rep. 571 — at 10 MHz, also in short distance propagation (about 1000 km) the predicted values are lower than those measured for minimum solar activity, and for maximum solar activity the exponent of cos x in the absorption equation is too low; — at 15 MHz on the WWVH-Hiraiso path (about 6300 km), good agreement is seen, except in spring when the predicted values are lower (less than 5 dB) than those measured; — at 15 MHz on the WWV-Hiraiso path (about 10 000 km), good agreement is also seen. However, measured intensities suggest the improvement of calculation of MUF, since high field intensities are observed even when the operating frequency exceeds standard MUF (EJF) of the circuit. It is to be noted that a part of this discrepancy can be removed when the contribution of the long path signal to the received field intensities is taken into account. For operating frequencies exceeding the classical MUF (JF) a field-strength prediction method was developed by the Telecommunications Engineering Centre of the Federal German Post Office (FTZ) [C.C.I.R., 1966-1969]. This is derived from numerical maps by MUF = A.EJF where K is an empirical factor of which an improved version has been given [C.C.I.R., 1970-1974c]. Field-strength measurements of WWV made in Hiraiso, Japan, differ by less than 20 dB from the improved field-strength predictions. This difference is less than 10 dB if the EJF5 data measured in Japan are applied instead of the EJF data given by the numerical maps. Comparisons have been made by “ DEUTSCHE WELLE ”, Federal Republic of Germany, between predictions (using the interim method of Report 252-2) and SINPO assessments of the signal strength of broadcasts received at distances between 6000 and 23 000 km during the period from 1971 to 1973. In general, there was good agreement at distances up to 12 000 km, but the predicted values were systematically low at longer ranges. R eferences B a r g h a u se n , A.F., F in n e y , J.W., P r o c t o r , L.L. and S c h u l t z , L.D. [1969] Predicting long-term operational parameters of high frequency skywave telecommunication systems. Technical Report ERL 110 ITS 78 (U.S. Environmental Science Services Administration). B ra d l e y , P.A. and H o w a r d , D.R. [1973] Transmission loss at high frequencies on 3260 km temperate-latitude path. Proc. IEE, 120, 173-180. C.C.I.R. [1966-1969] Doc. VI/57, Federal Republic of Germany. C.C.I.R. [1970-1974a] Doc. 6/89, U.S.S.R. C.C.I.R. [1970—1974b] Doc. 6/65, Japan. C.C.I.R. [1970-1974c] Doc. 6/60, Germany (Federal Republic of). E l l in g , W., G e isw e id , K.H. and S c h w e n t e k , H. [1973] The southern boundary region of the winter anomaly in ionospheric absorption in winter 1971/1972 observed on board the cargo vessel “ Hanau ” of Hapag-Lloyd moving between 10° and 55° N. Mitteilung aus dem Max-Planck-Institut fiir Aeronomie, 51. G eorge, P.L: and B r a d l e y , P.A. [1973] Relationship between HF absorption at vertical and oblique incidence. Proc. I.E.E., 120, 1355-1361. L u c a s, D.L. and H a y d o n , G.W. [1966] Predicting statistical performance indexes for high frequency ionospheric telecommuni cations systems. Technical Report ITSA-1 (U.S. Environmental Science Services Administration). P ig g o t t , W.R. [1959] The calculation of the medium skywave field-strength in tropical regions. Radio Research Special Report No. 27, H.M.S.O., London. R ottger, J. and S c h w e n t e k , H. [1974] A numerical description of the winter anomaly in ionospheric absorption for a sunspot cycle. J. Atmos. Terr. Phys., 36, 363-366. Rep. 572 — 58 — REPORT 572* ESTIMATION OF SKY-WAVE FIELD STRENGTH AND TRANSMISSION LOSS AT FREQUENCIES BETWEEN 2 AND 30 MHz (A proposed plan for revision of the C.C.I.R. interim method) (Study Programme 11 A-2/6, Decision 6) (1974) 1. Introduction Report 252-2, which was adopted by the C.C.I.R. at New Delhi, 1970, describes an interim method for estimating sky-wave field strength and transmission loss at frequencies between the approximate limits of 2 and 30 MHz. Interim Working Party 6/1 is charged with the task of developing a more accurate method than that described in Report 252-2, and the purpose of the present Report is to set out the intentions of the Working Party in this respect. Briefly, it is considered that the proposed improved method, to be outlined below, should be in the form of a complete revision, since although the general principles of the existing interim method will continue to be followed, the component loss factors of this system are to some extent inter-related and should therefore not be modified independently of each other. The primary aim will be to produce a method of estimating the monthly median sky-wave field strength, and the day-to-day amplitude distribution, for specified individual modes of propagation, when 1 kW of power is radiated isotropically. Either vertical or horizontal polarization may be independently specified for transmission and reception. In practice, antenna directivity is clearly important since it has a marked effect on received signal strengths, and may determine the dominant mode of propagation. A secondary aim of the system will therefore be to enable the calculations for individual modes to be made, and for their summation at the receiver, when using transmitting and receiving antennae with known polar diagrams. It is also clear that for a final estimation of the signal quality to be expected in any communication circuit, evaluation of signal strength alone is insufficient, and must be supplemented by quantitative information about other parameters, especially noise, as available in Reports 322-1 and 258-2. However, since such considerations involve system characteristics such as types of modulation and signal processing, they are regarded as being outside the scope of the present Report. 2. The proposed method The calculations will proceed in the following steps, further details of which are discussed in § 3: A. Specification of relevant ionospheric characteristics. B. Determination of the necessary vertical distributions of electron concentration from these ionospheric characteristics. C. Determination of active modes and angles of elevation from these vertical distributions. D. Determination of transmission losses and signal variability. 1. Free-space attenuation. 2. Absorption (deviative and non-deviative). 3. Absorption (auroral). 4. Horizon focusing. * Adopted unanimously. — 59 — Rep. 572 5. Polarization-coupling loss. 6. Ground loss on multihop modes. 7. Sporadic-E obscuration and reflection loss. 8. Loss associated with propagation above the MUF. 9. Day-to-day distribution of hourly median amplitudes. In the following section, the way in which it is proposed to carry out each of the above steps is discussed. In some cases, well-documented procedures are available, and it is intended to incorporate these procedures as described. In other cases, it is indicated that further work is necessary to identify the best procedure to be used. However, although refinements to the various parts of the system could undoubtedly continue to be made for a very long time ahead, it is the firm intention of the Working Party not to allow this process to delay the issue of the revised method beyond the time of the next Interim Meeting of Study Group 6. It is considered that sufficient progress has been made in the relevant studies to enable a worthwhile improvement over the existing interim method to be made. 3. Details of the proposed method A. Specification of relevant ionospheric characteristics The characteristics needed are those required for the models of electron concentration and for Es screening determination. These should include foF2, foE, M(3000)F2, foEs, and may also include foFl, h'F, h/F2. C.C.I.R. numerical coefficients should be used for median foF2, M(3000)F2, foEs, h'F, h'F2. Those for foF2 should be the Oslo (3-dimensional) numerical coefficients, noting that these foF2 values may saturate at high solar activity levels. Median foFl and foE should be from the equations of Report 340-2. For day-to-day variability of foF2, the x2 distribution should be assumed to apply, with values as given in Report 252-2. Coefficients for decile values of foEs should be those referred to in Report 340-2, Part 4. Fixed percentage standard deviations should be taken for foE and foFl; day-to-day variations of other characteristics should be ignored. B. Determination of vertical distributions of electron concentration There is an urgent need to improve the model distribution used in Report 252-2. Such an improved model is available, and is described in detail in Part 9 of Supplement No. 1 to Report 340-2. C. Determination of active modes and angles of elevation Great-circle propagation should be assumed; allowance should be made for different ray apogees of different hops; ionospheric tilts should be included. Mirror reflection from heights equal to the virtual heights of reflection at “ equivalent ” frequencies for vertical incidence should be assumed. Ways of deducing virtual heights at vertical incidence in the model ionospheres by analytical equations or by numerical integration should be compared to establish the more efficient means. Homing procedures appear more economical on computation time for fixed path lengths than the production of tables of elevation angles and path lengths with interpolation as appropriate. An efficient homing procedure is available [Bradley and Murphy, 1974]. D l. Free-space attenuation This should be treated as inverse distance attenuation based on the equivalent ray path deduced in C above. Rep. 572 — 60 — D2. Deviative and non-deviative absorption The absorption of oblique-incidence high-frequency radio waves in the ionosphere may be deduced either from vertical-incidence absorption measurements, mainly available from experiments using the Al method or from direct oblique-incidence measurements.’ Both of these methods have limitations. The vertical-incidence absorption measurements are very accurate, but there are problems in converting them to oblique incidence. The interpretation of oblique-incidence absorption measurements is uncertain because of difficulties in knowing the active propagation modes, elevation angles and the unabsorbed field strengths. A method for predicting oblique-incidence ionospheric absorption, based on vertical-incidence measurements, has been developed by George and Bradley [1974], This method is based on maps of vertical-incidence absorption [George, 1971] which include the effects of the winter anomaly of absorption, which extend in geographical coverage almost to the auroral zones, and which include both deviative and non-deviative absorption. The conversion from vertical to oblique incidence is made using empirical equations which have been found to be consistent with the results of ray tracing calculations through model ionospheres [George and Bradley, 1973]. Studies of the absorption winter anomaly have also been made in the Federal Republic of Germany [C.C.I.R., 1970-1974a]. The values of absorption given by the formula presented in this document should be compared with those given by the United Kingdom method described above; it should be noted, however, that the German work is of limited application in respect of frequency and geographical coverage. The revised absorption method should give results which are consistent with those of Wakai [1961] for night-time. D3. Auroral absorption There is a recognised need to develop a reliable method for including the effects of auroral absorption. The only developments on this subject that have been reported were made some years ago in the United States of America [Agy, 1971]. Work on this problem is being carried out in Argentina, France and the United Kingdom and should lead to improved methods for estimating and predicting auroral absorption. At present the best available method of allowing for auroral absorption is to use the values of Yp given in Report 252-2, less 9 dB. D4. Horizon focusing Suitable expressions are available [Bradley, 1970]. D5. Polarization-coupling loss Polarization-coupling loss is often small, but under some conditions it is not. Suitable methods of calculation are available [Bradley and Bramley, 1971]. D6. Ground loss on multihop modes Ground reflection coefficients using standard Fresnel theory should be combined with polarization-coupling losses, taking separate account of the O and X waves, as in D5. D7. Sporadic-E obscuration and reflection loss A method of calculation based on a normal distribution of irregularities is available for both . obscuration and reflection [Lloyd, 1973]. Empirical estimates of reflection loss have been made [Miya and Sasaki, 1966]. In deciding the optimum method to be adopted the relevance of recent experimental studies of sporadic-E reflection coefficient [Turunen, 1974] should also be considered. — 61 — Rep. 572 D8. Loss associated with propagation above the MUF The methods of ITS [Lloyd, 1973] and FTZ [Beckmann, 1967] should be compared. If they give similar results the ITS method which is simpler should be adopted. If they differ significantly the FTZ method which has practical justification should be adopted. D9. Day-to-day distribution of hourly median amplitudes The Si values of Report 252-2 should continue to be used. 4. Antenna polar diagrams, and addition of active modes The foregoing description relates to the calculation of the transmission loss and field strength of individual modes. Before their summation at the receiver can be effected it is necessary to allow for the way in which the amplitude of each mode is affected by the vertical polar diagram of the transmitting and receiving antennae. A comprehensive collection of antenna polar diagrams is that of Barghausen et al. [1969], and this should be incorporated in the computerized method. The summation of active modes at the receiver should then be carried out assuming phase incoherence between modes, that is, by addition of the power in each mode. 5. Form of computer output The computer programme for the improved method should be written so that the user can obtain quantitative information on any parameter derived in the course of the calculations. 6. Computer language In the revision of the computer programme of Report 252-2 it would be desirable to take account of the work accomplished in the field of standardization of computer language. 7. Manual method of estimating field strength and transmission loss Decision 6 notes that Interim Working Party 6/1 is planning to prepare tabulations or charts for manual usage. Substantial progress in this direction has been made in the United Kingdom by the provision of charts and nomograms which permit the evaluation of the field strengths of individual modes, assuming an isotropic source. The method is based on a simpler ionospheric model than that proposed in § 3B above, and includes the transmission losses as described in §§ Dl, D2 and D4. It is described in more detail in C.C.I.R. [1970—1974b] and in C.C.I.R. [1970-1974c]. The results given by this method should be compared with those of the full computerized method, and the manual method further developed as considered necessary. R eferences A g y , V. [1971] AGARD Conference on Radar Propagation in the Arctic. B a r g h a u se n , A.F., F in n e y , J.W., P r o c to r , L.L. and S c h u l t z , L.D. [1969] Predicting long-term operational parameters of high-frequency sky-wave telecommunication systems. ESSA Technical Report ERL 110-ITS 78. B e c k m a n n , B. [1967] Notes on the relationship between the receiving-end field strength and the limits of the transmission frequency range MUF, LUF. NTZ-Comm. J. 6, 37-47. B r a d l e y , P.A. [1970] Focusing of radio waves reflected from the ionosphere at low angles of elevation. Electronics Letters, 6, 457-458. B r a d l e y , P.A. and B ram ley, E.N. [1971] Wave polarization and its influence on the power available from a radio signal propagated through the ionosphere. Proc. I.E.E., 118, 1190-1196. Rep. 572 — 62 — B r a d l e y , P.A. and D u d e n e y , J.R. [1973] A simple model of the vertical distribution of electron concentration in the ionosphere. J. Atmos. Terr. Phys., 35, 2131-2146. B r a d l e y , P.A. an d M u r p h y , J.A. [1974] To be published. C.C.I.R. [1970-1974a] Doc. 6/189, Germany (Federal Republic of). C.C.I.R. [1970—1974b] Doc. 6/191, United Kingdom. C.C.I.R. [1970-1974c] Doc. 6/325, United Kingdom. G eorge, P.L. [1971] The global morphology of the quantity / Nv dh in the D- and E-regions of the ipnosphere. J. Atmos. Terr. Phys., 33, 1893-1906. G eorge, P.L. and B r a d l e y , P.A. [1973] Relationship between h.f. absorption at vertical and oblique incidence. Proc. I.E.E., 120, 1355-1361. G eorge, P.L. and B r a d l e y , P.A. [1974] A new method of predicting the ionospheric absorption of high frequency waves at oblique incidence. Telecommunication Journal, 41, 307-312. L l o y d , J. [1973] To be published. M iy a , K. and S a sa k i, T. [1966] Characteristics of ionospheric Es propagation and calculation of Es signal strength. Radio Science, 1, 99-108. W a k a i, N. [1961] Non-deviative absorption at night. J. Radio Research Labs., Japan, 8, 37, 213. T u r u n e n , T. [1974] To be published. — 63 — Rec. 372-1 SECTION 6D: ATMOSPHERIC AND MAN-MADE RADIO NOISE RECOMMENDATIONS AND REPORTS Recommendations RECOMMENDATION 372-1 USE OF DATA ON RADIO NOISE (1951 - 1953 - 1956 - 1959 - 1963 - 1974) The C.C.I.R. UNANIMOUSLY RECOMMENDS that the information contained in Reports 258-2 and 322-1 should be used in assessing the intensity and/or other characteristics of man-made and natural radio noise until new information to justify revision of these Reports is made available. Rep. 254-3 — 64 — 6D: Reports REPORT 254-3 * MEASUREMENT OF ATMOSPHERIC RADIO NOISE FROM LIGHTNING (Study Programme 7B/6) (1959 - 1963 - 1966 - 1970 - 1974) 1. Noise measurements and results An extensive series of measurements of noise has been made since 1957 using a standard equipment (ARN 2) at a network of stations throughout the world [Crichlow et al., 1959-1967]. All stations have measured noise power but some stations have been additionally equipped for the measurement of other noise parameters descriptive of the noise structure. An account of this measurement programme has been prepared by U.R.S.I. [U.R.S.I., 1962]. The regular forwarding of data to the Data Collection Centre at Boulder, Colorado, U.S.A. has now ceased from all but three stations situated in Northern Europe, the subtropical Western Pacific and Southern Africa. Nevertheless sufficient data now exist for a major revision of Report 322-1, this being based on measurements prior to 1963. Results from many stations now cover a complete solar cycle. U.R.S.I.’s opinion is that a significant improvement in the data contained in the Report could be made, but the scientific interest in such a revision does not constitute a sufficiently strong incentive for the work to be done [C.C.I.R., 1970—1974a]. At some frequencies systematic differences, up to 4 dB in noise power, have been obtained by different techniques at the same site. Reports of measurements at high frequencies suggest that the data in Report 322-1 are too low during daytime in some tropical regions [C.C.I.R., 1963-1966 a, b, c; C.C.I.R., 1966-1969a]. The discrepancies may be up to 10 dB or more in the lower part of the HF band, and these regions need particular examination in a revision of the Report. Other published comparisons between predicted and measured noise values will also need to be taken into account [Herman, 1961; Ibukun, 1964]. Results of a special study of atmospheric noise at YLF have been published in the form of charts and curves [Watt, 1967]. Results for VLF and ELF are also available [Maxwell, 1967; Remizov et al., 1971]. In addition, recent work toward the development of an improved VLF noise-prediction model has been reported [C.C.I.R., 1970-1974a]. In a report from India it has been stated that, in tropical countries, atmospheric noise at VHF is of practical importance [C.C.I.R., 1966—1969b]. Data are furnished which should be useful to designers of VHF communication systems in the tropical regions. 2. Statistical characteristics of noise Measurements of amplitude-probability distributions of noise have been made at many locations [U.R.S.I., 1962] and the data have been used in Report 322-1. It has been shown that allowance can be made for the noise structure in estimating required signal-to-noise ratios. * Adopted unanimously. — 65 — Rep. 254-3 Statistical noise data for bands 5 (LF), 6 (MF) and 7 (HF) have become available from measure ments made in India over two decades. They are based on a technique which emphasizes the importance of the bursts of noise which are the main source of noise interference to many services. The influence of the noise on voice communication, broadcasting and data transmission has been studied [C.C.I.R., 1966-1969c]. Some information is available on the short- and long-term statistical variations of noise [Crichlow et al., 1960; Remizov et al., 1967; Watt, 1967; Watt and Maxwell, 1957; Spaulding et al., 1962]. The results of some recent measurements of the amplitude and time statistics and of the mathematical modelling of these statistics have been reported by Disney and Spaulding [1970]. In addition, recent attempts to produce laboratory simulators of atmospheric noise have been reported by Bolton [1971]. 3. Measurements with directional antennae Measurements have been made with directional antennae of the type used in high-frequency, long-distance communication. There are insufficient data for a full report to be made, but the results of some investigations have been published [Kronjager and Vogt, 1959; Bradley and Clarke, 1964] and some preliminary information is contained in Report 322-1. In addition, more recent measurements carried out in Canada are reported in [C.C.I.R., 1970-1974b]. 4. Lightning-flash counters and direction-finding networks The object of lightning-flash counter observations, in so far as they are required for noise studies, is to determine the number of lightning discharges occurring per unit area of the Earth’s surface, in different places and at different times. Although the expansion of counter networks has proceeded rather slowly, further experience has been gained in their use, and in the comparison between the C.C.I.R. types and other types. Data from counters of a type which has been adopted by C.I.G.R.E. (Conference internationale des grands reseaux electriques) and which has been used extensively, can be correlated with data from the C.C.I.R. counters provided that appropriate sensitivities have been used; work has been in progress to establish the effective range of the C.I.G.R.E. counter with improved accuracy [Friihauf et al., 1966]. Experiments with another counter, operating on the high-frequency energy from atmospherics have led to similar conclusions, regarding the incidence of discharges, as those from the C.C.I.R. counter. Experiments in the United Kingdom [Horner, 1960] and others, reported in a C.C.I.R. document from Sweden [C.C.I.R., 1963] have resulted in different estimates of the effective range of the C.C.I.R. counter, and further investigation is required. However, the more precise evaluation of this quantity need not delay the collection of further data which will show at least the relative values in different geographical locations. A lightning-flash counter operating in the high frequency band has been developed in India and is claimed to be cheap, compact and portable, and to have a well-defined range of detection [Aiya and Hari Shankar, 1969]. An analysis has been made of data from lightning-flash counters of the C.C.I.R. recommended design, used in the United Kingdom and in Singapore. The results, which have been published [Horner, 1967] include comparisons between counter data and noise data which are particularly relevant to the work of the C.C.I.R. These comparisons illustrate the conditions under which estimated noise intensities, based on counts and on the known electrical characteristics of lightning flashes, agree or disagree with the direct measurements of noise which are mentioned in § 1. In broad terms agreement is good when the measured noise can reasonably be expected to originate mainly in relatively near storms, provided that these are sufficiently numerous to produce a continuing succession of atmospherics. Conditions which give rise to discrepancies between the two methods of estimating noise are: — at VLF, when the hourly count is less than 30, the measured noise is higher than would be calculated from the counts. The difference is presumably due to the reception of noise from an area larger than that for which the counter records can be regarded as representative; Rep. 254-3 — 66 — — at MF and the lower parts of the HF band, when the hourly count is less than 500 per hour, the measured noise is lower, sometimes by up to 30 dB, than the value expected from the counts. This is believed to occur because the dynamic range of the noise measuring equipment, though large, is still insufficient for the recording of the mean power of the most impulsive type of noise. The errors in the measurements of noise at MF and HF occur only during local storms and when the hourly count is less than 500. They therefore affect only a small proportion of the data in Report 322-1. However, in tropical regions the measured monthly median and the upper and lower decile values of noise can all be affected, depending on the intensity of storm activity during the month. The data in Report 322-1 need to be used with caution in any application in which noise from local storms may be important. Counter data, if available, probably provide a better guide in such circumstances than do measurements of noise intensity. The work has shown that counters are at least a valuable supplement to noise measurements and help to distinguish genuine high levels of noise from those caused by interference. Although a large volume of data from atmospherics direction-finding networks exists, no reports are to hand which show how these data could be used in noise studies. The problems involved are considerable, but, since these networks are potentially capable of providing information leading to knowledge of the noise generated in oceanic areas, where adequate networks of lightning-flash counters cannot be established, early attention should be given to a study of whether the potentialities are likely to be realized in practice. Some recent work bearing on results obtained by direction-finding on atmospherics is contained in [C.C.I.R., 1963—1966d; Horner, 1971]. 5. Intensity and nature of noise from individual lightning discharges Atmospherics from near storms have been recorded, in such a way that the effects of propagation can be allowed for, and an energy spectrum derived, which is effectively a characteristic of the source alone [Watt and Maxwell, 1957; Taylor and Jean, 1959; Horner, 1964; Horner and Bradley, 1964; Pierce, 1967]. The spectrum has been used in conjunction with lightning-flash counters and propagation data, to give estimates of integrated noise at certain locations [Horner, 1967]. These have been compared with measured noise intensities and the agreement is sufficiently close to justify further investigation of this method of approach. 6. World distribution of thunderstorms Estimates of the probability of a thunderstorm occurring at any location for each hour of the day for each of the four seasons have been prepared [Crichlow et al., 1971] based on the W.M.O. thunderstorm day reports. R eferences A iy a , S.V.C. an d H a r i S h a n k a r , M. [1969] A lightning flash density gauge. I.T.E. Journal, 15, N o . 10. B o l t o n , E . [1971] Simulating atmospheric radio noise from low frequency through high frequency. Review of Scientific Instruments, 42(5), 574-577. B r a d l e y , P.A. and C l a r k e, C . [1964] Atmospheric radio noise and signals received on directional aerials at high frequencies. Proc. I.E.E., 111, 1534. C.C.I.R. [1963] Doc. 183, Sweden, Geneva. C.C.I.R. [1963-1966a] Doc. VI/79, India, Geneva. C.C.I.R. [1963-1966b] Doc. VI/201, India, Geneva. C.C.I.R. [1963-1966c] Doc. VI/202, India, Geneva. C.C.I.R. [1963—1966d] Doc. VI/67 (Rev. 1), Federal Republic of Germany, Geneva. — 67 — Rep. 254-3 C.C.I.R. [1966-1969a] Doc. VI/86, India, Boulder. C.C.I.R. [1966-1969b] Doc. VI/175, India, Geneva. C.C.I.R. [1966-1969c] Doc. VI/173, India, Geneva. C.C.I.R. [1970-1974a] Doc. 6/184, U.R.S.I., Geneva. C.C.I.R. [1970-1974b] Doc. 6/270, Canada, Geneva. C r ic h l o w , W.Q., D isn e y , R.T. and Je n k in s , M.A. [1959-1967] Quarterly radio noise data. NBS Tech. Notes Nos. 18 and 18-2 to 18-26, and ESSA Tech. Reports IER 18-27 to 18-32, Institute for Environmental Research, Boulder, Colo., U.S.A. C r ic h l o w , W.Q., S p a u l d in g , A .D ., R o u b iq u e , C.J. and D isn e y , R.T. [1960] Amplitude probability distributions for atmospheric radio noise. NBS Monograph 23, Government Printing Office, Washington, D.C. C r ic h l o w , W.Q., D a v is, R.C., D isn e y , R.T. and C l a r k , M.W. [1971] Hourly probability of world-wide thunderstorm occurrences. Telecommunications Research Report OT/ITS RR 12, U.S. Department of Commerce, Washington, D.C. D isn e y , R.T. and S p a u l d in g , A.D. [1970] Amplitude and time statistics of atmospheric and man-made radio noise. ESSA Technical. Report ERL 150-ITS-98, U.S. Department of Commerce, Washington, D.C. F r u h a u f , G., A m berg , H. and W ur ster, W. [1966] Wirkungsweise und Reichweite von Blitzzahlern (Functioning and range of lightning flash counters). Beihefte (Supplement) der ETZ, 6. H e r m a n, J. [1961] Reliability of atmospheric radio noise predictions. NBS Journ. o f Research, 65D, 565. H o r n e r , F . [1960] The design and use of instruments for counting local lightning flashes. Proc. I.E.E., 107B, 321. H o r n e r , F. [1964] Radio noise from thunderstorms. Advances in Radio Research. Ed. J.A. Saxton, 2, 121, Academic Press, London. H o r n e r , F . [1967] Analysis of data from lightning flash counters. Proc. I.E.E., 114, 916. H o r n e r , F. [1971] Scientific activities of (U.R.S.I.) Commission VIII (in Progress in Radio Science, gen. ed. C.M. Minnis, 1966-1969, Part 1, 253-257, U.R.S.I., Brussels). H o r n er , F. and B r a d l e y , P.A. [1964] The spectra of atmospherics from near lightning discharges. J. Atmos. Terr. Phys., 26, 1155. I b u k u n , O. [1964] Measurements of atmospheric noise levels. Radio and Electronic Eng., 28, 405. K ro njag er, W. and V o g t , K. [1959] Ober das Aussengerauschkommerzieller Antennenanlagen (On the external noise of commercial antenna installations). NTZ, 12, 371. M a x w e l l , E.L. [1967] Atmospheric noise from 20 Hz to 30 kHz. Radio Science, 2, 637-644. P ierce, E.T. [1967] Atmospherics—their characteristics at the source, and propagation. Progress in Radio Science, 1963-1966, Part 1, Proc. XV, General Assembly of U.R.S.I. R em izo v , L .T ., O le in ik o v a , I.V ., K oroliev, A .N ., V isk revtso v, I.G. and T cherniavsky , A.F. [1971] Statistical characteristics of atmospheric noise at V L F (in Russian). Radiotekhnika i Elektronika, 16, 1191-1199. R em izo v , L.T., P o ta po v, A.V. and V ik h r o v , B.R. [1967] Probability distribution of time intervals between atmospherics (in Russian). Radiotekhnika i Elektronika, 12, 706-708. S p a u l d in g , A.D . R o u b iq u e, C.J. and C r ic h l o w , W.Q. [1962] Conversion of the amplitude-probability distribution function for atmospheric radio noise from one bandwidth to another. NBS Journ. o f Research, 66D, 713. T a y l o r , W.L. and Je a n , A.G. [1959] Very low frequency radiation spectra of lightning discharges. NBS Journ. of Research, 63D, 119. U.R.S.I. [1962] The measurement of characteristics of terrestrial radio noise. Special Report No. 7, Elsevier. W a t t , A.D. [1967] VLF radio engineering. Pergamon Press, New York. W a t t , A.D. and M ax w e l l , E.L. [1957] Characteristics of atmospheric noise from 1 to 100 kc/s. Proc. IRE. 45, 787. Rep. 258-2 — 68 — REPORT 258-2 * MAN-MADE RADIO NOISE (Study Programme 7C/6) (1963 - 1970 - 1974) In the solution of telecommunication problems, it is highly desirable to be able to estimate the radio noise at any location as caused by different types of noise sources. At certain locations, unintended man-made noise may be dominant. Since such noise can arise from a number of sources, such as power lines, industrial machinery, ignition systems, etc., it varies markedly with location and time [Herman, 1971; Horner, 1971]. Current information is not available for estimating man-made noise intensities under all conditions but from limited observations [J.T.A.C., 1968; Disney, 1972] it is possible to derive typical values. Median values of man-made noise power expressed in terms of Fa (dB above thermal noise at 288 K, see Report 322-1) attributed to man-made sources are shown in Fig. 1 for business, residential, rural and quiet rural areas. Measurements from 103 locations in the United States during 1966-1971 inclusive were used in the determination of the upper three curves. Business areas are defined as the core centres of large cities and urban areas as the residential sections of large cities as well as the suburban areas of large population centres. Rural areas are defined as small communities and farms, while the curve for quiet rural areas corresponds to the values of man-made noise at a quiet site as reported in Report 322-1. The dashed curve of galactic noise obtained from Report 322-1 is included for reference. Note. — The C.C.I.R. Secretariat is requested to convey the text of this Report to Study Group 1 for information. R eferences D isn e y , R.T. [1972] Estimates of man-made radio noise levels based on the Office of Telecommunications ITS data base. Proc. IEEE International Conference on Communications, Order No. 72CH0622-1-COM, 20-13/20-19. H e r m a n , J.R. [1971] Survey o f man-made radio noise. Progress in Radio Science, Vol. I, 315-348, U.R.S.I., Brussels. H o r n e r , F . [1971] Techniques used for the measurement o f atmospheric and man-made noise. Progress in Radio Science, Vol. II, 177-182, U.R.S.I., Brussels. J.T.A.C. [1968] Unintended radiation. Suppl. 9 to Spectrum Engineering—The Key to Progress, IEEE, New York. * Adopted unanimously. — 69 — Rep. 258-2 Frequency (MHz) F ig u r e 1 Mean values of man-made noise power for a short vertical lossless grounded monopole antenna A: Business D: Quiet rural B: Residential E: Galactic C: Rural Rep. 322-1 — 70 — REPORT 322-1 WORLD DISTRIBUTION AND CHARACTERISTICS OF ATMOSPHERIC RADIO NOISE (Study Programme 7B/6) (1963 - 1974) Report 322-1, which was adopted unanimously, includes Report 322, together with Supplement No. 1, and has been published separately. This supplement contains the addenda 1 and 2 to Report 322. — 71 — Rec. 434-2 SECTION 6E: IONOSPHERIC MAPPING RECOMMENDATIONS AND REPORTS Recommendations RECOMMENDATION 434-2 C.C.I.R. ATLAS OF IONOSPHERIC CHARACTERISTICS (1966 - 1970 - 1974) The C.C.I.R, CONSIDERING (a) that Interim Working Party 6/3 has defined specifications, both for the production of the C.C.I.R. Atlas of Ionospheric Characteristics and for the studies required for its development; (b) that a first edition and supplements of the C.C.I.R. Atlas (Report 340-2) are available in alternative forms, either as a set of contour charts or as a set of coefficients for use in a computer; (c) that a master set of punched cards and master magnetic tapes containing the coefficients, together with programmes for computing the functions to which the coefficients apply, have been deposited with the C.C.I.R. Secretariat for purposes of reproduction and sale; (d) that the computer versions of the Atlas can be kept up to date relatively cheaply, while further editions of the contour chart form of the Atlas are expensive, so that the computer versions may include later information than the contour charts; UNANIMOUSLY RECOMMENDS 1. that Administrations and organizations, also the I.F.R.B, having access to a computer, should use computer versions of the Atlas; 2. that, in other cases, the Atlas in contour chart form should be used. Note. — The Director, C.C.I.R, is requested to keep computer programmes, punched cards and magnetic data tapes available for distribution. Rep. 340-2, 430-1 — 12 — 6E: Reports REPORT 340-2 C.C.I.R. ATLAS OF IONOSPHERIC CHARACTERISTICS (1966 - 1970 - 1974) Report 340-2, which was adopted unanimously, includes Report 340, together with Supplement No. 2, Geneva, 1974, and has been published separately. Supplement No. 1, New Delhi, 1970, is cancelled and replaced by Supplement No. 2, Geneva, 1974. REPORT 430-1 * IMPROVEMENT IN THE WORLD-WIDE IONOSPHERIC OBSERVING PROGRAMME FOR NUMERICAL MAPPING PURPOSES (Study Programme 2A-2/6) (1970- 1974) 1. Introduction Improvements in long- and short-term predictions of ionospheric conditions depend upon the availability of new sources of ionospheric observations as well as upon improved methods of ionospheric mapping and predictions. Significant improvement might be possible by supplementing vertical-incidence measurements with data obtained from satellites and by other appropriate techniques. 2. Geographical distribution of ground-based ionosondes The existing ionosonde network has provided a reasonable sample of basic data for about one-third of the earth’s surface. Gaps in ionosonde coverage are the ocean areas and sparsely populated land areas, especially the equatorial anomaly region, the South Atlantic magnetic anomaly region, the Antarc tic region, and the sub-auroral region. It might be possible to cover these regions using unattended ionosonde equipment which would automatically measure, scale, process and transmit pertinent iono spheric parameters in digital form. Ionospheric data from back-scatter and oblique-incidence sounding programmes may also be of value. 3. Quality of data Constant vigilance is required for adequate quality control of data from ionospheric sounding stations. The statistical significance of data reported from a given ionospheric station depends entirely on careful following of the established rules for scaling and reporting, and adequate continuity of records to provide sufficient data for meaningful monthly medians. * Adopted unanimously. The Ionospheric Network Advisory Group (I.N.A.G.) of U.R.S.I. Commission 3 is responsible for considering the scientific problems associated with the ground-based vertical-incidence network, and, when necessary, for recommending appropriate action. I.N.A.G. [Piggott and Rawer, 1972] has recommended a number of modifications in the rules previously used for scaling ionograms. Use of top-side sounding and other satellite data The availability of top-side data from satellite soundings can provide possible improvements in iono spheric predictions by establishing information on: — mean gradients of foF2 and hmF2 in order to improve the extrapolation of data from ground iono sonde stations to remote locations; — geographic or magnetic coordinate systems which would enable ionospheric characteristics to be represented more accurately by numerical mapping methods; — the extent to which the changes in vertical incidence ionospheric characteristics depend upon changes in the positions of localized features such as the peaks and troughs in foF2 in equatorial and high- latitude regions. Ancillary data such as electron density and temperatures, total ionospheric electron content and ionization gradients may also be applicable to mapping problems, especially if these measurements are simultaneous. Many of the features observed by top-side sounders and relevant to ionospheric mapping problems have been reviewed in the Proceedings of the IEEE [IEEE, 1969]. Australia and Japan using data from the Alouette satellites [Nishizaki and Nagayama, 1969] and Australia, Japan and the United Kingdom using data from the ISIS-1 and ISIS-2 satellites are examining the compatibility of ground-based and top-side sounder data. Sample maps of foF2 at constant local times have been produced from such top-side sounder data at equatorial, middle and polar latitudes [Burge et al., 1973]. Numerical mapping techniques developed in the United States of America [Jones and Obitts, 1970; Jones and Stewart, 1970] permit the integration of ground based data with top-side sounder data. Plasma frequency observations from the Ariel 3 satellite [Piggott, 1970; Sayers, 1970] indicate that the ratio of plasma frequency to foF2 at different longitudes for the same local time is approximately constant from day-to-day. This feature is of potential use in deducing ionospheric conditions in remote areas in terms of available ground-based observations. On the other hand, a small number of theoretical calculations suggest that the extrapolation of satellite data to values of foF2 is markedly dependent on electron temperature and local time [King, 1973]. A theoretical relationship has been established between neutral-air winds and electron density distributions [Rishbeth and Kelley, 1971]. The theory is being used to deduce changes in height of maximum ionization with longitude and thereby to yield values of foF2 from the observed plasma frequencies at the satellite. Bottom-side satellites and satellite airglow data could provide additional useful information on layer heights. Total electron content Maps of total electron content (TEC) are needed for predicting ionospheric effects on satellite-Earth links. Measurements of total electron content have been made at many stations by observations of the Faraday rotation or the Doppler effect on signals from low orbiting and geostationary satellites. Maps of total electron content have been prepared for a mid-latitude ionosphere [Klobuchar, 1973; Amayenc et al., 1971; C.C.I.R., 1970-1974a]. Observations of the latitudinal variations of TEC in different equa torial regions have been described [Physical Research Laboratory, 1969; C.C.I.R., 1970-1974b; Golton and Walker, 1971]. Accuracy of the C.C.I.R. numerical maps of foF2 and hmF2 Comparisons made [Burge et al., 1973; King, 1973] between satellite observations of plasma fre quencies and the numerical maps of foF2 reveal that average discrepancies are generally small, but that Rep. 430-1 — 74 — in some ocean regions, notably the South Pacific and in the southern hemisphere, the predictions are systematically in error. Studies in the United States of America [C.C.I.R., 1970-1974c] and the U.S.S.R. [C.C.I.R., 1966- 1969] comparing the predicted monthly median foF2 with observations from ground-based ionospheric sounding stations indicate that the discrepancies in the predictions are usually less than ±15% except in high latitudes where they are sometimes as high as 40%. A deficiency in the numerical maps of foF2 appears to be an assumption that a limited portion (five years) of solar cycle 19 can accurately predict the ionospheric state for other solar cycles. R eferences A m a y e n c , P ., B e r t in , F., P a pet-L e p in e , J. [1971] Sur revolution latitudinale du contenu electronique de l’ionosphfere (On the latitudinal evolution of the electron content of the ionosphere). Ann. de geophys., 27, 345-357. B u r g e , J.D., K in g , J.W. and S la t er , AJ. [1973] Mapping of foF2 by means of top-side sounder satellites. Telecommunication Journal, 40, 356-363. C.C.I.R. [1966-1969] Doc. VI/191, U.S.S.R. C.C.I.R. [1970-1974a] Doc. 6/81, Germany (Federal Republic of). C.C.I.R. [1970-1974b] Doc. 6/182, United Kingdom. C.C.I.R. [1970-1974c] Doc. 6/305, Interim Working Party 6/3. G o l t o n , E. and W a l k e r , G.O. [1971] Observations of ionospheric electron content across the equatorial anomaly at sunspot minimum. J. Atmos. Terr. Phys., 33, 1-11. IEEE [1969] Topside sounding and the ionosphere. Proc. IEEE, 57, 859-1171. Jo nes, W.B. and O bitt s, D.L. [1970] Global representation of annual and solar cycle variation of foF2 monthly median 1954-1958. Telecommunications Research Report OT/ITSRR3, Boulder, Colo., U.S.A. Jo nes, W.B. and St e w a r t , F.G. [1970] A numerical method for global mapping of plasma frequency. Radio Science, 5, 773-784. K in g , J.W. [1973] The determination of foF2 and hmF2 from satellite-borne probe data. Telecommunication Journal, 40, 364-368. K l o b u c h a r , J.A. [1973] Total electron content studies of the ionosphere. Air Force Cambridge Research Labs. Report TR-73-0098. N is h iz a k i, R. and N a g a y a m a , M. [1969] Analysis of observational data obtained by Alouette II, V. Comparison of foF2 from two observational methods of top-side sounding and ground-based sounding. J. Radio Res. Labs., Japan, 16, 227-234. P h y sic a l R esearch L a bo r a to r y, Ahmedabad [1969] Proc. of Third International Symposium on Equatorial Aeronomy, India. P ig g o t t , W.R. [1970] The use of satellite data for prediction purposes, in ionospheric forecasting. AGARD Conference Proceed ings N o. 49. P ig g o t t , W.R. and R a w e r , K. [1972] U.R.S.I. handbook of ionogram interpretation and reduction (2nd Ed.), World Data Center A for Solar-Terrestrial Physics. Report UAG 23. NOAA, Boulder, Colorado, 80302, U.S.A. Spanish text available from L.I.A.R.A. (Argentina), and French text available from CNET (France). R ish b eth , H. and K e lley, D.M. [1971] Maps of the vertical F-layer drifts caused by horizontal winds at mid-latitudes. J. Atmos. Terr. Phys., 33, 539-545. S ayers, J. [1970] In situ probes for ionospheric investigation. J. Atmos. Terr. Phys., 32, 663-691. — 75 — Rep. 251-1 SECTION 6F: PROBLEMS OF INTERFERENCE RECOMMENDATIONS AND REPORTS Recommendations There are no Recommendations in this section. Reports REPORT 251-1 * INTERMITTENT COMMUNICATION BY METEOR-BURST PROPAGATION (1959 - 1963 - 1966) 1. Introduction The subject of VHF propagation by reflections from columns of ionization produced by meteors (meteor trails) has been referred to in Report 259-3, where signals inadvertently so transmitted were considered in terms of their ability to cause interference. It is now clear that this propagation mode can be used for communications. 2. Summary of available information 2.1 Complete information is not available on the geographical distribution of the meteors important in this mode of transmission and of their preferred directions. Some data exist on the distribution of meteor-ionization over parts of Canada, the Federal Republic of Germany, the United Kingdom, the United States of America, Australia, New Zealand and Italy. The data available are probably adequate for prediction within 10 dB, but this possibility has not yet been demonstrated. 2.2 Theoretical computations indicate that, for a given transmitter power, the average information rate of an intermittent system can increase with increasing bandwidth. 2.3 Experimental single-channel two-way telegraph circuits have been operated in the 30 to 40.MHz frequency range, over distances of 600 to 1300 km with transmitter powers of 1 to 3 kW. One-way transmissions of voice and facsimile have also been made with transmitter powers of 1 kW and 20 kW respectively. Experimental results have indicated the possibility of useful transmission, in bandwidths up to 100 kHz, for time intervals of the order of one second and duty cycles of the order of 5%. However, it should be noted that the duty cycle decreases with increasing frequency. 2.4 Reflections from meteor-ionization trails are highly dependent upon path geometry. This characteristic results in propagation which is dependent on geographical location, path orientation, operating frequency and time. Optimum conditions usually occur a few (5° to 10°) degrees off the great circle path. * This Report, which was adopted unanimously, represents the termination of work on Study Programme 15A/6 which has been cancelled. Rep. 251-1, 259-3 — 76 — 2.5 During the burst interval, the system loss is significantly less than that associated with ionospheric scatter. This suggests the use of meteor-burst propagation for mobile and airborne installations (requiring less power and antenna gain). Meteor-burst propagation may also be useful in geographical regions where ionospheric-scatter losses are unusually high. 2.6 Meteor-burst propagation is known to be subject to abnormal attenuation, as for example, during polar-cap absorption events and sudden ionospheric disturbances. As would be expected, the absorption effects are less at higher frequencies as shown by measurements on 104 MHz and 41 MHz. But note that attenuation as high as 30 dB at 100 MHz has been observed on rare occasions during polar-cap absorption events. 2.7 Few data are available at frequencies above 100 MHz. B ibliography Some general references on the characteristics of meteors are : Lo vell, A.C.B. [1954] Meteor Astronomy, Oxford. M cK in l e y , D.W .R . [1961] Meteor Science and Engineering. McGraw-Hill Book Co., Inc. Characteristics of special interest to radio transmission are summarized in : M a n n in g , L.A. and E sh e l m a n , V.R. [1959] Meteors in the ionosphere. Proc. IRE, 47, 186-199. V il l a r d , O.G. et al. [1955] The role of meteors in extended-range VHF propagation. Proc. IRE, 43, 1473. These papers contain numerous references to earlier work. Most of an issue of the Proceedings of the IRE was devoted to this subject (Proc. IRE, 45 (December, 1957)). More recent papers are : E y f r ig , R.W ., H ess, H.A. and R a w e r , K . [1962] Ober die Haufigkeit von Meteorechos (Concerning the frequency of occurrence of meteor echoes). Archiv der elektrischen Vbertragung, 16, 609. H o r n b a c k , C.E. et al. [1960] The NBS meteor-burst propagation project, a progress report. NBS Technical Note 86. M a y n a r d , L.A. [1961] Propagation of meteor burst signals during the polar disturbance of November 12-16, 1960. Canadian Journ. Phys., 39, 628. S u g a r , G.R. [1964] Radio propagation by reflection from meteor trails. Proc. IEEE, 52, 116. REPORT 259-3 * VHF PROPAGATION BY REGULAR LAYERS, SPORADIC E OR OTHER ANOMALOUS IONIZATION (Study Programme 4B-2/6) (1951 - 1953 - 1956 - 1959 - 1963 - 1966 - 1970 - 1974) 1. Regular layers VHF propagation by way of the regular E layer is unlikely at any time. Propagation by reflection from the FI layer may take place at frequencies greater than 30 MHz but the highest standard MUF will usually be for F2 and not FI propagation. * Adopted unanimously. — 77 — Rep. 259-3 Near the peak of the solar cycle long-distance transmissions via the F2 layer in temperate latitudes can occur for a significant fraction of the time up to 50 MHz or more. In low latitudes regular transmission will occur around 30-40 MHz, and such transmission can occur at 60 MHz and above. The lowest frequency at which interference due to such propagation becomes so infrequent as to be negligible is about 60 MHz for stations in temperate latitudes, and 70 MHz for stations at low latitudes (see Report 260-2). 2. Sporadic E The most familiar form of sporadic E, so-called temperate zone Es, appears as an intensification in ionization in the form of a horizontal sheet of about 1 km average thickness and a horizontal dimension of the order of 100 km. The height is commonly 100 to 120 km. The sheets appear in a random manner but show distinct preference for certain times of day and months of the year. The occurrence of sporadic-E propagation decreases with frequency. It may be considered a negligible factor above 90 MHz. Sporadic-E propagation above 30 MHz rarely occurs at distances of less than 500 km ; the limiting distance for one hop is about 2000 km. Multihop transmission of VHF by sporadic E is possible, but unlikely. 2.1 Geographical and temporal variations There are three major sporadic-E zones: equatorial, temperate, and auroral. The equatorial zone is a belt centred on the magnetic equator and extending 6° in magnetic dip on either side of it. A highly transparent form of sporadic E (q-type) is a regular occurrence on the day side of the belt and can be shown to be associated with the equatorial electrojet. Propagation of VHF signals via q-type Es is by scattering. The temperate zones cover most of the Earth and extend from the equatorial zone to a geomagnetic latitude of about 60° (actually to the line at which the occurrence of aurora is 15% of its maximum). The most dominant feature in the temperate zone is the summer maximum in occurrence of intense sporadic E, which becomes more distinct as one moves to higher latitudes until it is sharply altered by auroral zone influences. Sporadic E rarely occurs between the hours of midnight and 0600 local time. It normally peaks around 1000 local time and in some areas exhibits a second peak in the afternoon or the evening. The sporadic-E layer is often very dense in the daytime. There are considerable longitude as well as latitude variations in the occurrence of sporadic E in the temperate zones. A mechanism associated with wind shear appears most promising as an explanation for intense sporadic E at temperate latitudes. In the auroral zones, the dominant feature is the night-time peak in the occurrence of sporadic E. The summer peak observed at temperate latitudes disappears entirely at the zone of maximum visible aurora. The auroral zones, like the equatorial zone, are regions of heavy current flow in the lower ionosphere. In addition, they are regions where ionized layers can be formed through the precipitation of charged particles. It appears that both of these phenomena may be responsible for the production of sporadic E. 2.2 Estimating Es signal strength from vertical-incidence data The only comprehensive and continuing source of sporadic-E data is the world-wide network of ionosonde stations. It is important therefore to develop quantitative methods for estimating the oblique-incidence transmission characteristics from the vertical-incidence data. A consideration of this question for the United States is found in Davis et al. [1959]. A study of this problem in Japan has been carried out by Miya and Sasaki [1966]. Sporadic E is first classified by its reflection coefficient, T, as observed on VHF oblique-incidence circuits. The study Rep. 259-3 — 78 — suggests that if T < 4 5 dB, then the sporadic-E reflections are specular in nature, while if 45 dB < T < 70 dB, a scattering mechanism is involved. Curves are available for calculating V in terms of the ratio / / foEs, w here/is the transmission frequency in question and foEs is the vertical-incidence critical frequency of the ordinary wave for sporadic E taken from ionosphere observations. Estimates are available for both single-hop (0 to 2100 km) and two-hop (2400 to 4000 km) sporadic-E modes. An important feature of the method is the inclusion of antenna-to-medium coupling loss. 2.3 Occurrence of oblique sporadic-E propagation Oblique-incidence pulse soundings between Greece and the Federal Republic of Germany (1700 km) [Reinisch, 1965] showed that the MOF was often determined by Es reflections from May through September, the average daytime occurrence (0600 to 1800 hours local time) being about 10% at 22 MHz decreasing to almost zero at 30 MHz. An examination of the oblique-incidence data obtained on the Ottawa-The Hague (5500 km) and Winnipeg-Resolute Bay (2800 km) circuits indicate that the sporadic-E mode at times controls the operational MUF, but sporadic-E layer propagation seldom occurs at frequencies greater than 30 MHz over these paths [Stevens, 1968]. Since 1962, the E.B.U. has recorded data on 23 paths [Lari et al., 1967], varying in length between 900 and 2510 km, and which are situated in different locations in Western Europe. Daily measurements were taken at five frequencies in the range 41 to 59 MHz between 0800 and 2300 GMT from April through October of each year. The results may be summarized as follows: — considerable year-to-year variation exists, differences in field strengths of up to 35 dB were observed; — the month of the maximum occurrence of sporadic E varied between May and August; — for a given path this month varies from year-to-year, and for a given year it changes for the various paths; — studies of the variation in field strength with distance showed maximum field strengths (exceeded 1% of the time) occurring at a distance of approximately 1400 to 1500 km; at these ranges the field strength exceeded for 5 % of the time was about 15 to 30 dB lower. At 2500 km the corresponding field strengths were only 10 to 20 dB less than those at 1500 km. This result suggests that, at VHF, atmospheric refraction and the use of elevated antennae may extend the maximum range for one-hop ionospheric propagation beyond that normally experienced at lower frequencies. The observations made during a complete solar cycle of 11 years, indicate that there is no simple long-term correlation between solar activity * and VHF propagation via Es. However, it became apparent that the yearly variations are rather regular and tend to be similar for different paths. Table I shows the years when maxima and minima of the field strength were observed. T a b le I Path /M H z Field-strength Field-strength maxima minima Divis—Enkoping 41-465 1964, 1966,1971 1965, 1967/8 Divis—Chatonnaye 41-465 1964, 1966, 1971 1965, 1967/8 Meldrum—Enkoping 58-215 1963/4, 1966 1965 Divis—Monte-Lauro 41-465 1964, 1968/9, 1971 1963, 1965/6, 1970 The E.B.U. also undertook a brief study on three paths (Table II) for June of 1971 of the duration of the periods of Es propagation (each period is denoted as an “ event ”). The reference level was defined as the mean of field-strength values exceeded for 1 % of the time for the period 0800-2300 GMT during June in the years 1965 through 1970. * Sunspot-maximum: 1968/69; minimum: 1964/65. — 79 — Rep. 259-3 T a b le II Transmitter Reception site /M H z D km Divis Monte-Lauro 41-465 2510 Divis Morlupo 41-465 1930 Meldrum Morlupo 58-215 2000 In these cases it could be seen that: — the events having the shortest duration occur most frequently: some three quarters of the events had durations of less than two minutes (Fig. 1); — the total duration of the events is due largely to the duration of the longest events, and only one quarter of the total duration is contributed by the events lasting less than three minutes. These results, which are still too limited, cannot be considered to be definitive until supplementary studies have been undertaken. This would also require the assessment of the subjective effects on viewers of television broadcasts with respect to this special type of interference. 2.4 Characteristics of oblique sporadic-E propagation Studies by the Max-Planck-Institute [Moller, 1963] using HF pulse transmissions with different transmitter powers over a 1965 km path in middle latitudes indicate the possibility that propagation by the sporadic-E layer may be power sensitive. This could result, for a 10 dB change in transmitter power, in modest differences (6 to 8 %) in Es MOF. During the summer months, the power sensitivity appears to vary as a function of the time of day, indicating a possible correlation with the physical structure of the sporadic-E layer (see Report 344-2). As a consequence of a study made in the U.S.S.R. on oblique-incidence propagation over paths of about 1000 km in length and at frequencies of 4-5 to 40 MHz, with ionosondes near the path midpoints, the following conclusions were drawn [Kerblai et al, 1973]: — the fading rate during Es propagation by specular reflection is slow (0 01 to 01 Hz), but increases when the scattering type of reflection sets in (0-05 to 0-4 Hz); — while the secant law relating vertical-incidence values of foEs to oblique-incidence conditions is in satisfactory agreement in the HF band, it gives values 1-2 to 1-3 times too low at VHF. 3. Propagation by other anomalous ionization 3.1 Meteoric ionization Studies have been made in Canada, the Federal Republic of Germany, the United Kingdom, the United States of America and elsewhere, of the reflections which occur from meteor trails. Report 251-1 gives a bibliography on this subject. 3.2 Trans-equatorial propagation Recent studies indicate that strong transmission can occur, particularly during high sunspot years, over long North-South paths spanning the magnetic equator. Most observations have been made by radio amateurs at a frequency of 50 MHz for paths of the order of 4000 to 9000 km; paths between ^ South America-North America, Africa-Europe and Japan-Australia have been noted. From the observation of echoes due to trans-equatorial propagation at frequencies of 32, 48 and 72 MHz carried out between Japan and Australia during a half-cycle of solar activity (1964-1968), it was found that trans-equatorial propagation occurred on 32 MHz during a large part of the day (except for a few hours in the morning) even in years of low sunspot activity. The hours of reception Rep. 259-3 — 80 — are expected to increase with solar activity. At frequencies of 48 and 72 MHz, trans-equatorial propagation occurs mostly at night during the equinoctial periods. It seems that propagation in the daytime is by the 2F mode, based on the equatorial anomaly; the night-time signal which exhibits rapid shallow fading is due to a scattering process associated with the equatorial spread-F [Tao et al., 1970]. 3.3 Ionization of other kinds The studies indicate that there may, at times, occur bodies of ionization at heights different from those of any of the recognized ionospheric layers. Such ionization irregularities may occasionally give rise to reflections of waves in the 30 to 300 MHz range, the principal case being that of reflection from the edges or sides of magnetic-field aligned irregularities which occur within or near the auroral zone. Such reflections may constitute a source of interference to stations working in the 30 to 300 MHz range [Czechowsky, 1970]. Reflections have been observed via the F region, at frequencies appreciably above the F2 MUF but at intensity levels well below free-space. These reflections have been observed in the Far East, in South America and in Africa and appear to be a phenomenon of high sunspot years [Bateman et al., 1959]. The equinoctial months appear to be favoured. 3.4 Ground and ionospheric side-scatter VHF radio waves propagated via the ionosphere have been observed to arrive at azimuths which differ considerably from the great circle direction. The signals are predominantly the result of multihop modes of propagation in which side-scatter occurs from irregularities at the surface of the Earth (Report 429-1). The intensity levels of the signals are well below free-space values and exhibit fading frequencies which are less than one hertz. — 81 — Rep. 259-3 4. Main causes of interference to stations working at frequencies between 30 and 300 MHz Cause of Latitude Period of Approximate Approximate Approximate interference zone . severe highest frequency range o f' interference frequency above which distances with severe interference affected interference is negligible (km) (MHz) (MHz) Regular Temperate Day, equinox- 50 60 E-W paths F-layer winter, solar- 3000-6000 reflections cycle maximum or N-S paths 3000-10 000 Low Afternoon to 60 70 late evening, solar-cycle maximum Sporadic-E Auroral Night 70 90 500-4000 reflections Temperate Day and 60 90 evening, summer Equatorial Day 60 90 Sporadic-E Low Evening up to 60 90 Up to 2000 scatter midnight Reflections All Particularly May be important anywhere in Up to 2000 from meteoric during the range ionization showers Reflections Auroral Late afternoon from magnetic and night field aligned columns of auroral ionization Scattering in the Low Evening through 60 80 1000-4000 F region midnight, equinox Special Low Evening through 60 80 4000-9000 transequatorial midnight effects Rep. 259-3 — 82 — F ig u r e 1 Distribution of durations of Es events at frequencies of 40-60 MHz (June, 1971) t = duration of “ events N /N to i = proportion of all events, having a duration less than t. • Divis — Monte-Lauro A Divis — Morlupo ° Meldrum — Morlupo R eferences B a t e m a n , R., F in n e y , J.W., Sm it h , E.K., T v et en , L.H. and W a tts, J.M. [1959] IGY observations of F-layer scatter in the Far East. J. Geophys. Res., 64, 403-405. C z e c h o w sk y , P. [1970] Berechnung eines Stromsystems in der polaren E-region (Calculation of a current system in the polar E-region). Zeitschrift fur Geophysik, 3 6 , 647-650. D a v is, R.M ., S m it h , E.K. and E llyett, C.D. [1959] Sporadic E at VHF in the U.S.A. Proc. IRE, 47 , 762-769. K e r b l a i, T.S., N o so va, G .N ., M i n u l i n , R.G. and K u r g a n o v , R.A. [1973] Periody fluktuatsii signalov s chastotoi 27-8 MGts, otrazhennykh ot intensivnykh sloev Es (Fluctuation periods of signals with a frequency 27-8 MHz reflected from intense Es layers). Collection of articles “ Questions concerning short-wave propagation ”, Izmiran. L a r i, G., T a g h o lm , L.F. and B ell, C.P. [1967] Research on VHF ionospheric propagation (Bahd I). Techn. 3085-E, EBU Technical Center, Brussels. M iy a , K. and S a sa k i, T. [1966] Characteristics of ionospheric Es propagation and calculation of Es signal strength. Radio Science, 1, 99-108. M oller, H.G. [1963] Impulsiibertragungsversuche mit schrager Inzidenz und veranderlicher Frequenz uber Entfernungen zwischen 1000 km und 2000 km (Pulse transmission tests at oblique incidence and variable frequency made over distances between 1000 km and 2000 km). Forschungsbericht des Landes Nordrhein-Westfalen Nr. 1149. R e in is c h , B. [1965] Die Bedeutung von Echos an der sporadischen E-Schicht bei Impulsfernubertragung uber 1700 km (Athen- Breisach) (The influence of sporadic-E echoes using pulse transmission for a distance of 1700 km (Athens-Breisach)). AEU, 19, 361-366. S te vens, E.E. [1968] The significance of sporadic-E propagation. Ionospheric Radio Communications. Ed. K. Folkestad, Plenum Press, New York, 289-293. T a o , K., O c h i, F., Y a m a o k a , M., W a t a n a b e , S., W a t a n a b e , C. and T a n o h a t a , K. [1970] Experimental results on VHF trans equatorial propagation. J. Radio Res. Labs. (Tokyo), 17, 83-101. — 83 — Rep. 259-3, 260-2 B ibliography B ailey, D.K. [1962] Ionospheric forward-scattering. Monograph on Ionospheric Radio. Ed. W.J.G. Beynon, Elsevier, 189-199. C o l l in s, C. and F o r syth e, P.A. [1959] A bistatic radio investigation of auroral ionization. J. Atmos. Terr. Phys., 13, 315-345. D ie m in ge r, W., R ose, G. and W id d e l , H.U. [1963] On the probability of co-channel interference in VHF bands I, II and III in Africa, caused by scatter and reflection from the E and F layers. Telecommunication Journal, 30 , 251-256. REPORT 260-2 * IONOSPHERIC-SCATTER PROPAGATION (1956 - 1959 - 1963 - 1966 - 1970) 1. Introduction Considerable experience has been gained since the first high-speed communication circuits employing ionospheric-scatter propagation came into regular service in 1953. The circuits already operating during the low sunspot minimum of 1954, above latitude 40°N in the United States of America, have now been supplemented by operational and experimental links which extend the range of observation from the tropics to the Arctic and through a period of solar activity with an unprecedently high maximum (1957-1958). The newer operational links have, in accordance with earlier mid-latitude height measurements, been designed, and at least operated initially, to use midpoint scattering from a height of 85 km. New terminal equipment has been developed to include, for instance, advances in modulation techniques and in antenna design, and considerable attention has been given to the problem of reliability. Experience with ionospheric-scatter propagation, derived both from experimental studies and practical communication systems, has clarified a number of features of the observed propagation such as the important advantages that may be obtained practically by realization of the contribution from aspect- sensitive meteor ionization and magnetic field-aligned ionization structures. The implications of these contributions from the viewpoint of interference are negligible compared with the occasional occurrence of propagation by the sporadic-E mode. Report 259-3 gives information regarding propagation by sporadic E at the appropriate frequencies. Report 109-2 may be consulted in connection with this Report for a more detailed discussion of system aspects of ionospheric-scatter propagation. In the following sections, a series of topics is discussed in the light of recent experience. 2. Geographic distribution of signal intensity The records show that, without regard to path orientations and the scattering mechanisms, the signal intensities are high, both in a region within about 20° of the magnetic pole, and in the region of the magnetic equator, but that there is a fairly deep minimum at about 20° to 30° from the magnetic equator. In this trough, it appears that for a large fraction of the time, the received signals are those that have been scattered from meteoric ionization. The location of this minimum has been fixed most accurately in the central Pacific Ocean region north of the equator, but it may be assumed that its geographic latitude will be a function of longitude. There is evidence in all latitudes, that certain types of ionospheric irregularity are aligned with the direction of the Earth’s magnetic field. Indeed in the region of the minimum just described, communication systems employing antennae directed to take advantage of scattering from field-aligned ionization have exhibited greatly improved performance in comparison with otherwise substantially identical facilities with antennae directed towards the path midpoint. * This Report, which was adopted unanimously, represents the termination of work on Study Programme 13A-1/6, which has been cancelled. Rep. 260-2 — 84 — 3. Influence of the solar cycle A long-term variation in signal intensity related to the solar cycle would be expected and there is some evidence of it during the daytime. At times of high solar. and magnetic activity, daytime median-signal intensities are higher by several decibels than during the minimum of the solar cycle. The differences are not constant, but vary with the path, time of day and season. The weaker signal intensities are almost independent of solar activity, in accordance with the suggestion that they are representative of conditions when meteoric ionization plays a relatively important part in the scattering process. In middle latitudes the results are similar, though the effects of magnetic disturbances are less noticeable. At low latitudes, propagation by means of the intense field-aligned ionization existing near the magnetic equator has been found to increase in intensity with decreasing sunspot number, as might be expected from the negative correlation between the intensity of the equatorial electrojet and sunspot number. The solar cycle dependence is far from simple to evaluate quantitatively, since it is complicated in high latitudes, for example, by the effect of absorption which tends to reduce signal intensities selectively at times of high solar and geomagnetic activity. Moreover, its true nature may well be obscured by long term variations in meteoric ionization unrelated to the solar cycle. 4. Influence of magnetic activity While extensive analyses of the influence of magnetic activity have been based on the planetary magnetic AT-index, other indices, such as the local magnetic indices and the occurrence of blackouts in high latitude ionograms, have also been used. It is important to recognize that the X-index provides a fair measure of solar corpuscular radiation. In high latitudes, for frequencies in the 40 to 50 MHz range, it is well established that high A-indices are accompanied by abnormally high signal intensities. For somewhat lower frequencies, the relationship is less clear, because of the increased importance of ionospheric absorption at times of high magnetic activity. In middle latitudes, the relationship is much less striking, as might be expected, since solar corpuscular radiation seldom penetrates to such latitudes. At low latitudes, very little correlation exists between the signal intensity resulting from the normally random scattering process and magnetic activity. However, the propagation by way of the intense field-aligned ionization at the magnetic equator shows a remarkably close correlation during daytime with even small variations in the local horizontal magnetic intensity such as magnetic bays. The dependence on magnetic activity of the field-aligned propagation, that is of particular operational significance at locations removed 20° to 30° from the magnetic equator, remains to be determined. 5. Equatorial ionospheric-scatter It has been suggested that the strong signal intensities associated with equatorial ionospheric- scatter propagation are related to the equatorial or q-type sporadic E. Because of the very close correlation between variations of the intensity of scatter signals and variations of local magnetic intensity during the daytime that are associated with the equatorial electrojet, an association with the electrojet seems also indicated. These associations are strengthened by observations which show that the scattering sources responsible for the strong signals lie between the heights of 100 and 110 km, within which the electrojet occurs. Oblique scatter signal intensities, however, have been observed at night without q-type sporadic E, that are of nearly equal intensity to daylight signal intensities when q-type sporadic E is present. 6. F-region scatter Recent experience in sub-equatorial regions in the central and western areas in the Pacific Ocean has shown that, at times, there is scattering from the F region of the ionosphere. No serious difficulties in the operation of ionospheric-scatter links have resulted from the simultaneous presence of F-region — 85 — Rep. 260-2 scattering. The geographical limits of F-region scattering have not been clearly established, but it has been observed many Jiundreds of kilometres from the equator. 7. Multipath signals Short-delayed multipath signals, up to a few milliseconds, arise from meteoric reflections. Long delayed multipath signals from F2 reflections may disturb the ionospheric scatter circuit, even at times of low solar activity, when lower frequencies of VHF are used. Such echoes may be delayed up to 40 milliseconds or more [Koch, 1956]. It has been shown [Kanaya and Isomura, 1966] that one of such multipath signals is the radio signal propagated by F2 reflection and side-scatter in the neigh bourhood of a continent and of islands. The time, rate of occurrence and the bearings of such echo signals can be predicted by calculating the MUF for the echo signals, using a suitable method based on the appropriate assumptions. A more comprehensive discussion of ground and ionospheric side- scatter at HF and VHF is given in Report 429-1. Observed and calculated results show that the rate of occurrence of echo signals increases as solar activity ascends. The bearings of the echo signals, in general, differ from that of the main signal, but it can be predicted that the bearings of the echo signals will spread over a wide range including the bearings of the main signal, when solar activity becomes very high. 8. Useful range of frequencies It is now usually accepted that the useful range of frequencies for communication by the ionospheric- scatter mode of propagation extends from about 30 to 60 MHz at the lower end of the VHF band (band 8). The lower limit is set by the increasing ionospheric absorption associated with decreasing frequency during SID and polar cap events and also by the desirability of avoiding interference from signals propagated by reflection between the Earth and the F2 layer. At times, when the circuit is operating on the scatter mode, interference may be caused by transmitters from other directions where the propagation of the F-layer mode is possible. Although mutual interference between the scatter circuit and other circuits may sometimes be mitigated by the use of highly directional antennae, this type of interference can only be satisfactorily avoided by careful planning in the allocation of frequencies. An especially important consideration in the selection of frequencies is the possibility of self interference, as mentioned in § 7, by energy scattered back from the ground and propagated by the F2 reflection mode. This situation arises primarily at times of high solar activity but may also be trouble some at low levels of solar activity in certain areas of the world. The charts, Figs. 1 to 3, discussed in § 9 below, provide a guide for choice of frequencies to avoid such interference. Special modulation techniques have now also been developed (see Report 109-2) to reduce the effect of back-scattering so that it may be no longer a source of difficulty for telegraphic communications. The fundamental limitation of the use of lower frequencies is thus the increasing absorption. At high latitudes, extreme absorption during times of polar-cap absorption can render the circuit unusable for a rapidly increasing fraction of the time, as the operating frequency is decreased. The absorption can be fairly serious for a significant fraction of the time, even in the 30 to 40 MHz range, at times of high solar activity, if extreme reliability of operation is sought. The upper limit of frequency is fixed by mainly economic considerations. Since, as a rough guide, the signal intensities may be taken as varying inversely as the l l/2 power of the frequency, for scaled antennae with equal input power, while the cosmic noise intensities vary inversely as about the l l/2 power of the frequency, the signal-to-noise ratio varies inversely as approximately the 5th power of the frequency. With this frequency dependence of the signal-to-noise ratio and using scaled antennae, 15 dB more transmitter power is required at 60 MHz than at 30 MHz to give the same signal-to-noise ratio. Rep. 260-2 — 86 — 9. Choice of operating frequencies The above consideration of the factors which decide the useful range of frequencies suggests that the choice of frequencies for given ionospheric-scatter circuits, may be grouped under three categories of service: 9.1 Single-frequency circuits in the low part of the range (say 30 to 45 MHz) Such circuits would be suitable in temperate latitudes for providing reliable telegraph and facsimile services when special antennae and modulation techniques are used to suppress the self-interference from back-scattered multipath echoes. For amplitude-modulated speech, such provisions are not necessary, as the intelligibility is not seriously affected by this form of interference. Continuous single-frequency operation, of course, implies the possibility of long-range interference by F2-layer propagation, as with HF circuits, and also by sporadic-E reflections. To avoid this latter type of interference, ionospheric-scatter circuits should have their transmitting and receiving terminals separated geographically by at least 2500 km from other circuits capable of causing such interference. As a rough guide to the probability of interference by F2-layer propagation, Figs. 1, 2 and 3 show contour maps of the maximum frequencies reflected by the F2 layer, which are exceeded for 1 % of the time during the December solstice, June solstice and Equinox periods respectively, at sunspot maximum. A circle of radius 2000 km centred on the receiving antenna of an ionospheric-scatter circuit gives the frequencies for which interfering signals can arrive from various directions over a 4000 km path for 1 % of the time. The percentage of time is smaller for paths longer or shorter than 4000 km. 9.2 Single-frequency circuits above 45 MHz These circuits would be used for services requiring the highest reliability, and which can be of limited channel-capacity or are designed for the high power necessary for greater channel capacity. They have the advantage that the antennae and modulation techniques used can be simpler than those for single-frequency working below 45 MHz. 9.3 Two-frequency circuits Particular interest is being taken in two frequency operations, using one frequency in the 30 to 45 MHz range and the other between 45 and 60 MHz. This category would contain services requiring the highest obtainable reliability, which are designed for the minimum power needed for normal operation at the lower frequency, but which can work at the higher frequency on greater power, or with reduced transmission capacity, if it is necessary to assure continuity of service through occasional periods of intense absorption, or of interference by long-range F2-layer propagation at the lower frequency. The need for the higher frequency will be greatest at times of high solar activity, but it would be available at all times, ready to take over from the lower frequency at the onset of difficult conditions. The actual choice of frequencies in the two ranges will depend on geographical location. 10. Modulation techniques and automatic error-correction Apart from the modulation techniques designed to overcome the effects of self-interference from back scattering, special frequency-modulation techniques for telegraphy have been adopted to combat the Doppler effects associated with reflections from meteoric ionization. These use time-division rather than frequency-division multiplex, and 16-channel systems of 100 words per minute are now in use. Ionospheric-scatter circuits are not economically capable of providing sufficient bandwidth for extensive voice transmission and are not likely to be used for more than one or two speech channels. — 87 — Rep. 260-2 Automatic error-correction techniques, which are particularly effective when the natural character error-rate is moderately high, can be made highly efficient when applied to telegraph traffic passed over an ionospherie-scatter circuit. 11. Antenna design The problems of antenna design centre round the need to obtain, for a given available input power to the transmitting antenna, the maximum protection at the receiving antenna against harmful interference from multipath signal components and, to a more limited degree, from other transmitters; and to achieve the best possible signal-to-noise ratio, in view of the inherently high transmission loss associated with ionospheric-scatter propagation. Large rhombic antennae, which were much used in the original installations, are now disappearing in favour of more compact arrays of less gain and greater beamwidth, but with higher back-to-front ratios. Under conditions of low signal intensity, space-diversity reception is useful. 12. Antenna orientation While it is usual to direct the antennae of an ionospheric-scatter circuit in the great-circle plane to favour the reception of signals scattered from near the midpoint of the path, the importance of scattering from aspect-sensitive ionization, which is usually most effective in azimuths somewhat different from the great-circle plane, is now recognized. The effectiveness of such ionization as a scattering source varies greatly with the time of day, season, meteor activity, and with path position, length and orientation. By using antennae which take advantage of scattering from field-aligned ionization in regions of the ionosphere other than above the midpoint of the path, the signal-to-noise ratios obtained can, at certain times, be significantly increased. Such increases may be of particular. importance in improving the usability of a circuit at certain seasons, times of day and geographical locations, particularly where very low signal-to-noise ratios for scattering from the midpoint region occur for much of the time. 13. Reliability of ionospheric-scatter circuits In conclusion, it may be stated that, although much remains to be discovered about the mechanism of ionospheric scattering, the technique of communication by the scatter mode has so far progressed that the problems affecting the reliability of the services lie, not in the changing propagation conditions, but in the electrical, mechanical, electronic and human factors that enter into the running and the maintenance of the circuits. R e f e r e n c e s ' K a n a y a , S. and I s o m u r a , E. [1966] On the echo signals in the ionospheric-scatter VHF radio circuit between Japan and Taiwan. Published by the P.G.R.P., Inst. Elec. Commun. Engrs. of Japan. K o c h , J.W. [1956] Modulation technique VHF ionospheric scattering. IR E Trans, on Comm. Sys., CS-4. B ibliography A b e l , W.G. et al. [1955] Investigations of scattering and multipath properties of ionospheric propagation at radio frequencies exceeding the MUF. Proc. IRE, 43, 1255-1268. B a il e y , D.K. [1962] Ionospheric “ forward ” scattering. General survey to the XHIth General Assembly of U.R.S.I., London, September 1960, in Monograph on ionospheric radio. 189-199, Elsevier. B a il e y , D .K . et al. [1952] A new kind of radio propagation at very high frequencies observable over long distances. Phys. Rev., 86, 141-145. Rep. 260-2 — 88 — B a il e y , D .K ., B a t e m a n , R. and K i r b y , R.C. [1955] Radio transmission at VHF by scattering and other processes in the lower ionosphere. Proc. IRE, 43, 1181-1230. B a i n , W.F. [1963] Solar-induced effects on VHF ionospheric propagation at low magnetic latitudes. Agardograph 59. The effect of disturbances of solar origin on communications. 185-192, Pergamon. B l a i r , J.C., D a v is , R.M. and K i r b y , R.C. [1961] Frequency dependence of D-region scattering at VHF. NBS Journ. of Research, 65-D, 417-425. B r a y , W J. et al. [1956]. VHF propagation by ionospheric scattering and its application to long-distance communications. Proc. I.E.E., 103, Part B, 236-260. H a l l e y , P., D u b o c , J. and T h u il l i e r , J. [1961] Etude des limites superieures des frequences susceptibles d’etre r6fl6chies par l’ionosphere en Europe (Study of the upper limits of frequency that can be reflected by the ionosphere in Europe). Ann. des telecomm., 16, 5-6. R o se, J. et al. [1961] The Pacific scatter communication system. Convention Record, 5th National Symposium on Global Com munication, 110-113. S a s a k i , I . and I s o m u r a , E. [1962] On space diversity effect in ionospheric-scatter propagation at VHF. P.G.R.P., Inst. Elec. Commun. Engrs. of Japan. In addition to the above, attention is drawn to substantial parts of issues of journals as follows: IRE Transactions on Communications Systems [March, 1956] Vol. CS-4 Proc. I.E.E. [May, 1958] 105, Part B, containing, under Session I, the papers and discussion of a symposium on “ Ionospheric forward scatter propagation ”. Proc. IR E [October, 1955] 43. Proc. IR E [January, 1960] 48, 7-31. Report of JTAC on “ Radio transmission by ionospheric and tropospheric scatter ”. 80* 100* 120® 140* I60®e 180* w 160" 140* 120* 100® 80® 60® 40® 20® w 0® E 20® 40® 60® 60® 80® '00® 120® 140° 160° E IB0°» '60® 140° 120® 100° 80° 60° 40® 20° w 0® E 20® 40° 60® F ig u r e 1 F2 4000 MUF exceeded during 1% of hours — December solstice; sunspot maximum Rep. 260-2 60® 80® 100® 120® 140® 160® E 180® w 160® 140® 120® 100® 80® . 60® 40® 20® » 0® E 20® 40° 60® Vo © <50®80® 100® 120® 140” 160" E I80®« 160® 140® 120® 100” 80” 60® 40® 0® E 20® 40® F ig u r e 2 F2 4000 MUF exceeded during l°/0 of hours — June solstice; sunspot maximum F ig u r e 3 F2 4000 MUF exceeded during 1% of hours — Equinox; sunspot maximum Rep. 341-2 — 92 — REPORT 341-2* HF PROPAGATION BY DUCTING ABOVE THE MAXIMUM OF THE F REGION (Question 5-2/6) (1966 - 1970 - 1974) 1. Existence of HF propagation by ducting along field aligned ionization High frequency propagation along ionization irregularities aligned with the Earth’s magnetic field was suggested at almost the same time by Wagner [1928] and by Pedersen [1929]. Later, Obayashi [1959] and then Gallet and Utlaut [1961] made a similar interpretation of observations of long-delay echoes in HF radar. Further observations with HF radar seemed to confirm the interpretation [du Castel, 1963, 1965a; Petit, 1966], while others appeared to contradict it [Budden and Yates, 1952; Thomas et al., 1963]. In the development of space techniques, rocket observations were also interpreted in terms of propagation by ducting [Knecht and Russel, 1962]. Then satellite observations brought more extensive confirmation [Muldrew, 1963; Calvert and Schmid, 1964; du Castel, 1965b; Dyson, 1967; Loftus et al., 1966; Ramasastry and Walsh, 1969]. 2. Interpretations of observations The theory of propagation along field aligned irregularities was developed by several authors [Voge, 1961, 1962; Booker, 1962; Muldrew, 1963; du Castel, 1965c; Walker, 1966]. Some models of the irregularities assume a relative variation in ionization and others an absolute variation. Thus, the sizes of the irregularities and the gradient of the electron density deduced from these models will differ to some extent. There is difficulty in reconciling the results of satellite and ground based radar observations. In both cases, the guided propagation path may correspond to a line of force lower than that at the obser vation point, which complicates the interpretation of results. However, the density of the irregularities may be tentatively taken as a few percent of the ambient ionization density, and the dimensions perpendicular to the magnetic field seem to be a few kilometres [Loftus, et al., 1966]. A maximum in the frequency of occurrence is found near sunrise, and a secondary maximum near sunset [Loftus, et al., 1966; Muldrew, 1967; Petit, 1966; and du Castel, 1965c]. Initial studies using data gathered over a large region indicate that the occurrence of ducting seems to be independent of the level of geomagnetic disturbance [Muldrew, 1967; Loftus, et al., 1966]. However, a positive correlation with geomagnetic disturbance was found at northern midlatitudes (around 45° to 55° invariant latitude) for medium solar activity [Nishizaki and Matuura, 1972]. An analysis of satellite data from five low latitude telemetry stations indicates that the frequency of occurrence of ducting has a lunar semi-monthly oscillation with a maximum at about 6 and 21 days after a new moon [Sharma and Muldrew, 1973]. 3. Conclusions Further studies and measurements are necessary to reconcile the differences in the size of the irregularity, and the gradient of the electron density obtained from satellite and ground based observations. It is possible that the discrepancy results from observations made at different times and * Adopted unanimously. — 93 — Rep. 341-2 locations, and also from the use of different theoretical models for the irregularities in the data analyses. Additional work is necessary to establish a sound basis for quantitative estimates of the effects on radiocommunications. R e f e r e n c e s B o o k e r , H.G. [1962] Guidance of radio and hydromagnetic waves in the magnetosphere. J. Geophys. Res., 6 7, 4135. B u d d e n , K.G. a n d Y a tes, G.G. [1952] A search for radio echoes of long delay. J. Atm. Terr. Phys., 2 , 272. C alv ert, W. and S c h m id , C.W. [1964] Spread F observations by the Alouette topside sounder satellite. J. Geophys. Res., 69, 1839. du C astel, F. [1963] Recherche experimentale sur le guidage geomagnetique d’ondes decametriques. Ann. des telecomm., 18, 177. du C astel, F. [1965a] Mise en evidence experimentale d’un- guidage geomagnetique d’ondes decametriques. C.R. Acad. Sci., Paris, 2 6 1 , 1057. du Castel, F. [1965b] Sur certains echos guides observes par sondages ionospheriques en satellite. Space Research V, 216, North-Holland Publ. Co. du Castel, F. [1965c] Projet d’observation par satellite des irregularites d’ionisation alignees sur le champ magndtique dans la magnetosphere. Plan. Sp. Sc., 13, 1255. D y s o n , P.L. [1967] Magnetic field aligned irregularities at mid geomagnetic latitudes. J. Atm. Terr. Phys., 28 , 857-869. G allet, R.M. and U t l a u t , W.F. [1961] Evidence of the laminar nature of the exosphere obtained by means of guided high frequency wave propagation. Phys. Rev. Letters, 6, 591. K n e c h t , R.W. and R u s s e l , S. [1962] Pulsed radio soundings of the topside ionosphere in the presence of spread F. J. Geophys. Res., 67, 1178. L o ft u s, B.T., v a n Z a n d t , T.E. and C a lv er t, W. [1966] Observations of conjugate ducting by the fixed frequency topside sounder satellite. Ann. Geophys., 2 2, 530. M u l d r e w , D.B. [1963] Radio propagation along magnetic field aligned sheets of ionization observed by the Aloutte topside sounder. J. Geophys. Res., 68, 5355. M u l d r e w , D.B. [1967] Medium frequency conjugate echoes observed on topside-sounder data. Can. J. Phys., 4 4 , 3835. N is h iz a k i, R. and M a t u u r a , N. [1972] Conjugate ducted echoes observed on Alouette II ionograms. J. Radio Res. Labs., Japan, 19, 99. O b a y a sh i, T. [1959] A possibility of the long distance HF propagation along the exospheric field aligned ionization. Rep. Ion. and Space Res. in Japan, 13, 177. P e d e r sen, P.O. [1929] Wireless echoes of long delay. Proc. IRE, 17, 1750-1785. P e t it , M. [1966] Etude du guidage geomagnetique d’ondes decametriques entre points conjugues au sol. Diplome E.S., Fac. des Sc. Paris. R am asa st r y, J. and W a l sh , E.J. [1969] Conjugate echoes observed by Alouette II topside sounder at the longitudes of Singapore. J. Geophys. Res., 7 4 , 5665. S h a r m a , R.P. and M u l d r e w , D.B. [1973] A lunar effect in the occurrence of conjugate echoes on topside sounder ionograms. J. Geophys. Res., 7 8 , 8251. T ho m as, J.A., M cI n n e s, B.A. and C r o u c h l e y , J. [1963] High frequency exospheric duct propagation experiments. J. Atm. Terr. Phys., 2 5 , 613. V o g e , J. [1961, 1962] Propagation guidee le long d’un feuillet atmospherique ou exospherique. Ann. des telecomm., 16, 288 and 17, 34. W a g n e r , K.W. [1928] Beobachtungen uber die Reflexion von Hertzschen Wellenim Weltraum, weit ausserhalb der Erde (Obser vations on the reflection of radio waves in space, distant from the Earth). Elektr. Nachr. Techn., 5, 483. W a l k e r , A.D. [1966] The theory of guiding of radio waves in the exosphere. J. Atm. Terr. Phys., 28, 1039. Rep. 343-2 — 94 — REPORT 343-2 * SPECIAL PROBLEMS OF HF RADIOCOMMUNICATION ASSOCIATED WITH THE EQUATORIAL IONOSPHERE (Question 6-2/6) (1966 - 1970 - 1974) 1. Introduction The use of HF radiocommunications encounters special problems in equatorial regions since the equatorial ionosphere differs markedly from that at temperate latitudes. The principal differences are: — that the F layer is very thick; it has relatively large plasma frequencies and often has large spatial and temporal ionization gradients; — that irregularities are often present in the night-time F layer; — that a particular type of Es layer, associated with the equatorial'electrojet, is a regular daytime occurrence; — that ionospheric absorption is high. In equatorial regions the Earth’s magnetic field lines have low angles of dip and hence the wave polarization and the scattering intensity associated with irregularities which are field-aligned are dependent on the azimuth of the propagation path. The enhanced atmospheric thermal convection at low latitudes results in the generation of greater thunderstorm activity; atmospheric noise intensities in some tropical regions thus exceed those at temperate latitudes and present a higher background threshold for satisfactory signal reception. It should be noted that, with the exception of thunderstorms, all the above-mentioned phenomena are related to the magnetic dip; hence they are not “ tropical ” but “ low-dip ” phenomena. Though the well-known (ionized) layers and rather regular diurnal variations are found, the dip equator thus appears as a singularity for HF propagation. Therefore, propagation conditions along and transverse to the magnetic dip equator can be appreciably different. The unique equatorial characteristics have an important influence on the normal diurnal propagation features of the equatorial region. 2. Morphology of the equatorial F layer It is well known from both ground-based and satellite observations that significant latitudinal gradients exist in the F-layer ionization at low latitudes and that the form of the transequatorial distribution of foF2 changes markedly with local time. Shortly after sunrise, a single maximum of ionization exists near the magnetic equator, whereas, later, the “ equatorial anomaly ” develops such that a “ trough ” of low critical frequencies is found above the equator and “ crests ” of high values occur where the magnetic dip is approximately i 30°. The following summary of the structure and behaviour of the equatorial F layer is based partly on published literature [Rastogi, 1959; Croom et al., 1960; Lyon and Thomas, 1963; Vila, 1966; Eccles and King, 1969] and partly on work currently in progress in the United Kingdom. Fig. 1 shows an example of the structure of the topside ionosphere when the anomaly exists, together with the corresponding latitudinal variations of NmF2 and hmF2. It shows that the anomaly crests occur where a magnetically field-aligned arch of enhanced ionization intersects the peak of the layer; this arch extends into the topside ionosphere to altitudes of the order of 1000 km. The height * Adopted unanimously. — 95 — Rep. 343-2 of the layer peak is greatest (approximately 400 km) above the magnetic equator and decreases rapidly on either side to about 300 km near the location of the crests. It is not known at the present time whether the field-aligned arch in the topside ionosphere extends below the layer peak and, if so, in what manner. During the development phase of the anomaly, the arch of enhanced ionization moves to higher magnetic field lines such that its top extends to greater altitudes and the crests move away from the equator. The decay phase of the equatorial anomaly, however, has not been fully investigated, although there is evidence [Rastogi, 1959; Eccles and King, 1969] which suggests that the top of the arch decreases in altitude and the crests move back towards the equator. The time at which the anomaly develops and decays in different longitude sectors varies appreciably and has not been finally established. The topside sounder observations [King et al., 1964] made at longitudes near 105°E show that the anomaly exists between 1000 LMT and 2000 LMT. Near 75°W, however, it appears [Lockwood and Nelms, 1964] to form at any time between 1100 LMT and 1800 LMT; it is thus only consistently observed after 1800 LMT and sometimes persists until after midnight. Electron density data obtained by means of a probe on the Ariel 3 satellite indicates that the anomaly first appears and also disappears about two hours earlier in the Asian sector than in the American sector. Marked differences can occur between the latitudinal distributions of foF2 observed on a particular day at the same local time, but separated in longitude by only 30°. While, in some cases, the observed behaviour is very similar to the C.C.I.R. predictions in Report 340-2, at other locations the observed critical frequencies at the anomaly crests, and also the ionization gradients, are much higher than those predicted. The latitudinal variation of foF2 across the equator is sometimes asymmetrical, that is the critical frequencies at the two crests are appreciably different [Lyon and Thomas, 1963]. This asymmetry appears to be due [Bramley and Young, 1968] to thermospheric winds blowing across the equator from the sub-solar latitude; in general, the crest which lies farther from the sub-solar latitude will be the greater. King et al. [1967] have reported that on magnetically disturbed days the anomaly crests are generally significantly closer to the magnetic equator than they are on quiet days. 3. F-layer irregularities During the evening, irregularities develop in the F-region ionization inside a belt extending approximately between geomagnetic latitudes of 30°N and 30°S. The nature of these irregularities has been investigated by means o f: — ground-based and satellite-borne ionosonde data which show the existence of spread-F echoes produced by irregularities; — trans-equatorial VHF propagation data; — radio-star and satellite scintillation data; — HF back scatter data; — fixed-frequency HF signals and oblique sounder data. Details of the observational methods have been given by Clemesha and Wright [1967]. 3.1 Diurnal variation of the occurrence of irregularities The times of occurrence of irregularities have been inferred from studies using the above-mentioned types of data, but conflicting conclusions have been reached by different workers. Some apparent inconsistencies can be ascribed to the different height ranges involved in the various probing techniques. The irregularities appear to be associated with the change in the height of the F layer during the evening [Booker and Wells, 1938; Osborne, 1951; Wright, 1959; Lyon, Skinner and Wright, 1961]. The spread-F develops about 1 hour after the layer starts to rise and persists for several hours. According to Rao [1966], spread-F is seen in the evening whenever h'F exceeds 400 km. Lomax [1967] has observed that the onset of spreading occurs later at locations farther from the magnetic equator. Various authors Rep. 343-2 — 96 — including Smith and Finney [1960] have reported that evening enhancements of VHF scatter signals are more frequent on trans-equatorial paths in the Far Eastern sector than in the American sector. 3.2 Seasonal variation of the occurrence of irregularities Generally, the irregularities are more widespread during the equinoxes when the layer heights rise most rapidly [Osborne, 1951], but the seasonal variations of the occurrence of spread-F appear to be different in different longitude sectors; for example, Lyon, Skinner and Wright [1960, 1961] have concluded that there is no seasonal variation in Ibadan. Reber [1956] has reported the existence of a “ spread-F equator ” separating those locations which experience minimum spread-F in northern summer from those in which minimum spread-F occurs in southern summer. This equator does not coincide with the magnetic equator and, as noted by Singleton [1963], its position shifts during the solar cycle. 3.3 Solar-cycle variation of the occurrence of irregularities The way in which the occurrence of irregularities varies during the solar cycle is not clear. Increased spread-F during sunspot minimum has been reported for Maui and Rarotonga [Reber, 1956] and for Ahmedabad [Kotadia, 1959]. Rangaswamy and Kapasi [1964] concluded that at Baguio and Kodaikanal, however, spread-F occurs most frequently at sunspot maximum and that the belt containing the irregularities is narrower at sunspot minimum. At Ibadan and Singapore, however, no clear dependence on solar cycle is observed. 3.4 Effects o f magnetic activity on the occurrence of irregularities Within the equatorial zone, the incidence of spread-F is inversely correlated with magnetic activity [Lyon, Skinner and Wright, 1958]; this behaviour appears to be opposite to that observed at geomagnetic latitudes above about 30° [Lyon, Skinner and Wright, I960]. Koster and Wright [1960] have reported that at sunspot maximum the incidence of radio-star scintillations at Accra is also inversely correlated with magnetic activity, but there appears to be no clear correlation at sunspot minimum. The maximum amount of spreading on vertical incidence ionograms occurs later in the evening [Singleton, 1963] on disturbed days. 3.5 The height at which irregularities occur The existence of irregularities over a wide range of heights has been reported. On vertical incidence ionograms, spread traces are first seen at frequencies reflected at heights near the base of the F-layer, but as the evening progresses the heights of the irregularities increase and the spreading becomes restricted to higher frequencies [Wright, 1959]. Cohen and Bowles [1961] deduced, from experiments using narrow beam trans-equatorial VHF transmissions, that irregularities occurred at all heights from 100 km below the base of the layer to 450 km. From HF back-scatter measurements made in Ghana, Clemesha [1964] inferred that the heights of the irregularities are 400-450 km in the autumn and 300-400 km in spring. The heights of the irregularities deduced from satellite scintillation data range from 200 to 600 km. 3.6 The size and orientation of individual irregularities VHF forward-scatter and HF back-scatter data reveal that individual irregularities are magnetically field-aligned. Cohen and Bowles [1961] report transverse dimensions of more than 10 m and lengths along the field lines of more than 1 km. Axial ratios of about 20 to 1 have been deduced from satellite scintillation data [Kent and Koster, 1967]. 3.7 The thickness and size o f patches of irregularities The thickness of patches of irregularities has been estimated by Cohen and Bowles [1961], from observations of VHF signal pulse durations, to be typically 50 km. Thicknesses of 50-100 km have, — 97 — Rep. 343-2 however, been reported by Kent and Koster [1961, 1967] from analyses of satellite scintillations. H F back-scatter data also suggest [Clemesha, 1964] a vertical extent of 100 km. Irregularity patches sometimes cover large areas. Kotadia [1959] has reported good day-to-day correlation of spread-F echoes between Ahmedabad, Kodaikanal and Singapore during the equinox, but not the solstice, months. Cohen and Bowles [1961] have studied the motion of irregularity patches using successive vertical incidence ionograms and concluded that they extend up to 1000 km in the E-W direction. Satellite data suggest patch sizes of 100-2000 km in the N-S direction and 500-3000 km in the E-W direction. On the other hand, HF back-scatter data [Clemesha, 1964] indicate patch sizes of 30-400 km, with a mean value of 100 km, in the E-W direction. Calvert and Schmid [1964] have concluded from topside sounder data that not only individual irregularities, but also entire patches of irregularities, are field-aligned. 4. Equatorial (q-type) sporadic E In a narrow zone near the dip equator, a special type of sporadic E appears regularly during daylight hours [Matsushita, 1962; Knecht and McDuffie, 1962]. The width of this zone is ± 6° in magnetic dip and it is thus quite narrow in comparison with the width of the equatorial anomaly. This “ equatorial ” sporadic-E layer is only observed during daylight, is highly transparent and reaches high top frequencies; values of about 10 MHz are regularly observed using ionosondes near the dip equator. The structure of the sporadic-E irregularities, which are magnetically field-aligned, has been inferred by Cohen et al. [1962] from radar observations at Jicamarca, Peru. It appears that the origin of the irregularities is associated with the equatorial electrojet [Mayaud, 1965]. Theories involving plasma instabilities have been proposed [Farley, 1963; Waldteufel, 1965] to account for their formation. 5. Ionospheric absorption Shirke and Henry [1967] have reported the existence of an equatorial absorption anomaly. This anomaly has been further studied by George [1971] who showed that, when normalized to an overhead sun, the total vertical incidence non-deviative absorption is a maximum at the equinoxes in latitude zones where the magnetic dip is approximately 25-30°. In these zones the absorption (expressed in decibels) is about 80 % greater than that at the magnetic equator. It should be appreciated, none the less, that owing to the greater solar zenith angles, the absorption at the equator is greater than that at middle latitudes. 6. Influence of equatorial geophysical phenomena on radiocommunication 6.1 Propagation via the regular F layer During certain hours, steep horizontal gradients of ionization result in: — asymmetric ray paths; — deviations from great-circle propagation; — single hop propagation over long distances without intermediate ground reflections (see Report 250-3); — observed classical M UF’s in excess of the predicted standard M UF’s, particularly on N-S paths. Single hop propagation over distances greater than 10 000 km may arise [Fulton et al., 1960; Muldrew and Maliphant, 1962]. In such cases, the rays are successively reflected from the F layer without intermediate ground reflections [Stein, 1958]. When this happens focussing can occur and, if the rays do not pass through the absorbing region near the middle of the path, the received field strengths will be high. Low angles of elevation (less than 10°) are involved in these cases [Gerson, 1968]. 6.2 The Earth's magnetic field At low latitudes, where the magnetic field lines are horizontal, it is important for E-W paths— particularly at lower frequencies—to use horizontally polarized antennae since these result in maximum Rep. 343-2 — 98 — coupling into the ordinary wave. Large polarization losses, associated with a failure to couple to the ordinary wave after ground reflection, can arise in cases of multihop N-S propagation [Bradley and Bramley, 1971]. The aspect sensitivity of the scattering from irregularities, which are magnetically field-aligned will discriminate against forward- or back-scattered signals propagating in the N-S as opposed to the E-W directions. Clemesha [1964] has confirmed that at 18 MHz direct back-scatter returns in Ghana were strongest from directions to the East and West. According to Obayashi [1959], trans-equatorial propagation is theoretically possible along exospheric field lines at frequencies in excess of those which normally propagate via the equatorial F-layer (see Report 341-2). 6.3 Propagation via irregularities in the F region Both advantages and disadvantages for radiocommunication result from the existence of F-layer scattering irregularities. Satisfactory communication via scatter modes sometimes occurs at frequencies well above the predicted standard MUF—sometimes up to more than 100 MHz [Nielson, 1968]. On the other hand, irregularities can result in interference signals. The main disadvantage accompanying the use of scatter signals at HF is that movements of the irregularities cause rapid and deep fading (flutter fading). Fading rates are produced of up to 10 Hz, depending on the frequency used. Changes in the azimuth of the arriving signal of up to ± 50° have been observed on trans-equatorial paths, associated with the presence of spread-F [Crochet, 1972; Rottger, 1971]; this is further discussed in Report 429-1. The systematic movement of a group of irregularities results in a Doppler shift of the transmitted frequency (up to about 30 Hz at 20 MHz) while random motions of individual irregularities lead to Doppler spreading (up to about 20 Hz). The observed fading rates are different for different wave frequencies. Davies and Barghausen [1967] have reported that, for a path from Monrovia to Accra, the fading rates at 10 MHz are about six times greater than those at 20 MHz. These fading rates are consistent with the radio-star scintillation rates reported by Chivers [1960] and Lawrence et al. [1964] for frequencies below 30 MHz. On the other hand, Koster [1963] studied signals received in Ghana from London at frequencies of 7, 12, 15 and 21 MHz and concluded that the flutter fading rate was directly proportional to the wave frequency. He also observed that signals propagated along N-S paths experience more flutter fading than those along E-W paths. 6.4 Propagation via irregularities in the E-region Propagation in a narrow equatorial belt at frequencies up to and above the classical MUF is highly influenced by equatorial sporadic-E. The reflecting properties of the irregularities involved depend, of course, on the direction of propagation; side scatter may also occur. The fading rates of HF signals reflected from Es clouds are greater than those reflected from the E or F layers, but smaller than those scattered from F-layer irregularities; rates of 0-2-5 Hz are typical [Skinner and Wright, 1962; Rastogi et al., 1965]. An additional propagation mode, due to waveguide propagation within the field-aligned Es structure, has been observed [Mewes, 1971] and may cause multipath delays of 1-5 to 5 ms on paths which are closely perpendicular to the magnetic equator. 6.5 Thunderstorms and atmospheric noise World maps of atmospheric noise are given in Report 322-1 and current investigations of atmospheric noise in tropical regions are discussed in Report 303-1. — 99 — Rep. 343-2 R e f e r e n c e s B o o k e r , H.G. and W e l l s, H.W. [1938] Scattering of radio waves by the F-region of the ionosphere. Terr. Magn. and Atmos. Elec., 43, 249-256. B r a d l e y , P.A. and B r a m l e y , E .N . [1971 ] Wave polarization and its influence on the power available from a radio signal propagated through the ionosphere: Part 3—two-hop modes. Proc. I.E.E., 118, 1190-1196. B r a m l e y , E.N. and Y o u n g , M. [1968] Winds and electromagnetic drifts in the equatorial F2-region. J. Atmos. Terr. Phys., 30, 99-111. C a l v e r t , W. and S c h m id , C.W. [1964] Spread-F observations by the Alouette topside sounder satellite. J. Geophys. Res., 69, 1839-1852. C h iv e r s , H.J.A. [1960] The simultaneous observation of radio-star scintillations on different radio frequencies. J. Atmos. Terr. Phys., 17, 181-187. C l e m e s h a , B.R. [1964] An investigation of the irregularities in the F-region associated with equatorial spread-F. J. Atmos. Terr. Phys., 26, 91-112. C l e m e s h a , B.R. and W r i g h t , R.H.W. [1967] A survey of equatorial spread-F in the volume Spread-F and its effect upon radio wave propagation and communication. Ed. P. Newman and R. Penndorf (Technivision, England). C o h e n , R. and B o w l e s , K.L. [1961] On the nature of equatorial spread-F. J. Geophys. Res., 66, 1081-1106. C o h e n , R., B o w l e s , K.L. and C a l v e r t , W. [1962] On the nature of equatorial slant sporadic-E. J. Geophys. Res., 67, 965-972. C r o c h e t , M. [1972] Propagation transequatoriale en dehors du grand cercle en ondes decametriques. Part 1: Mise en evidence et morphologie. Ann. de geophys. 28 (1), 27-35, Part 2: Application: l’etude par diffusion des irregularites, 28 (2), 381-388. C r o o m , S., R o b b in s , A. and T h o m a s , J.O. [1960] Variations of electron density in the ionosphere with magnetic dip. Nature, 185, 902-903. D a v ie s , K. and B a r g h a u s e n , A.F. [1967] The effect of spread-F on the propagation of radio waves near the equator. Spread-F and its effect upon radio wave propagation and communications. Ed. P. Newman and R. Penndorf (Technivision, England). E c c l e s , D. and K i n g , J.W. [1969] A review of topside sounder studies of the equatorial ionosphere. Proc. IEEE, 57, 1012-1018. F a r l e y , D.T. [1963] A plasma instability resulting in field-aligned irregularities in the ionosphere. J. Geophys. Res., 68, 6083-6097. F u l t o n , B., S a n d o z , O. and W a r r e n , E . [1960] The lower frequency limits for F-layer radio propagation. J. Geophys. Res., 65, 177-183. G e r s o n , N.C. [1968] Ray tracing over a trans-equatorial path. Scatter propagation of radio waves. AGARD Conference Pro ceedings No. 37 (N.A.T.O., Neuilly-sur-Seine). G e o r g e , P.L. [1971] The global morphology of the quantity JNv dh in the D- and E-regions of the ionosphere. J. Atmos. Terr. Phys., 33, 1893-1906. K e n t , G.S. and K o s t e r , J.R. [1961] Height of night-time F-layer irregularities at the equator. Nature, 191, 1083-1084. K e n t , G.S. and K o s t e r , J.R. [1967] Some studies of night-time F-layer irregularities at the equator using very high frequency signals radiated from earth satellites. Spread-F and its effect upon radio wave propagation and communication. Ed. P. Newman and R. Penndorf (Technivision, England). K i n g , J.W., R e e d , K .C ., O l a t u n j i , E.O. and L e g g , A.J. [1967] The behaviour of the topside ionosphere during storm conditions. J. Atmos. Terr. Phys., 29, 1355-1363. K i n g , J.W., S m it h , P.A., E c c l e s , D ., F o o k s , G.F. and Helm, H. [1964] Preliminary investigation of the structure of the upper ionosphere as observed by the topside sounder satellite Alouette. Proc. Roy. Soc., A.281, 464-487. K n e c h t , R.W. and M c D u f f ie , R.E. [1962] On the width of the equatorial Es belt. Ionospheric sporadic-E. Ed. E.K. Smith and S. Matsushita, 215-218 (Pergamon, Oxford). K o s t e r , J.R. [1963] Some measurements on the sunset fading effect. /. Geophys. Res., 68, 2571-2578. K o s t e r , J.R. and W r i g h t , R.W.H. [1960] Scintillation, spread-F and trans-equatorial scatter. J. Geophys. Res., 65, 2303-2306. K o t a d i a , K.M. [1959] Spread-F in the ionosphere over Ahmedabad 1954-1957. Proc. Indian Academy of Sciences, 24, 259-271. L a w r e n c e , R.S., L ittle, C .G . an d C h iv e r s, H.J.A. [1964] A survey of ionospheric effects upon Earth-space radio propagation. Proc. IRE, 52, 4 -27. L o c k w o o d , G.E.K. and N e l m s , G.L. [1964] Topside sounder observation of the equatorial anomaly at 75°W longitude. J. Atmos. Terr. Phys., 26, 569-580. L o m a x , J.B. [1967] Spread-F in the Pacific. Spread-F and its effects upon radio wave propagation and communication. Ed. P. Newman, 29-58 (Technivision, Slough, England). L y o n , A.J., S k i n n e r , N.J. and W r i g h t , R.W.H. [1958] Equatorial spread-F and magnetic activity. Nature, 181, 1724-1725. L y o n , A.J., S k i n n e r , N.J. and W r i g h t , R.W.H. [1960] Equatorial spread-F and F-layer heights. Nature, 187, 1086-1088. L y o n , A.J., S k i n n e r , N.J. and W r i g h t , R.W .H. [1961] Equatorial spread-F at Ibadan, Nigeria. J. Atmos. Terr. Phys., 21,100-119. L y o n , A.J. and T h o m a s , L . [1963] The F2-region equatorial anomaly in the African, American and East Asian sectors during sunspot maximum. J. Atmos. Terr. Phys., 25, 373-386. Rep. 343-2 — 100 — M a t su sh it a , S. [1962] Lunar tidal variations of sporadic-E. Ionospheric Sporadic-E. Ed. E.K. Smith and S. Matsushita, 194-214 (Pergamon, Oxford). M a y a u d , P.N. [1965] Morphological analysis of the day-to-day variability in the “regular” daily variations Sr of the Earth’s magnetic field. Ann. de geophys., 21, 369-401. M ew es, E.R. [1971] Kurzwellenausbreitung in der aquatorialen E-region (HF propagation in the equatorial E-region). FTZ, Darmstadt, Kleinheubacher Berichte, 14, 263-267. M u l d r e w , D.B. and M a l ip h a n t , R.G. [1962] Long-distance one-hop ionospheric radio-wave propagation. /. Geophys. Res., 67,1805-1815. N ie l s o n , D.L. [1968] A review of VHF trans-equatorial propagation. Scatter propagation of radio waves. AGARD Conference Proceedings No. 37 (N.A.T.O., Neuilly-sur-Seine). O b a y a s h i, T. [1959] A possibility of the long-distance HF propagation along the exospheric field-aligned ionizations. Reports Ion. and Space Res. in Japan, 13, 177-186. O sbo r n e, B.W. [1951] Ionospheric behaviour in the F-2 region at Singapore. J. Atmos. Terr. Phys., 2, 66-78. R a n g a sw a m y , S. and K a pa si, K.B. [1964] Equatorial spread-F and solar activity. J. Atmos. Terr. Phys., 26, 871-878. R a o , B.C.N. [1966] Control o f equatorial spread-F by the F-layer height. J. Atmos. Terr. Phys., 28, 1207-1217. R a st o g i, R.G. [1959] The diurnal development of the anomalous equatorial belt in the F2-region of the ionosphere. J. Geophys. Res., 64, 727-732. R a st o g i, R .G ., K a u sh ik a , N .D . and D e sh p a n d e , M .R . [1965] Fading characteristics of Es reflections over the magnetic equator in Thumba (India). Proc. 2nd Symposium on Equatorial Aeronomy, Sao Jose dos Campos, 98-100. R eber, G. [1956] Worldwide spread-F. J. Geophys. Res., 61, 157-164. R o tt g er , J. [1971] An application of the Monte Carlo method to remote sensing systems. AGARD Conf. Proc. on “ Propagation limitations in remote sensing ”. Paper 34. S h ir k e , J.S. and H e n r y , G.W. [1967] Geomagnetic anomaly in ionospheric absorption at low latitude observed on board USNS Croatan. Ann. de geophys., 23, 517-525. S in g l e t o n , D .G . [1963] The spread-F equator. J. Atmos. Terr. Phys., 25, 121-149. S k in n e r , N .J . and W r i g h t , R .W .H . [1962] The reflection coefficient and fading characteristics of signals returned from the Es-layer at Ibadan. Ionospheric Sporadic-E. Ed. E.K. Smith and S. Matsushita, 37-49. S m it h , E.K. and F in n e y , J.W. [1960] Peculiarities of the ionosphere in the Far East: a report on IGY observations of sporadic-E and F region scatter. J. Geophys. Res., 65, 885-892. S t e in , S . [1958] The role of the ionospheric layer tilts in long-range high-frequency radio propagation. J. Geophys. Res., 63, 217-241. V il a , P. [1966] Experimental study of the equatorial ionospheric anomaly in Africa at sunspot minimum. Ann. de geophys. 22, 396-404. W a l d t e u fe l , P. [1965] Study of the instability associated with the eqhatorial sporadic-E layer. Ann. de geophys. 21, 579-604. W r i g h t , R .W .H . [1959] Geomorphology of spread-F and characteristics of equatorial spread-F. J. Geophys. Res., 64, 2203-2207. — 101 — Rep. 343-2 Magnetic dip tti Geographic latitude F ig u r e 1 Data obtained from ISIS-1 ionograms recorded on 27 October, 1969, at 160° E longitude, 1505 LM T Upper section: latitudinal variations of NmF2 (continuous curve) and electron concentration at fixed heights in the topside ionosphere (broken curves). The thin line joins points which lie on a magnetic field line. Lower section: the corresponding variation of hmF2. Rep. 344-2 — 102 — REPORT 344-2 * PREDICTION OF SPORADIC E ** (Study Programme 4A-2/6) (1966 - 1970 - 1974) 1. Introduction Some of the physical processes responsible for sporadic-E ionization are outlined in Report 573; this Report describes the structure and occurrence of Es in the three major zones: auroral, temperate and equatorial. Equatorial Es is also discussed in Report 343-2, while the possibility of VHF propagation by way of sporadic-E ionization is discussed in Report 259-3. 2. Long-term predictions of sporadic E Because of the highly variable nature of the occurrence and intensity of sporadic-E ionization, predictions must be statistical in character. Maps of foEs for various portions of the solar cycle have been prepared, for example in the United States of America [Leftin et al., 1968] and the U.S.S.R. [Kerblai, 1964]. An analytical description of the world-wide distribution of characteristics of the Es layer [Chernishyev, 1968; Bazarzhapov and Chernishyev, 1968] makes possible the calculation of the probability of appearance of an Es layer with a specified top frequency, and of the frequency of the wave reflected with a given probability. Equatorial sporadic E is more consistent than other types and should be included in MUF predictions [Kift, I960]. Detailed geographical studies of the position of the equatorial zone have been made in Peru, in West Africa, in India and in the Philippines. Predictions for other equatorial areas are subject to somewhat higher errors. The application of vertical incidence data to oblique incidence should be undertaken with caution. Experimental studies in the U.S.S.R. [Nosova, 1973] and in the Federal Republic of Germany [Mewes and Moller, 1974] indicate that the ratio of the oblique-incidence MUF to the Es top frequency at vertical incidence is often greater than that expected from the secant law. 3. Short-term predictions of sporadic E No method currently exists which will forecast the occurrence of a sheet of sporadic E. The neutral wind is associated with shears up to 100 metres per second per km; the vertical wavelength is a few km at 105 km, and perhaps 70% of the total movement is due to tidal or general drift and is, therefore, predictable in principle. In order to improve the possibility of predicting temperate zone sporadic E from theory, it will be necessary for the theory itself to be more firmly established as well as our knowledge of the ambient neutral wind. Using the wind-shear theory and knowing the tidal shears and background ionization, it may be possible to make significant improvements in the prediction of sporadic E. In a study made over a solar cycle in South America, Giraldez and Mesterman [1973] have indicated that the occurrence of Es during a day depends upon the virtual height of the Es observed near sunrise. * Adopted unanimously. ** Note. — The Director, C.C.I.R., is invited to thank the International Union of Radio Science (U.R.S.I.) for its recent contribution on this subject and also to request further comments. — 103 — Rep. 344-2, 573 Conditions in the magnetosphere, such as electron density and temperature, appear to be controlling factors in the basic pattern of particle dumping and electric fields and thus the most useful way of predicting auroral Es may be via satellites in operation for magnetospheric studies. R eferences B a z a r z h a p o v , A .D . and C her n ish y ev , O.V. [1968] Analytical description of the planetary distribution of ionospheric charac teristics (in Russian). Geomagnetism i Aeronomiia, 3. C h er n ish y ev, O.V. [1968] foEs distribution curves (in Russian). Geomagnetism i Aeronomiia, 6. G ir a l d e z , A.E. and M esterm an, I. [1973] Physical processes and guides for predicting Es (in Spanish). L.I.A.R.A. C-21, Argentina. K er b la i, T.S. [1964] Instructions for calculating high frequencies reflected from the Es layer. Annex to Monthly forecasts of radio propagation (in Russian). Institute for Geomagnetism, and Study of the Ionosphere and Radio Propagation (IZMIR), U.S.S.R. (Akademiia Nauk, Moscow). K ift, F. [1960] The propagation of high frequency radio waves to long distances. Proc. I.E.E., 107B, 127-140. L eftin, M ., O st r o w , S.M. and P resto n , C. [1968] Numerical maps of foEs for solar cycle minimum and maximum. ESSA Technical Report ERL 73-ITS 63, Supt. of Documents. U.S. Govt. Printing Office, Washington, D.C. 20402. M ew es, E.R. and M oller, H.G. [1974] Der Einfluss der sporadischen E-Schicht auf eine transaquatoriale Kurzwellen-Obertragung (The influence of sporadic E on transequatorial HF propagation). FTZ, Darmstadt, Kleinheubacher Berichte, 17, 443-447. N o so va, G.N. [1973] Kharakter zamiranii signalov pri otrazhenii i rasseyanii ot spradicheskogo sloya E (Character of fadings of signals reflected and scattered from the sporadic E layer). Collection of articles “ Questions concerning short wave propa gation ”, Izmiran, U.S.S.R. REPORT 573 * THE CHARACTERISTICS OF SPORADIC E (Study Programme 4B-2/6) (1966 - 1970 - 1974) 1. Introduction The physical processes responsible for the major proportion of sporadic E in the different geographic zones can be assumed with considerable assurance to be quite different. For example, in the temperate zone, wind shear plays the major role. At the magnetic equator, where q-type sporadic E is dominant, the two-stream plasma instability mechanism appears to offer a satisfactory explanation. In the auroral zone, energetic particles play a significant role. A comprehensive world-wide survey of sporadic E has been prepared by Whitehead [1970]. More recent progress is reviewed in a special issue of Radio Science [Matsushita and Smith, 1972]. Descriptions and definitions of the various types of Es appearing on ionograms will be found in the U.R.S.I. handbook of ionogram interpretation and reduction [Piggott and Rawer, 1972]. * Adopted unanimously. Rep. 573 — 104 — 2. Temperate zone sporadic E 2.1 Structure Sporadic E occurs usually in the form of thin layers 500 m to 2 km in thickness, as has been shown by many direct measurements by means of rockets. The electron density is often many times that of the ambient E-region value. Sporadic E forms clouds of all sizes from 1000 km to a few metres which tend to drift towards the equator at about 80 metres per second. The heights at which Es is observed are predominantly 95 to 135 km with a peak at about 110 km. Three types of sporadic E were defined for temperate latitudes for the IGY in terms of the height relative to the regular E region at which the sporadic E occurred during the day. These merge into a single fourth type at night. The peak electron density observed in a rocket flight corresponds most closely to the blanketing frequency of sporadic E (fbEs) measured from a vertical incidence ionogram [Bross et al., 1967] whereas the oblique-incidence MUF is probably correlated with the critical frequency (foEs) by the secant law. However, the measurements of foEs may be affected by the fact that reflections from transparent sporadic E often occur from off-vertical directions. Ground-based VHF measurements indicate that both specular and scatter propagation can take place from the Es layer. Miya et al. [1961], for example, have categorized sporadic-E reflections observed on VHF oblique incidence circuits in the Far East in terms of a parameter T, effectively the reflection coefficient (apparent loss in decibels) of the layer, as follows: T < 45 dB: specular reflections from sporadic E (appears like a layer reflection); 45 dB < r <70 dB: scattered reflections from Es; r> 70 dB: sporadic E masked by normal D-scatter. 2.2 Occurrence While the occurrence patterns of sporadic E vary considerably in different parts of the temperate zone, at middle latitudes the dominant characteristics of sporadic-E occurrence are: — a dominant maximum in the months of the summer solstice. The seasonal minimum is usually near the spring equinox. This seasonal behaviour changes abruptly at 60° geomagnetic latitude; — a diurnal pattern of occurrence where the majority of sporadic E is seen during the daylight hours. One peak is normally found in the mid-morning hours and a second near sunset. These two peaks appear to lag the growth and decay of the regular E layer. The times of occurrence of high foEs values correspond to the times of easterly or westerly drifts in the E layer, the periods of the foEs variations are half the corresponding periods of E-layer drifts [Harnischmacher and Rawer, 1972]; — a dependence on sunspot cycle which results in maxima of foEs at both sunspot maxima and minima [Harnischmacher and Rawer, 1970]. The seasonal' variation appears to be related to the behaviour of upper atmosphere winds [Matsushita and Reddy, 1968] and possibly to the behaviour of the magnetosphere [Smith, 1968]. The phase lag (approximately two hours) of sporadic E relative to the regular E layer can be explained in terms of the slower recombination rates of the meteoric ions appearing in the layers. Progress is being made toward understanding the sunspot cycle variation of Es. The critical frequency foEs increases at low latitudes, and decreases at higher latitudes, with magnetic activity. The lunar tidal amplitude of foEs is ~ 0-3 MHz, while that of fbEs seems to be much less [Tarpley and Matsushita, 1972]. More data are required to establish the association with airflow, nuclear explosions and meteors. There is no established association between Es and thunderstorms or weather. — 105 — Rep. 573 2.3 Theoretical explanation An attractive explanation for the formation of sporadic E at temperate latitudes is found in the wind-shear theory. According to this theory, an intensification in ionization over a very narrow height range can be achieved under appropriate conditions, but the role of the metallic ions and the electron temperature variations is not clear. The metallic ions in the E region have total contents of ~ 1 0 14 in a square metre column, and may form layers with peak densities of ~ 1 0 10 m-3. It is difficult to relate the world-wide production and loss of metallic ions to the observed sporadic E [Whitehead, 1971]. Gradients and the resulting instabilities do not give rise to thin layers, but may cause the fine structure. Turbulence and ionization by energetic particles probably do not play significant roles. Many attempts have been made to measure the wind vector, as a function of height, simultaneously with measurements of the vertical structure of the sporadic-E layer. Agreement of the observations with the predictions of the simple wind-shear theory have so far been only fair. 3. Auroral sporadic E 3.1 Structure Auroral sporadic E is related to visual aurora in a complex manner. Considerable uncertainty exists regarding its electron structure and height although radio aurora seem to arise from heights ranging from 85-130 km with 110 km, perhaps, most likely. Auroral Es may occur as a thin layer, a thick layer, resulting in large group retardation, or as a field-aligned irregularity exhibiting aspect sensitivity. Sporadic-E clouds have been observed to move with velocities of 200 to 3000 metres per second toward the west in the evening and eastwards in early morning, corresponding in this behaviour to the motions of visual auroral forms during the auroral sub-storm phenomenon. 3.2 Occurrence Auroral sporadic E occurs mainly at night at geomagnetic latitudes greater than about 60° with a maximum near 69°. The dependence on solar cycle is not clear. Auroral Es occurs more frequently during periods of high magnetic activity and has been noticed to follow the sudden commencement associated with a solar flare. Around the North Pole Es is highly non-symmetrical and variation with solar zenith angle and dip angle is not clear. 3.3 Causes of auroral sporadic E Energetic particles play a vital role in the production of auroral Es. Bailey [1968] has shown that electron precipitation, characterized by exponential energy spectra with e-folding energies between 3 and 15 keV, results in type-r Es. The d-type Es in the 80 to 90 km height region may, however, be caused by similar electron energy spectra with e-folding energies as high as 60 keV. Type-r Es and the so-called night-E with critical frequencies less than about 4 MHz may also result from soft proton precipitation associated with the hydrogen aurora. Thin layers of ionization cannot themselves be produced by particles having broad energy spectra and arriving from many directions. Horizontal gradients of ionization combined with large electric fields may lead to the gradient instability suggested for the equatorial region. 4. Equatorial sporadic E Equatorial sporadic E occurs within about 4° of the dip equator as a belt which varies in width and slightly in position. The term is used synonymously with q-type sporadic E. This type of sporadic E is transparent and is essentially a daytime phenomenom. It is clearly related to the equatorial Rep. 573 — 106 — current belt (electrojet). A generally accepted explanation for the observed radio reflections has been presented by Farley [1963] in terms of the two-stream plasma instability. The blanketing forms of temperate latitude sporadic E decrease by about a factor of 10 in occurrence at the magnetic (dip) equator, a phenomenon which has been predicted to occur as a feature of the wind-shear theory of sporadic E [Axford and Cunnold, 1966]. It seems possible that the gradient instability may give rise to the weaker reflections at VHF and to a substantial part of the HF reflections. An additional discussion of this type of Es is found in Report 343-2. R e f e r e n c e s A x f o r d , W.I. and C u n n o l d , D.M. [1966] The wind-shear theory of temperate zone sporadic E. Radio Science, 1 (New Series), 191-197. B a il e y , D.K. [1968] Some quantitative aspects of electron precipitation in and near the auroral zone. Reviews of Geophysics, 6, 289-346. B r o s s , P., J a c o b s , K.G. and R a w e r , K. [1967] Direct electron density measurements in the ionosphere below 200 km. Space Research, VII, 1, 467-476. F a r l e y , D.T. [1963] A plasma instability resulting in field-aligned irregularities in the ionosphere. J. Geophys. Res., 68, 6083-6097. H arnischmacher , E. and R a w e r , K. [1970] Precursor events for possible forecasting of sporadic E and increased absorption. AGARD Conference Proceeding No. 49, 32-1 through 32-12. H arnischmacher , E. and R a w e r , K. [1972] Relation entre la direction des vents ionospheriques et la couche E sporadique. C.R. Acad. Sci., Paris, 274, B, 298-300. M a t s u s h it a , S. and R e d d y , C.A. [1968] The variations of neutral wind-shears in the E region as deduced from blanketing Es. J. Atmos. Terr. Phys., 30, 747-762. M a t s u s h it a , S. and S m it h , E.K. (Eds.) [1972] Special issue of Radio Science. M iy a , K ., S a s a k i, T . and I s h ik a w a , M . [1961] Observations of F-layer and sporadic-E scatter at VHF in the Far East. NBS Journ. o f Research, 65D, 93-99. P ig g o t t , W .R. and R a w e r , K. [1972] U.R.S.I. Handbook of ionogram interpretation and reduction—2nd Ed. Report UAG-23, Nov., World Data Center A for Solar-Terrestrial Physics, NOAA, Boulder, Colo. U.S.A. Spanish text available from L.I.A.R.A. (Argentina), and French text available from CNET (France). S m it h , E.K. [1968] A suggested conjugate point dependence for intense temperate latitude sporadic E. Radio Science, 3 (New Series), 687-690. T a r p l e y , J.D. and M a t s u s h it a , S. [1972] Lunar influences on sporadic E. Radio Science, 7, 411-416. W h it e h e a d , J.D. [1970] Production and prediction of sporadic E. Rev. Geophys. Space Phys., 8, 65. W h it e h e a d , J.D. [1971] Difficulty associated with wind-shear theory of sporadic E. J. Geophys. Res., 76, 2523. — 107 — Rec. 435-2 SECTION 6G: FIELD STRENGTHS BELOW ABOUT 1-5 MHz RECOMMENDATIONS AND REPORTS Recommendations RECOMMENDATION 435-2 * PREDICTION OF SKY-WAVE FIELD STRENGTH BETWEEN 150 AND 1600 kHz (1966 - 1970 - 1974) The C.C.I.R., CONSIDERING (a) that there is a need to give guidance to engineers in the planning of broadcast services in bands 5 and 6 (LF and MF bands); (h) that it is important, for stations working in the same or adjacent frequency channels, to determine the minimum geographical separation required to avoid interference resulting from long-distance ionospheric propagation; (c) that the method given in § 7 of Report 575 and its Annex is based on the statistical analyses of field- strength measurements for 217 paths distributed throughout the world, supplemented by the results of analyses for geographical areas from which individual path data is not available; RECOMMENDS that the method provided in § 7 of Report 575 and its Annex be adopted for provisional use, taking particular note of the cautions on accuracy in its application to certain regions stated therein. * The People’s Republic of China reserves its opinion as regards this Recommendation. Rep. 264-3 — 108 — 6G: Reports REPORT 264-3 * SKY-WAVE PROPAGATION CURVES BETWEEN 300 km AND 3500 km AT FREQUENCIES BETWEEN 150 kHz AND 1600 kHz IN THE EUROPEAN BROADCASTING AREA (Study Programme 17A-2/6) (1959 - 1963 - 1966 - 1970 - 1974) 1. Introduction The purpose of this Report is to present a set of propagation curves for the prediction of nocturnal field strengths in the frequency range 150 kHz to 1600 kHz at distances between 300 km and 3500 km in the European Broadcasting Area. The wider problem of extending these curves for shorter and longer distances and of providing curves for other areas, and if necessary for daytime conditions, is considered in Report 431-1. Several field-strength prediction methods for application in other regions or on a world-wide basis are described in Report 575. The possibility of providing curves for frequencies below 150 kHz is discussed in Report 265-3. The nocturnal field strength at low frequencies (band 5) and medium frequencies (band 6) has been the subject of a measurement campaign organized by the European Broadcasting Union (E.B.U.) since 1952 and by the International Broadcasting and Television Organization (O.I.R.T.) since 1958. These measurement campaigns were undertaken independently of one another; the number of hours recorded were greater in the case of the E.B.U. than that of the O.I.R.T. but the O.I.R.T. measurement campaign covers more frequencies. This Report is based on the field-strength recordings made by the two organizations in the European Broadcasting Area up to 1962; the total number of hours recorded exceeds .67 000. It is intended for forecasting the nocturnal field strength in this area, such forecasts being necessary to solve any problem relating to the assignment of frequencies. It does not, therefore, propose to give a scientific explanation of the physical phenomena that underlie the variations of the nocturnal field strength, and it does not seek to justify the methods used; it has been intentionally designed as a working instrument. The organization and analysis of the measurements are described in detail elsewhere [Ebert, 1962; C.C.I.R., 1963]. Note J. — As these predictions are based on data obtained only in the European Broadcasting Area, they do not necessarily apply to other areas. Note 2. — Administrations are encouraged to examine the methods in the light of information at their disposal, with a view to extending them to other areas. 2. Annual median value of field strength The field strength can be represented by the following general expression: F{ = F0 + Aa (1) where F\ is the annual median value of the field strength (dB rel. 1 fV/m ) which is defined as the median observed during a time interval of 30 minutes, centred round a reference hour, for a power of 1 kW radiated by the transmitting antenna, reception being by a small loop antenna. * Adopted unanimously. — 109 — Rep. 264-3 F q is the annual median value of the field strength (dB rel. 1 jj-V/m) which is defined as the median which would be observed during a time interval of 30 minutes, centred round a reference hour, for a reference transmitting antenna giving, over perfectly conducting ground, a field strength of 3 X105 p,V/m at a distance of 1 km in all directions above the horizon, reception being by a small loop antenna. A^ is a correction factor (dB) for the transmitting antenna to account for the radiation pattern; if this antenna radiates a power of 1 kW, the factor Aa is equal to the ratio (dB) between the field strength for a given angle of departure in the vertical plane at a distance of 1 km and a field strength of 3 X105 p.V/m. The correction factor A^ may be deduced from the horizontal and vertical radiation patterns of the transmitting antenna assumed to be placed on a perfectly conducting earth. Fig. 1 gives values of A a as a function of the distance between the transmitting and receiving points in the theoretical case of: ■— a lossless unloaded vertical antenna of height (h), — a reflection at a virtual height of 100 km above a spherical earth. (The height of 100 km corresponds to propagation via the E region of the ionosphere, which is usually the case, except occasionally at frequencies approaching 1600 kHz, for the later hours and for the shorter distances.) The discontinuity of the curves at 2200 km corresponds to the distance beyond which there are at least two hops. The value of F0 is given by: Fq = 80-2 - 10 log D - 0-0018/ ° '26 D (la) where D = the distance (km); / = the frequency (kHz). The values of Fx and F0 are valid for the following conditions: — the frequency range is from 150 to 1600 kHz; — the magnetic dip at the mid-point of the path is 61°; — the sunspot number (Wolf number) is S = 0; (In this Report, the letter S is used instead of R to indicate the annual mean value of the sunspot number (Wolf number). This has been done to avoid confusion with the symbol R, used for the field-strength ratio in § 4 of this Report); — the reference hour is local midnight at the mid-point of the propagation path; — the distances D are between 300 and 3500 km. Fig. 2 shows a family of propagation curves for computed from equation (la), for frequencies of 150, 200, 300, 500, 700, 1000, 1300 and 1600 kHz, so that the family covers the whole of the broadcasting band to which the curves apply. The following equation * should be used to obtain the annual median values for more general conditions: Fh (50) = Fj + P + A7 + Ah (50) — 0 • 02 (2a) The symbols in this formula have the following significance: F h (50) is the annual median value of the vertical electric field strength (dB rel. 1 p.V/m) at H hours local time at the mid-point of the propagation path, for: — a transmitting antenna characterized by A^ ; — a sunspot number, S; — a magnetic dip, I, at the mid-point of the propagation path. Fx is the value given by equation (1); P is the power radiated from the transmitting antenna (dB rel. 1 kW); * On the basis of a re-analysis of its measurements the E.B.U. feels that a better correction for solar activity in band 6 (MF) is given by replacing the term —0 025 in equation (2a) by —SD x 10-6 where D is the transmitter-receiver distance in km [C.C.I.R., 1970-1974]. Rep. 264-3 — 110 — A/ is the correction to be applied to take account of the magnetic dip I, at the mid-point of the propagation path. The value of this correction (dB) is given by the curves shown in Fig. 3. The magnetic dip considered is given by the maps shown in Figs. 4 and 5; A# (50) is the correction to be applied when the local time H at the mid-point of the path differs from midnight. The value of this correction (dB) is given in Fig. 6. Note. — If the concept of propagation loss is used (see Recommendation 341-1 and Report 112), formulae (la) and (2a) become respectively: Lpo = 10 log 19 + 20 lo g / + 0-0018/ ° '26 D - Aa - 7-75 (lb) L„„ (50) = Lta - A, - A„ (50) + 0-02 S (2b) Lpo denoted the annual median value of propagation loss between the transmitting antenna and a small loop receiving antenna, and LpH (50) denoted the annual median value of propagation loss at a local time //a t the mid-point of the propagation path. The symbols in the two formulae and the conditions in which the formulae are used are the same as for formulae (la) and (2a) respectively. For reception with other types of receiving antenna, equation (lb) may still be used, if an additional term of the type A^ is subtracted to allow for the directional pattern of the receiving antenna in azimuth and elevation. Equation (1) cannot be generalized in a similar way, since the field strength is independent of the kind of antenna used for its reception. 3. Statistical variations of the field strength or of the propagation loss 3.1 Distribution of hourly medians Observed values, particularly when recorded over short periods of time, may be expected to depart significantly from the long-term values given in this Report. The curves of Fig. 7 give the order of magnitude of the statistical variations of the hourly medians (it is shown by Ebert [1962] that the long-term distribution of the half-hourly medians is very nearly the same as that of the hourly medians) of the field strength, taking into account both the time at the mid-point of the path and the percentage of the nights considered. They make it possible to determine, approximately, the correction that has to be applied to the annual median value, to ascertain the field strength during a givenpercentage of the nights of a year. Thus (2a) and (2b) become: FH(T) = FH (50) + M T ) (3a) LpH (T)= LpII (50)-Z h (T) (3b) Note. — Fig. 7 shows the values (T) = AH (T) — AH (50), where AH (T) are the values given by Ebert [1962]. 3.2 Distribution within an hour In Report 266-3 it is suggested that the short-term variations (within a half hour or an hour) follow the Rayleigh distribution. All the statistical variations considered above refer to the hourly median values. To assess, in a'more complete fashion, the possibilities of interference, the quasi-maximum value of the field strength during the course of an hour may well have to be considered. At any given point of reception, the annual median value of the ratio between the hourly quasi maximum (10%) value and the hourly median value, varies little from one year to another. This ratio is, however, a function of the distance and the frequency. The study of the median value of this ratio, based on a large number of measurements made during the course of several years, shows that it increases with the frequency and that it decreases when the distance increases. Depending upon the distance and the frequency, this value varies between approximately 6 dB and 3 dB for medium frequencies (band 6) and between 4-5 dB and 2 dB for low frequencies (band 5). However, for distances where single-hop propagation is no longer possible (above about 2000 km), this ratio no longer obeys an obvious law, but its median value remains in general below 6 dB for medium frequencies and around 2 dB for low frequencies. — Ill — Rep. 264-3 4. Formula for estimating the wanted-to-interfering signal ratio R Consider a particular receiving location, at a distance Du from the wanted transmitter of power Pu and at a distance Dn from the interfering transmitter of power Pn, and consider an interval of one hour, the mid-point of which corresponds to the local times Hu and Hn of the mid-points of the path of the wanted and unwanted transmissions, then the ratio R(T) in dB between the wanted hourly median signal level and the interfering hourly median signal level, exceeded for a percentage T greater than 50% of the hours of the year when the value R is exceeded, can be calculated for a non-directional receiving antenna from the following formula: R (T) = F«u (50) - FHn (50) - |/ S*„2 (T) + SH„4 (100 - T) + 2 pSff„ (T) (100 - T) (4) where p represents the correlation between the changes in hourly median values for the wanted and interfering signal propagation paths. In the absence of measurements of this factor p, it is suggested that it be set equal to 0*5 in using equation (4). It should be noted that BHn and Sffu always have opposite signs and that the minus sign before the radical in (4) is associated with the practical situation normally encountered, where the time availability T of satisfactory service is greater than 50%. Strictly speaking, equation (4) is applicable only to the extent that a log-normal distribution describes the data. However, for the distributions encountered in practice, the formula is an adequate approximation. It neglects the rapid variations of both the wanted and interfering signals. 5. Temporal variations of the field strengths 5.1 Combined influence of the hour and season Fig. 6 provides a correction of the hourly median as a function of the hour at the midpoint of the path. But this correction term is itself no more than a yearly median derived from results obtained with different frequencies and at all times of the year. The spread of the correction, shown in Fig. 7 is, therefore, very great. Its value can, however, be rendered more precise and its dispersion reduced by bringing to the fore the seasonal factor obtained through statistical study carried out separately for each of a number of frequencies. A first study has been effected using as a basis recordings made at 845 kHz throughout the night, over a period of three consecutive years (1959, 1960 and 1961), on the Rome-Wittsmoor, Rome- Darmstadt, Rome-Belgrade and Rome-Tel Aviv paths, the lengths of which vary between 700 and 2200 km. This made it possible to establish the family of curves in Fig. 8, giving the median value of the correction AH (50) to be made, as a function of the month and the hour, to the field strength calculated with the method described above. The curves clearly show two seasonal maxima, in March and in September, the latter being the greater. This seasonal effect is quite apparent on each of the paths used to establish the curves. The present study of data obtained for other frequencies enables it to be assumed already that, for the European Broadcasting Area at least, a similar effect occurs throughout the medium-frequency broadcasting band and that the family of curves in Fig. 8 is also valid for the high end of the medium- frequency band. However, the completion of this study must be awaited to know whether the same results will be found at lower frequencies, including the low-frequency band, and to determine the effect of the geographical position of the path. An analysis of the seasonal variations of the median values of the field strength over the period 1959-1965 was carried out by Taumer and Starick [1969]. The transmitters used were Allouis (164 kHz), Rome (845 kHz) and Monte Carlo (1466 kHz) over one-hop propagation paths at distances between 1000 and 1150 km (Figs. 9 (a), (b) and (c)). The seasonal variations of night-time field-strength medians have the following trend: — at all three frequencies, the semi-annual variation in sky-wave field strength is clearly pronounced. In the MF band the maximum field strengths appear in the equinoctial months, while in the LF band they arise in the summer and winter months; — the sunspot activity has an effect at all three frequencies, where a steady increase in field strength from sunspot maximum to sunspot minimum may be seen; Rep. 264-3 — 112 — — the year-to-year trend in field strength at all three frequencies is not comparable, so that it does not seem admissible to combine seasonal variations over a whole sunspot cycle. The slight field-strength maximum in the second half of the night, to be seen on Fig. 6 relating to the whole of one year, is also found on each of the curves established each month for the four paths indicated earlier for 845 kHz, and its position in the course of the night seems to be sensibly independent of the time of sunset. The final objective of this study is to replace Fig. 6 by several families of curves giving the values Ah (50), Ah (90) and AH (10) and valid for the whole medium-frequency band in the European Broad casting Area. In certain applications, these families of curves might replace all those in Figs. 6 and 7. 5.2 Influence of the angle, o f elevation o f the Sun A provisional method for giving a correction A'H (50) in place of AH (50) so as to obtain median values for the actual time of year rather than the yearly median value, has been proposed, based on an analysis of measurements. It takes account of the changes in the times of sunrise and sunset during the year and the decreasing attenuation during the night, arising from the decay of D-region ionization, and is given by: A7# (50) = 9 [fix) — 1] + 0*47 t (5) where t is the time in hours relative to midnight, X is the negative angle of elevation of the Sun (beneath the horizon) and / ( X ) = sin 2 x when x < 45° = 1 when x > 45°. The validity of this correction is restricted to the period from three hours before midnight (t = — 3) to three hours after midnight (t = 3), as most measurements were made in this period. Charts are available for obtaining x as a function of latitude and time of year [Muller and Taumer, 1965]. 6. Properties of sky-wave field strength distribution in bands 5 and 6 (LF and MF) The analysis of the properties of the sky-wave field strength distribution in the LF and MF bands on three frequencies in the European Broadcasting Area was carried out by Taumer and Sulanke [1967 and 1968]. An explanation of the distribution function valid in these bands is given in Report 266-3. The e, yj distribution function determined is valid for the half-hourly and hourly median night-time sky-wave field strength in an undisturbed lower ionosphere (D and E regions). 7. Influence of finite ground conductivity on the vertical polar diagram of the antenna The finite ground conductivity near a transmitting or receiving antenna modifies the vertical polar diagram, as compared with the diagrams for perfectly conducting ground. The measurements used for deriving the propagation curves were not adjusted for this, and F0 therefore includes the effect of ground of average conductivity at both sending and receiving ends of the path. The correction for other cases depends on the angle of elevation of the path, and, to a lesser extent, on the type of antenna. As a rough guide for paths exceeding 1000 km, good conductivity at the transmitter (10-2 S/m) gives a greater sky-wave field strength by about 1-5 to 2 dB, while poor conductivity (10~3 S/m) gives a reduction of the same order. An extreme case is a transmitter sited near to the sea, with very good conductivity maintained for many wavelengths in the direction of the receiving point. An increase of about 6 dB is expected for this case, and a similar correction would also apply for sky-wave signals received at sea. In all cases, the correction for ground conductivity becomes less as the path length is reduced below 1000 km (see Report 401-2). — 113 — Rep. 264-3 8. Remarks concerning the reliability of the predictions The preceding results have been obtained from measurements with path lengths between 360 and 3200 km. The measurements towards the upper limit of the distance range were less numerous and refer only to frequencies below 845 kHz. The predictions for the higher frequencies and greater distances are thus less reliable even within the indicated limits of validity. In this respect, it would be useful to consult Report 431-1. R eferences C.C.I.R. [1963] Doc. 209 (O.I.R.T.), Geneva. C.C.I.R. [1970-1974] Doc. 6/99, E.B.U. E bert, W. [1962] Ionospheric propagation on long and medium waves. E.B.U. Review, Part A, Technical, 7 1 , 7 2 , and 7 3 ; also Technical Monograph No. Techn. 3081, E.B.U. Technical Centre, 32, Avenue Albert Lancaster, Brussels 18, Belgium. M u ll er , K. and T a u m e r , F. [1965] Eine Methode zur Vorausbestimmung der nachtlichen Raumwellenfeldstarke im Frequenz- bereich von 150-1500 kHz fur Entfernungen zwischen 300 und 3000 km (A method of predicting nocturnal sky-wave field strength in the frequency range from 150 to 1500 kHz for distances between 300 and 3000 km). Techn. M itt. RFZ, 9, 85-89. T a u m e r , F. and S t a r ic k , E. [1969] Jahreszeitliche Feldstarkevariationen im Lang- und Mittelwellenbereich (Seasonal field strength variations in the bands 5 and 6). Techn. M itt. RFZ, 13, 16-19. T a u m e r , F. and S u l a n k e , H. [1967-1968] Die Eigenschaften von Haufigkeitsverteilungen der Raumwellenfeldstarke im Frequenz- bereich von 150-1600 kHz (The properties of probability distribution of the sky-wave field strength in the frequency band 150-1600 kHz). Techn. M itt. RFZ, 11, 4; 12, 20-30. ANNEX EXAMPLE OF A COMPLETE CALCULATION OF FIELD STRENGTH Calculate the probable field strengths that are exceeded during 50% and 10% of the nights during a year, under the following conditions: Length of path: D = 1500 km Magnetic dip at the mid-point: / = 66° Local time at the mid-point of the path: H = 2130 h Wolf number: S = 100 Frequency: / = 800 kHz Antenna height: h = 150 m, non-loaded antenna Radiated power: p = 100 kW Rep. 264-3 — 114 — The successive stages of the calculation are as follows: Value Stage Parameters Figures Term calculated (dB) ([TV/m) / 800 kHz 2 33 1 D = 1500 km = 2 p = 100 kW P = 10 log 100 = ‘ 20 3 S = 100 - 0-02 S = - 2 4 1 = 1 f = 800 kHz } h = °'4 5 I = 66° 3 A / = - 1 6 H = 2130 h 6 Ah (50) = - 2 7 F h (50) = 49 280 8 T = 10% 7 8 h (T) = 6 9 F h (T) = 55 560 It is, of course, assumed here that the factors /, D and H have been determined in advance. In particular, the local time H at the mid-point of the path is obtained normally from the local time at the point of reception. < 1000 2000 5000 Distance, D, (km) F ig u r e 1 Correction factor A a of vertical transmitting antennae of various lengths as a function of the distance D from the point of reception (The figures on the curves represent values of the parameter h/X) Rep. 264-3 dB (fjtV/m) Family o f basic curves to determine the annual median value o f the field strength F0 strength field the f o value median annual the determine to curves basic f o Family for the frequencies (k H z), indicated on the curves the on indicated z), H (k frequencies the for Distance, Distance, 16 — 116 — F gure r u ig D, 2 (km) fV/m \ A / dB orcin ( Correction A/J A/J Magnetic dip, dip, Magnetic o ae con ofte antc dip magnetic the f o account take to 17 — 117 — F gure r u ig I, 3 (degrees) e. 264-3Rep. 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I I JI zC> m o ~ o ~ o ~ o ~ o ~ ~ ~ ID ~ ~ ~ ffl ~ N N Latitude FIGURE 4 Map of Europe on a system of rectilinear coordinates having equally-spaced lines of geographic latitude and longitude. The curves are lines of equal magnetic dip, I Longitude The curves are lines of equal magnetic dip; those below the 0° curve correspond to negative dip, i.e. a dip a i.e. dip, negative to correspond curve 0° the below those dip; magnetic equal of lines are curves The ytmofrciier oriae egahc aiueadlniuefr h woe world whole the for longitude and latitude f geographic o coordinates rectilinear f o System downwards towards magnetic south magnetic towards downwards SMP : South magnetic pole magnetic South : SMP NM P: North magnetic pole magnetic North P: NM 19 — 119 — F Latitude gure r u ig 5 Rep.264-3 Rep.264-3 AH (50) dB Correction orcin 8 Correction A h h (50) to take account o f the time at the mid-point o f the path the f o mid-point the at time the f o account take to (50) (T ) to take account o f the percentage o f the nights o f a year a f o nights the f o percentage the f o account take to ) (T iea temdpit ftept ( path the of mid-point the at Time Time at the mid-point of the path ( path the of mid-point the at Time 10 — 120 — F F gure r u ig gure r u ig 6 7 ) H H ) — 121 — Rep. 264-3 15 16 17 18 19 20 21 22 23 24 01 02 03 04 05 06 07 Time at the mid-point of the path (H ) F ig u r e 8 An example of correction to take account of the time at the mid-point of the path H and of the season of the year (Frequency: 845 kHz) Rep.264-3 Median value of night-time field strength (dB) (b) Rome-Kolberg (845 kH z), between 1959 and 1965, (c) Monte-Carlo-Kolberg (1466 kH z), between 1961 and 1965 and 1961 between z), kH (1466 Monte-Carlo-Kolberg (c) 1965, and 1959 between z), kH (845 Rome-Kolberg (b) ESEWESEWES EWESEWESEWESEWESEW esnlvrain ein ausofngttm il tegh npt () loi-obr (6 Hz), kH (164 Allouis-Kolberg (a) path on strength field night-time f o values median f o variation Seasonal 99 90 91 92 93 94 1965 1964 1963 1962 1961 1960 1959 99 90 91 92 93 94 1965 1964 1963 1962 1961 1960 1959 — itr = Eunxs — Summer — S Equinoxes = E Winter — W 91 92 93 94 1965 1964 1963 1962 1961 — t ain ta r e c n u d n e tr —— — 12 — 122 — F gure r u ig (b) (a) (c) 9 — 123 — Rep. 265-3 REPORT 265-3 * SKY-WAVE PROPAGATION AT FREQUENCIES BELOW 150 kHz WITH PARTICULAR EMPHASIS ON IONOSPHERIC EFFECTS ** (Study Programme 17B-1/6) (1959 - 1963 - 1966 - 1970 - 1974) 1. Introduction Radiocommunication at frequencies below about 150 kHz is limited by the available bandwidth and certain practical considerations, further discussed below. In this frequency range, at distances where the surface wave becomes too weak to be useful (a distance which depends on frequency, time of day, season, epoch of the solar cycle, and earth surface condition (distances of the order of 500 to 1500 km)), propagation continues between the Earth and the lower boundary of the ionosphere to great distances and is characterized by good reliability and stability. For a given radiated power, increased field strength is obtained with decreasing frequency, down to frequencies of 10-20 kHz [Watt, 1967] and the diurnal seasonal and solar cycle variations become smaller. Because of the stability of propagation (amplitude and phase), frequencies in the VLF range are use ful not only for communication but also for standard frequency and time broadcasts [Blair et al., 1967], as well as navigation systems employing CW phase comparison [Casselman; 1959; Pierce, 1964; Williams, 1951] or direction-finding techniques. In particular, it has been found that the stability of transmission permits frequency comparison to within a few parts in 1012, which is four orders of magnitude better than is possible at HF, thus permitting long range navigation utilizing phase comparison between spaced atomic frequency controlled or phase-locked transmissions [Pierce, 1964]. At frequencies above 100 kHz, where it becomes practical to isolate, in some circumstances, the ground wave by pulse and sampling techniques, highly accurate time and navigation systems can be devised [Doherty, 1961]. Owing to the long wavelengths involved, fading is characterized by spatial variations as the waves or modes add vectorially. Marine communications use LF and VLF, which are also used for point-to-point communications at times of severe ionospheric disturbances to HF circuits, since the lower frequencies are not in general adversely affected [Belrose, 1962], an exception being solar proton events affecting long range transpolar VLF circuits which traverse arctic or antarctic ice caps [Crombie, 1967; Reder, 1968]. Communication to points under the sea or beneath the ground can be achieved on these frequencies [Moore, 1967; Tsao, 1967]. Propagation has been assumed to be essentially by way of vertical polarization, which at long distances is well preserved [Bracewell, 1951]. A limitation, which is partly economic, is the need for large transmitting antenna installations and high transmitter powers. Another limiting factor is the increasing level of atmospheric noise with decreasing frequency (noise generated in the Earth’s ionosphere can also become important at the lowest frequencies on some occasions in high latitudes) and thus, apart from the small total bandwidth available, services generally use a fairly narrow bandwidth and hence low communication rates [Belrose, 1959]. VLF transmissions (3 to 30 kHz) are normally used for very long-distance (world-wide) communi cation; LF transmissions (30 to 300 kHz) are useful for communications to 3000 km or so. * Adopted unanimously. ** Note. — The Director, C.C.I.R., is requested to transmit this text to the International Union for Radio Science (U.R.S.I.) for comment. Rep. 265-3 — 124 — In recent years research has been concentrated on the VLF range and to a lesser extent on the LF range. In describing propagation, it will be found useful to consider whether the transmission is by way of a once-reflected sky wave together with the surface wave, or whether waves reflected more than once from the ionosphere contribute to the received field strength. These considerations depend on frequency, distance, earth surface conditions, and ionospheric conditions. Two methods are available for theoretically calculating the field strength of VLF and LF signals. 1.1 The “wave-hop ” method in which electromagnetic energy paths between a given transmitter and receiver are represented geometrically as is done in the case of HF (Fig. 1). This method treats radio transmission as taking place along certain paths defined by one or more ionospheric reflections, depending on whether the propagation in question involves one or more “ hops ”. The total field is then the vectorial resultant of the fields due to each path. In view of the long wavelengths concerned, the diffraction of the waves by the earth’s surface must be taken into account, which is not the case for HF. The wave-hop method may be justified by the fact that, with oblique incidence, the dimensions of the section of altitude in which propagation takes place are equal to or greater than several wavelengths. With this method it is necessary to know the values of the reflection coefficients of the incident wave on the ionosphere. Since this varies greatly in altitude in comparison with the wavelength, the theoretical calculation of the reflection coefficients may be carried out in two different ways: — if the ionosphere is regarded as homogeneous and as having a sharp lower edge, the reflection coefficients on the air-ionosphere interface can be calculated by the theory of rays, taking into account the polarization of the incident wave. One thereby obtains transmission and reflection matrices because, with an incident plane wave, the polarization of the reflected wave is usually elliptical; — if we wish to use a real ionospheric model, which varies with altitude, the ionosphere must be divided into slices thin enough for the medium to be regarded as homogeneous. Since the thickness of each slice will then be small in comparison with the wavelength, the differential equations obeyed by each component of the electromagnetic field must be written out and solved below the ionosphere, taking account of boundary conditions at high altitude. If the solutions for the up and down paths are separated, the reflection coefficients can be determined. This method is usually called the “ full- wave ” theory. 1.2 The “ wave-guide mode ” method is that in which propagation is analyzed as the sum of the waves corresponding to each of the different types of propagation in the earth-ionosphere wave-guide, analogous to a mode as defined for wave-guides in the microwave region. The selection of the method to be used for field calculation depends on practical considerations of numerical calculation. In the case of VLF at distances of less than 1000 km and for LF in general, the series of modes are slightly convergent and the calculations then require adding together vectorially a large number of components. The wave-hop theory, on the other hand, requires only a limited number of paths, including the ground wave, and it is particularly convenient for the long-distance propagation of LF, taking account, if possible, of the diffraction. For VLF at distances of more than 1000 km, the wave-hop theory requires the vectorial addition of the fields due to a large number of paths whereas, since the series of modes converge rapidly, sufficient accuracy can be obtained by adding together only a small number of modes. The mode theory, therefore, is better suited to this type of propagation. 2. “ Wave-hop” propagation theory 2.1 General description According to this theory, the sky-wave field (strength and phase) at a point is treated as the resultant of the fields created by different waves propagated directly from the transmitter in one or more hops. — 125 — Rep. 265-3 The total field at this point is then the resultant of the field due to the wave diffracted by the ground and of the field due to the sky wave. The sky-wave field is calculated by applying the theory of rays in the regions where the methods of geometric optics are applicable and by integrating the effects of diffraction [Rydbeck, 1944; Bremmer, 1949; Wait, 1961] or by applying the full wave theory [Budden, 1961b; Johler, 1962; Krasnushkin, 1963; Pitteway, 1965] in regions where optics are no longer valid. The geometry of a path comprising a single hop is shown in Fig. 1. The surface of the Earth is defined by r = a and a smooth reflecting ionospheric layer located at r = a + h. It is convenient to distinguish three eases. In the first, the receiving antenna located at R < is illuminated by the once-reflected sky wave for the transmitting antenna located at T < where ig is less than tc/2. In the second, the two antennae at Tc and Rc are located at the critical points where ig = iz/2. In the third case, the antennae are located at T > and R > beyond the critical points such that the first sky-wave hop propagates into the diffraction or shadow zone. 2.2 Calculating the ray-path field strength The vertical component of electric field radiated from a short vertical electric dipole may be written as: Eu ~ 300 |fp t mV/m at 1 km (1) where Pt is radiated power in kW. The field strength of the downcoming sky wave, before reflection at the ground in the vicinity of the receiving antenna, is given by: E{ = ^ cos <\i “ R ” J) Ft mV/m (2) where L = sky-wave path length in km, “ R ” = ionospheric reflection coefficient for vertically polarized waves, D = ionospheric focusing factor, Ft = transmitting antenna factor, ^ = the angle of departure and arrival of the sky wave at the ground. If reception is by a small loop antenna located on the surface of the Earth, the effective field strength of the sky wave is: 2 E Es = — - cos “ R ” D Ft Fr mV/m (3) L For reception by a short vertical antenna (3) becomes: 2 E Es = — - (cos where FR is the appropriate receiving antenna factor. 2.2.1 Angles of elevation and ionospheric incidence The ray path geometry for determining the angles of departure and arrival of the sky wave at the ground (^) and ionospheric angles of incidence (i) are shown on Figs. 2 and 3. These angles are given in Fig. 2 for an effective reflection height of 70 km which corresponds to typical daytime conditions and in Fig. 3 for an effective reflection of 90 km which corresponds to typical night-time conditions. The effects of atmospheric refraction [Norton, 1960] on the departure and arrival angles are included and shown by the dashed curve although they are probably not valid for frequencies below about 50 kHz. Rep. 265-3 — 126 — 2.2.2 Path length and differential time delay To calculate L the sky-wave path length and estimates of the diurnal phase changes, Fig. 4 is used. This shows the differential time delay between the surface wave and the one, two or three hop sky wave for ionospheric reflection heights of 70 and 90 km, corresponding to daytime and night time conditions. A propagation velocity of 3 X 105 km/s is assumed. 2.-2.3 Focusing factor The ionospheric focusing factor, D, for a spherical Earth and ionosphere [Wait, 1962a] is shown in Fig. 5 for daytime conditions and in Fig. 6 for night-time conditions. 2.2.4 An tenna factors The antenna factors, FT and FR, which account for the effect of the finitely conducting curved earth on the vertical radiation pattern of the transmitting and receiving antennae [Wait and Conda, 1958] are shown in Figs. 7 to 9. Factors are calculated for land, sea and ice conditions which are defined by their electrical properties (conductivity and dielectric constant), as shown in the following table: C°1 s S hy ° Permittivi* Sea water 5 80s0 Land . . 2 X10-3 15e0 Polar ice 2-5 x 10-5 3s0 where s0 is the permittivity of free space. The curves were calculated assuming an effective earth’s radius, 8480 km, which is 4/3 of its actual value to take account of atmospheric refraction effects. The factors are the ratio of the actual field strength to the field strength that would have been measured if the Earth were perfectly conducting. Negative values of ^ refer to propagation beyond the geometric optical limiting range for a one hop sky wave (see Figs. 1-3). 2.2.5 Ionospheric reflection coefficients Values of the ionospheric reflection coefficient, “R ” are shown in Fig. 10 for solar cycle maximum and Fig. 11 for solar cycle minimum. “R ” refers to the component electric fields parallel to the plane of incidence before and after reflection from the ionosphere. To take account of frequency and distance changes, the values of “i?” are given as a function of/ cos i, where/is the transmitted frequency and i is the ionospheric angle of incidence [Belrose, 1957; Allcock, 1955; - Piggott, et al., 1965], Curves are shown for night and day conditions during three seasons of the year. Measured values at vertical and oblique incidence are indicated based upon the results given in numerous reports [Allcock, 1955; Alpert, 1967; Bain, 1952; Belrose, 1957, 1963; Bickel, 1957; Bracewell, 1952, 1953; Crichlow, 1946; Elling, 1960; Hopkins, 1954; Straker, 1955; Weeks', 1952]. The concept of an effective frequency / cos i for which reflection coefficients are constant cannot, however, always be relied on [Jones and Wand, 1969]. The curves in Figs. 10 and 11 are derived from data obtained at steep incidence (d < 200 km) and at more oblique incidence {d > 500 km) the/ cos i concept is likely to be approximately correct for such distances. At intermediate distances, however, the concept of an equivalent frequency is likely to lead to substantial errors in reflection coefficient, because in such circumstances the reflection coefficient and polarization of the wave change rapidly with distance. 2.3 Calculating the full wave field strength This method uses the full wave solutions for the various hops composing each path, the calculation of which has now been made possible by the existence of high-speed large scale computers [Wait, 1961; Berry, 1964]. Berry [1967] compares the full wave method with the method described above. A report [Berry and Christman, 1965] provides curves of amplitude and phase and demonstrates a process for the separation of the effects of ionospheric reflections from those due to the diffraction propagation — 127 — Rep. 265-3 in a manner similar to that described for the ray-path method. A computer programme is available for these computations of field strength by means of the wave-hop method [Berry and Herman, 1971]. 3. The wave-guide mode radio propagation theory 3.1 General description The mode theory, which is used extensively at YLF, is a full wave theory that includes diffraction and surface wave propagation. The waves are considered to propagate between the Earth and the ionosphere as normal modes, analogous to microwave .propagation in a lossy wave-guide [Alpert, 1967; Budden, 1961a; Galejs, 1967a; Galejs, 1967b; Pappert, 1968; Volland, 1966; Wait, 1962b; Wait, 1964]. For frequencies above about 30 kHz the wave-guide is many wavelengths high, and for short distances many propagating modes must be considered, but at VLF and distances greater than 1000 km only a few modes need be considered. While the physical picture of wave-guide mode theory is less easy to visualize compared with ray-paths it provides a better explanation of certain features of VLF propagation which are not readily explicable on ray-path/wave-hop theory. VLF transmitters in common use radiate a vertically polarized field. Although the ionosphere and terrestrial magnetic field may in principle introduce a horizontally polarized component, there is no experimental evidence which demonstrates that such fields are significant at great distances. Theoretical calculations [Snyder and Pappert, 1969] suggest that at night, in low latitudes, there may be considerable horizontal polarization. 3.2 Calculating field strength in a spherical homogeneous guide The field strength from a transmitter at a distance d km, on the ground, can be calculated by [Wait, 1962]: £ = . t \ - ‘ 9* + * /4 > £ A„e- mV/m (5) sja sin dja h n where Pt = radiated power in kW, a = radius of the Earth in km, X = free space wavelength in km, k = 27t/X, A n = excitation factor for the nth mode, kSn = propagation constant, d = path distance in km, h = height of ionosphere (70 km-day; 90 km-night). In general, the terms A n and Sn are complex. The excitation factor A n gives the relative amplitude and phase of each mode of order n excited in the Earth-ionosphere wave-guide by the source. The real part of the propagation constant kSn contains the phase information for each mode while the imaginary part determines the attenuation rate. In order to obtain the field strength the contributions of each mode must be summed giving proper attention to the relative phases of the terms. Modification is required near the antipode when dja ^ 7t. The field strength for mode n is: 300 \j Pt s j\ —1 (O/j kd Re Sn) \ri ' ' A„ e e mV/m ^ \Ja sin d/a h Rep. 265-3 — 128 — where: A„ (excitation factor) = | A„ | e ^ n aw (attenuation factor) = ImS„. ~ A = -&6%.^ImSn dB/km A Im.Sn and Re.Sn denote the imaginary and real parts of Sn respectively. The phase velocity V„ = c/ReSn, where c is the free space velocity. In most cases it is practical to derive field strength versus distance curves [Watt, 1967]. The recom mended method is to start with appropriate values of An and Sn and perform the calculations in (5). Where more than one mode is present, it is necessary to interpret (5) in amplitude and phase. In most cases, it is unnecessary to consider more than three modes, but exceptions do occur [Bickel et al., 1970; Snyder and Pappert, 1969]. The field strength due to each mode n depends on A n and Sn. These factors depend on wavelength ionosphere height, the ground electrical properties, and the spherical reflection coefficient of the ionosphere. The ionosphere reflection coefficients depend on the vertical distribution of electron density and collision frequency, the direction and magnitude of the Earth’s magnetic field, the frequency, and angle of incidence. The electron density distribution is a function of latitude, season, solar cycle, time of day and whether or not ionospheric disturbances are present. The influences of these various parameters can be broadly summarized in the following way [Alpert, 1967; Budden, 1961a; Volland, 1966; Wait, 1964], — Ground conductivity: In general the effect of reducing the ground conductivity is to increase the attenuation rate of all modes. However, some calculations [Wait, 1965] show that when the conduc tivity is very low (e.g. polar ice-caps) the attenuation rate may approach a maximum and then decrease as the conductivity is lowered further. For moderate conductivities the magnitude of the excitation factor for the first order mode usually increases somewhat as the conductivity is reduced. Further more, as the conductivity is reduced, the phase velocity of each mode is also reduced. — Direction ofpropagation with respect to the Earth's magnetic field: The effect of the Earth’s magnetic field is to cause greater attenuation for propagation to the magnetic west than for propagation to the magnetic east. Propagation in north-south directions gives intermediate attenuation rates. At the magnetic equator, there is little dependence of the phase velocity on direction of propagation. At slightly higher latitudes, there is some evidence [Crombie, 1966] supported by theory [Galejs, 1967b] that the difference in the phase velocity of the first two modes is greater for west-to-east propagation than for propagation in the opposite direction at night. Calculations indicate that the excitation factor of the first mode is greater for east-to-west propagation than for west-to-east propagation, again tending to compensate somewhat for the increased attenuation. — Use of conductivity parameter: It has been shown [Wait, 1965] that useful calculations of wave-guide mode propagation can be performed by assuming the height variation of a conductivity parameter co, (z) to be given by co, (z) = co, Qi) exp [(3 (z-h)] (7) where cor = co02/v for v > co; co, v and co0 are the wave angular frequency, the collision frequency, and plasma angular frequency respectively; z is the height, h is a reference height at which co, ~ 2*5 X105. The term (3 gives the vertical gradient of cor. Under daytime conditions, h is about 70 km and [3 ph 0*3, while at night h is around 90 km and (3 ph 0*5. These two parameters, h and (3, provide a convenient, but approximate, means of describing the ionosphere for use in YLF calculations. 3.3 Calculating field strength in a non-homogeneous guide The above calculations refer to conditions where the path properties are independent of distance. When the electron density distribution with height along the path is constant but the ground conductivity or magnetic field angle changes, it is approximately correct to use average values of the attenuation rate and phase velocity in performing the computation. For example, if the attenuation rates and phase velocities over a portion of the path of length dm are cnnm and c/ReSnm then — 129 — Rep. 265-3 ocd 2 u.nmdm (8) Re.Snd = S Re.Snmdm (9) m When the reference heights of the ionosphere at the transmitter and receiver are different (near dawn or dusk), this procedure is not very accurate. It is then necessary, in addition, to replace A n and h in equation (5) by \l AnT A nR and \/hT hR respectively, where the subscripts T and R refer to the transmitter and receiver locations [Wait, 1964; Watt, 1967]. The amplitude and phase of a single mode change smoothly in a non-oscillatory manner with distance. When two or more modes with different phase velocities are present, the field strength changes in an oscillatory manner as the various modes go in and out of phase. If only one mode is present = d>„ — kd ReSn. When more than one mode is present, it is suggested that calculations start with small distances and calculate new values of At present there is no suitable procedure for calculating the diurnal field strength changes when signals are transmitted in an east-to-west or west-to-east direction and portions of the path are in daylight and darkness. However, it can be shown that the signal amplitudes and phases are proportional to the combined day and night excitation factors, AD,N, attenuation rates; aDtN, and phase velocities, V„, as follows: Such an approach appears to account for the gross diurnal changes which are observed on long paths 10 000 km) at frequencies below about 18 kHz. The total phase delay is the sum of the phase delays along the> sunlit and dark portions of the path. As the terminator (i.e. the sunrise or sunset line) moves along the path, the phase in this simple single mode model changes in proportion to the length of path in darkness giving rise to the well known trapezoidal diurnal variation [Crombie, 1958]. At higher frequencies or on shorter paths, however, this approach is no longer adequate since it is found that the diurnal changes of phase may almost disappear, or even be reversed (i.e. phase delays are greater during the day than at night) or that the changes at sunrise and sunset may be in the same direction [Burgess, 1967]. However, when two or more modes are included in equation (5) and the calculations of phase delay are made for appropriate daytime and night-time parameters the observed patterns are obtained. Another phenomenon [Crombie, 1964; Lynn, 1967; Walker, 1965] observed at sunrise or sunset is that instead of the phase changing smoothly, it changes in a series of steps which are accompanied by deep nulls in the field strength. This feature is observed at all frequencies in the VLF band and is not masked by the above-mentioned irregular behaviour at the lower frequencies and on long paths. The phase steps are a result of conversion of energy from one mode into another at the sunrise or sunset terminator. For example, consider the case of propagation west to east at sunrise. The transmitter excites several modes of which at least the second and first are of significant amplitude at the terminator. However, because they travel with different phase velocities, they go in and out of phase along the path. Values of the mode parameters are best derived by fitting (5) to measurements of field strength versus distance taken during day and night conditions over a wide variety of paths [Wait, 1962b; Watt, 1967; Bickel, 1967; Rhoads, 1967]. 4. Future work The theory of wave propagation at VLF and LF, necessary to estimate field strengths, is now becoming fairly well understood. Two competing theories are used to describe propagation in these bands: the wave-hop theory and wave-guide mode theory. Rep. 265-3 — 130 — The solutions obtained from the wave-hop theory are generally most suitable for short distances and the higher (LF) frequencies, while the wave-guide mode theory is most suitable for longer distances and the lower (VLF) frequencies. More precise mode field calculations will require inclusion of transverse electric (TE) as well as transverse magnetic (TM) modes and the conversion between these types of modes. 4.1 Wave-hop theory This theory describes the propagation using terms that correspond to physically real quantities that the radio engineer can easily visualize. While many data have been incorporated into the curves given in Figs. 10 and 11, which show that the ionospheric reflection coefficient varies with time of day (midnight and noon), season, and epoch of the solar cycle, more work is needed to define the curves more precisely. In particular the range of values about the mean is needed; winter data show a greater spread of values than summer and equinox values; and the significance of various discontinuities in the curves needs to be verified. The sharp change in the values for effective frequencies 35-45 kHz in sunspot maximum years (summer), and for effective frequencies ^ 125 kHz in sunspot minimum years (winter and equinox) need verification. Also the reflection coefficients are larger in sunspot maximum years at the very low frequencies, whereas at low frequencies they are smaller in sunspot maximum years. It now seems possible, at least for paths at VLF, that better propagation predictions could be made from full-wave calculations using model ionospheres than is possible by reading curves such as those in Figs. 10 and 11. Efforts to construct such model ionospheres for the D-region should be encour aged—night and noon models for various seasons, latitudes and solar activities should first be produced. There is currently a lack of propagation data from which revised ionospheric reflection coefficients as a function of frequency, distance, time of year, solar and geomagnetic activity and other parameters can be derived. Because of this, the various theoretical treatments described below have not been adequately compared with experimental data and their usefulness is not yet fully achieved. In view of these limitations in reliable propagation data, a measurement programme has begun in Canada where frequency-stabilised transmissions are being made from Ottawa on 23-7, 32-8, 46-0, 65-1, 80-0, 89-7, 130-8 and 184-0 kHz with receivers located at distances ranging from 100 to 2500 km. More measurements are needed to establish differences between middle, low and high latitudes. For example, it is shown [Watt and Plush, 1959] that marked effects occur due to the low ground conductivities encountered in Arctic areas. Estimates of field strength agree well with the observed results at middle latitudes, providing the effects of the poorly conducting ground are properly taken into account. However, at high latitudes there may be significant dependence on seasonal and solar cycle changes in ionospheric reflectivity. A more comprehensive study should be made by comparing the ray-path solutions (with diffraction corrections) and the wave-hop solutions in order to establish the validity of the approximations inherent in the former approach. 4.2 Wave-guide mode theory The wavq-guide mode theory seems to describe better certain features of VLF propagation, e.g. the phase steps at dawn which are observed on long east-to-west transmissions, than does the ray-path/ wave-hop theory. The calculation of field strengths can be simpler using the wave-guide mode theory than the wave-hop method for many cases. More comprehensive comparisons should be made between solutions using the wave-guide mode method and wave-hop method. It has been shown that there is limited agreement between theoretical calculations and measurement. However, further experimental observations of field strength (and phase) versus distance in a variety of directions with respect to the Earth’s magnetic field, at several frequencies, between 10 and 30 kHz, over sea water and land of various conductivities, and at representative latitudes are needed. The obser vations need to be repeated sufficiently often to demonstrate their stability. With such measurements it will be possible to determine values of the excitation factors, attenuation rates and phase velocities which in themselves could be used as a basis for calculation of field strength and phase over different — 131 — Rep. 265-3 paths. More importantly, however, these experimental propagation parameters should be compared with theory and, if necessary, the theory should be refined or its input parameters adjusted. It is likely that greater discrepancies will be observed for night-time than for daytime conditions. Research employing atmospherics as natural VLF sources on frequencies below 10 kHz is being carried out by various groups. A statistical method of analysing atmospherics in this frequency range has been developed at the Heinrich-Hertz Institute in Berlin [Volland et al., 1967] which permits the determination of the location of individual thunderstorms as well as of the transmission function of the ionospheric wave-guide in amplitude and phase [Frisius et al., 1970; Harth, 1971]. In the United States of America multi-frequency pulsed and phase coherent transmissions between 9-4 and 31-25 kHz are being used for propagation studies over very short and medium paths and over the long-distance path from Hawaii to California. A desirable way to carry out these measurements would be to use an aircraft flying along great circles between transmitters and field receiving stations. In this way the mode structure at the receivers could be determined, especially at night for paths up to 5000 to 7000 km depending on frequency. The fixed receivers which would monitor the signals continuously could then be used to determine changes in the propagation parameters resulting from diurnal and seasonal variations and changes caused by differences in solar activity as well as mode conversion factors. The other approach which is also desirable is to use measurements by rockets or ground-based techniques to determine the vertical variations of electron density in the lower ionosphere. The resulting profiles should then be used to calculate ionospheric reflection coefficients. These would then be used to calculate the VLF mode parameters for comparison with the experimental VLF results, or for direct calculation of the mode parameters. Electron density measurements are particularly required at heights below 70 km where there are large discrepancies in existing observations, at night-time and at low latitudes. R eferences A ll c o c k , G. McK. [1955] Ionospheric absorption at low and medium frequencies. The Phys. o f the Ionosphere (The Phys. Soc. of London), 14-19. A l pe r t, Ya. L., G u se v a , E.G. and F lig el, D.S. [1967] Propagation of LF electro-magnetic waves in the earth-ionosphere waveguide. Academy of Sciences of the U.S.S.R. B a in , W.C., B r a c e w ell, R .N ., S t r a k e r , T.W. and W escott, C.H. [1952] The ionospheric propagation of frequency 16 kc/s over distances of about 540 km. Proc. I.E.E. Monograph, 37, 99 (Pt. IV), 250. B elrose, J.S. [1957] Some investigations of the lowest ionosphere. Ph. D. Thesis Univ. of Cambridge, first copy, 1956; revised copy. B elrose, J.S. [1963] The oblique reflection of low-frequency radio waves from the ionosphere. Propagation of radio waves at frequencies below 300 kc (ed. Blackband), Agardograph 74, 149-165, Pergamon Press. B elrose, J.S., H a t t o n , W.L., M cK e r r o w , C.A. and T h a in , R.S. [1959] The engineering of communication systems for low frequencies. Proc. IRE, 47, 661. B elrose, J.S. and Ross, D.B. [1962] Observations of unusual LF propagation made during polar cap disturbance (PCD) events. J. Phys. Soc. Japan, 17, Supplement A-II (Int. Conf. on cosmic rays and the earth storm), 126-130. B err y, L.A. [1964] Wave-hop theory of long distance propagation of low frequency radio waves. Radio Science, 68D, 1275-1284. B err y, L.A. [1967] Wave-hop radio propagation theory. Conf. on MF, LF and VLF radio propagation, I.E.E., Conf. Pub., 36, 63-69. B e r r y , L.A. and C h r ist m a n , M.E. [1965] Numerical values of the path integrals for low and very low frequencies. NBS Technical Note No. 319 (for sale by Superintendent of Documents, -U.S. Govt. Printing Office, Washington, D.C.). B er r y , L.A. and H e r m a n , J.E. [1971] A wave-hop propagation program for an anisotropic ionosphere. OT Telecom. Res. Rept. 11 (GPO Stock No. 0300-0316). B ic k e l, J.E. [1957] A method for obtaining reflection coefficients and its application to 135-6 kc/s data in the Alaskan area. J. Geophys. Res., 62, 373-381. B ic k e l , J.E. [1967] VLF attenuation rates deduced from aircraft observations near the antipode of NPM. Radio Science, 2 (New Series), 575-580. Rep. 265-3 — 132 — B ic ke l, J.E. and F e r g u so n , J.A. [1970] Experimental observations of magnetic field effects on V L F propagation at night. Radio Science, 5, 19-25. B l a ir , B.E., C r o w , E.L. and M o r g a n , A.H. [1967] Five years of VLF world-wide comparison of atomic frequency standards. Radio Science, 2 (New Series), 627-636. B r a c e w e l l , R.N. [1952] The ionospheric propagation of radio waves of frequency 16 kc/s over a distance of about 200 km. Proc. I.E.E., 99 (Pt. IV), 217. B r a c e w e l l , R .N ., B u d d e n , K.G., R atcliffe, J.A., S t r a k e r , T.W. and W eekes, K. [1951] The ionospheric propagation of low and very low-frequency radio waves over distances less than 1000 km. Proc. I.E.E., 98 (Pt. Ill), 221-236. B r a c e w ell, R .N ., H a r w o o d , J. and S t r a k e r , T.W. [1953] The ionospheric propagation of radio waves of frequency 30-65 kc/s over short distances. Proc. I.E.E., Monograph 84. B rem m er, H. [1949] Terrestrial radio waves. Elsevier Publ. Co. B u d d e n , K.G. [1961a] The waveguide-mode theory of wave propagation. Prentice Hall. B u d d e n , K.G. [1961b] Radio waves in the ionosphere. Cambridge University Press. B u r g ess, B. [1967] VLF phase delay variability and the design of long range navigation aids. Paper presented at AGARD/E.P.C. Symposium on instability in electromagnetic propagation, Ankara,Turkey. C asselm an, C.J., H eritag e, D.P. and T ib ba l s, M.L. [1959] VLF propagation measurements for the Radux-Omega navigation system. Proc. IRE, 47, 829-839. C r ic h l o w , W.Q., H erbstreit, J.W., Jo h n so n , E.M. and S m ith , C.E. [1946] Measurement technique and analysis of low frequency Loran system. Operational Res. Staff, Washington, D.C., Report No. ORS-P-23-S. C rom bie, D.D. [1964] Periodic fading of VLF signals received over long paths during sunrise and sunset. Radio Science, NBS Journ. o f Research, 68D, 27-34. C rom bie, D.D. [1966] Further observations of sunrise and sunset fading of VLF signals. Radio Science, 1 (New Series), 47-51. C rom bie, D.D. [1967] The waveguide mode propagation of VLF radio waves to great distances. I.E.E. (London) Conference Publication, 36. C rom bie, D .D ., A l l a n , A.H. and N e w m a n , M. [1958] Phase variations of 16 kHz transmissions from Rugby, observed in New Zealand. Proc. I.E.E., 105B, 301-304. D o h erty, R.H ., H efley, G. and L in fe l d , R.F. [1961] Timing potentials of Loran-C. Proc. IRE, 49, 1659-1673. E l l in g , W. [1960] Scheinbare Reflexionshohen und Reflexionsvermogen der Ionosphare fiber Tsumeb, Sfidwest-Afrika, ermittelt mit Impulsen im Frequenzband von 350 bis 5600 kHz (Pulse measurements of virtual heights and reflection coefficients of the ionosphere over Tsumeb, South West Africa, in the frequency range from 350 to 5600 kHz). Arch. Elektr. Ubertragung, 15, 115-124. F r isiu s, J., H e y d t, G. and H a r t h , W. [1970] Observations of parameters characterising the VLF atmospherics activity as a function of the azimuth. J. Atmos. Terr. Phys., 32, 1403-1422. G alejs, J. [1967a] Propagation of VLF waves below an anisotropic stratified ionosphere with a transverse static magnetic field. Radio Science, 2 (New Series), 557-574. G alejs, J. [1967b] Propagation of VLF waves below anisotropic ionosphere models with a dipping static magnetic field. Radio Science, 2 (New Series), 1497-1512. Geophysikalische Messreihen des Observatoriums ffir Ionospharenforschung Kfihlungsborn, DDR, monthly bulletins giving results of measurements by Dr. E.A. Lauter and his colleagues, since 1953. H a r t h , W. [1971] Sudden anomalies of spectral atmospherics parameters during SID’s. Proc. of the VLF Symposium 0do- Sandefjord. H o p k in s , H .G . and R e y n o l d s, L .G . [1954] An experimental investigation of short distance ionospheric propagation at low and very low frequencies. Proc. I.E.E., 101 (Pt. HI), 21-34. Jo h ler, J.R. [1962] Propagation of low frequency radio signal. Proc. IRE, 50, 404-427. Jo n es, T.B. and W a n d , I.C. [1969] Validity of the / cos i theorem for the absorption of very low frequency radiowaves in the ionosphere. Nature, 222, 462-463. K r a s n u s h k in , P.Y. and Y a b l o c h k in , N .A . [1963] The propagation theory of ultra-long waves. Mathematical Institute, Academy of Sciences, Moscow. L y n n , K.J.W. [1967] Anomalous sunrise effects observed on a long trans-equatorial VLF propagation path. Radio Science, 2 (New Series), 521-530. M o ore, R.K. [1967] Radio communication in the sea. IEEE Spectrum, 4, 42-51. N o r t o n , K.A. [1960] Low and medium frequency propagation. Electro-magnetic wave propagation. 375-444, Academic Press. P a p pe r t , R.A. [1968] A numerical study of VLF mode structure and polarization below an anisotropic ionosphere. Radio Science, 3 (New Series). P ierce, J.A., P alm er, W., W a t t , A .D . and W o o d w a r d , R.H. [1964] OMEGA- a world-wide navigational system.Pickard and Burns Electronics, P & B Publ. No. 886. — 133 — Rep. 265-3 P ig g o t t , W .R., P it te w a y , M.L.V. and T h r a n e , E.V. [1965] The numerical calculation of wave fields, reflexion coefficients and polarizations for long radio waves in the lower ionosphere. II, Phil. Trans. Roy. Soc., A-257, 243. P it t e w a y , M.L.V. [1965] The numerical calculation of wave fields, reflexion coefficients and polarizations for long radio waves in the lower ionosphere. I, Phil. Trans. Roy. Soc., A-257, 219-241. R ed e r , F.H. and W e st e r l u n d , S. [1968] VLF signal phase instabilities produced by propagating medium — experimental results. Proc. of AGARD/EPC Symp. on phase and frequency instability in radio propagation. R h o a d s, F.J. and G a r n e r , W.E. [1967] An investigation of the modal interference of VLF radio waves. Radio Science, 2 (New Series), 539-546. R y d b e c k , O.E.H. [1944] On the propagation of radio waves. Trans. Chalmers Univ. Technol. Gothenberg, 3 4 . S n y d e r , F.P. and P a ppe r t , R.A. [1969] A parametric study of VLF transequatorial propagation anomaly. Radio Science, 4, 213-226. S t r a k e r , T.W. [1955] The ionospheric propagation of radio waves of frequency 16 kHz over short distances. Proc. I.E.E., Monograph 114. T sao , C a r so n , H.K. and d e B et ten c o u r t , J.T. [1967] Subsurface radio communication. Conf. on MF, LF and VLF Propagation. I.E.E. Conf. Publ., 36, 313-317. V o l l a n d , H. [1966] Die Ausbreitung langer Wellen (Propagation of long waves). Verdffentlichungen des NKGG der DDR, Reihe II, Heft 2, 3-126. V o l l a n d , H ., H e y d t , G. and H a r t h , W . [1967] The statistical measurement of the spectral amplitudes and phases of atmospherics in the VLF range. 13th AGARD Symposium. Conference Proceedings No. 33. W ait, J.R. [1961] A diffraction theory for LF sky wave propagation. J. Geophys. Res., 66, 1713-1724. W a it , J.R. [1962a] Introduction to the theory of VLF propagation. Proc. IRE, 50, 1624-1647. W a it , J.R. [1962b] Electromagnetic waves in stratified media. Pergamon Press, New York. W a it , J.R. [1964] Two-dimensional treatment of mode theory of the propagation of VLF radio waves. Radio Science, NBS Journ. o f Research, 68D, 81-93. W ait, J.R. and C o n d a , A.M. [1958] Pattern of an antenna on a curved lossy surface. IRE Trans. Antennas and Propagation, AP-6, 348. W a it , J.R. and S pies, K.P. [1965] Influence of finite ground conductivity on the propagation of VLF radio waves. Radio Science, NBS Journ. of Research, 69D, 1359-1373. W a l k e r , D. [1965] Phase steps and amplitude fading of VLF signals at dawn and dusk. Radio Science, NBS Journ. of Research, 69D, 1435-1443. W att, A .D . [1967] VLF radio engineering. Pergamon Press, New York. W a t t , A .D . and P l u s h , R.W. [1959] Power requirements and choice of an optimum frequency for a world-wide standard frequency broadcasting station. NBS Journ. of Research, 63D (Radio Prop.), 35-44. W eekes, K. and S t u a r t , R.D. [1952] The ionospheric propagation of radio waves with frequencies near 100 kc/s over short distances. Proc. I.E.E., 99 (Pt. IV), 29. W il liam s, C a r a d o c [1951] Low-frequency radiowave propagation by the ionosphere, with particular reference to long-distance navigation. Proc. I.E.E., 98 (Pt. HI), 81-89. B ibliography D eeks, D.G. [1966] D-region electron densities in middle latitude deduced from the reflection of long radio waves. Proc. Roy. Soc., A291, 413-437. K n ig h t , P. [1967] The effect on medium-frequency sky-wave propagation of the conductivity of the ground near the transmitting aerial. Conf. on MF, LF and VLF radio propagation. I.E.E. Conf. Publ., 36, 106-110. S pr e n g e r , K. and L a u t e r , E.A. [1966] Verfahren zur Messung der ionospharischen Absorption (A3-methode) im Lang- und Mittelwellenbereich (Traffic measurement of ionospheric absorption (A3 method) in the LF and MF bands). Verdffent lichungen des NKGG der D D R , Reihe II, Heft 1, 47-66. S t a r ic k , E. and T a u m e r , F. [1966] A method of field-strength computation of the daily propagation of long waves. O.I.R.T. Radio and Television, 3, 28-36. W a it , J.R. [1968] Mode conversion and refraction effects in the earth-ionosphere waveguide for VLF radio waves. J. Geophys. Res., 73, 3537-3548. W a it, J.R. and S pies, K.P. [1964] Characteristics of the earth-ionosphere waveguide for VLF radio waves. NBS Tech. Note N o. 300. W a it , J.R. and S pies, K.P. [1968] On the calculation of mode conversion at a graded height change in the earth ionosphere waveguide. Radio Science, 3, 787-791. W a t t , A .D ., M a x w e l l , E.L. and W h e l a n , E.H. [1959] Low-frequency propagation paths in Arctic areas.\NBS\Journ. of Research, 63D, 99-112. Rep. 265-3 — 134 — ANNEX EXAMPLE OF A COMPLETE CALCULATION OF FIELD IN AMPLITUDE AND PHASE It is required to calculate the summer daytime expected field during solar cycle minimum under the following conditions using short vertical dipole transmitting and receiving antennae: Length of path d = 1911 km Frequency f = 80 kHz a - 2 X10-3 ! Transmitting location on land e = 15 e0 a = 5 S/m Receiving location on sea water }:£ = 80 e0 Radiated power Pt = 0-4 kW The successive stages of the calculation are as follows: Stage Parameters Figures Terms calculated Values 1 Pt = 0-4 kW Eu = 300j/04 = 190 mY/m 2 d - 1911 km 2 -0-36° i - 81° 3 * = -0*36° 8 Ft = 0-36 4 = -0-36° 7 F* = 0-67 5 d = 1911 km 4 L-d = 46 [xs c = 3 x 105 km/s L = 1911 + (46 X 10_6.3 X 105) = 1925 km 6 d = 1911 km 5 D = 2-16 7 f = 80 kHz /cos i = 80 cos 81° = 12.5 kHz i - 81° 8 /cos i— 12*5 kHz solar cycle 11 “ R ” = 0-11 minimum day (summer) 9 Es = ll-4 x iO -3mV/m 10 h = 70 km (day) 4 differential time = 67-47 h = 90 km (night) delay = 20 (xs d = 1911 km (1 hop) = 1*6 cycle (i.e. 576°) at 80 kHz — 135 — Rep. 265-3 F ig u r e 1 Ray path geometry for the wave-hop radio propagation theory (first hop sky wave) e. 265-3Rep. i or 4» angles, (degrees) eatr n ria nls n oopei nlsoficdne (i) incidence f o angles ionospheric and angles arrival and Departure h ahdcre nlds h fet topei refraction atmospheric f o effects the includes curve dashed The o tpcldyiecniin (h conditions daytime typical for Distance, Distance, : eaie ^ negative A: 16 — 136 — F gure r u ig d, 2 (km) = 0k ). km 70 / / or angles, (degrees) eatr n ria nls <> adinshrcage niec (i) incidence f o angles (<\>)ionospheric and angles arrival and Departure The dashed curve includes the effects o f atmospheric refraction atmospheric f o effects the includes curve dashed The o tpclngttm odtos (h conditions night-time typical for Distance, Distance, : eaie tjj negative A: 17 — 137 — F gure r u ig d, 3 (km) = 0k ). km 90 Rep. 265-3Rep. Time delay — 138 — in microseconds Rep.265-3 ifrniltm ea ewe ufc wv n n, w adtrehp k waves sky hop three and two one, and wave surface between delay time Differential B: 2 hop D : Limiting range Limiting : D hop 2 B: : hp : hop 1 C: hop 3 A: Ionospheric focusing factor focusing Ionospheric Distance, Distance, F F gure r u ig gure r u ig d, (km) 5 4 — — daytime — 139 — Rep. 265-3 500 1000 1500 2000 2500 Distance, d, (km) F ig u r e 6 Ionospheric focusing factor — night-time e. 265-3Rep. Antenna factor Ft or F r nlsaoehrzn (degrees) horizon, above Angles a = a e = 80 e0 80 = e S/m 5 = Antenna factors Antenna 4/3 x 6360 km 6360 x 4/3 10 — 140 — Sea water Sea F / gure r u ig 7 Antenna factor Ft or F r nlsaoehrzn (degrees) horizon, above Angles a 2 = ct e = 15 e0 15 = e = 4/3 4/3 = Antenna factors Antenna 11 — 141 — F gure r u ig X X Land X X 08 S/m 10-8 6360 km 6360 8 Rep.265-3 e. 265-3Rep. Antenna factor Ft or F r nls bv oio, (degrees) horizon, above Angles Antenna factors Antenna 2 x 03 S/m 10-3 x 025 0 e0 3 — 4°C at Ice 4/3 x 6360 km 6360 x 4/3 12 — 142 — F gure r u ig 9 — 143 — Rep. 265-3 / cos i kHz F ig u r e 10 Ionospheric reflection coefficients — solar cycle maximum Letters designate vertical incidence measurements; numbers designate oblique incidence measurements @ : night (all seasons) (D : day (winter) © : day (equinox) (p) ; day (summer) Rep. 265-3 — 144 — / cos i kHz F ig u r e 11 Ionospheric reflection coefficients — solar cycle minimum Letters designate vertical incidence measurements; numbers designate oblique incidence measurements ® : night (all seasons) (§) : day (winter) (g) : day (equinox) (gj : day (summer) — 145 — Rep. 431-1 REPORT 431-1 * ANALYSIS OF SKY-WAYE PROPAGATION MEASUREMENTS FOR THE FREQUENCY RANGE 150 kHz TO 1600 kHz (Study Programme 17A-2/6) (1970- 1974) 1. Introduction The propagation curves in Report 264-3, which form the basis of Recommendation 435-2, refer only to nocturnal field-strength values and the range of distances from 300 to 3500 km in the European Broadcasting Area. Study Programme 17A-2/6 and Decision 8 mention the need to extend these curves for shorter and longer distances, and to provide curves for other areas and, if necessary, for daylight conditions in which there is significant reflection from the ionosphere. However, realizing the urgency of the problem as expressed in Resolutions No. 614 (May, 1967) No. 635 (May, 1968) and No. 652 (May, 1969) of the Administrative Council, Interim Working Party 6/4, set up under the terms of reference in Decision 8, is actively pursuing this study. Several field-strength prediction methods for either restricted or world-wide application have, however, been developed in some countries. A method drawing elements from two of these forms the subject of § 7 and Annex of Report 575. As a further measure, it is desirable to have some indication of the progress of the work in terms of the areas where experimental work is being done and for which field-strength measurements exist. Such a review follows, together with some discussion on measurements made at distances shorter than 300 km and longer than 3500 km and suggestions for future work. 2. Australia 2.1 Prediction curves A study of medium-frequency sky-wave characteristics has been conducted by the Australian Broadcasting Control Board since March, 1953. Measurements made during this study have been the subject of publications [Dixon, 1960, 1962] and contributions to the work of the C.C.I.R. [C.C.I.R., 1962a, 1963—1966b]. The field-strength curves of Figs. 1, 2, 3 and 4 are derived from the latter document which contains the results of two major measurement campaigns conducted in 1958-1959 and 1963-1965. These figures show the hourly median field strength exceeded on 50% and 10% of the nights of a year for an unattenuated field strength of 3 X105 p.V/m at 1 km in the direction of propagation and at the angle of departure appropriate to one-hop E propagation. Figs. 1 and 2 refer to the field strength at the second hour after sunset for paths which are predominantly north-south, Fig. 1 being for the period of high sunspot activity and Fig. 2 for low sunspot activity. Fig. 4 also applies to the second hour after sunset and the period of low sunspot activity, but relates to long paths which are predominantly east-west. Fig. 3 relates to the field strength at 2330 hours during the period of low sunspot activity, for paths which are predominantly north-south. Corrections given in Fig. 1 of Report 264-3 should be applied to field strength predictions from Figs. 1, 2, 3 and 4 to compensate for the transmitting antenna vertical radiation pattern and to enable the predictions to be made on the basis of a radiated power of 1 kW. The inverse of this correction was applied to measured field strengths from which the basic prediction values were derived. Corrections given in Fig. 1 of Report 264-3 are considered to be sufficiently accurate, except where the transmitting antenna environment departs significantly from that of the transmitting stations used in the sky-wave measurement campaigns. Most of these transmitting sites have a ground conductivity between 5 X 10~3 and 10-2 S/m. * Adopted unanimously. Rep. 431-1 — 146 — 2.2 Interpretation of predictions 2.2.1 When applied to individual cases, these predictions may be in error for the following reasons: 2.2.1.1 The random variation from night to night which produces a standard deviation of 2 dB over a period of one month for measurements made at a particular time of night. 2.2.1.2 The variation which occurs from hour to hour throughout the night. 2.2.1.3 The seasonal variation which produces a standard deviation of 2-5 dB for measurements made at the second hour after sunset. 2.2.1.4 The variation which occurs from site to site in the one locality. For reception in an urban area, this variation can be negligible at one frequency and up to i 6 dB at another frequency. Predictions in Figs. 2 and 3 were developed from measurements processed in such a way that this variation was eliminated. Figs. 2 and 3 therefore refer to the field strength exceeded at 50 % of locations. An additional increase in field strength of the order of 6 dB may occur, where the receiving antenna is in very close proximity to domestic electric wiring. The predictions are not applicable to reception sites with sea water in the foreground. In this connection, it should be noted that the selection of reception sites for MF sky-wave field strength measurement campaigns is usually dominated by considerations of convenience for the staff who make the measurements, rather than by considerations based upon technical features of the site environment. It is not uncommon for such measurements to be made at radio receiving centres surrounded by large rhombic antennae and adjacent to antenna feeder lines. This presents no major difficulty provided the system is calibrated by measurements made at a clear remote site. However, where this is impos sible, a more elaborate calibration method is required. This was the case at the reception site near Melbourne, used in the Australian MF sky-wave measurement campaign of 1963-1966 [C.C.I.R., 1966-1969a]. Tests were conducted around the Melbourne site which revealed the spatial variation of field strength in the locality, and the degree of correlation between instantaneous measurements made at separate sites within the locality. From this information, site correction factors were determined which convert values obtained at the reception site to values exceeded at 50 % of the sites in the locality. This method of calibration reduces the variance of annual median values measured at the site and therefore increases the accuracy of field strength curves derived from these measurements. The degree of correlation between instantaneous measurements is usually moderately strong out to a distance of two miles, but it becomes quite poor at a distance of four miles. 2.2.1.5 The variation between sunspot minimum and sunspot maximum. 2.2.1.6 All other variations. These produce a standard deviation in annual median values of 1 dB. 2.2.2 The type of variation mentioned in §§ 2.2.1.2 and 2.2.1.3 can be seen in Fig. 5, which shows the hourly median field strength exceeded at 50 % of locations on 50 % of the nights of a month, in dB relative to the hourly median field strength at 50 % of locations on 50 % of the nights of a year (April, 1964-March, 1965) at the second hour after sunset. Fig. 5 may be used in conjunction with the predictions given in Fig. 2 to determine the field strength at any time of night and in any month during the period of low sunspot activity, for a transmission frequency of 1000 kHz. When Figs. 2 and 5 are used in this manner, the predictions will be more accurate for a distance of 700 km than for a distance of 1600 km. 2.3 Comment on the development of world-wide prediction methods In connection with the development of world-wide prediction methods a comparison has been made [C.C.I.R., 1970-1974a], on a time-of-night/month-of-year plot, of hourly median field-strength values for the European and Australian regions, given in Fig. 8 of Report 264-3 and Fig. 5 of this Report, respectively. The same information is repeated here in simplified form as Fig. 6 with values for Australia shifted by six months to make the seasons in both hemispheres coincide. — 147 — Rep. 431-1 There are significant characteristic differences between the two sets of results, as will be seen from Fig. 6; these differences must pose certain difficulties in developing world-wide predictions within conventional prediction methods. It is suggested that it may be preferable to adopt a different approach to the whole question of sky- wave prediction. Features in accord are the maximum monthly median levels in any one hour and the season in which these maxima occur at mid-latitudes. In comparison with existing methods which refer to annual distributions it is probable that prediction of this maximum level would: — be more meaningful in regard to interference calculations as it refers to the maximum monthly median irrespective of the season or time of night, — be in close agreement from region to region, — involve less work if collection of additional field-strength data is necessary, as the observation period is reduced from a minimum of twelve months to a minimum of one or two months. Apart from the variation with time of night and season, there are other ways in which medium- frequency sky-wave field strength varies (sunspot activity, distance, location and direction) most of which warrant further study. 2.4 New transmission system A method of sky-wave field-strength reduction, which exploits the high absorption of extraordinary waves for transmission frequencies near the gyrofrequency, was proposed in 1965. The transmitting antenna for this system is required to radiate a signal polarized in such a manner that waves entering the ionosphere do so exclusively through extraordinary modes. The system is termed “ orthogonal transmission ” (see also Report 461). Propagation tests [Dixon, 1965, 1968] conducted in 1965 and 1967 indicate that the median value of the sky-wave field strength from a broadcasting transmitter operating in band 6 (MF) may be reduced by 16 dB on paths to the north in the southern hemisphere, when vertically polarized transmission is replaced by orthogonal transmission. No significant change in this reduction was evident on south- north paths extending from 243 km to 695 km. The reduction decreased on paths with eastward or westward components due to features in the design of the transmitting antenna, which did not provide the polarization ellipse tilt required on such paths. A field-strength reduction of 13 dB was measured on paths which were 19° to the east or west of the bearing of the target area (magnetic north). When the reduction in sky-wave field strength is examined in relation to its variation with time, sky-wave absorption, and path bearing, the results are found to be consistent with: — a small variation in the limiting polarization of the ionosphere at night; — propagation along paths which depart from the most direct path when the signal fades during vertically polarized transmission; — the expected variation of the reduction with change in non-deviative absorption; — characteristics of the transmitting antenna. 3. New Zealand 3.1 Detailed study A detailed study of medium-frequency sky-wave propagation was commenced by the New Zealand Administration in February 1969 across the sea path between Australia and New Zealand at distances between 2000 km and 3200 km. The field-strength measurements were taken at the second hour after sunset with the transmitter-receiver relationship being west-east in all cases. Measurements in the range 550-1500 kHz are to continue through a period of one sunspot cycle [C.C.I.R., 1966—1969b]. Rep. 431-1 — 148 — 3.2 Interpretation of results While there are at present not sufficient data for propagation curves to be prepared, Fig. 7 represents median values of results so far from a total of 240 measurement hours. In comparing results with those'obtained in Australia (see § 2 above) the measured values are 6 to 7 dB higher at 3200 km. Assuming about a 4 dB variation of field strength over a sunspot cycle, as found by the European Broadcasting Union, it appears that the difference in measured values is due to sea reflection for the 2-hop E-layer propagation paths between Australia and New Zealand, whereas the Australian measurements were taken over land propagation paths. 4. India The field strengths of transmitters operating at frequencies between 550 kHz and 1560 kHz and over distance ranges from 185 km to 1760 km have been recorded in India. Measurement results covering paths in different azimuthal directions indicate that field strengths in the north-south direction are about 4-6 dB higher than the corresponding field strengths in the east-west direction due to high polarization coupling losses on east-west propagation paths at the lower latitudes. Measurement values obtained from north-south paths tend to be about 6 dB higher than the corresponding predicted values for the European Broadcasting Area and about 3 dB lower than those predicted in Australia [C.C.I.R., 1970—1974b]. 5. Japan Radio waves transmitted from a number of MF broadcasting stations in Asia at 57 frequencies in the range 548 kHz to 1550 kHz have been received since 1961 at several receiving stations in Japan. West-east path median field-strength values obtained as a result of point-to-point measurements at frequencies centred around 600 kHz are plotted, as a function of distance, in Fig. 8 [C.C.I.R., 1970-1974c]. Results of two series of mobile MF field-strength measurements covering west-east/east-west and north-south propagation paths, which were carried out in June and July, 1973, in the Pacific Ocean area on board ships, are shown in Fig. 9 [C.C.I.R., 1970-1974c]. All results, regardless of-the direction of propagation paths, tend to follow rather closely the N-S Cairo curve [C.C.I.R., 1937 and 1938], if the latter is adequately converted from quasi-maximum to median values by subtraction of 9 dB. 6. United States of America The F.C.C. standard broadcast curves of yearly median field strength [F.C.C., 1968], derived from measurements made in the United States of America, have been available for many years and considerable use has been made of them in other countries. The measurements refer to a period of low solar activity, and the presentation adopted for the curves is based on the second hour after sunset and shows how they vary with latitude. A detailed comparison [Barghausen, 1966] has been made between the curves of Report 264-3 and the F.C.C. curves referred to above, which shows, for the same conditions, that good agreement is obtained when the F.C.C. curves are applied in the European Broadcasting Area (at least for field strengths exceeded for 10% of the time). On the other hand, when the curves of Report 264-3 are compared with the F.C.C. curves in the middle latitudes of the northern part of the western hemisphere, differences of the order of 5 dB at 1800 km and 25 dB at 3200 km are indicated for both the 10% fields and the 50% fields. The higher values are predicted by the F.C.C. curves in the area for which they were developed. It is believed that these differences are due, in a large measure, to the use of the correction term A7 for the magnetic dip latitude at the mid-point of the path shown in Fig. 3 of Report 264-3. 7. Africa A joint measurement campaign was organized in Africa by the three broadcasting organizations— O.I.R.T., E.B.U. and U.R.T.N.A.—for the African LF/MF Broadcasting Conference. The campaign lasted in all for about one year, from June, 1963 to July, 1964, but many paths were studied for less v than one year. Unfortunately, it was too short for sufficiently reliable propagation curves to be prepared. — 149 — Rep. 431-1 However, 760 measurement sheets (one per path and per evening) were collected, representing a total of 2700 measurement hours distributed over 23 paths, and the results apparently correlate fairly well on the whole with what could have been gathered from the curves for the European Broadcasting Area, except for a relatively large spread which could not be accounted for. The results, together with additional data on sea-paths provided by a member of E.B.U., are shown in Fig. 10, together with a curve of field strength against distance for a frequency of 1 MHz at 2200 hours local time at the midpoint of the path, as deduced from the curves in Fig. 1 of Report 264-3. The plotted points are the median values of the field strength measured for the various paths in Africa, corrections of the type valid in Europe having been applied to convert them to 1 MHz at 2200 hours. (This reference time was chosen because it corresponded closely to most observations, thereby minimizing errors due to uncertainty about the time correction for Africa.) It appears that, in the absence of other data, the curves of the European Broadcasting Area could be used temporarily for African planning requirements, but with some caution. For instance, near the magnetic equator, the measurement results seem to confirm that propagation loss is greater along east-west as compared with north-south paths, as might be predicted from theoretical considerations of wave polarization [Phillips and Knight, 1965]. Also, the variation of the hourly median value of the field strength during the night may differ from that found in Europe and may depend considerably upon the magnetic dip. 8. Field strengths at distances below 300 km 8.1 Field-strength predictions As a contribution to the African Broadcasting Conference (1964, 1966), Interim Working Party 6/4 prepared ionospheric field-strength curves for distances below 300 km for use in the African Broadcasting area. They were based on pulse measurements at vertical incidence carried out in Tsumeb (South-West Africa) [Elling, 1961] in the MF band. The data obtained were extrapolated for oblique incidence both in the E and the F region. From these calculations two families of curves for the case of either E or F reflection were obtained at frequencies between 300 and 1100 kHz. These curves were combined with those in Report 264-3 for planning purposes by drawing curves in the transition region with deviations up to ± 1*5 dB from the two sets of curves so combined. These composite curves were used during the African LF/MF Broadcasting Conference (I.T.U., 1966). The interpretation of the shape of these curves was given by Taumer [1968]. The field-strength prediction curves for distances below 300 km are now referred for further consideration to Interim Working Party 6/4. 8.2 Measurements 8.2.1 Pulse measurements at vertical and oblique incidence For the study of short-distance sky-wave propagation, it will be recalled that a knowledge of the possible values of the ionospheric reflection coefficients at vertical and oblique incidence (see Study Programme 17A-2/6) can be very helpful in giving a complete understanding of the physical phenomena underlying medium-frequency modes of propagation. The work done in South-West Africa can be mentioned in this connection. Measurements at a number of frequencies between 350 kHz and 5600 kHz have been made at times of high and low solar activity [Elling, 1961 and C.C.I.R., 1966-1969c]. At both times, values of the mean night-time attenuation ranging from 6 dB at 1070 kHz to 17 dB at 365 kHz were found. Daytime values are much higher and at one hour before sunset and one hour after sunrise they may be several times higher than the night-time values. However, the relationship between the vertical-incidence measurements and the propagation of medium-frequency waves arriving in the ionosphere at oblique incidence is not very clear. Vertical-incidence reflections may occur at a height of approximately 100 km Rep. 431-1 — 150 — (at which height obliquely arriving waves are generally reflected) and at around 250 km (F layer) and even at intermediate heights (150 km), or even at several heights simultaneously. Observations at the same station in South-West Africa have also shown daytime reflections in the D region between 70 and 100 km. In general, maximum absorption at vertical incidence was found in the region of 500 kHz, apparently associated with a change of reflection level. Thus, the lower frequencies were reflected in the D region where, due to the small gradient of ionization, the deviative absorption increases with frequency. Reflections from the E region appear at about 500 kHz; here the ionization gradient is steeper, especially when sporadic E is present, and at higher frequencies only non-deviative absorption is important. In the evening after sunset, for vertical-incidence reflection at frequencies below about 500 kHz, the ordinary component is more highly attenuated than the extraordinary component, while at higher frequencies the reverse is true [C.C.I.R., 1963-1966c]. Ionograms made by the NBS at Boulder [Belrose, 1965] actually show that the “ 100 km stratum ” has a very irregular structure and, since the origin of this “ stratum ” is unknown, its irregularity and its partial transparency to vertically-incident medium-frequency waves complicate extrapolation to obliquely-incident waves. The “ 150 km stratum” seems, in actual fact, to be the real nocturnal E layer [Belrose, 1965]. This fact is of interest for the physics of the nocturnal ionosphere, but probably this layer has no effect on the propagation of obliquely- incident medium-frequency waves. Systematic analysis [Wakai, 1967, 1968] of low-frequency and medium-frequency ionograms (50-2000 kHz) at vertical incidence obtained at Boulder by the ESSA reveals the nocturnal variation as well as solar-cycle dependence of the night-time E layer as below: — many stratifications can usually be observed on the ionograms. The lowest stratum, however, at a virtual height of less than 100 km, is less sensitive to geomagnetic activity and varies in a regular manner during the night. Stratifications at virtual heights less than about 135 km show more or less similar characteristics to those of the lowest stratum. These may be identified as the night-time E layer. On the contrary, it is sufficient to classify the higher stratification having a virtual.height of about 150 km as an independent layer (so-called intermediate layer) which is separated from both the F layer and the normal night-time E layer by valleys of ionization. This layer usually exhibits close association with the geomagnetic activity; — the variation with local time of the night-time E layer shows different features in summer and winter; — the peak electron density of the night-time E layer is related linearly to the relative sunspot number. Analysis of conventional ionograms at 24 sounding stations in the American meridional zone shows that night-time ionization in the E region varies with latitude during both geo- magnetically quiet and disturbed nights. Evidence of night-time plasma frequencies in the E region exceeding 1 MHz is found during quiet nights above 52° in the geomagnetic latitude. On the other hand, a smooth increase in critical frequency of the night-time E layer is seen up to 52° in latitude during disturbed nights. Above this latitude, high critical frequencies behave dis- continuously up to about 80°. Somewhat lower frequencies may be observed in the polar cap region above 80°. Pulse measurements in the MF broadcasting band have been carried out in the Peoples’ Republic of Poland. A comparison of the first results with those of vertical ionospheric sounding indicates that MF propagation is often controlled by Es ionization [C.C.I.R., 1970-1974d]. 8.2.2 Field-strength recordings for distances below 300 km There are much less experimental data for distances below 300 km than for greater distances. This is partly due to the fact that in the European Broadcasting Area at least, only in exceptional cases is the reception free from interference. Another reason is the difficulty of obtaining the sky-wave field strength from broadcast transmitters because, in general, the field strength has a contribution from the ground wave. — 151 — Rep. 431-1 A reduction of the ground-wave component can be achieved by using a loop antenna at the transmitting or at the receiving site, or at both sites. Moreover, a method of analysis of the recordings of the combined ground-wave and sky-wave field strength has been developed [Muller, 1965] which enables the sky-wave field strength to be derived from the amplitude distribution. This method can, however, only be used when the ground wave is not the predominant component of the total field strength because otherwise small errors in the measurements would give rise to considerable uncertainties in the sky-wave component so derived. With the use of more than one receiving point at different ranges (e.g. from 100 to 300 km), the sky-wave field strengths can be compared and, taking into account the vertical radiation diagram of the transmitting antenna, the presence of reflections from the E or F regions can be distinguished. Measurement results for a distance range of 38 km have been obtained [C.C.I.R., 1963—1966d] by means of a loop receiving antenna. They show an average reflection loss of about 12 dB at 827 kHz for the period from two hours after sunset to 2200 hours. There is, however, an important seasonal variation with stronger reflection in summer. In April 1965, field-strength recordings were made in the Federal Republic of Germany [C.C.I.R., 1966—1969d] on 30 consecutive nights between 0100 and 0720 hours CET. The same transmitter, which operated successively at 620, 889, 1160 and 1529 kHz for a period of 50 minutes at each frequency was received simultaneously at 4 receiving sites located on a straight line at distances between 60 and 275 km. The transmitting antenna was of a small vertical type, whereas the receiving antenna was a loop at all receiving sites. At 63 km the ground-wave component was predominant at the lower frequencies and could not be eliminated by the analytical method [Muller, 1965]. For comparison purposes the average of the nocturnal median values of the recorded field strength was expressed in dB relative to 1 ;rV/m related to the unattenuated field of a reference antenna giving a field strength of 3 x 105 (TV/m at a distance of 1 km in all directions above the horizon. Fig. 11 shows the measurement results and a reference curve which can be obtained from formula (la) of Report 264-3 for distances greater than 300 km or, for smaller distances, from formula (4) of Report 575 entitled “ Methods for predicting sky-wave field strengths at frequencies between 150 kHz and 1600 kHz ”. The latter formula is based on the hypothesis that the ionospheric reflection coefficient remains constant for reception at distances below 300 km. The mean values of the standard deviation were 3-5 dB at 128 km, 3-6 dB at 185 km and 4-6 dB at 274 km. Further measurements were carried out on at least three successive days from 2200 to 2300 hours CET at 822 kHz at a distance of 11-6 km (May-June, 1968) and at 30, 50, 100 and 370 km (October-November, 1968). Loop antennae were used at both terminals. In Fig. 12, curve A shows the median values obtained in the measurements, curve C is the corresponding reference curve, and curve B is obtained as the annual mean if the seasonal variations observed [C.C.I.R., 1966—1969d] are used for a correction of curve A. An analysis of the Muller method of recording total field strength in the range 150 to 230 km from a broadcast transmitter on 191 kHz in Sweden has shown good agreement with the expected reflection losses according to the method of calculation described in Report 265-3. 9. Measurements made at distances above 3500 km 9.1 E.B.U. measurements Measurements of long-distance propagation between 3200 and 9300 km at frequencies between 620 and 1345 kHz have been carried out as part of the activities of the E.B.U. [C.C.I.R., 1966-1969e and C.C.I.R., 1970-1974g]. The Federal Republic of Germany has carried out measurements at Tsumeb, South-West Africa, on three north-south paths at distances of about 7000 km and frequencies of 164 kHz (Allouis), 845 kHz (Rome) and 1602 kHz (Ismaning) [C.C.I.R., 1970-1974e, 1970-1974h]. In Fig. 13 these measurements are compared with measurements made before 1938, but the latter have been reduced by 9 dB to convert quasi-maximum values to median values. Fig. 13 also shows Rep. 431-1 — 152 — the curves adopted by the C.C.I.R. at Cairo [C.C.I.R., 1937 and 1938], but with field-strength values reduced by 9 dB. The results show that field strengths observed at distances exceeding 3500 km, especially on north-south paths, are considerably higher than values obtained by extrapolating the curves of Fig. 2 of Report 264-3. The Cairo curves for north-south paths shown in Fig. 13 appear to be satisfactory for predicting the highest field strengths which are likely to be observed when the transmitting and receiving antennae are situated on ground of average conductivity. Higher field strengths will be observed if either, or both, terminals are situated near the sea (see Report 401-2). 9.2 U.S.S.R. measurements Measurements have been made in the U.S.S.R. on frequencies between 150 and 1500 kHz at distances between 600 and 4500 km [C.C.I.R., 1966—1969f] and are shown in Figs. 14, 15 and 16. . The results show that at the highest frequencies in this band the field strengths may already be higher than those shown in Fig. 2 of Report 264-3 even at a distance of 2000 km, and that between 2000 and 4500 km the propagation curves would approach progressively towards the extrapolated curve for the frequency of 200 kHz as indicated in Figs. 15 and 16. The U.S.S.R. results show that the magnetic dip correction curves of Report 264-3 are valid for paths up to 2200 km. For longer paths characterized by multihop propagation, it is better to determine the correction separately for each hop and to add up the results. 9.3 A.B.U. measurements A series of long-distance sky-wave propagation measurements in band 6 (MF) have been conducted since 1971. In Fig. 17, the maximum, median and minimum field strength values are shown for each of the 25 paths under study. The values have been derived from either half-hourly or hourly cumulative distributions [C.C.I.R., 1970-1974c]. It is apparent that the entirety of these long-term median field strengths is very close to the N-S Cairo curve [C.C.I.R., 1937 and 1938], when the latter is adequately converted from quasi-maximum to median values. 10. Daytime propagation of the sky wave Within the framework of studies carried out by the E.B.U. on medium-wave propagation, the Norddeutscher Rundfunk’s receiving and measuring station at Wittsmoor near Hamburg has made daytime measurements. The transmitters used for these measurements were Strasbourg (1160 kHz, 150 kW) at a distance of 610 km and Mainflingen (1538 kHz, 270/300 kW), at a distance of 405 km. The recording period was from November 1963 to November 1964, and the recordings were made between 0500 GMT (beginning of transmission) and 2200 GMT (close-down). 107 recordings are available of the transmission from Strasbourg and 128 from Mainflingen [C.C.I.R., 1963-1966a]. From the results obtained from these recordings it may be concluded that, for the frequencies and the propagation paths concerned, the annual mean value of the sky-wave field strength at noon, at the midpoint of the path, is about 45 dB lower than the corresponding value at midnight. An analysis of the seasonal variations of the median values of the daytime field strengths over the period 1963-1965 was carried out by Taumer and Starick [1969]. Their results show at daytime an increase in absorption, that is a decrease in field strength, from winter to summer, of about 18 dB at 845 kHz and 31 dB at 1466 kHz. The difference between daytime and night-time median field strengths at 845 kHz varies between about 42 dB in winter and 54 dB in summer, and is even greater in the higher part of the MF band. At the frequency 1466 kHz the difference lies between 40 dB in winter and 65 dB in summer. The daytime field strength at a frequency of 750 kHz and for a path length of 740 km has been recorded in Japan since 1969. The seasonal variation of the field strength at noon is similar to that of the zenith angle of the sun. The difference of the annual median field strengths at noon and at midnight during the moderate period of the sunspot cycle is 42 dB. The correction to be applied to the annual mean reference field strength valid for R12 — 0 is about —0*055 Rn dB, where R12 denotes the 12-month running average of the sunspot number [C.C.I.R., 1970—1974f]. — 153 — Rep. 431-1 11. Future work Future work should deal with several aspects of the question, namely: 11.1 to widen the geographical area in which propagation curves similar to those prepared for the European Broadcasting Area could be available and to associate such curves with one another, in accordance with various values of some parameters of geographical situation. In theory, plans for assigning and sharing medium-frequency channels over very wide areas, such as Europe or Australia or the United States of America, can be dealt with independently of one another, but there are bound to be problems in adjoining regions in the case of areas such as Europe, Africa and Asia; also, comparisons between measurements made at different parts of the world might give very useful information about geographical factors affecting field-strength curves. After the first measurement campaign mentioned in § 7, O.I.R.T., E.B.U. and U.R.T.N.A: have jointly undertaken a new measurement campaign lasting for about one year in 1965-1966; 11.2 to expand the range of propagation-path lengths for which most measurements have so far been made, both for short distances (below 300 km) and for very long distances (above 3500 km). One problem with short distances is the difficulty of separating ground-wave effects from sky-wave effects, and the curves prepared so far have been based, not on direct measurements of the sky-wave field, but on the theoretical deductions from knowledge of the coefficient of ionospheric reflection at vertical incidence. However, most reflection data coefficients are derived from measurements of vertically-incident waves at a relatively small number of stations, and since little work has been done up to now on the critical frequency of the E layer at night, it is highly conjectural to extrapolate the vertical-incidence data to cover obliquely incident waves and therefore the short-distance calculation of sky waves. There is therefore a need to develop studies on the critical frequency of the E layer at night and on the reflection coefficient of the E and F layers for various incidences of medium-frequency and low-frequency waves and also to study further general methods of total-field recordings to enable the sky-wave term to be derived from them. If possible, direct short-distance measurements of sky waves should be made. As regards long-distance measurements, they should help towards determining inter alia the influence of sunspot activity, and there are appreciable differences between over-sea paths and over-land paths. It is suggested that as early as possible the material dealing with measurements in this Report should be separated from that on the prediction methods so far available, to form a special report on field-strength predictions in the LF and MF bands both at distances below 300 km and above 3000 km. 11.3 There is a lack of detailed data on daytime ionospheric propagation between 150 and 1600 kHz; this form of propagation may be considerable, particularly in winter months. 11.4 It will be useful to obtain more accurate data on the influence of rapid fading on the value of the field strength for a given percentage of time. 11.5 Still further statistical studies* based on known data or, even better, on new results, are required to reduce the area of uncertainty of predictions based on the curves available at present or to reduce the area of uncertainty by better knowledge, either of parameters affecting the field or of the correlation between the field and other parameters. Some Administrations or authorities are at present undertaking studies of this kind. R eferences B a r g h a u se n , A.F. [1966] Medium frequency sky-wave propagation in middle and low latitudes. IEEE, Transactions on Broad casting, B C -12,1-14. B elrose, J.S. [1965] The low ionosphere regions in Physics of the Earth's upper atmosphere. (Ed. C.O. Hines, et al.,) 66-67, Prentice Hall Inc. C.C.I.R. [1937] Docs. 4th Meeting, Bucharest, May-June, 1937, Vol. 1. C.C.I.R. [1938] Documents de la Conference Internationale des Radiocommunications du Caire, Tome I, 425-433, Fig. 3. (See also Report of Committee on Radio Wave Propagation, Proc. IRE, 26, 1193-1234, Fig. 3.) Rep. 431-1 — 154 — C.C.I.R. [1962a] Doc. VI/88, Australia. C.C.I.R. [1963-1966a] Doc. VI/252, E.B.U. C.C.I.R. [1963-1966b] Doc. VI/205, Australia. C.C.I.R. [1963—1966c] Doc. VI/191, Federal Republic of Germany. C.C.I.R. [1963-1966d] Doc. VI/208, P.R. of Bulgaria. C.C.I.R. [1966-1969a] Doc. VI/171, Australia. C.C.I.R. [1966-1969b] Doc. VI/213, New Zealand. C.C.I.R. [1966—1969c] Doc. VI/2, Federal Republic of Germany. C.C.I.R. [1966-1969d] Doc. VI/186, Federal Republic of Germany. C.C.I.R. [1966-1969e] Doc. VI/208, E.B.U. C.C.I.R. [1966-1969f] Doc, VI/206, U.S.S.R. C.C.I.R. [1970—1974a] Doc. 6/177, Australia. C.C.I.R. [1970-1974b] Doc. 6/295, India. C.C.I.R. [1970-1974c] Doc. 6/289, Japan. C.C.I.R. [1970—1974d] Doc. 6/291, Poland (People’s Republic of). C.C.I.R. [1970—1974e] Doc. 6/280, Germany (Federal Republic of). C.C.I.R. [1970—1974f] Doc. 6/273, Japan. C.C.I.R. [1970-1974g] Doc. 6/97, E.B.U. C.C.I.R. [1970—1974h] Doc. 6/86, Germany (Federal Republic of). D ix o n , J.M. [1960] Some medium frequency sky-wave measurements. Proc. IRE, Australia, 21, 407-409. D ix o n , J.M. [1962] Attenuation of medium frequency sky-wave signals in Australia following the Mid-Pacific high altitude nuclear explosion in August, 1958. / . Geophys. Res., 67, 123-125. D ix o n , J.M. [1965] The absorption of medium frequency sky-waves by close coupling to the extraordinary mode. Proceedings, I.R.E.E., Australia, 26, 369-380. D ix o n , J.M. [1968] Experimental tests with orthogonal transmission. Proc. I.E.E., 115, 1755-1761. E l l in g , W. [1961] Scheinbare Reflexionshohen und Reflexionsvermogen der Ionosphare fiber Tsumeb, Sfidwest-Afrika, ermittelt mit Impulsen im Frequenzband von 350 bis 5600 kHz (Pulse measurements of virtual heights and reflection coefficients of the ionosphere over Tsumeb, South-West Africa in the frequency range from 350 to 5600 kHz). A.E.U., 15, 115-124. F ed e r a l C ommunications C om m ission [1968] Rules and Regulations, Part 73, Radio Broadcast Services. Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. M u l l e r , K. [1965] Determination of the sky-wave field strength at short and medium distances from recordings of the total field on long- and medium-wave transmitters. Techn. M itt. RFZ, 9, 89-95. P h il l ips, G.J. and K n ig h t , P. [1965] Effects of polarization on medium-frequency sky-wave service, including the case of multihop paths. Proc. I.E.E., 112, 31. T a u m e r, F. [1968] Uber die Ableitung von Ausbreitungskurven im Mittelwellenbereich in der afrikanischen Rundfunkregion (On the derivation of propagation curves in the MF band for use in the African Broadcasting Area). Techn. M itt. RFZ, 12,178-182. T a u m e r , F. and S t a r ic k , E. [1969] Jahreszeitliche Feldstarkevariationen im Lang- und Mittelwellenbereich (Seasonal variations of field strength in the LF and MF bands). Techn. M itt. RFZ, 13, 16-19. W a k a i, N. [1967] Quiet and disturbed structure and variations of the night-time E region. J. Geophys. Res., 72, 4507. W a k a i, N. [1968] Mean variations of the night-time ionospheric E layer. J. Radio Res. Labs. Japan, 15, 109. I Hourly median values o f the field strength exceeded at 50 % o f locations on 50 on locations f o % 50 at exceeded strength field the f o values median Hourly dB (fxV/m) 34 36 40 37 35 39 38 44 45 41 42 43 46 47 49 50 48 600 700 Nrhsuhpts ana en uihsnptnme 180) number sunspot Zurich mean annual paths; (North-south 800 900 at the second hour after sunset after hour second the at 1000 15 — 155 — F 1100 D gure r u ig km 1 1200 1300 1400 % and 10 and 1500 % o f the nights o f a year a f o nights the f o 1600 1700 Rep. 431-1Rep. 1700 1600 1500 1400 1300 1200 2 km ig u r e D F 1100 — — 156 — 1000 a year at the second hour after sunset 900 800 (North-south paths; annual mean Zurich sunspot number 10) 700 600 h a t !) aP Hourly median values ofthe field strength exceeded at 5 0 / oflocations on 5 0 / and 1 0 / ofthe nights of Rep. 431-1 Rep. orymda vle h il tegh xedda 0 oain n5% ad 0, h ihsof o nights the f o and. 10%, 50%, on locations f o 50% at exceeded strength field the f o values median Hourly dB (fxV/m) 600 700 Nrhsuhpts ana enZrc uso ubr 10) number sunspot Zurich mean annual paths; (North-south 800 900 1000 a year at 2330 hrs 2330 at year a ______10% 17 — 157 — F 1100 D gure r u ig km 50% 3 1200 1300 1400 1500 / 1600 1700 Rep. 431-1Rep. Rep. 431-1 — 158 500 1000 1500 2000 2500 3000 3500 D km F ig u r e 4 Hourly median values of the field strength exceeded on 50 % and 10 % of the nights of a year at the second hour after sunset. The number of the paths investigated was not sufficient to obtain an accurate prediction over the dashed section. (East-west paths; annual mean Zurich sunspot number 20) — 159 — Rep. 431-1 F ig u r e 5 Field-strength variation with time o f night and month of the year Contours show the hourly median values of the field strength exceeded at 50 % of locations on 50 % of the nights of a month, in decibels above the hourly median value of the field strength exceeded at 50% of locations on 50% of the nights of a year (April, 1964-March, 1965) at the second hour after sunset. The values shown are accurate for quasi-longitudinal transmission on a path of 700 km for transmission frequencies in the range 900 kHz to 1100 kHz. Dashed lines show the times of sunset and sunrise. Rep.431-1 Contours for Australia have been shifted by six months to make the seasons in both hemispheres coincide hemispheres both in seasons the make to months six by shifted been have Australia for Contours Local time at the mid-point of the path (h) Zero dB contours are the maximum field strength contours given for both regions both for given contours strength field maximum the are contours dB Zero «. _____ orymda il-tegh values field-strength median Hourly _ - Australia, 900 to 1100 kHz, 700 km 700 kHz, 1100 to 900 Australia, - Europe, 845 kHz, 700 to 2200 km 2200 to 700 kHz, 845 Europe, otu nevli 6 dB 6 is interval Contour 10 — 160 — F Month gure r u ig 6 1500 2000 2500 3000 3500 4000 D km F ig u r e 7 Field strength received for 50 % and 10 % of the time for 50 % of locations (West-east paths: mean Zurich sunspot number 90) A: 10% o = 10% values B : 50 % A = 50 % values C: See Fig. 2 of Report 264-3 Rep.431-1 dB (jiV/m) Night-time sky-wave field strengths measured at frequencies around 600 kH z, as a function o f distance f o a function as z, kH 600 around frequencies at measured strengths field sky-wave Night-time Annual medians of hourly or half-hourly medians half-hourly or hourly of medians Annual Rslsfo on-opit measurements) point-to-point from (Results : - Cio curve Cairo N-S A: Distance (km) Distance 12 — 162 — Wpaths -W E F gure r u ig 8 Half-hourly medians of recordings made around midnight local time at the mid-point of the propagation path (Results obtained (Results path propagation the of mid-point the at time local midnight around made recordings of medians Half-hourly dB OV/m) dB (fiV/m) Night-time sky-wave field strength measured at band 6 (M F) as a function o f distance f o a function as F) (M 6 band at measured strength field sky-wave Night-time ntePcfc ca, nbadsis i ueadJl, 1973) July, and June in ships, board on Ocean, Pacific the in CioNS curve N-S Cairo : A Distance (km) Distance Distance (km) Distance 13 — 163 — Wpaths -W E N -S paths -S N F gure r u ig 9 a 870 a 1440 ■ 1290 □ 770 o • 530 KHz 530 • a 810 a 770 o 1290 □ KHz 570 • 950 ■ 1530 x Rep.431-1 950 870 Rep. 431-1 — 164 — 40 30 g £>:L 20 i 10 0 -10 500 1000 1500 2000 2500 3000 3500 D km F igure 10 Medium-frequency measurements in Africa (median field strength corrected for 1 M H z, time 2200 hours and a transmitting antenna giving, over perfectly conducting ground, 3 X 10s [xV/m at 1 km in all directions above the horizon). : F '0 = 80-2— 10 log £>—0 0018 D / ° ' 26 + A 2200 (50) • : paths over land, x : paths over the sea (shore-sited transmitter) Key: / == frequency (kHz), I = magnetic dip at mid-point, N o. /(k H z ) I (deg.) (deg.) N o. /(k H z ) I (deg.) (deg.) 1 1 2 0 0 8 245 - 17 818 25 325 2 620 38 335 18 818 2 0 335 3 818 38 335 19 940 9 105 4 809 - 6 4 205 2 0 1300 i - 1 6 215 5 940 - 4 70 2 1 555 / 6 728 - 1 5 2 1 0 2 2 1300 \ - 1 9 240 7 665 - 6 3 235 •23 555 / 995 - 6 3 0 24 1300 i 8 245 9 1484 39 300 25 555 / - 2 1 1 0 940 7 105 26 555 - 7 195 1 1 791 - 1 5 190 27 1300^ - 2 0 250 1 2 1484 - 2 0 145 28 555 / 13 940 - 1 7 340 29 1300 \ - 1 9 245 14 1 2 0 0 1 295 30 555 / 15 935 28 1 0 31 1300 i - 3 4 275 16 935 29 35 32 555 / — 165 — Rep. 431-1 50 100 200 500 1000 D km F igure 11 Average of median values o f field strength, as a function o f distance Curves A and B are calculated according to formulae (4) of Report 575 and (la) of Report 264-3 respectively dB (ptV/m) 0 0 0 0 20 0 1000 500 200 100 50 20 10 Median values o f field strength, F0, at 822 kH z as a function o f distance f o a function as z kH 822 at F0, strength, f field o values Median Curve B: corrected for seasonal variations seasonal for corrected B: Curve uv : esrd values measured A: Curve Curve C: reference curve reference C: Curve F igure D km 12 e. 3- — 166 — 431-1 Rep. — 167 — Rep. 431-1 D km F igure 13 Sky-wave field strengths measured at distances greater than 3000 km A: N-S, Cairo 1938; B: E-W, Cairo 0 N/S -e- E/W U.E.R. ♦ N/S ♦ E/W U.I.R. Nov. 1936 - Jan. 1937 1 Riyad - Helsinki 2 1 Swan Island - 001 Northern Ireland - Ottawa 013 Pittsburgh - Buenos Aires 2 Batra - Wittsmoor Helsinki 0 0 2 Rennes - Ottawa 014 Buenos Aires - 3 Batra - Helsinki 2 2 Swan Island - 003 New York - London Strathburn (CAN) 4 Dakar - Limours Jurbise 0 0 4 Northern Ireland - 015 Buenos Aires - Ottawa 7 Batra - St Denis 23 Swan Island - Monza Washington 016 Toulouse - Buenos Aires 8 Harrar - Monza 24 Fort de France - 00 5 Rennes - New York 017 Rennes - Buenos Aires 10 Roma - St Denis Jurbise 0 0 6 New York - Bruxelles/ 018 Nice - Buenos Aires 15 Sackville - Limours 25 Kuwait - Monza Eindhoven 019 Buenos Aires - London 16 Sackville - 30 Masirah - Limours 00 7 Rennes - Washington 020 Paris - Buenos Aires Chatonnaye 32 Masirah - Leucate 00 8 New York - Berlin 021 Northern Ireland - 17 Moncton - 33 Bangkok - Helsinki 00 9 New York - M oscow Buenos Aires Chatonnaye 34 lsmaning - Tsumeb 0 1 0 Buenos Aires - Washington 022 Buenos Aires - Bruxelles 20 Poro - Helsinki 35 Roma - Tsumeb 011 Buenos Aires - New York 023 Buenos Aires - Berlin 36 Allouis - Tsumeb 0 1 2 New York - Buenos Aires Rep.431-1 ....poaain uvsfr20ad10 Hzadrslsofmaueet i te .... t97 94 n 9 Hz kH 998 and 944 917, at U.S.S.R. the in measurements f o results and z kH 1000 and 200 for curves propagation C.C.I.R. dB ((xV/m) dB (fxV/m) 20 10 20 30 40 50 6000 5000 4000 3000 2000 1000 0 ------r-l ....poaain uvsadrslsofmaueet n U.S.S.R. in measurements f o results and curves propagation C.C.I.R. rpsdcre : 94 2 16, : 96 4 1967 4: 1966, 3: 1965, 2: 1964, 1: curve proposed ----- 1 (Frequencies in kHz shown beside each dot) each beside shown kHz in (Frequencies 18 — 168 — e r u g i F e r u g i F D D km km 15 14 -- -=» --- Rep. 431-1 Rep. ^ ------ 16 km igure D F — 169 — 1:1965,2:1966,3:1967 — 1 ------0 1000 2000 3000 4000 5000 6000 ■30 ■30 (rn/Ari) C.C.I.R. propagation curvesfor 200 and 1500 kHz and results ofmeasurements in the U.S.S.R. at 1546 kHz Rep. 431-1 — 170 — Distance (km) N-S E-W MAX " MED " MIN F ig u r e 17 Night-time sky-wave field strengths measured in band 6 (M F) as a function o f distance Results from the regular survey o f the A.B.U. a: Inverse distance referred to 3 x 105 [xV/m at 1 km. b: N-S curve, Cairo 1938. c: E-W curve, Cairo 1938; EBU curve for 300 km < Distance < 3500 km at 1 MHz is approximated by this curve, d : U.S.S.R. curve at 1 MHz and for geomagnetic latitude 37°. — 171 — Rep. 432 REPORT 432 * THE ACCURACY OF PREDICTIONS OF SKY-WAYE FIELD STRENGTH IN BANDS 5 (LF) AND 6 (MF) (Decision 8) (1970) 1. Introduction The field strength of sky-wave signals in bands 5 (LF) and 6 (MF), observed at the surface of the Earth varies continuously with time and may be predicted only in terms of statistical methods. It has become common practice to base predictions on field-strength measurements without prior knowledge of all the factors influencing the measurement. If instrumental and processing errors are - small, the accuracy of such predictions depends upon the reception site environment and the assumption that conditions in the propagation medium will follow much the same pattern as in the past. Although this is a reasonable assumption, it does not permit the accuracy of prediction to be identified completely with the accuracy of determining a field-strength statistic, until the morphology of the propagation medium and the source of variations within the medium are better understood, or until measurements are made over a very long period. Provided these limitations are recognised, it may be said that the accuracy of prediction is determined by the spread of statistics around a common characteristic. During the past decade, there have been numerous contributions to Study Programme 17A-2/6 from the E.B.U., O.I.R.T., United States of America, India, Australia and the Federal Republic of Germany. As detailed results are readily available for the E.B.U., O.I.R.T. and Australian field-strength measuring campaigns, these three sources of data are used exclusively in this Report to determine the accuracy of low frequency and medium frequency sky-wave field-strength predictions. Different operating conditions prevailing in Europe and Australia resulted in different approaches to the measurement of sky-wave signals. Predictions for Europe are based on the combined results from the E.B.U. and O.I.R.T. measurement campaigns. The E.B.U. campaign produced data from twenty-four receiving sites which provided fifty-three distance-frequency combinations from five broadcasting stations (operating on 164 kHz, 602 kHz, 845 kHz, 1178 kHz and 1466 kHz) over distances from 325 km to 3360 km. On the other hand, the O.I.R.T. campaign produced data from six receiving sites and twenty-one broadcasting stations which provided thirty distance-frequency combinations. The Australian measuring programme involved four receiving sites which provided thirty-eight distance-frequency combinations from twenty-eight broadcasting stations operating on frequencies between 550 kHz and 1510 kHz observed over distances from 620 to 3340 km. Field-strength measurements from Europe are therefore available for numerous receiving sites, propagation paths and transmitting stations, whereas Australian results are available for numerous transmitting stations and propagation paths but for very few receiving sites. These different approaches do not prevent a comparison of the results either with respect to field-strength values or the variance inherent in the measurement. It is not the purpose of this Report to enquire into the difference in absolute values, but rather to study the accuracy of predictions, the information required to achieve this accuracy, and the significance of different variances. 2. General aspects of the prediction of sky-wave field strength in band 6 (MF) The nature of fading of the sky-wave field strength in band 6 (MF) suggests that the distribution of instantaneous values may be determined with sufficient precision from observations of half an hour * Adopted unanimously. Rep. 432 — 172 — to one hour duration, except during periods of rapid general change, such as that following sunset and that preceding sunrise when a period of one quarter hour is more appropriate. The distribution of instantaneous values is not constant but is appreciably more stable than other field-strength statistics from night to night. A convenient indication of this distribution is obtained by extracting the hourly median, upper decile and lower decile values from recordings. In the medium frequency band, the ratio of upper decile to median has a mean value of 5 dB and a standard deviation * of 1-1 dB to 2 dB. If median or upper decile values are plotted against the time of night, a general characteristic pattern will be evident for any particular frequency, distance and month of the year. Hourly median values for any one transmission and reception site, at a given time of night and month are distributed with a standard deviation of 2*0 dB to 4-5 dB. The standard error * of a mean determined from such hourly medians is 0*6 dB to 1-4 dB for a sample of 15 values or 1-1 dB to 2-6 dB for a sample of 4 values. Monthly means of hourly medians may therefore be determined with considerable accuracy and are used to study the variation of field strength throughout a year. If the measurement is made independent of sunset or sunrise variations, hourly median values for any one transmitting station and any one reception site are distributed throughout a year with a standard deviation ranging from 3*0 dB to 5*0 dB. The former value refers to some transmission paths observed at the second hour after sunset in Australia which display a normal distribution of hourly median field strength in dB (p,V/m). The latter value refers to measurements made at midnight in Europe where the distribution is excessively skew. In order to apply normal statistical methods, it is necessary to transform these values into a form which will produce a normal distribution. A satisfactory trans formation is obtained if hourly median values are given in dB (fxV/m). Results from some paths observed in Australia also display an excessively skew distribution which on a dB scale has a standard deviation of 4-2 dB. This represents a significant departure from that observed in Europe. The standard error of the annual mean of hourly medians at the second hour after sunset or at midnight is therefore between 0-5 dB and 0-7 dB for a sample of 52. If annual mean values for several transmissions are plotted against distance, a single characteristic may be drawn for each transmission frequency about which annual means are distributed with a standard deviation of 1-0 dB to 2-5 dB. The former value refers to observations made in Australia during 1958/1959 in which there was no significant variation with transmission frequency except at the shortest distance (650 km). The latter figure refers to weighted annual means used to compute E.B.U. predictions. At any one distance, the standard error of the characteristic derived from annual means is therefore 0-5 dB, 1*25 dB for a sample of 4 values and 0-7 dB, 1-8 dB for a sample of 2 values. More importance is attached to the distribution of annual mean values as these include errors of measurement associated not only with receiving sites, but also with transmitting sites. It should be understood that, with our present knowledge of sky-wave signal variations, such a statement relates to the error of a sample mean derived from an assumed single population. An examination of the statistics will show that this assumption is incorrect, but in determining propagation curves, the variance of annual means (between sample variance) represents an error. On the other hand, this error is very much a part of sky-wave field strength prediction if it is primarily due to the reception site environment. Although the reception sites used to obtain the Australian 1958/1959 results were in the same vicinity as rhombic antennae employed for short wave reception, particular care was taken to ensure that the influence of these antennae and their associated transmission lines was reduced to a minimum. Standard deviations and standard errors shown in Table I apply only for rural reception as they do not include variations due to overhead or underground wires. It may be inferred that no one source * Standard deviation and standard error values, in decibels, refer to the ratios (standard deviation + mean)/mean and (standard error of mean + mean)/mean. — 173 — Rep. 432 of independent variation, such as the error in the determination of the constant product of field strength times distance or the influence of the reception site environment, produced a standard deviation exceeding 0*8 dB. A particular feature of the E.B.U. results is the extent of the variation between samples as indicated by the spread of weighted annual means. The significance of these variations can be gauged by comparing the standard deviation between samples (2-5 dB) with the standard error of individual means, which for a sample of 52 values would be 0-7 dB. A departure of 2-5 dB by a sample mean from a population would occur only once in a thousand trials if the departure occurred purely by chance. The origin of this variance has not been determined, but it is probably associated with several factors, including the influence of the reception site environment. In order to reduce the amount of work involved in future propagation tests, methods of measurement should be employed which reduce the standard error of estimate to a minimum. The possibility of reducing the influence of the reception site environment has been demonstrated by applying corrections to the measurement. 3. General aspects of the prediction of sky-wave field strength in band 5 (LF) It is not surprising to find that comments, similar to those in the previous section, may be made in respect to predictions of sky-wave field strength in band 5 (LF). However, there are some significant departures in terms of the actual degree of variation. In Europe the ratio of upper decile to median sky-wave field strength decreases from 4-5 dB at short distances to 1*6 dB at long distances. The standard deviation of hourly median values at midnight over a period of one year for any one transmitting station and reception site is not significantly different from that observed at medium frequencies (5*0 dB). Low frequency annual means display a considerably greater variation than medium frequency annual means and therefore more propagation paths (or reception sites) have to be investigated to obtain the same precision in locating propagation curves. The standard deviation of annual means about a common characteristic is 4-9 dB and the standard error of estimate is therefore 1-3 dB for a sample of 14 values. 4. Information required to determine sky-wave propagation curves There will be an accuracy of prediction beyond which planning authorities will have little interest and therefore, from a practical point of view, there is no need to exceed this accuracy. It is difficult to set an absolute limit, but the error should be less than that which would cause a significant change in listener satisfaction within the most critical region of desired protection ratios.* It is suggested that this limit lies between i 1-5 dB and i 3 dB. Mean values may be determined within this accuracy by selecting samples of sufficient size as indicated in Tables I, II and III. There appears to be no justification in taking more than one recording per week for a particular combination of distance and frequency, because a sample of this size will provide an annual mean with a standard error of 0-5 dB to 0-7 dB and a monthly mean with a standard error of 1-1 dB. These errors relate only to measurements made at individual receiving sites. When predictions are made for other sites or when a whole area is considered, an additional variance is introduced due to changes in site environment. Where electric power lines are suspended overhead in urban areas and where receiving antennae may be in close proximity to household electric wiring, the median field strength at individual locations may vary up to 6 dB from the median value for the area. Measurements made in a locality at any one frequency give no indication of the variation from site to site at another frequency. It is not yet clear what procedure should be adopted if the number of reception sites is to be reduced to a minimum when making measurements in an urban area, but if the site-to-site variation is accepted as an inevitable aspect of such measurements, the standard error of estimate is 0*7 dB in band 6 and * For co-channel interference, an increase of 5 dB in the level of an interfering signal will reduce the percentage of listener satisfaction from 90% to 50%. Rep. 432 — 174 — 1*4 dB in band 5 for a sample of 13 values. For any particular combination of distance and frequency, the standard error of estimate is 1-25 dB in band 6 and 2-5 dB in band 5 for a sample of 4 values. Where measurements are made in a rural area away from the influence of electric power lines, two or more stations should be recorded for each range of distance investigated. This figure should be increased to four or more stations where there is evidence of a variation with frequency. Prediction accuracy depends jointly on the accuracy of measurement at receiving and transmitting sites. Unattenuated values may be determined by field-strength measurements along the propagation path at the transmitter end. These results are used to locate an appropriate curve (ground-wave field strength E times distance d) which gives the unattenuated field strength at unit distance, when extended back to zero distance. The standard error in locating such a value is less than 0-2 dB for a sample of 14 values. In regions with a small temporal variation of ground conductivity, measured values of (E.d) do not alter substantially over a period of five years, provided normal operating conditions are maintained during the measurement and provided the transmitting antenna-earth system is not subjected to chemical change. In order that a number of recordings may be considered collectively, the extraction of a single statistic from each recording requires considerable attention, as the time involved in the analysis of the records can be the determining factor which limits the scope of a recording programme. This may not be important where continuous or automatic analysers are available but when recordings have to be analysed by inspection, the time spent on the analysis can be appreciable. A means of reducing this time is found in the selection of a suitable field-strength statistic. Results of many measurements in the medium frequency band have shown that the variance of upper decile values is only slightly greater than the variance of median values. It therefore follows that the mean of upper decile values is determined to almost the same accuracy as that of median values for samples of equal size. The extraction of upper decile values is accomplished in considerably less time than the extraction of median values, and provides a means of measurement not significantly less accurate than the measurement of medians. Alternatively, the standard deviation of the ratio E10/E50 is less than the standard deviation of En over the period of one month or of one year. Since these variables are not correlated, the additional error appearing in a mean F50 calculated from a measured mean E10 and mean E10/E50 (taken from a small sample), will be of no consequence. In most cases, it will not be necessary to take a sample of the ratio E10/E50, as an assumed value of 5 dB will be sufficiently accurate for the medium frequency band. T a bl e I Standard deviation Standard error of mean Recordings of one hour duration from transmissions in the medium frequency (dB) (dB) band over distances between 650 km and 1450 km Origin A B A B EwlEso 1 -1 0-5 for a Australia sample o f 6 1958/1959, 1963 E50 for any one transmitting station and any one receiving site at a particular time 2 0 1 -1 for a Australia of night over a period of one month sample of 4 1963 £ 5 0 for any one transmitting station and any one receiving site at the second hour 3 0 4-2 0-5 for a 0 - 6 for a Australia after sunset over a period of one year sample of 52 sample of 52 1958/1959 ' (£50) annual mean, for any transmission over the period of one year 1 0 0*5 for a Australia sample of 4 1958/1959 Values under heading A refer to normal distributions (ptV/m) where the standard deviation and standard error are given as ratios in decibels of (standard deviation + mean)/mean and (stan dard error of mean + mean)/mean. Values under heading B refer to transformed skew distributions (dB (ptV/m)). e. 432 Rep. Rep. 432 T a b l e II Standard deviation Standard error of mean or (dB) standard error of estimate (dB) Recordings of half-an-hour or one hour duration Band Origin A B A B E10IE50 MF LF E50 for any one transmitting station and any one receiving site MF at a particular time of night over a period of one month LF ' E 50 for any one transmitting station and any one receiving site MF 5 0 0-7 for a E.B.U. (Rdme, 845 kHz, to at midnight over a period of one year sample of 52 Darmstadt and Monte-Carlo, 1466 kHz to Jurbise) LF 5-0 0-7 for a E.B.U. (Allouis, 164 kHz to sample of 52 Enkoping) (E50) annual mean, for any transmission over the period of one year MF 2-5 E.B.U. 845 kHz, 1178 kHz, 1466 kHz LF 4-9 1-3 for a E.B.U. 164 kHz sample o f 14 Values under heading A refer to normal distributions (pV/m) where the standard deviation and standard error are given as ratios in decibels of (standard deviation + mean)/mean and (stan dard error of mean + mean)/mean. Values under heading B refer to transformed skew distributions (dB (pV/m)). T a b le III Standard deviation Standard error of mean (dB) (dB) Recordings of half-an-hour duration Band Origin A B A B E J E so MF 1-7 to 2 0 O.I.R.T. (Rome, 845 kHz) LF 1 - 6 (Allouis, 164 kHz) E50 for any one transmitting station and any one receiving site MF 2-5 to 4 -5 0 O.I.R.T. (Rome, 845 kHz) at a particular time of night over a period of one month LF 3-7 to 5 -7 0 (Allouis, 164 kHz) E 50 for any one transmitting station and any one receiving site MF 3-7 to 4 0 O.I.R.T. (Rome, 845 kHz) at midnight over a period of one year LF 5 1 to 5*50 (Allouis, 164 kHz) (£ 50) annual mean, for any transmission over the period of one year 0 Varies with the season. Values under heading A refer to normal distributions (pV/m) where the standard deviation and standard error are given as ratios in decibels of (standard deviation + mean)/mean and (stan dard error of mean + mean)/mean. Values under heading B refer to transformed skew distributions (dB (pV/m)). Rep. 432, 574 — 178 — B ibliography C.C.I.R. [1956] D oc. XII/356 (India), Warsaw. C.C.I.R. [1962] Doc. VI/8 8 , Australia. E b e r t , W. [1962] Ionospheric propagation on long and medium waves. E.B.U. Review, Part A, 71, 3-11, 72, 68-80, 73, 109-121. S u l a n k e H. and T a u m e r F. [1967] Calculation of standard deviations and errors for medians of medium and long-wave field strength recordings. Contribution to the work of International Working Party VI/4 of the C.C.I.R. REPORT 574 * IONOSPHERIC CROSS-MODULATION ** (Question 23-1/6) (1974) 1. Introduction The basic theory of non-linear phenomena in a plasma as developed by Ginsburg and Gurevich [1960] and Gurevich [1971] extends the earlier work of Bailey [1937] on wave interaction or “ cross modulation ”. Briefly stated, the propagation of strong modulated waves through a plasma produces perturbations in the plasma which causes changes to take place in the electron temperatures which in turn affects the collision frequency, ion chemistry and electron density; and therefore the conductivity and permittivity of the medium. The result of these changes in the medium produced by one modulated intense radio wave is the superimposition of its modulation on the carrier of another wave propagating through the same region. Because of the large number of transmissions using the HF, MF and LF bands which propagate through the D and E regions, this wave interaction or ionospheric cross modulation is difficult to distinguish from co-channel interference and even more difficult to measure [Huxley et al., 1950,1955; Fejer, 1955; Ratcliffe and Shaw, 1948; Shaw, 1951; Bell, 1966; Hibberd, 1964; Cutolo, 1964, etc.]. 2. Measurements made when the bands were less crowded have been summarized by Knight [1972], and show cross-modulation depths less than 7 % when standardized to a power of 100 kW. The standardized measurements are shown in Fig. 1 of Report 460-1. For the communications engineer concerned with estimating the interference caused by cross modulation, the main features of a treatment given by Huxley and Ratcliffe [1949] in a paper designed to give a simple physical explanation of the phenomenon are presented. 2.1 The electron collisional process The free electrons which are mainly responsible for the reaction of the ionosphere on radio waves are located in D and lower E regions and may be regarded as a gaseous constituent statistically in thermal equilibrium with the far more numerous molecules in the atmosphere. Each electron may be considered to have a thermal energy Q0 and a velocity v0 related to the temperature 0O of the atmosphere by the gas equation: * Adopted unanimously. ** Note. — The Director, C.C.I.R., is requested to transmit this text to the International Union of Radio Science (U.R.S.I.) for comments. — 179 — Rep. 574 Qo = J mv0 = J k 0o 0 ) where m is the mass of the electron (9*1 x 10-31 kg), k is the Boltzmann’s constant (1-37 X 10~23 joules per kelvin) and 0O is in kelvins, Q0 and v0 being in M.K.S.A. units. If at the point under consideration the equilibrium is disturbed by increasing the velocity of the electrons to v, Q0 and 0O change to Q and 0 in accordance with (1), the temperature of the surrounding atmosphere being unchanged. The mean free path of the electrons is unaltered so that the collisional frequency v is increased from its equilibrium value v0 proportionately with v. Thus from (1): Q = v 2 = i r Qo V (2) At each collision some energy is transferred to the surrounding atmosphere, the amount being proportional to the energy excess and equal to G (Q — Q0) where G is a constant that from laboratory experiments on nitrogen is found to have a value of about 10-3. The equilibrium is disturbed by the passage of a radio wave, and if the energy extracted from the wave by an electron in a collision at the time t is Qe; the energy equation for the electron is: d Q - ^ = vQe - vG (Q - <20) which from (2) may be written — = — (v2 - V2) (3) dt 2 Q0 2 1 °J K) If Qe were constant v would ultimately take up a value v given by: Qe \ y2 v M 1 + GQo ) ^ except possibly for very high fields, Qe -C GQ0 and therefore v-v0 <^v0. Thus with the removal of Qe> v-v0 decays exponentially to zero with a time-constant of 1 /Gv0. In the region of the ionosphere under consideration v0 is of the order of 106 collisions per sec, so that this time constant is about 10-3 sec corresponding to a frequency of 1000 Hz. It therefore follows that the collisional frequency is unable to respond to the changes at radio frequency in the wave. As, however, Qe is proportional to the power density in the wave and hence to the square of the electric field E, the collisional frequency can respond to the r.m.s. value of the field. If this r.m.s. value is amplitude-modulated at audio frequency, the collisional frequency will in some measure be able to follow the modulation for frequencies not greatly in excess of 500 Hz. 2.2 The modulation process In general the modulation on the radio wave will contain many audio frequencies, but to estimate the level of cross-modulation it is sufficient to consider a single frequency co/27t and only the component of this frequency in the square of the field. Thus writing the r.m.s. radio field as: E — E0 (1 + M cos cot) (5) the square will be taken as: M 2 E 2 = E2q (1 + _ ) + El 2M cos co/ (6) where M — 1 for 100% modulation. The mean value of E 2 is thus significantly above E£, and it is this increased value that determines the mean value of v in (4), while v takes the form: Rep. 574 — 180 — v = v (1 +M J (7) where Afv is the modulation derived from the modulation term in (6). Writing Qe as Q e = C E 2 (8) where the constant of proportionality will be found by considering the attenuation of the radio wave by the electron collisions as it passes through the ionosphere, and taking the modulation component of v in (7) as v0 Mv, the modulation equation derived from (3), (6) and (8) becomes: dMv vo CEq 2M cos a t G _ . „. x v»d T = ------2 a ------T 2*o W or dMv , „ . , v0C El M cos tot _ + G,0M = ------ whence CElM M J = 0 (9) Q,G 'A 1 + l ^ The denominator shows how the response drops off at higher audio frequencies. 2.3 The absorption process As the radio wave passes through the ionosphere, the loss of the energy extracted by the electrons in the collisional process causes the wave to be exponentially attenuated according to an amplitude reduction factor exp (— //cds) where the integral is taken over the transmission path and k is an absorption coefficient. By considering the absorption of the wave passing through a thin slab of unit cross-section, it can be shown that the energy Qe extracted by each electron in a collision is: « • - where [x is the real part of the refractive index of the ionosphere given by the Appleton-Hartree equation, N is the electron density (number per cu.m.) and Z0 is the impedance of free space, the field E being in V/m. As the radio frequency may be in the neighbourhood of the gyromagnetic frequency f H, it is important to include the effect of the earth’s magnetic field by considering the case for which it is greatest, when the propagation is along the direction of the earth’s field. The value of k is then known to be: Ne2Z0 v K~ 2 [x m [4n2 ( / ± f Hf + v2] (11) where e is the electron charge (1-60 x 10-19 coulomb), / is the radio frequency in Hz, and the + and — signs refer to the ordinary and extraordinary waves respectively. Thus from (8), (10) and (11): C = ______(1 2 1 m [4tc2 ( f ± f Hf + v2] The v2 term in the denominator can be neglected in the present estimate of cross-modulation if, say, 4 t t 2( / ± / h ) 2 > 1 0 v 2 or (/dz/n)> y. For high frequencies this criterion is well enough obeyed and at medium frequencies, except for the extraordinary wave near to the gyrofrequency, which is of the order of 1 MHz, where a resonance occurs increasing the value of C. Under these resonance conditions, especially for high power transmissions, the value of v — v0 may be comparable with v0, and (4) from (8) and (12) should be regarded as a quadratic equation in v"2. At lower frequencies — 181 — Rep. 574 where / is of the order of or less than v, a much more sophisticated full-wave treatment of the whole problem is really needed. The value of C in (12) used in (9) gives 2e2E02M I Af J = ------co Vi (13) 3 m/cO„ 4^2 ( / ± f H)2 + v20 1 + W j where Q0 has been given its value in (1) and in the denominator v2 has been approximated by v2, in keeping with the assumption made in deriving (9) that C is a constant. 2.4 Demodulation and cross-modulation As the attenuation of the wave has the form of an amplitude reduction factor, the direct dependence of k in (11) upon v implies that the modulation transferred from the wave to the collisional frequency reacts back on the amplitude of the- wave. Actually this produces some demodulation of the wave, which is a phenomenon of considerable interest when using modulation experiments to investigate the physical structure of the lower regions of the ionosphere. For this purpose a much more detailed statistical analysis of the electron motion than has been adopted here is really necessary, with a close study of the gyromagnetic resonance. More importantly here, however, the modulation imposed on k can similarly be transferred to the amplitude reduction factor of another wave of different frequency passing through the modulation region, giving rise to cross-modulation. Apart from its physical significance, this phenomenon is a source of interference which it is here the purpose to assess. In principle the effect is mutual, each wave to some extent being demodulated and modulating the other, but it is now desirable to distinguish between the frequencies and to regard one transmission as wanted, with its own modulation on to which some modulation is imposed from an unwanted or disturbing transmission. Calling the frequencies fw and f D respectively, the frequency / i n (13) must now be interpreted as f D. If the amplitude reduction factor for the wanted wave is p, it is then given from (7) and (11) by Ne2Z0 v (1 + Mv ) ds exp - (14) 2 \xm [ 4 tc 2 (fw ± f H)2 -f- v 2] The integration is extended over the region of cross-modulation, which is of limited size because of the attenuation suffered by the disturbing wave in passing through the ionosphere, whereby the modulation in (13) imposed on the collisional frequency decreases with the decrease in E0. The region may, however, be sufficiently extended for the increase of N and the decrease of v with height in the ionosphere to be significant in estimating p in (14). Eventually the product Nv decreases, but initially it can increase with height. The time of passage of a wave-front through the region is so short that (14) may be regarded as an integration across the region at a moment of time on the audio-frequency time scale. Thus Mv is held constant with respect to time during the integration, and the consideration of the time variation of Mv at the audio frequency shows that p becomes modulated and may be written in terms of a mean value and modulation M p by: P = p (1 + M p) (15) Thus In p = In f + In (1 + M p) or since it may be anticipated that M p where in the denominators the modulation on v2 has been neglected by using the mean value. The integral for Mp in (18) may now be estimated by taking Mv outside as a mean value while formally retaining the dependence of N and v on position inside the integral, so that from (17) in terms of amplitudes Mp I = — I Mv I In p (19) and hence from (13) 2e2E\M In p M j = CO2 2 Vl (20) 3 m k 0_ 4^(fn ± /ff)2 + vJ 1 + Wo * j with an estimated mean value of El within the modulation region. This is essentially the result given by Huxley and Ratcliffe. The determination of In ~p from (17) implies a knowledge of the distribution of N and v in the modulation region for an assumed model of the ionosphere. Huxley and Ratcliffe point out that the reduction factor "p may be derived from measurements of the reflection coefficient of the ionosphere for the frequency of the wanted wave qn the assumption that there is no deviative absorption near to the reflection point in the ionosphere, the absorption all occurring at the level of cross-modulation, remembering that the modulation may not occur on both the upgoing and downcoming paths. If p~ is expressed in terms of a positive decibel loss D, then D = —20 log p = —8*7 In p so that In p = — 0-115 D (21) If also the cross-modulation region is at a distance d km from the disturbing transmitter which has an e.i.r.p. of P kW in the direction of the region, then 0-1732 1Ip V/m (22) It will then be found from (20), (21) and (22) that with the numerical values of e, m and k already given, the value G = 1-3 x 10“3 quoted by Huxley and Ratcliffe and the value 0O = 300 K of the atmosphere at the modulation level: 0 31 PDM M j = 2-34 X l0 -5 2 Vl (23) d2 (J d ± f Hf + 0-225Vq 1 + ------2------f V0 M where /d ,/# , and v0 are in MHz and the modulation frequency fw — ^/2tt is in Hz. 2.5 Discussion The main purpose here has been to give a simple derivation of the cross-modulation equation in symbolic form in (20) and in numerical form in (23). It has been based on the concept of the collisional frequency as the link between the modulation on the disturbing wave and that transferred to the wanted wave. Report 460-1 lists some of the factors on which the depth of the cross-modulation depends. As is to be expected, the cross-modulation is directly proportional to the depth of modulation on the disturbing wave and on the radiated power in the direction of the modulation region in the ionosphere from the disturbing transmitter. The dependence upon the decibel loss suffered by the wanted wave in the modulation region stresses the fact that not only must the wave pass through the modulation region on its passage to the receiving point on the earth, but that it must also suffer absorption in the process. Thus although the cross-modulation does not depend intrinsically on the power of the wanted transmission, this power must be sufficient for the received signal to survive the loss by absorption in the region. A typical value of D would be 10 dB if the loss is not to be excessive, — 183 — Rep. 574 while it is clear that the phenomenon will not be observed on sufficiently high frequencies where the absorption is very small because of the large value of fw in (14). The direct effect of the earth’s magnetic field on the wanted wave is not evident in (23), but it is implicit in (21) in determining the value of D from p~ given by (17). It is greatest for the extraordinary wave for which under some conditions it may be effectively complete, leaving only the cross-modulation on the ordinary wave. The reception of cross-modulated signals therefore depends on the polarisation characteristics of the transmitting and receiving antennae and on the time of day. It has also to be remembered that the wanted transmission may be primarily intended for reception by the ground wave on a medium frequency at a distance where at night the sky wave may be a source of interference that will be rendered less tolerable if in addition it carries some modulation from a disturbing transmission. The part played by the earth’s field is more evident for the disturbing wave, as seen in (23). Here again the transmission may be intended for medium or low frequency ground wave propagation on the basis that the wave entering the ionosphere is heavily absorbed. It is evident that the extraordinary wave is particularly effective in modulating the collisional frequency when the disturbing frequency is near to the gyrofrequency. On the other hand the absorption of the wave restricts the volume of the cross-modulation region and hence the decibel loss of the wanted wave in traversing the region. The relation between M p and Mv in (19) is based on a mean value of Mv and this implies a mean value of v0 in (23) and a power P somewhat less than the actual e.i.r.p. of the disturbing transmitter. The collisional frequency has a controlling influence at the gyromagnetic resonance for the extraordinary wave, but otherwise the rapid decrease of v0 with increasing height mainly restricts the audio-frequency range over which cross-modulation is obtained, as seen from the square-root factor in the denominator of (23) which for /m = 100 Hz and v0 = 0*1 MHz (105 collisions per sec) has a value of 5. Writing v0 as 10w collisions per second, n decreases at night linearly with height, changing by unity in a height range of 13 km, with a value of 6 at about 81 km. These figures are derived from a graph given by Knight [1968] in a review containing details of the ionospheric parameters and the related attenuation characteristics at different heights and frequencies. It also contains a very useful discussion of the gyromagnetic effect. As the frequency of the disturbing wave is increased above the gyrofrequency, the cross-modulation eventually varies as f D~2, and Huxley and Ratcliffe writing in 1949 stated that the highest frequency that has so far been observed to produce cross-modulation was in the neighbourhood of the gyro frequency. With the availability now of much higher radiated powers, it is desirable to make an estimate of the power required to produce interference by cross-modulation on higher frequencies. For this purpose (23) can be put in the simple approximate form 3-2 | Mp | d2 f n kW (24) MD Assuming that a nuisance level of interference on the wanted wave occurs when |M P| — 0-05, and that the disturbing wave is 100% modulated so that M = 1, while d = 150 km corresponding to a wave incident on the ionosphere at about 45°, the value for P in (24) for D = 10 dB on a frequency of 2 MHz for the disturbing wave is 1400 kW or 1*4 MW. With the e.i.r.p. values now obtainable, it seems likely that cross-modulation interference troubles may arise from disturbing transmissions on frequencies well above the gyromagnetic frequency, especially as the simplified form in (24) excludes possible gyromagnetic resonance effects. 3. Fig. 1 shows some results of cross-modulation measurements, using the 60 kHz transmissions from WWVB as the wanted signal and a 7-4 MHz transmission from Platteville (United States of America) with an e.i.r.p. of 50 MW as the modifying signal [Utlaut, 1974]. The measurements were made at Bennett, Colorado. Both ordinary and extraordinary polarizations of the Platteville signal were used. The cross-modulation effects are, as expected, much stronger for extraordinary than ordinary. Rep. 574 — 184 — 4. Interpretation of cross-modulation effects has been largely confined to the changes in conductivity (i.e., modulation transfer as a result of changes in wanted signal attenuation) and has not considered changes in permittivity. The changes in permittivity that can result from the presence of signals from high power transmitters suggests that wanted signals involving modulation other than amplitude modulation may also be affected by a cross-modulation phenomenon. R eferences B a iley , V.A. [1937] On some effects caused in the ionosphere by electric waves. Part I, Phil. Mag., 23, 929-960. B ell, C.D. [1966] Investigation of problems encountered in wave interaction experiments at high power. Ionosphere Research Lab. Scientific Report No. 262, Ionosphere Research Lab., The Pennsylvania State University. C u t o l o , M. [1964] On some resonance phenomena near the gyrofrequency obtained during the propagation of a radio wave in plasma. NBS Technical Note No. 211, 2, 1-69. D a l g a r n o , A. and H e n r y , R.J.W. [1965] Electron temperatures in the D region. Proc. Roy. Soc., 288A, 521-530. F ejer, J.A. [1955] The interaction of pulsed radio waves in the ionosphere. J. Atm. Terr. Phys., 7, 322-332. G in z b u r g , V.L. and G u r e v ic h , A.V. [1960] Non-linear phenomena in plasma situated in an alternating magnetic field (in Russian). Usp. Soviet Phys., 3, 175-195. G u r e v ic h , A.V. [1971] Disturbance of the ionosphere by radio waves (in Russian). Geomagnetism i Aeronomiia, 11, 953 (English translation, 803). H ib b e r d , F.H. [1964] Wave interactions at oblique incidence near the gyrofrequency. NBS Technical Note No. 211, 4, 45-64. H u x l e y , J .G .H . [1950] Ionospheric cross-modulation at oblique incidence. Proc. Roy. Soc. A200, 486-511. H u x l e y , L.G.H. [1955] Comment on the theory of radio wave interaction. Proc. Roy. Soc., A229, 405-407. H u x l e y , L.G.H. and R atc liffe, J.A. [1949] A survey of ionospheric cross-modulation. P.I.E.E., Part III, 96, 433-440. Jo nes, T.B., D a vies, K. and W ie der, B. [1972] Observations of D region modifications at low and very low frequencies. Nature, 238, 33-34. K n ig h t , P. [1968] Ionospheric non-linearity. B.B.C. Research Department, Report No. RA-17. K n ig h t , P. [1972] L.F. and M.F. propagation: A study of ionospheric cross-modulation measurements. B.B.C. Research Department, Report No. 1972/23. R atc liffe, J.A. and S h a w , I.J. [1948] A study of the interaction of radio waves. Proc. Roy. Soc., A193, 311-343. S h a w , I.J. [1951] Some further investigations of ionospheric cross-modulation. Proc. Phys. Soc., B64, 1-20. U t l a u t , W.F. [1970] An ionospheric modifications experiment using very high power, high frequency transmission. J. Geophys. Res., 75, 6402-05. U t l a u t , W.F. [1974] A survey of ionospheric modification effects produced by high-power HF radio waves. AGARD Conference Proceedings 138, pp. 3-1 to 3-16 (Conference on non-linear effects in electromagnetic wave propagation). NTIS, 5285 Port Royal Road, Springfield, Virginia 22151, U.S.A. Percentage cross-modulation (amplitude) of WWVB (60 kHz) 30 0 2 50 25 0 aiu .... 5 M Pretg mdlto: 100% modulation: Percentage MW 50 e.i.r.p.: Maximum rqec: - MH Mdlto rqec : 0 Hz 10 : frequency Modulation Hz M 7-4 Frequency: rs-ouain asdb tePatvle transmitter Platteville the by caused Cross-modulation Percentage of full power full of Percentage B: extraordinary wave extraordinary B: wave ordinary A: F A B gure r u ig 1 5 100 75 ------ REPORT 575 * METHODS FOR PREDICTING SKY-WAVE FIELD STRENGTHS AT FREQUENCIES BETWEEN 150 kHz AND 1600 kHz (Study Programme 17A-2/6) (1974) Introduction The propagation curves contained in Report 264-3 were derived from measurements made in Europe. Report 431-1 ** draws attention to the fact that these curves may be seriously in error, especially when extrapolated, if used for paths outside Europe, or for paths longer than 3500 km. To overcome this difficulty, several field-strength prediction methods which can also be used outside the European Broadcasting Area have been proposed. The purpose of this Report is to describe these various methods. The Cairo curves An extensive series of measurements over distances between 5000 and 12 000 km was made in 1936 and 1937 at frequencies of about 1 MHz. These measurements were used to derive the so-called Cairo curves [International Radio Conference, 1938]. The original curves gave quasi-maximum field strengths; to enable comparisons to be made with other curves, and other prediction methods, all values have been reduced by 9 dB to give median field strengths. There are two Cairo curves. The east-west curve was derived from measurements made across the North Atlantic; these measurements may have been affected to some extent by auroral absorption. The north-south curve was derived from measurements between North and South America; one terminal (Buenos Aires) was situated near the sea. Method proposed by the U.S.S.R. To establish sky-wave propagation curves suitable for field-strength prediction throughout the U.S.S.R., methods for field-strength measurement and analysis have been developed [Gadden, 1964; Ivanov-Kholodny and Nikolsky, 1969; Vilenski et al., 1970; Udaltsov, 1971; Udaltsov and Shlyuger, 1972] [C.C.I.R., 1970-1974 a]. Measurements were made on 14 frequencies in the vicinity of 200, 500, 1000 and 1500 kHz at different latitudes. The measurements made in the U.S.S.R. were analysed both statistically and theoretically. The analysis has shown that the magnetic-dip corrections given in Fig. 3 of Report 264-3 are much too large, especially when applied to long paths at the higher latitudes. Statistical analysis of these measurements has enabled an empirical formula to be derived which gives the dependence of field strength on distance and frequency at a geomagnetic latitude of 37°, as follows: F0 = 105-3 - 20 log 10d - 1-9 X 10~3f 0 l5d (1) where F0 is the annual median field strength (dB([j.V/m)) when 1 kW is radiated from a short vertical antenna, d is the ground distance (km) and/ the frequency (kHz). The generalized r.m.s. deviation of the measured results from the propagation curves was found to be 3*63 dB. The reliability of the correlation between the experimental data and the propagation curves was found to be better than 0*999. In calculating field strengths at geomagnetic latitudes other than 37°, the following formula is used: Adopted unanimously. §2.3 of Report 431-1 also contains Australian comments on the development of world-wide prediction methods. — 187 — Rep. 575 F0 = 105-3 - 20 log10rf - 1-9 X 10-3/ 01V - 0-24 x 10~3f 0 4d [tan20> - tan2 37°] (2) where O is the geomagnetic latitude at the path mid-point. This formula is valid for values of d> between 37° and 60° and contains an additional frequency dependent term. Figs. 1 to 4 show sky-wave propagation curves, calculated from equation (2), for a range of frequencies and geomagnetic latitudes, for distances up to 6000 km. From these curves the annual median field strength for midnight local time at the path mid-point can be determined for standard conditions. To calculate field strengths for other times of day and for particular seasons, the correction coefficients given in Report 264-3 must be taken into account excluding that for geomagnetic latitude as this is contained in equation (2). Equations (1) and (2) were derived from measurements made at distances which are sufficiently great for the ground wave to be negligible. At short distances, the ground wave is strong and special steps have to be taken to distinguish between the ground wave and the sky wave. Consequently, when establishing single curves for both short and long distances, great care must be taken with the construction of the curves at distances between 300 and 500 km. The U.S.S.R. curves show a much smaller dependence of field strength on frequency at middle latitudes than do the curves of Report 264-3. At higher latitudes, ionospheric absorption increases rapidly as the frequency increases, resulting in a much greater dependence of field strength on frequency than is found at lower latitudes. 4. Methods proposed by the E.B.U. A contribution from the E.B.U. [C.C.I.R. 1970—1974b] draws attention to a field-strength prediction method for medium frequencies proposed by the B.B.C. [Knight, 1973]. In this method, called the wave-hop method, estimated losses due to all the ionospheric and terrestrial factors which affect a wave as it propagates from transmitter to receiver are subtracted from the field strength which would arise if losses were absent. This process is carried out for each propagation mode which is likely to make a significant contribution to the received signal; the contributions are then added on a power basis. The method is intended for paths of any length and for world-wide application with the exception, at present, of the auroral regions. Field strengths have been predicted for 152 paths in different parts of the world and compared with measured values. On paths shorter than 3000 km, 84 % of the differences are less than 10 dB and on longer paths 66% of the differences come within this range. A further contribution from the E.B.U. [C.C.I.R. 1970-1974c] uses the wave-hop method to calculate field strength arising from the use of horizontal transmitting antennae. Measurements have shown that transmissions from horizontal antennae, at the higher frequencies in the medium frequency band, give rise to field strengths at distances of about 2000 km which are comparable with those produced by transmissions from vertical antennae. The calculations show that propagation to this distance is probably caused by multi-hop high-angle modes, which are strongly excited by horizontal antennae. In planning exercises at medium frequencies, the E.B.U. has made use of the following empirical formula, known as the Minne formula: 396 F0 = ------40 (3) 4 + 0001 d where F0 is the annual median field strength (dB(+V/m)) when a semi-isotropic source radiates with a cymomotive force (c.m.f.) * of 300 volts, and d is the ground distance (km). The formula has been used for all ground distances up to 10 000 km. At distances greater than 5000 km it gives values which are about 10 dB less than those given by the Cairo north-south curve. * A semi-isotropic source radiating with a c.m.f. of 300 volts is a fictitious antenna which provides a convenient reference. A short vertical antenna which radiates 1 kW with an efficiency of 100% produces the same c.m.f. in the horizontal direction as the semi-isotropic source. At distances greater than i000 km, the semi-isotropic source and the short vertical antenna produce almost identical field strengths and may be regarded as equivalent. Rep. 575 — 188 — 5. Method proposed by the United Kingdom For frequency planning, there is a requirement for a simple prediction method applicable to paths in any part of the world. As a contribution to the work of Interim Working Party 6/4, the United Kingdom has produced a semi-empirical prediction method, which is based on physical principles but which contains coefficients derived from measured field strengths [C.C.I.R. 1970-1974d]. The basic formula includes terms which take account of terminal losses, inverse distance attenuation and the losses associated with the ground and ionospheric reflection processes. The prediction method includes corrections for proximity of terminals to the sea, for geomagnetic latitude, for polarization coupling loss, for solar activity and for diurnal variation. A correction, similar to that given in Fig. 1 of Report 264-3, is applied for the vertical directivity of the transmitting antenna. The method is intended for use in both the LF and MF broadcasting bands. At LF its use is restricted to paths of lengths up to 4500 km in the European area. At MF it can be applied to paths of lengths up to 12 000 km in any part of the world, but it should be used with caution for paths which pass through the auroral regions. The method shows no frequency dependence within each of the LF and MF bands since, on the basis of the analysis of the available measurements, no significant dependence was found. [See for example C.C.I.R., 1970-1974e, f.] 6. Prediction methods for short distances For distances smaller than 300 km which are not covered by formula (1 a) of Report 264-3 the following formula was derived in the Federal Republic of Germany: MM 0-26 F0 = 60-6 - 10 log 1 + 10175 I - 0-54 (4) 10 \200 where F0 is the annual median field strength (dB([xV/m)) obtained when a semi-isotropic source radiates with a cymomotive force of 300 V, d is the ground distance (km) and / is the frequency (kHz). This formula is based on the hypothesis that the ionospheric reflection coefficient remains constant for recep tion at distances below 300 km. The Minne formula, equation (3), and the United Kingdom method are also intended for use at short distances. Measurements, described in Report 431-1, have been made in various countries. 7. Method proposed by Interim Working Party 6/4 (modified U.S.S.R. method) Interim Working Party 6/4 has considered the prediction methods above and has compared in detail their accuracy of prediction. The Interim Working Party was able to show that the range of validity of the U.S.S.R. method can be extended to distances less than 300 km * and to regions beyond the U.S.S.R. if the following modifications are made to the U.S.S.R. formula: — the ground distance d is replaced by the slant-propagation distance p ; — the United Kingdom correction for sea-gain is added where applicable; — the United Kingdom correction for polarisation-coupling loss is applied in tropical regions; — paths longer than 3000 km are divided into two equal sections and the loss factors for the two sections calculated separately and averaged, as in the United Kingdom method. These modifications have negligible effect on field-strength predictions within the U.S.S.R. Outside the U.S.S.R., the United Kingdom and modified U.S.S.R. methods give similar results and agree reasonably well with field strengths measured in most parts of the world. * However, extension of the formula derived for paths longer than 300 km to distances less than 300 km requires additional study. — 189 — Rep. 575 The general opinion reached within the Interim Working Party is that the modified U.S.S.R. method and the United Kingdom method are in reasonable agreement with the measured data. Although the possibility of a frequency dependence, particularly at high latitudes, requires additional study, the inclusion of such an effect in the U.S.S.R. method did not dissuade the Interim Working Party from proposing the modified U.S.S.R. method described in the Annex. For the future, however, the Interim Working Party considers that the United Kingdom method holds considerable promise, and when more data are available, the United Kingdom method may prove to be more accurate. Figs. 5a and 5b show histograms of the differences between measured and predicted field strengths for 217 paths in various parts of the world for frequencies in bands 5 and 6. These histograms are so arranged as to provide direct comparison between the United Kingdom method, the U.S.S.R. method and the modified U.S.S.R. method proposed by the Interim Working Party. Further measurements in equatorial regions are desirable, as the accuracy of the modified U.S.S.R. method has not been sufficiently tested in these areas. Field strengths in these regions appear to differ to some extent from those predicted by the method. Suitable correction factors may need to be introduced, as has been done for Australia and New Zealand (§ 5 of the Annex). Field strengths measured on paths between North America and Europe are 15 to 20 dB higher than those predicted by the modified U.S.S.R. method. Further measurements on paths near and across the auroral zone are therefore desirable. The method contained in the Annex is essentially complete with Figs. 6, 7 and 8. The additional Figs. 9 to 15 are provided for convenience. R eferences C.C.I.R. [1970-1974a] Doc. 10/82, U.S.S.R. C.C.I.R. [1970-1974b] Doc. 6/277, E.B.U. C.C.I.R. [1970—1974c] Doc. 6/276, E.B.U. C.C.I.R. [1970-1974d] Doc. 6/330. C.C.I.R. [1970-1974e] Doc. 6/280, Germany (Federal Republic of). C.C.I.R. [1970-1974f] Doc. 6/289, Japan. G a d d e n , K.J. [1964] Magnito-ionnaya teoriya (Magneto-ionic theory). Sbornik, Geofizika i okolozemnoe kosmicheskoe pros- transtvo, Mir. International R a d io C onferenc e, C airo [1938] Vol. 1, 425-433. Iv a n o v -K h o l o d n y , G.S. and N ik o l sk y , G.M . [1969] Solntse i ionosfera (The Sun and the ionosphere). Nauka, Moscow, 338-351. K n ig h t , P. [1973] MF propagation: a wave-hop method for ionospheric field-strength prediction. B.B.C. Research Department Report 1973/13 (to be published in B.B.C. Engineering). U d a l t so v , A.N. [1971] O shirotnykh izmeneniyakh napryazhennosti poly ionosfernoi volny v nochnoe vremya (Latitudinal variations of the night-time field of the space wave). Geomagnetism i Aeronomiia, XI, 5, 912. U d a l t so v , A .N . and S h l y u g e r , I.S . [1972] Krivye rasprostranemiya ionosfernoi volny dlya radiovecktchatelnogo diapazona (Propagation curves for the ionosphere wave in the broadcasting band). Geomagnetism i Aeronomiia, 6 . Vil en sk i, I.M ., U d a l t so v , A .N . and S h ly u g e r , I.S. [1970] O napryazhennosti polya radiovoln diapazona 150-1600 kHz rasprostranyayushchiesya na bolshie rasstoyaniia ot peredatchika (Field-strength of radio waves at long distances from the transmitter, in the band 150-1600 kHz). Geomagnetism i Aeronomiia, X, 2, 262. Rep. 575 — 190 — Distance (km) F ig u r e 1 U.S.S.R. curves calculated for Distance (km) F ig u r e 2 U.S.S.R. curves calculated for ® = 43° — 191 — Rep. 575 F ig u r e 3 U.S.S.R. curves calculated for <&= 49° F ig u r e 4 U.S.S.R. curves calculated for I Number of paths 20 -0 1 2 3 -0 1 0 -10 -20 30 20 10 0 -10 0 -2 Rep.575 ^ ntd igo method Kingdom United Differences between predicted and measured field strengths field measured and predicted between Differences Peitdfed tegh — Maue il tegh, dB strength), field (Measured — strength) field (Predicted 20 -2 2 -0 1 2 3 -0 -10 -20 30 20 10 0 -10 -20 20 -2 20 -2 I ___ J L ___ -10 -10 -10 6 t j n years of an an of years nlss fdt o e opee b nei Wrig at 6/4; Party Working Interim by completed yet not data of Analysis .... method U.S.S.R. L gure 5 e r u ig F E f r r 12 — 192 — u f I I I I I a 1 1 1- J 1 I 1 I J ,1,-1 ya esrmn rgam aebe completed. been have programme measurement -year fia ad seso Iln paths Island Ascension and African 0 0 0 -20 30 20 10 0 0 0 20 -0 0 -10 0 -2 30 20 10 -10 -20 30 20 10 0 0 0 20 -‘ - 0 -2 30 20 10 Australia - New Zealand paths Zealand New - Australia I I I I ot Aeia paths American North uoen paths European utain paths Australian U L LU ehd rpsdb nei okn at 6/4 Party Working Interim by proposed Method L_L Mdfe USSR method) U.S.S.R. (Modified 10 -1 t j id t J E L - I I I I I I I I 0 0 30 20 10 0 0 30 20 10 30 20 10 0 0 30 20 10 0 0 30 20 10 j i i i i Number of paths 10 15 15 10 10 20 -0 1 2 3 -0 1 0 0 0 0 20 -0 1 2 30 20 10 0 -10 0 -2 30 20 10 0 -10 -20 30 20 10 0 -10 0 -2 2 -0 1 2 30 20 10 0 -10 -20 0 5 20 -0 1 2 3 - 1 0 0 0 0 20 -0 1 2 30 20 10 0 -10 0 -2 30 20 10 0 -10 0 -2 30 20 10 0 -10 0 -2 20 -0 1 2 3 - 1 0 -10 0 -2 30 20 10 0 -10 0 -2 5 5 r T T f r r J United Kingdom method Kingdom United __ -10 L x i 0 i i i TT 1 i I 1 TTTTI tru I l L l l I I 0 0 2 -0 1 2 3 - 1 0 0 0 30 20 10 0 -10 0 -2 30 20 10 0 -10 -20 30 20 Peitdfedsrnt)—(esrdfedsrnt) dB strength), field (Measured — strength) field (Predicted Differences between predicted and measured field strengths field measured and predicted between Differences 20 -0 1 2 3 - 1 0 0 0 30 20 10 0 -10 0 -2 30 20 10 0 -10 0 -2 n irTfrTTn T T r f T ir rn L I L- I l t n r n i r t n~T .... method U.S.S.R. TT n F m t JL gure r u ig 13 — 193 — i i i i i I I I I I I I I 11111rn 11111rn I I I I I I 5b L t n 0 0 0 2 -0 0 -10 -20 80 20 10 I I L I l I J - 1 I L I I 1 I J- I I —L I— I I I I EBU long distance paths distance long EBU “ Cairo ” east-west paths east-west ” Cairo “ ar ” ot-ot paths north-south ” Cairo I L I I J sa paths Asian LF paths LF ehdpooe yItrm okn at 6/4 Party Working Interim by proposed Method n ITT Mdfe USSR method)' U.S.S.R. (Modified i t n r r nTT t d j I t T t c ti i ...... k t n 0 0 30 20 10 Rep.575 i i i j .i i i i m i m i Rep. 575 — 194 — ANNEX SKY-WAVE FIELD STRENGTH PREDICTION METHOD FOR THE FREQUENCY RANGE 150 TO 1600 kHz Method proposed by Interim Working Party 6/4 (Modified U.S.S.R. method) List of symbols b Solar-activity factor given in § 2.6. d Ground distance between transmitter and receiver (km). Fq Annual median field strength at the reference time defined in § 2 (dB ([xV/m)). Ft Annual median field strength at time t (dB (jxV/m)). / Frequency (kHz). f A frequency defined in equation (6) (kHz). G0 Sea gain for a terminal on the coast (dB). Gh Transmitting antenna gain factor due to horizontal directivity (dB). Gs Sea gain for a terminal near the sea (dB). Gy Transmitting antenna gain factor due to vertical directivity (dB). h Transmitting antenna height (Fig. 6). hr Height of reflecting layer (km). I Magnetic dip angle (degrees). k Basic loss factor. k R Loss factor. LP Excess polarisation coupling loss (dB). L t Diurnal loss factor (dB). P Radiated power (dB (1 kW)). p Slant propagation distance (km). Q A sea-gain parameter given in § 2.3. R Twelve-month smoothed Zurich sunspot number. s Distance of terminal from sea, measured along great-circle path (km). t Time relative to sunset or sunrise (hours). V - Transmitter cymomotive force (dB (300 V)). 0 Direction of propagation relative to magnetic East-West (degrees). A Wavelength. 1. Introduction This method of prediction gives the night-time sky-wave field strength produced for a given power radiated from one or more vertical antennae, when measured by a loop antenna at ground level aligned in a vertical plane along the great-circle path to the transmitter. It applies for paths of lengths up to 12 000 km. However in band 5 it was only verified for paths of up to 5000 km. The accuracy of prediction varies from region to region and may be improved in certain regions by applying modifications such as those shown in § 5. In any case the method should be used with caution for geomagnetic latitudes greater than 60°. 2. Annual median night-time field strength The predicted sky-wave field strength is given by: Fo = V + G s -L p + 105*3 - 20 log10p - 10~3 kRp (1) where F0 = annual median of half-hourly median field strengths (dB above 1 [TV/m) at the reference time defined in § 2.1. V — transmitter cymomotive force, dB above a reference cymomotive force of 300 V, Gs = sea-gain correction, dB, LP = excess polarization-coupling loss, dB, p = slant-propagation distance, km, kR = loss factor incorporating effects of ionospheric absorption, focusing and terminal losses, and losses between hops on multi-hop paths. 2.1 Reference time The reference time is taken as six hours after the time at which the sun sets at a point S on the surface of the Earth. For paths shorter than 2000 km, S is the mid-point of the path. On longer paths, S is 750 km from the terminal where the sun sets last, measured along the great-circle path. 2.2 Cymomotive force The cymomotive force V is given as: V=P + Gv + GH (2) where P = radiated power, dB above 1 kW, Gv = transmitting antenna gain factor (dB) due to vertical directivity, given in Fig. 6, Gh = transmitting antenna gain factor (dB) due to horizontal directivity. For directional antennae, GH is a function of azimuth. For omnidirectional antennae, GH — 0. 2.3 Sea gain Gs is the additional signal gain when one or both terminals is situated near the sea. Gs for a single terminal is given by: = G0 - 10- ( ^ ) dB (3) where G0 is the gain when the terminal is on the coast, / is the frequency in kHz and 5 is the distance in km of the terminal from the sea, measured along the great-circle path. Q = 0-44 in band 5 and 1-75 in band 6. G0 is given in Fig. 7 as a function of d for bands 5 and 6. In band 6, G0 = 10 dB Rep. 575 — 196 — when d > 6500 km. Equation (3) applies for values of s such that Gs > 0. For larger values of s, Gs = 0. If both terminals are near the sea, Gs is the sum of the values of Gs for the individual terminals. 2.4 Polarisation coupling loss LP is the excess polarization coupling loss. In band 5, LP = 0. In band 6 at low latitudes, for |/| < 45° LP = 180 (36 + 02 + / T 1/* - 2 dB/terminal (4) where I is the magnetic dip in degrees at the terminal and 0 is the path azimuth measured in degrees from the magnetic E-W direction, such that 10| < 90°. For |/| > 45°, LP = 0. LP should be evaluated separately for the two terminals, because of the different 0 and I that may apply, and the two LP values added. The most accurate available values of magnetic dip and declination should be used in determining 0 and I. 2.5 Slant propagation distance For paths longer than 1000 km, p is approximately equal to the ground distance d (km) . For shorter paths p = (d2 + 4V ) 1/2 (5) where hr — 100 km iff < f and 220 km if/ > / ;, where f (in kHz) is given by / = 350 + [(2*8d)3 + 3003]1/3 (6) Equation (5) may be used for paths of any length with negligible error. 2.6 Loss factor The loss factor k R is given by kR = k + 10~2 bR (7) where R = twelve-month smoothed Zurich sunspot number. In band 5, b = 0. In band 6, b = 4 for North American paths, 1 for Europe and Australia and 0 elsewhere. k = 1-9/ 015 + 0-24/° '4 (tan2 where / = frequency (kHz). For paths shorter than 3000 km 4. = (4>r + /2 (9) where 3. Nocturnal variation of annual median field strength Ft = FQ- L t (12) where Ft = annual median field strength at time t, dB (fxV/m) Fq = annual median field strength at reference time defined in §2.1, dB ([TV/m), given by equation (1) Lt — diurnal loss factor, dB, given in Fig. 8. Fig. 8 shows the average of the annual median nocturnal variations for Europe and Australia, derived from Fig. 8 of Report 264-3 and Fig. 5 of Report 431-1 respectively. The time t is the time in hours relative to the sunrise or sunset reference times as appropriate. These are taken at the ground at the mid-path position for d < 2000 km and at 750 km from the terminal where the sun sets last or rises first for longer paths. 4. Dayrto-day and short-period variations of field strength The field strength exceeded for 10% of the total time on a series of nights, during short periods centred on a specific time is: 8 dB greater in band 5 10 dB greater in band 6 than the values of F0 and Ft given above. 5. Accuracy of the method This method is believed to be reasonably accurate in I.T.U. Regions 1 and 3. Comparison of predicted and measured values shows, however, that its accuracy in certain regions may be further improved by making the following corrections. 5.1 Since field strengths measured in Australia and New Zealand are 4 to 7 dB higher than those predicted by the method, a better prediction formula for this area is F0 = V + Gs — LP + 108 - 20 log10p - 0-8 X 10~*kRp (13) The field strength exceeded on band 6 for 10% of the total time on a seriesof nights, during short periods centred on a specific time, is only 7 dB greater than the annual median in this area. 5.2 The accuracy may'be improved in North America by subtracting 3 dB from field strengths predicted by the method. Rep.575 Gy dB — ______rnmtigatnagi atr y) G ( factor gain antenna Transmitting hT hr h Ground distance, distance, Ground = Antenna height Antenna = = 220 km (F layer reflection) layer (F km 220 = = 100 km (E layer reflection) layer (E km 100 = 18 — 198 — F gure r u ig 6 d, (km) — 199 — Rep. 575 Ground distance, d, (km) A : Band 6 B: Band 5 F ig u r e 7 Sea gain (G0) for a single terminal on the coast Time after sunset (hours) Time after sunrise (hours) F ig u r e 8 Diurnal loss factor (Lt) Rep. 575 — 200 — > T3PP Ground distance, d, (kra) F ig u r e 9 Basic field strength The curves show 105-3 — 20 logw p where p = (d2 + 4h2r) I — 201 — Rep. 575 F ig u r e 10 Basic loss factor k k = l- 9/o-is + o-24/0'4 (tan2 — tan2 37°) (0 < <£ < 60°) Rep.575 Lp (dB/terminal) ieto o rpgto rltv t mgei atws, 0 east-west, magnetic to relative propagation of Direction Excess polarization coupling loss Lp loss coupling polarization Excess rqec eie n qain (6) equation in defined Frequency = ' f Lp = Ground distance, distance, Ground ( 8/3 ]Vs 3] 0 0 3 + -8c/)3 [(2 + 0 5 3 36 + 02 )_/ - 2 - _1/* 2) / + 2 0 + 6 (3 0 8 1 22 — 202 — F F gure r u ig gure r u ig 11 2 1 d, (km) Latitude Geomagnetic latitudes Geomagnetic F Longitude gure r u ig 13 0 — e. 575 Rep. — 203 e. 7 — 204 — 575Rep. F ig u r e 14 Map of magnetic dip 0 — e. 575 Rep. —205 F ig u r e 15 Map of magnetic declination (dotted curves east declination, continuous curves west declination) PAGE INTENTIONALLY LEFT BLANK PAGE LAISSEE EN BLANC INTENTIONNELLEMENT — 207 — Rep. 266-3 SECTION 6H: FADING OF SIGNALS RECEIVED VIA THE IONOSPHERE RECOMMENDATIONS AND REPORTS Recommendations There are no Recommendations in this section. Reports REPORT 266-3 * IONOSPHERIC PROPAGATION CHARACTERISTICS PERTINENT TO RADIOCOMMUNICATION SYSTEMS DESIGN (Fading) (Study Programme 16A-2/6) (1953 - 1956 - 1959 - 1963 - 1966 - 1970 - 1974) 1. Introduction Experience has shown that information concerning the mean value of the received signal is not sufficient for planning radiocommunication systems. The variations in time, space and frequency, collectively described as fading, also have to be taken into consideration. Fading has a decisive influence on the performance of radiocommunication systems and on the type of modulation that may be used effectively. It is essential to know the severity and rapidity of fading to be able to specify the power required for transmitters, the necessary protection ratio to guard against interference and, with additional knowledge of the correlation of signals at separate antennae or frequencies, to be able to determine the most efficient and economical diversity or coding systems. A discussion of the importance of fading in connection with frequency utilization, together with a treatment of various aspects of fading, is given in Reports 413, 414 and 415. 2. General causes of fading Fading may be caused by several different effects, such as: — movement of the ionosphere and multipath changes causing interference fading; — rotation of the axes of the polarization ellipses; — variations of the ionospheric absorption with time; — focusing and temporary disappearance of the signal due to MUF failure [Davies, 1965]. On an 8000 km north-south transmission path, long-term fading has been attributed to atmospheric gravity waves [Rottger, 1973]. As also remarked by Davies [1965], the period of the fading cycle depends largely upon the cause of the fading. As a result, the period for interference and polarization fading may last for a time of * Adopted unanimously. Rep. 266-3 — 208 — the order of a fraction of a second to a few seconds, focusing may last something like 15 to 30 minutes, absorption fading may last for more than an hour; MUF failure is highly irregular and occurs at times of fade-in and fade out. Fading tends to be faster on high frequencies than on low, because a given movement in the ionosphere represents a greater phase shift at the shorter wavelengths. Motion of the ionized regions gives rise to selective fading and to a distortion of the modulation envelope of a signal. The motion produces changes in path length, and Doppler shifts of frequency on each of the individual contributing signal components. 3. Sampling periods For the purpose of communication system analyses, it is not always essential to identify the individual contributory effects; instead, it is possible to observe the resulting time series and characterize the fluctuations of signal levels in time as a random or stochastic process [Brennan, 1961], Charac terization of the time series requires, first, the selection of an observation period Ts, long enough to include a sufficiently large number of fluctuations of the signal level. The choice of Ts, while somewhat arbitrary, is generally made to suit the objectives of the analysis. Thus, for HF paths, samples from a few minutes to an hour in length have been found suitable where the interest was in the fast fading; sampling periods of a month have been used to estimate the random variations of each hourly mean period in a day. It is expedient to consider short-term and long-term fading separately. The short-term fading component includes phase interference between multipath propagation components, and rapid variations of signal strength caused by ionospheric irregularities. The long-term fading component arises from random changes in the short-term median values of the received signal. For example, long-term fading would include variations in the median field strength measured from day to day at a fixed time. Diurnal, seasonal and solar-cycle variations are of a more systematic nature and are usually not associated with fading. 4. Severity and rapidity of short-period fading The terms “ severity ” and “ rapidity ”, as used here, refer to the characteristics of the variations of the received signal amplitude when a steady tone is emitted from the transmitter. 4.1 Severity of fading The signal amplitude distribution function P(v0) conventionally employed gives the probability of finding the signal amplitude v greater than v0. It is related to the probability density function p(v) by: (1) Probability density functions, obtained analytically to describe the envelope of a fading signal, differ according to the different assumptions made with respect to the structure of the contributory signals. Among the most frequently used models is the one which assumes that the received signal before detection is composed of a steady sinusoidal component and a random Rayleigh component with a uniform phase probability density [McNicol, 1949; Bramley, 1951]. This leads to the Nakagami- Rice probability density function [Nakagami, 1943; Rice, 1944 and 1945]: P (v) = (2 v/vj) exp [— (vj + v2)/vj] IQ (2 vxv/vl) (2) where: I0(x): modified Bessel function of zero order, v : received signal envelope voltage/ j/2, vx : r.m.s. voltage of steady sinusoidal component, v„ : r.m.s. value of random voltage component. — 209 — Rep. 266-3 Fig. 1 gives the corresponding signal amplitude distribution function P(v0). Each curve represents a constant probability P(v0) that the level v0 (shown as the ordinate) is exceeded. The abscissa is the ratio vjvv the parameter that is needed to specify a particular Nakagami-Rice distribution. For instance, if the ratio of r.m.s. random voltage to r.m.s. steady voltage equals 1 (i.e. 0 dB), then a level that is 7-5 dB below the median will be exceeded 90% of the time. If v„/vx> 1, then (2) reduces to the Rayleigh density function P 0) = (2v/vJ) exp ( - v2/vl) (3) Fig. 1 shows, however, that the actual distribution approximates closely to the Rayleigh distribution provided vn/v1 > 2 (6 dB). If 1, then (2) reduces to the normal or Gaussian distribution function (v -v x)2 P 00 = exp (4) The signal then varies symmetrically about the median value vt with standard deviation v„/|/2. Fig. 1 shows that the distribution about the median value is almost symmetrical provided v„/v1 <0-1 (—20 dB); the distribution may therefore be regarded as approximately normal whenever this condition is satisfied. If the signal strength is expressed in decibels relative to a specified level the distribution referred to in (4) is known as the log-normal distribution. Both the normal and log-normal probability density functions may be expressed through one formula, as follows: 1 P (*) = exp (5) ax «J2tz 2 For the normal distribution, x is to be interpreted as (v— v,„) while for the log-normal distribution in dB, x is to be interpreted as 20 log10 (v/vm), where vm is the median value of v. In addition to the above probability densities, which are bound up with certain theoretical assumptions (stationary processes, random motion of secondary radiators), there are other probability density functions worth considering, because they contain several arbitrary parameters to which values may be assigned to fit the measured data. In this connection, mention may be made of the /^-distribution [Nakagami, 1960], which is similar to the y2 and gamma distributions often used in statistics. The simplest method of characterizing empirical signal distributions is by the statistics which give an indication of the incidence of specified levels. As in Fig. 1 a distribution can be characterized by stating the levels exceeded for a number of specified time-percentages. The following Table gives these levels for the theoretical distributions discussed earlier [Norton et al., 1955]. Level (dB) relative to median, exceeded for the following percentages of time Distribution 1 % 1 0 % 90% 99% Rayleigh 8 - 2 2 5-21 -8 -1 8 . -1 8 -3 9 Nakagami-Rice Vi = v„ 7-02 4-48 -7 -5 3 -17 -5 5 Vj = 3-16 v„ 3-54 2 - 1 2 -2 -8 0 -5 -9 8 Normal and log-normal 2-326(7* 1-282(7* -1-2820* —2*326o* Rep. 266-3 — 210 — Having discussed some of the mathematical formulae that may be used to describe fading, we pass on to some measured results. With short analysis intervals (3 to 7 minutes), distribution functions close to the Rayleigh distribution seem to predominate. On the other hand, during longer analysis intervals (30 to 60 minutes), the distribution seems to follow the log-normal law rather than Rayleigh. The fading range is often defined as the difference (in dB) between the signal levels exceeded for 10% and 90% of the time, and values of 13 fz 3*2 dB [Grosskopf, 1953] and 16-6 i 3-2 dB [Konopleva, 1964] have been given for long-distance HF paths. The value does not appear to vary greatly with path length in the range 1500-6000 km, with time of day or with season [Konopleva, 1964]. It is of interest to note that, although the form of the measured distributions may differ from the Rayleigh distribution, the observed fading range is of the same order as the value of 13-4 dB expected for the Rayleigh distribution. However, at high signal levels the fading range has been observed to fall below the Rayleigh value, possibly due to a strong constant component term arising from a specular reflection, and distributions of the Nakagami-Rice type will apply under these conditions. The properties of the sky-wave field-strength distribution in the bands 5(LF) and 6(MF) show that the log-normal and the Rayleigh distributions apply only in marginal cases and that the 30 min and 60 min recording intervals follow a different distribution law when performed over several years and during various seasons [Taumer and Sulanke, 1967, 1968]. Further information is contained in Report 264-3. 4.2 Rapidity of fading The rapidity of fading can be characterized in different ways [McNicol, 1949; Ratcliffe, 1956; Price, 1957; Rice, 1958]. One description of fading which is useful for a number of purposes is given by the channel auto-correlation function in time or by the corresponding power spectrum. With certain theoretical assumptions (normal distribution of the components of the velocities of the secondary radiators), a normal curve is to be expected for the auto-correlation function: R (t) & R (0) exp ( - t2/2 $ (6) where t0 is called the correlation (or coherence) time-constant of the fading channel. The corresponding power (i.e. Doppler) spectral density is proportional to exp (— t%f2/2), a Gaussian shape with standard deviation of l/t0. The latter is also known as the correlation (or coherence) bandwidth of the fading channel. However, it is questionable whether the assumption for the velocity distribution of the secondary radiators is always justified. Therefore, the possibility of other velocity distribution functions should also be taken into consideration. These other velocity distributions can lead to other forms of the auto-correlation function as well as different Doppler spreads. Other parameters have often been used for characterizing the rapidity of fading. There is, first of all, the fading rate defined as the number of positive crossings per unit time through any specified level. When the signal fades according to (2), the fading rate for all values up to and including level v, is given by: N(y) = ( fs v ]/4tt /vn) exp [ - (v\ + v2)/v2] IQ (2v1 v/vj) (7) where f s, the r.m.s. frequency of fading, is defined as follows. If the random variable Z denotes the number of crossings of the median per unit time, then f s is the square-root of the average value of Z 2. In the special case of the Rayleigh distribution, we can introduce the median value vm obtaining [Rice, 1958]: N(v) = 2-95f s (v/vj exp ( - 0-693 v2/v£) (8) where, if v = vm, it follows that N(vm) = 1-47 f s. Results similar to these can be derived for various distributions of signal fading, such as the normal and log-normal distributions (5) or others. The main result is the proportionality of N(vm) to f s. — 211 — Rep. 266-3 Fading rates up to and including the field-strength level exceeded for 90 % of the time, were measured in the Federal Republic of Germany by recording on station WWV (15 MHz). The fading rates varied between 6 and 16 per min, the mean value being 11-25 per min. On a certain proportion of days, HF paths in the tropics may show much more rapid fading than paths confined to higher latitudes [Osborne, 1952; Yeh and Villard, 1958; Bennington, 1960; Koster, 1963]. This phenomenon is associated with equatorial F-scattering (see Report 343-2) and usually occurs, starting about 2000 hrs local time, for some two to four hours. It is likely to affect transmission paths with a reflection point in the F region between magnetic latitudes ± 15°. Seasonal and sunspot-cycle variations occur, but may differ in nature with the path orientation. Long-distance paths appear to be affected more at the equinoxes and at sunspot maximum [Humby, 1959]. Besides increased fading rates, brief frequency changes of 20 to 30 Hz have been reported and there is a possibility that a lower frequency may be affected, but not a higher frequency [Davies and Barghausen, 1964]. More data are needed before a more exact account of the various fading characteristics can be given. 5. Long-period variations In the case of long-period variations, the random component is usually analyzed by taking hourly median values and evaluating the amplitude distribution over a long period. For simplicity, a log-normal distribution of the long-period variations is usually assumed, and often gives a good approximation to the actual distribution. A very full analysis, carried out in the Federal Republic of Germany [Grosskopf, 1953; 1955a; 1955b] on long-period variations, showed that distribution curves for a sun-rotation period were closely log-normal for 50 % of the 28 rotation periods examined; 25 % could be split into two log-normal distributions, each valid over a different range of field strengths. The remainder could only be described in terms of the log-normal distribution if split up into more than two ranges. Results for the spread of long-term variations have been analyzed in a number of countries. Expressed in terms of the standard deviation ax of the log-normal distribution (equation (5)), the analyses mentioned above gave an average result of 8 dB, with some indication of greater values at night than in the daytime. U.S.S.R. experiments at HF over paths of 1500, 3000 and 6000 km, for all seasons and times of day, have given values within the range 5 to 10 dB [Konopleva, 1964]. Any systematic change with season or between day and night appears relatively small. Results obtained in the United Kingdom for Accra-United Kingdom, Bombay-United Kingdom- and Colombo- United Kingdom paths showed values in the range 5-5 to 7 dB. In certain regions, higher values may be obtained, e.g. a value of 10 dB or more may be found for paths crossing polar areas with high absorption. Information about long-period variations in bands 5(LF) and 6(MF) is contained in Report 432. 6. Correlation of signals in space, time, frequency and polarization The study of correlation between two received signals as a function of their separation in position, time, frequency or polarization can provide useful information for the design of a communication system in the presence of fading. An alternative [Stein, 1966] to the use of increased transmitter power, etc., is the utilization of techniques of modulation and reception which are less vulnerable to fading. Most widely known are multiple-receiver combining techniques classified as diversity reception. Depending upon the mechanism of the propagation, there are a variety of ways for obtaining signals for which the periods of fading occur independently of one another, such as: — space (spaced antenna) diversity, — frequency diversity, — angle-of-arrival diversity, — polarization diversity, Rep. 266-3 — 2 1 2 — — time (signal repetition) diversity, — multipath diversity (Rake). Of these, the last two have been considered primarily for digital transmission. It is not within the scope of this Report to discuss the details of channel characterization and diversity methods. It can be remarked, however, that most diversity analyses have been based upon slow, non-selective fading. However, in recent years, increasing attention has been directed to fast, non-selective types of fading. Very useful data on multipath ionospheric propagation have been obtained by Balser and Smith [1962] using oblique-incidence pulse sounders, which have pulse lengths of the order of 10 to 100 microseconds. From typical data, it is deduced that any attempt to transmit serial digital streams over long distances via the ionosphere, at a rate exceeding 100 to 200 pulses/second, will tend to result in a severe intersymbol interference problem [Stein, 1966]. Operators attempt to minimize these undesirable multipath effects by using frequencies near the MUF. Another approach has been the use of highly directive automatically adjustable antenna arrays, such as the MUSA [Polkinghorn, 1940], which discriminate against certain multiple-hop paths in favour of others. For characterizing the degree of approximation to slow non-selective fading, the spread factor is a useful parameter [Stein, 1966]. For information pulse length T, fading is essentially non-selective if the multipath spread TM satisfies Tm < T (9) Similarly, if the Doppler spread (the width of the received power spectrum with an unmodulated carrier) is Bd , the requirement for essentially slow fading is (10) Then, to approximate slow non-selective fading, T should satisfy both (9) and (10), and, therefore, the spread factor L, where L ~~ BDTM (11) is required to be 1 (12) Considering now diversity, Stein [1966] lists among the common linear methods (which are applicable both to digital and to distortionless reception of analogue transmission): — combination of selected signals, — combination of signals having the maximum ratio of strengths, — combination of signals on an equal gain basis. He also discusses more sophisticated methods: decision-oriented diversity for digital transmission, in which signal fidelity is abandoned as an irrelevance, and instead minimum error-rate is established as the sole goal. It should be mentioned that limited tests in the United Kingdom of space-diversity reception of single-sideband telephone transmission indicated only a small advantage under the conditions of the test. The correlation bandwidth was too low; the “ instantaneous ” amplitude of the reduced carrier was a poor indicator of the speech-band transmission. On the contrary, diversity is very effective with double-sideband amplitude-modulated telephony (carrier transmitted), and on typical telegraph signals. Normally in ionospheric propagation, the single most important limitation is multipath spread Tm. A bandwidth, A/, may be defined in terms of the reciprocal of TM — 213 — Rep. 266-3 which is similar to “ coherence bandwidth ”, “ flat-fading bandwidth ” or “ selective-fading bandwidth ” (Stein and Jones, 1967). Multipath spread in ionospheric propagation stems from: — O- and X-ray transit-time differences, — high and low ray transit-time differences, — multi-mode transit-time differences, — irregularity (spread F, etc.) transit-time differences, — pulse broadening in normal reflection due to dispersion. The range of delays in ionospheric reflection have been studied by Bailey [1959] and Salaman [1962]. 6.1 Diversity reception Early investigations of space-, polarization- and frequency-diversity were carried out prior to 1940. However, the mathematical techniques for the investigation of the theoretical and empirical aspects of diversity reception have largely been developed since 1947 [Briggs et al., 1950; Booker et al., 1950; Glazer and Farber, 1953; Ratcliffe, 1956]. Based upon a simple, though not completely satisfactory, model of scattering from an inhomo- geneous and time-varying ionosphere, a spatial correlation function p(d), normalized to unity at d = 0, may be derived to show the dependence of CW signals at two antennae spaced at a distance d [Bramley, 1951, Grisdale et al., 1957; Brennan, 1960; Khmelnitsky, 1960] viz.: p (d) = exp [ - d2/2 x2] (13) The parameter x is a function of the structure size of the ionospheric inhomogeneities, of the path length, and the frequency. At a separation d = x, the correlation is 0-61 and at d = x]/2, it is 0-37. At even the shorter of these distances, experience and theory have shown that, for all practical purposes, substantially all the benefits of diversity have been obtained. For example, with two independently fading signals (p = 0), the diversity improvement was 14 to 15 dB at the 99-9% reliability level. For p = 0-61, the diversity improvement was 13 dB. For this reason, it appears justifiable to identify the distance d — x]/2 as the “ diversity separation distance ” or “ correlation distance ”. In the United Kingdom, tests made in the frequency range 6 to 18 MHz over distances of 2000 to 17 000 km indicates that the structure size, x, was between 150 and 400 m, implying diversity separation distances of 210 to 560 m (10 to 25 X). These findings were confirmed by recordings made in the Federal Republic of Germany on station WWV (15 MHz). In addition, it was shown that the separation distances required, when specular reflection at the ionosphere is predominant, are much greater than when a substantial random component is present'. In other United States measurements below HF, the average correlation distance at 540 kHz was 29-4 X i 17-1 X [Brennan and Phillips, 1957], while at 85 kHz [Bowhill, 1957], two distinct'fading periods were observed, one of 7 min and one of 1*5 min. The correlation distance was determined to be 5 km for 7 min fading and 1 km for the 1*5 min fading. Polarization diversity in the frequency range 6 to 18 MHz was studied in the United States of America and the United Kingdom. The antennae were at the same location, but arranged to respond to waves with mutually perpendicular polarization. The result was that the diversity action was about the same as that obtained with an equivalent space separation of 240 to 480 m [Grisdale et al., 1957]. Experiments in Australia and New Guinea with nearly-vertical incidence broadcasting in the MF and HF bands have indicated that a polarization-diversity system would significantly reduce fading and distortion. 6.2 Frequency correlation For the transmission of information, a band of frequencies is always required. It can be shown that, for analysis, it suffices to understand what happens to a pair of spaced tones transmitted simul Rep. 266-3 — 214 taneously [Stein, 1966]. In practice, with sufficient tone spacing, the cross-correlation of the fading fluctuations at the two tone frequencies decreases toward zero. The lack of correlation in the fading of spaced tones is called selective fading. An early investigation of selective fading by the use of multi-tone signals on a transoceanic path was reported in the classic paper by Potter [1930]. Other work was accomplished in the 1950’s [Briggs, 1951; Price and Green, 1958]. 7. Characterization of channels for modulated signals There has been a rigorous mathematical development of theory which encompasses the character ization of channels as time-varying filters [Zadeh and Desoer, 1963]. Such an empirical development for characterization of radio channels requires a more complete identification of the total radio environment than has been possible with the limitations in the design of propagation experiments carried out so far. Although much of the theoretical background has been developed by a number of investigators [Green, 1963; Bello, 1963, 1964], much remains to be done. Others have contributed to this field: [Turin, 1956; Brennan, 1959; Barrow, 1963; Staras, 1956; Pierce and Stein, 1960; Wozencraft, 1961; Baghdady, 1961; Bello and Nelin, 1963]. A number of models have been used for channel characterization and Report 549 of Study Group 3 deals in detail with HF ionospheric simulators. Watterson et al., [1970] describe one model and its experimental confirmation. Their paper also included a comprehensive list of references. The abstract of their paper is reproduced here, as an example of current work on this subject: “ Specially designed HF ionospheric propagation measurements were made and analyzed to confirm the validity and bandwidth limitations of a proposed stationary HF ionospheric channel model. In the model, the input (transmitted) signal feeds an ideal delay line and is delivered at several taps with adjustable delays, one for each resolvable ionospheric modal component. Each delayed signal is modulated in amplitude and phase by a gain function for each baseband tap and the delayed and modulated signals are summed (with additive noise) to form the output (received) signal. Statistical specifications for the tap-gain functions involved three hypotheses : (1) that each tap-gain function is a complex-Gaussian process that produces Rayleigh fading, (2) that the tap-gain functions are indepen dent, and (3) that each tap-gain function has a spectrum that in general is the sum of two Gaussian functions of frequency, one for each magneto-ionic component. Statistical tests were performed on day time and night-time measurements confirming the validity of the three hypotheses, and thereby the validity of the model. For practical applications, the model can be considered valid over a bandwidth equal to about one fourth of the reciprocal of the effective (weighted) time spreads on the ionospheric modal components. The model should be useful both in theoretical analyses of communication system performance and for channel simulator designs. ” Another approach has been made by considering propagation path models which makes more precise the quantitative assessment of the possibilities for information transmission [Konopleva and Khmelnitsky, 1972; Konopleva and Khmelnitsky, 1970]. The accuracy of this method of assessment and the possibility of basing the calculations on real channel characteristics, with regard to the variation in both wanted signal [Khmelnitsky, 1970] and unwanted signal [Khmelnitsky, 1969] parameters should be verified using circuits of various orientations and lengths. 8. Further studies desirable Continued studies, along the lines of §§ 6 and 7 above, particularly as applied to ionospheric paths in various parts of the world, are desirable. For example, among the physical causes of fading, it is felt that focusing and defocusing effects [Fok, 1950; Liakhova, 1965; Rawer, 1952; Kerblai, 1963] merit more intensive study. Since these phenomena show considerable regularity, it may be that their description can be improved over that given by present random statistical models. — 215 — Rep. 266-3 With regard to improving knowledge of the severity and rapidity of fading, different theoretical approaches to the important practical problem of longer-lasting fades and field increases associated with ionospheric irregularities are needed to supplement the random statistical theory approach. Auto correlation methods should be helpful in studying phenomena which can be described as being between regular and random. R e fe r e n c e s B a g h d a d y , E.J. [1961] Lectures on communication system theory. Chapters 5 and 6 , McGraw-Hill, New York. B a il e y , D.K. [1959] The effect of multipath distortion on the choice of operating frequencies for high-frequency communication circuits. IR E Trans. AP-7, 397-405. B a l s e r , M. and S m it h , W.B. [1962] Some properties of pulsed oblique HF ionospheric transmissions. NBS Journ. of Research, 6 6 D, 721. B a r r o w , B.B. [1963] Diversity combination of fading signals of unequal mean strengths. Trans. IEEE, CS11, 73. B e l l o , P.A. [1963] Characterization of randomly time-variant linear channels. Trans. IEEE, CS11, 360-393. B e l l o , P.A. [1964] Measurement of the complex time-frequency channel correlation function. NBS Journ. of Research, 6 8 D, 1161-1165. B ello, P.A . an d N e l in , B.D. [1963] The effect of frequency selective fading on the binary error probabilities of incoherent and differentially coherent matched filter receivers. Trans. IEEE, CS11, 170. B e n n i n g t o n , T.W. [1960] Equatorial ionospheric effects. Wireless World, 6 6 , 501. B o o k e r , H.G ., R a t c l if f e , J.A. and S h i n n , D.H. [1950] Diffraction from an irregular screen with applications to ionospheric problems. Phil. Trans. Royal Soc. (London), Series A, 242, 579. B o w h il l , S.A. [1957] Ionospheric irregularities causing random fading of very low frequencies. J. Atmos. Terr. Phys. 11, 91-101. B r a m l e y , E.N. [1951] Diversity effects in spaced-aerial reception of ionospheric waves. Proc. I.E.E., 98, Part III, 19. B r e n n a n , D .G . [1959] Linear combining techniques. Proc. IRE, 47, 1075. B r e n n a n , D.G. [1960] The extrapolation and interpolation of spatial correlation functions. Statistical methods in radio wave propagation. W.C. Hoffman, ed., 296-305 (Pergamon Press). B r e n n a n , D.G. [1961] Probability theory in communication system engineering (Chapter 2); Analysis of long-term variability (Chapter 20), in Lectures on communication system theory. Ed. E. Baghdady, McGraw-Hill, New York. B r e n n a n , D .G . and P h il l ip s , M.L. [1957] Phase and amplitude variability in medium frequency ionospheric transmission. Lincoln Lab., M.I.T., TR-93. B r ig g s, B.H. [1951] Investigation of certain properties of the ionosphere by means of a rapid frequency change experiment. Proc. Phys. Soc., B63, 255. B r i g g s , B.H., P h il l ip s , G.J. and S h i n n , D.H. [1950] The analysis of observations on spaced receivers of the fading of radio signals. Proc. Phys. Soc., B63, 106-121. D a v ie s , K. [1965] Ionospheric radio propagation. NBS Monograph No. 80. D a v ie s , K. and B a r g h a u s e n , A.F. [1964] The effect of spread F on the propagation of radio waves near the equator. AGARD Ninth Meeting of the Ionospheric Research Committee, Copenhagen. F o k , V.A. [1950] Obobchtchenie otrajatelnykh formul na slutcha'i otrajeniia proizvolno'i volny ot poverkhosti proizvolnoi formy (Generalization of reflection formulae for the case of the reflection of an arbitrary wave by a surface of arbitrary shape). Journal eksperimentalnoi teoretitcheskol fisiki, 20, 961. G l a z e r , J. and F a r b e r , L. [1953] Evaluation of polarization diversity performance. Proc. IRE, 41, 1774-1778. G r e e n , Jr., P.E. [1963] Time-varying channels with delay spread. Monograph on radio waves and circuits. Ed. S. Silver (Elsevier, London). G r is d a l e , G.L., M o r r is , J.G. and P a l m e r , D.S. [1957] Fading of long-distance radio signals and a comparison of space- and polarization-diversity in the 6-18 Mc/s range. Proc. I.E.E., 104B, 13, 39. G r o s s k o p f , J. [1953] Statistical studies of short-wave transmission paths. FTZ, 6 , 393. G r o s s k o p f , J. [1955a] Feldstarkemessungen im Kurzwellenbereich, I. Teil (Field-strength measurements in the short-wave band, Part I). FTZ, 8 , 114. G r o s s k o p f , J. [1955b] Feldstarkemessungen im Kurzwellenbereich, II. Teil (Field-strength measurements in the short-wave band, Part II). FTZ, 8 , 146. Rep. 266-3 — 216 — H u m b y , A.M . [1959] Equatorial sunset effect. Wireless World, 65, 342. K e r b l a i, T.S. [1963] Vliianie gorizontalnykh gradientov elektronnoi kontsentratsii v ionosphere na velitchinou MPTCH (Influence of horizontal electron-concentration gradients on the value of the MUF). Geomagnetism i Aeronomiia, 3, 772. K h m e l n it s k y , E.A. [1960] Raznessennyi priem i opredelenie ego effektivnosti (Diversity reception and assessment of its effective ness). Sviazizdat, Moscow. K h m el n it sk y , E.A. [1969] Veroyatnost sboya na kv liniyakh svyazi pri pomekhakh ot sosednikh po chastote stantsii (Probability of error on short-wave circuits due to interference from stations operating on adjacent frequencies). Elektrosvyaz, 4. K h m e l n it s k y , E.A. [1970] Otsenka kachestva kv kanala svyazi dlya peredachi diskretnykh signalov (Assessment of the quality of a short-wave channel for discrete signal transmission). Elektrosvyaz, 10. K o n o pl e v a , E.N. [1964] Reliability of communications and necessary signal/noise ratio in short-wave radio channels (in Russian). Elektrosvyaz, 5. K o n o pl e v a , E.N. and K h m el n it sk y, E.A. [1970] Vozmozhnye sostoyaniya kv kanala svyazi po usloviyam rasprostraneniya radiovoln (Possible states of a short-wave channel depending on propagation conditions). Elektrosvyaz, 12. K o n o pl e v a , E.N. and K h m e l n it sk y , E.A. [1972] Modeli rasprostraneniya radiovoln na kv liniyakh svyazi neoptimalnoi proty- azhennosti (Propagation models for short-wave circuits of non-optimum length). Elektrosvyaz, 1. K oster, J.R. [1963] Some measurements on the sunset fading effect. J. Geophys. Res., 68, 2571-2578. L i a k h o v a , L .N . [1965] Otsenka effekta fokoussirovky pri rasprostranenii radiovoln po rikochetirouemoi traiektorii (Evaluation of the focusing effect in the propagation of radio-waves over ricochet paths). Geomagnetism i Aeronomiia, 5, 351. M c N ic o l , R.W.E. [1949] The fading of radio waves of medium and high frequencies. Proc. I.E.E., Part III, 96, 517-524. N a k a g a m i, M. [1943] Statistical character of short-wave fading. J. Inst. Elec. Comm. Eng., Japan, 27, 145. N a k a g a m i, M. [1960] The m-distribution: a general formula of intensity distribution of rapid fading. Statistical methods in radio wave propagation. Ed. W.C. Hoffman, 3-36 (Pergamon Press). N o r t o n , K.A., V o g le r , L.E., M a n sfie l d , W.V. and S h o r t , P.J. [1955] The probability distribution of the amplitude of a constant vector plus a Rayleigh distributed vector. Proc. IRE, 43, 1354-1361. O sbo r n e, B.W. [1952] Note on ionospheric conditions which may affect tropical broadcasting after sunset. J. Brit. IRE, 12, 110. P ierce, J.N. and S t e in , S. [1960] Multiple diversity with non-independent fading. Proc. IRE, 48, 89. P o l k in g h o r n , F.A. [1940] A single-sideband M USA receiving system for commercial operation on transatlantic radio telephone. Proc. IRE, 28, 157. P otter, R.K. [1930] Transmission characteristics of a short-wave telephone circuit. Proc. IRE, 18, 581-648, P r ic e, R. [1957] The autocorrelogram of a complete carrier wave received over the ionosphere at oblique incidence. Proc. IRE, 45, 879-880. P r ice, R. and G r ee n , P.E. [1958] A communication technique for multipath channels. Proc. IRE, 46, 555-570. R atc liffe, J.A. [1956] Some aspects of diffraction theory and their application to the ionosphere. Reports on Progress in Physics, 19, 188, Proc. Physical Society, London. R a w e r , K. [1952] Calculation of sky-wave field strength. Wireless Engineer, 29, 287-301. R ic e, S.O. [1944, 1945] Mathematical analysis of random noise. B.S.T.J., 23, 24. R ic e , S.O. [1958] Distribution of the duration of fades in radio transmission: Gaussian noise model. B.S.T.J., 37, 581. R o tt g er, J. [1973] Langperiodisches Fading von Kurzwellensignalen (Long-periodic fading of shortwave signals). Kleinheu bacher Berichte, 16, 167-178. S a l a m a n , R.K. [1962] A new ionospheric multipath reduction factor (MRF). IRE Trans. Comm. Syst. CS-10, 220-221. S ta r a s, H. [1956] Diversity reception with correlated signals. J. Appl. Phys., 27, 93. S t e in , S. [1966] Part III o f Communication systems and techniques, by Schwartz, Bennett and Stein, McGraw-Hill, New York. Stein, S. and Jones, J.J. [1967] Modern communication principles. McGraw-Hill Book Co., New York, London. T a u m e r , F. and S u l a n k e , H. [1967 and 1968] Die Eigenschaften von Haufigkeitsverteilungen der Raumwellenfeldstarke im Frequenzbereich von 150-1600 kHz (The properties of sky-wave field strength distribution in the bands 5 and 6). Techn. Mittlg. des RFZ, 11, 4, 192-195 and 12, 1, 32-39. T u r i n , G.L. [1956] Communication through noisy, random-multipath channels. IRE Nat. Conv. Record, Part 4, 154. W a tt er so n , C.C., Ju r o sh e k , J.R. and B ensem a, W.D. [1970] Experimental confirmation of an HF channel model. IEEE Trans. Comm. Tech., 18, 791. W o z e n c r a f t , J.M. [1961] Sequential reception of time-variant transmissions. Ch. 12 in Lectures on Comm. Theory, ed. E. Baghdady, McGraw-Hill. — 217 — Rep. 266-3 Y e h , K.C. an d V il l a r d , O.G. [1958] A new type of fading observable on high-frequency radio transmission propagated over paths crossing the magnetic equator. Proc. IRE, 46, 1968-1970. Z a d e h , L.A. and D esoer, C.A. [1963] Linear system theory. McGraw-Hill, New York. Vn/Vi dB F ig u r e 1 Distribution function P ( v0) for the Nakagami-Rice distribution (The values of F(v0) are shown on the curves) PAGE INTENTIONALLY LEFT BLANK PAGE LAISSEE EN BLANC INTENTIONNELLEMENT — 219 — Rep. 262-3 SECTION 6S: TOPICS RELATED TO SPACE SYSTEMS RECOMMENDATIONS AND REPORTS Recommendations There are no Recommendations in this section. Reports REPORT 262-3 * VLF PROPAGATION IN AND THROUGH THE IONOSPHERE (Question 18-1/6) (1963 - 1966 - 1970 - 1974) 1. Historical It has been known for many years that very low frequencies can be propagated through the ionosphere by a mode which for historical reasons has been called the “ whistler mode ”. In the original studies, the signals originated in lightning flashes and the frequencies observed were within the audio range and were termed “ musical atmospherics ” [Eckersley, 1925; Barkhausen, 1919]. These frequencies were, moreover, subject to a dispersion in time, the velocity of propagation decreasing with frequency so that the received signals had a characteristic whistle of decreasing frequency over a period of a second or more. They also consisted of a series of well-separated echoes with increasing frequency dispersions. These remarkable properties were explained theoretically in terms of propagation, by an extraordinary mode of low velocity and small attenuation, along a path closely confined around a line of the Earth’s magnetic field, the signals travelling to and fro between conjugate points on the surface of the Earth [Helliwell and Morgan, 1959; Morgan et al., 1956; Storey, 1953]. This mode is known as the “ whistler mode”. A comprehensive survey of the history and present state of whistler research has been recently published [Helliwell, 1965]. 2. Use of VLF transmitters More recently, this mode of propagation has been studied by using high-power VLF transmitters. With the increasing interest being shown in the problem of transmission on various frequencies for terminal points on the surface of the Earth and in or above the ionosphere, it is now preferable to think of VLF propagation in general and to restrict the term “ whistler ” to transmissions covering a frequency spectrum such as is originated by a lightning flash for which the frequency dispersion characteristic of whistlers can be observed. It is interesting to note, that the times when relatively strong signals have been obtained between conjugate points with VLF transmitters have shown only poor correlation with the times when whistlers were prevalent. Thus, although experiments using natural sources are of great scientific interest in unravelling some of the complexities of VLF propagation through the ionosphere, they must be supplemented with experiments directly related to propagation on given frequencies from chosen trans mitting points. * Adopted unanimously. Rep. 262-3 — 220 — 3. Frequency limits and intensity considerations It has been found that when both terminals are on the surface of the Earth, the whistler mode is effective at frequencies as low as 400 Hz and as high as 35 kHz. It has been observed at most locations between geomagnetic latitudes 20° and 80°, and there is a well-defined cut-off frequency which is approxi mately 0-6 of the minimum gyro-frequency along the path. Observations at middle latitudes show that, at night, signals by a single traverse between conjugate points can be obtained by the whistler mode for more than half the time and can approach in strength on occasion to within 10 dB of the conventional waveguide signal between the Earth and the ionosphere. Little is known about the intensity of daytime whistler-mode signals, but it appears that they are severely attenuated in the D region of the ionosphere. Several factors seem to be important in the calculation of intensity, including: — the polarization and directivity of the radiator, — the properties of the path between the end-points of the duct in the ionosphere and the terminal points on the ground, — the transmission coefficient for the propagation through the lower regions of the ionosphere, — the spatial divergence in the duct, — multipath effects resulting from the presence of more than one duct, — the possible amplification, or absorption, of signal energy through interaction with charged particles in the plasma. 4. Propagation analysis A general computer programme has been developed, for studying with a full-wave treatment, the fields in a horizontally stratified, ionosphere for waves incident from above or below for a wide range of the relevant parameters [Pitteway, 1965]. A new reciprocity relation has been established between the transmission coefficient and the limiting polarization, for a whistler-wave incident on the ionosphere from above and the corresponding parameters for a wave incident from below. The analysis has been applied to a large amount of LF and VLF data available from south-east England and to various models including one based on waye interaction experiments at Oslo, from which the distributions of electron density and collisional frequency with height have been determined with some accuracy up to about 85 km [Piggott et al., 1965; Deeks, 1966]. For any given model, it is necessary to extrapolate these distributions upwards through the E layer, in a manner consistent with the observed frequency dependence of reflection height and absorption at HF [Piggott and Thrane, 1964; Thrane, 1966]. A survey has been made of the whistler mode characteristics for day and night. In some of the models used the effects of sporadic-E layers have been included [Pitteway and Jesperson, 1966]. Near 14 kHz the transmission coefficient for upgoing waves varies only slowly with angles of incidence between ± 85° at middle latitudes, whereas at 100 kHz efficient transmission is possible only at angles within a cone having its axis along the direction of the magnetic field; this influence of the magnetic field is more noticeable at all frequencies at lower latitudes [Thomas and Horowitz, 1971]. Both by day and night, the transmission coefficient is mainly determined by losses in the ionosphere, provided that steep gradients of electron density, similar to those in a sporadic-E layer, are absent. The whistler wave absorption, which occurs chiefly in the lower ionosphere, increases with frequency and with decrease in latitude [Thomas and Horowitz, 1971; Wieder, 1967]. Both by day and night, sporadic E can give very high internal reflection coefficients for waves incident from above [Pitteway and Jesperson, 1966]. This is important when only the distant end of the guiding magnetic field line is in daylight, since very efficient two-hop whistler-propagation may then be possible. It has also been shown that a large part of the whistler wave energy can be reflected in the absence of sporadic E provided the wave-normal direction is outside the acceptance cone for penetration to the Earth’s surface [Thomas and Smeathers, 1971]. This is important in the interpretation of rocket measurements of whistlers. — 221 — Rep. 262-3 5. Satellite observations The study of VLF propagation in and through the ionosphere has been considerably advanced by using satellites, whereby it has become possible to make measurements within the ionosphere itself, as well as below and above it. For a terminal point within the ionosphere, consideration must be given to the effect of a highly anisotropic medium on the antenna characteristics and thus on the transmission loss. In this connection, an interesting series of experiments has been undertaken in the l o f t i satellite programme. This programme was designed to study both theoretically and experimentally the degree and extent of VLF radio wave penetration of the ionosphere. One aspect of the l o f t i programme includes an experiment to measure the impedance of dipoles and loops embedded in the ionosphere. To date, two space vehicles have been launched under the l o f t i programme, l o f t i I indicated that useful amounts of VLF radio energy can penetrate the atmosphere/ionosphere interface and, in par ticular, that 18 kHz signals from ground-based transmitters were detected as far as 10 000 miles away in the ionosphere with substantially less attenuation at night than by day, as expected. The observed time delays ranged from 10 to 200 ms, indicating that the VLF propagation velocity in the ionosphere is much lower than in free space [Leiphart et al., 1962]. In the l o f t i II experiment, in addition to signal strength, measurements were made of dipole impedance in the ionosphere. The measured impedance was found to be radically different from that in free space. Theoretical analyses have been made by various groups. These also indicate that the dipole impedance in a magneto-ionic medium is radically different from that in free space. Alouette I contains a VLF receiver covering the frequency range 0*4 to 10 kHz using an electric dipole. While the observations do not yield amplitude information the VLF signals received at the satellite are very strong indeed. The satellite moves in a nearly circular orbit, at an inclination of 80° to the equator and at a height of about 1000 km. Short fractional hop (SFH) whistlers, i.e. those which have been propagated from a lightning flash only once through the ionosphere, are strongly received and often overload the receiver. Transmissions are received from high-power ground-based transmitters and even from low-power transmitters (100 W) at 10 kHz, but no study has yet been made of these signals. Concerning the propagation of very low frequencies, these observations emphasize two special points not hitherto considered in any detail: — the importance of propagation within the ionosphere transverse to the direction of the Earth’s magnetic field, no clear evidence of which has been obtained with ground-based equipment; — the importance of ions on the propagation of the lowest frequencies. With reference to the first, it is noted in the Alouette I satellite that long fractional hop (LFH) whistlers, i.e. those thought to be propagating down the line of force from the opposite hemisphere, can often be observed having the same dispersion characteristics during a full ten-minute telemeter recording. This implies that a particular duct is active throughout this period in propagating the whistlers which, after leaving the duct, can travel some thousands of kilometres transversely to reach the satellite receiver. Besides this fact, not previously noted in the literature, horizontal propagation of SFH whistlers can lead to reflection of these signals near the height of the satellite [Barrington and Belrose, 1963; Carpenter et al., 1964; Smith, 1964]. Moreover, a VLF plasma resonance (the lower hybrid resonance for the transverse mode of propagation) is evident in Alouette I VLF data as a cut-off frequency for a VLF noise band [Brice and Smith, 1964], and its identification is providing a source of information on the mean ionic mass at the height of the satellite [Barrington et al., 1965]. The evidence for the importance of ions at the lowest frequencies is concerned with the identification of ion cyclotron whistlers [Smith et al., 1964; Barrington et al., 1966], which are also now being inves tigated as a means of studying ions at the height of the satellite [Gurnett et al., 1965; Barrington et al., 1966; Barrington and McEwen, 1967]. A useful review of satellite observations of VLF phenomena in the magnetosphere has been published [Rycroft, 1967]. Rep. 262-3 — 222 — 6. The magnetosphere The fact that propagation paths for the whistler mode can extend far from the Earth to distances of several earth-radii shows that a measure of ionization must exist out to these distances. As mentioned above, satellite experiments are revealing the part played by electrons and ions in the Earth’s magnetic field as a major factor in VLF propagation, and the whole subject of the movement of charged particles in the magnetosphere is of interest in this connection. It has been shown that, not only the forces due to the movement of the particles in the Earth’s magnetic field includingthe effects of collisional interaction, but also those due to gravity are important in determining the limits between which the particles spiral around the field lines [Chvojkova, 1965]. Such considerations are thought to explain why the density of electrons and ions, as deduced from investigations of the whistler mode of propagation, may differ from the value expected from simpler assumptions about the movement of charged particles in the magnetosphere. Furthermore, effects inside the magnetosphere (e.g. the impact of a relatively narrow stream of cosmic particles penetrating into the magnetosphere from the region of the neutral sheet) may result in an increase or decrease of ion density along the whole of the field “ line ”, producing the preferred channels for whistler mode of propagation [Chvojkova, 1967]. 7. Conclusions In conclusion, much is being learned experimentally about VLF propagation in and above the ionosphere, and the theory of VLF propagation is advancing as rapidly as is the discovery of new phenomena. The study of VLF phenomena by means of satellites has proved to be a most powerful technique for investigating ion densities in the magnetosphere. It is clear that an understanding of propagation in and above the ionosphere, not specifically at VLF frequencies but more generally at frequencies below the local gyro-frequency (particularly the gyro-frequency in the lower ionosphere when considering propagation through the ionosphere), is required to assess the practicability of employing these frequencies for communication to and between space vehicles. A knowledge of the effect of the ionosphere on these frequencies is also necessary in order to interpret satellite observations of noise in this frequency range (see Report 342-2). Finally, it may be pointed out that studies of the propagation of whistlers along magneto-ionic ducts may throw light on the nature of the ionospheric irregularities aligned along the Earth’s magnetic field, which are responsible also for the trapping of MF and HF waves (an important propagation mechanism for transmitters and receivers located above or immediately below the maximum in electron density in the ionosphere) (see Study Programme 18A-2/6 and Question 5-2/6). R e fe r e n c e s B a r k h a u s e n , H. [1919] Zwei mit Hilfe der neuen Verstarker entdeckte Erscheinungen (Two phenomena discovered with the use of new amplifiers). P h y s. Z., 20, 401-403. B a r r in g t o n , R.E. and B e l r o se , J.S. [1963] Preliminary results from the very low frequency receiver aboard Canada’s Alouette satellite. N a tu re , 198, 651. B a r r in g t o n , R.E., B e l r o se , J.S. and N e l m s, G.L. [1965] Ion composition and temperature at 1000 km as deduced from simultaneous observations of a VHF plasma resonance and topside sounding data from the Alouette I satellite. J. Geophys. R e s ., 70, 1647-1664. B a r r in g t o n , R.E., B e l r o se , J.S. and M a t h e r , W.E. [1966] A helium whistler observed in Canadian satellite Alouette II. N a tu re , 210, 80. B a r r in g t o n , R.E., B e l r o se , J.S. and N e l m s, G.L. [1966] Ion composition and temperature at 1000 km as deduced from VLF plasma resonances and topside ionograms. Electron density distributions in the ionosphere and exosphere, edited by J. Frihagen, North-Holland Publ. Co., Amsterdam. Barrington, R.E. and McEwen, D.J. [1967] Ion composition from VLF phenomena observed by Alouette I and II. S p a c e Research VII. B r ic e , N.M . and S m it h , R.L. [1964] Recordings from satellite Alouette I—a very low frequency plasma resonance. N a tu re , 203, 926. — 223 — Rep. 262-3, 263-3 C a r p e n t e r , D .L ., D u n c k e l , N. a n d W a l k u p , J.F. [1964] A new very low frequency phenomenon— whistlers trapped below the protonosphere. J. Geophys. Res., 69, 5009. C h v o j k o v a , E. [1965] Geometry of spiralling particle-paths of high speeds affected by gravitation. Bulletin o f the Astronomical Institute o f Czechoslovakia, BAC, 16, 2, 63-69. C h v o j k o v a , E. [1967] Charged particles spiralling in an extended gravity field and the magnetosphere. J. Atmos. Terr. Phys. 29, 465-469. D e e k s , D.G. [1966] D-region electron distributions in middle latitudes deduced from the reflection of very long waves. P ro c. R o y . S o c ., Series A, 291, 413-437. E c k e r s l e y , T.L. [1925] A note on musical atmospheric disturbances. P h il M a g ., 49, 1250-1260. G u r n e t t , D .A ., S h a w a n , S.D., S m it h , R.L. and B r ic e , N.M . [1965] Ion cyclotron whistlers. J. Geophys. Res., 70, 1665-1688. H e l l iw e l l , R.A. [1965] Whistlers and related ionospheric phenomena. Stanford University Press. H e l l iw e l l , R.A. a n d M o r g a n , M.G. [1959] Atmospheric whistlers. P ro c. I R E , 47, 200-208. L e ip h a r t , J.P., Z e e k , R.W., B e a r c e , L.S. and T o t h , E. [1962] Penetration of the ionosphere by very low frequency radio signals. Interim results of the LOFTI I experiment. P ro c. I R E , 50, 6-17. M o r g a n , M .G., D i n g e r , H.E. and A l l c o c k , G.McK. [1956] Observations of whistling atmospherics at geo-magnetically conjugate points. N a tu re , 177, 29-31. P ig g o t t , W.R. and T h r a n e , E.V. [1964] Electron density distribution in the ionosphere. 13-14. Ed. E.V. Thrane, North-Holland Publ. Co., Amsterdam. P ig g o t t , W .R., P it t e w a y , M.L.V. and T h r a n e , E.V. [1965] The numerical calculation of wave fields reflection coefficients and polarizations for long waves in the lower ionosphere. Part II, Phil. Trans. Roy. Soc., Series A, 257, 243-271. P it t e w a y , M.L.V. [1965] The numerical calculation of wave fields reflection coefficients and polarizations for long waves in the lower ionosphere. Part I, Phil. Trans. Roy. Soc., Series A, 257, 219-242. P it t e w a y , M.L.V. and Je s p e r s o n , J.L. [1966] A numerical study of the excitation, internal reflection and limiting polarization of whistler waves in the lower ionosphere. J. Atmos. Terr. Phys., 28, 17-43. R y c r o f t , M.J. [1967] A review of satellite observations of VLF phenomena in the magnetosphere, in MF, LF a n d VLF propagation. I.E.E. 'Conf. Publ., 36, 267-294. S m it h , R.L. [1964] An explanation of sub-protonospheric whistlers. J. Geophys. Res., 69, 5019. S m it h , R.L., B r ic e , N .M ., K a t s u f r a k is , J., G u r n e t t , D .A . S h a w a n , S.D., B e l r o s e , J.S. and B a r r in g t o n , R.E. [1964] An ion gyrofrequency phenomenon observed in satellites. N a tu re , 204, 274. S t o r e y , L.R.O. [1953] An investigation of whistling atmospherics. Phil Trans. Roy. Soc., Series A, 246, 113-141. T h o m a s , L. and H o r o w it z , S. [1971] The ionospheric absorption of downgoing whistler waves during night-time. J . A tm o s . Terr. Phys., 33, 879-888. T h o m a s , L. and S m e a t h e r s , J.R. [1971] The internal reflection of whistler waves in the lower ionosphere. J. Atmos. Terr. Phys., 33, 959-962. T h r a n e , E.V. [1966] The electron densities in the E and D region above Kjeller. J. Atmos. Terr. Phys., 28, 467-479. W ie d e r , B. [1967] Transmission of VLF radio waves through the ionosphere. Radio Science, 2, 595-605. REPORT 263-3 * IONOSPHERIC EFFECTS UPON EARTH-SPACE PROPAGATION (Study Programme 18B-1/6) (1963 - 1966 - 1970 - 1974) I. Introduction Any radio signal which is to penetrate the ionosphere will be modified by the medium due to the presence of free electrons and the earth’s magnetic field. The effects include scintillation, absorption Adopted unanimously. Rep. 263-3 — 224 — variation in the direction of arrival, propagation delay, frequency change and polarization. These effects at frequencies above about 30 MHz are treated in the succeeding sections. 2. Scintillation 2.1 Introduction Scintillations, as addressed by this Report, are variations of amplitude, phase, polarization, and angle of arrival produced when radio waves pass through electron density irregularities in the ionosphere. The scintillations can become quite severe and may represent a practical limitation for some commu nication systems. Most of the data used refer to amplitude scintillations since measurements of phase, polarization, and angle-of-arrival fluctuations have been sparse. Most scintillation data have been collected in the VHF band with some data at 400 MHz, 1500 MHz, 4 GHz, and 6 GHz. Under most ionospheric conditions, the magnitude of the amplitude scintillations decreases with frequency for frequencies above 100 MHz. Amplitude scintillation can be classified as varying from weak to strong depending upon the value of the scintillation index 54 [Briggs and Parkins, 1963]. For weak scintillation, *S4 is less than 0-6. In the limit of weak scintillation for frequencies below 1*5 GHz, varies as X1'5. Strong scintillation refers to the other limit, namely, 54 ~ 1. In this limit »S4 is independent of frequency. For a scintillation event, strong scintillation may occur at low and inter mediate frequencies and become progressively weaker at higher frequencies. Because of the saturation effect, low frequency data cannot be used to infer the magnitude of scintillation at higher frequencies. Radio star data collected since 1947 have only limited applicability to the problem of scintillation of satellite beacon signals because the measurements were only qualitative and the radio sources used had relatively large angular diameters (F for Cygnus A and A' for Cassiopeia). However measurements of radio star scintillations have yielded worthwhile contributions to the morphology of the irregularity regions at high and equatorial latitudes. Aside from frequency, the magnitude of scintillations depends upon the length of the path through the ionosphere, increasing with increasing zenith angle. It also depends on the distances from the irregularity region to the source and to the observer, on time and geomagnetic latitude of the path’s penetration of the irregularity region, on phase of the sunspot cycle and on magnetic index. Some scintil lations are produced in the E layer, but most are produced in the F layer. Comparison of the times of occurrence of scintillation and of spread F (the spreading of F-region echoes as observed on vertical-incidence ionograms) has led to the conclusion that the two phenomena are closely associated [Chivers, 1963]. However, spread F determined by normal ionosonde reduction is not quantitative. In addition during periods of intense auroral or polar cap absorption, soundings may be wiped out. Finally top-side spread F must be added to bottom-side measurements to determine whether or not spread-F conditions exist for a trans-ionospheric signal. The occurrence patterns of bottom-side spread F yield worthwhile background information for long periods of solar activity and for seasonal variations. The geographic distribution of spread-F occurrences as observed on ground-based ionosondes during both high and low solar activity periods indicates that the phenomenon of spread F is mainly a night-time one [Davis, 1973]. Occurrences during the daylight hours are found primarily in the auroral and polar regions. There is an apparent increase of spread F in the temperate regions during the period of low solar activity. The variations in scintillation can be divided into components that depend primarily on latitude and time. In the band of latitudes centered on the geomagnetic equator, particularly severe scintillations occur at night, varying with season and with epoch of the solar cycle. At mid-latitudes, there is a variation with time of day and with latitude but only modest variation with season or solar activity. High-latitude scintillation shows a sharp equator-ward boundary, the position of which depends on time of day and — 225 — Rep. 263-3 solar activity. Poleward of the boundary, scintillation is always strong, showing variation with latitude, geomagnetic activity and possible variations with solar epoch. Fig. 1 [Aarons et al., 1971] is a generalized map of the night-time hemisphere showing depth of fluctuations as a function of local meridian time for 1968, a year of relatively high solar activity. The depth of fluctuation is proportional to the shading density of the figure. The map of Fig. 1 was derived primarily from measurements made at 136 MHz. Observations of scintillation at 136 and 400 MHz show that there is a high correlation between the fading on right- and left-handed circular polarization, and between vertical and horizontal linear polarization at middle latitudes [Koster, 1966; Whitney and Ring, 1971; Crane, 1974]. 2.2 Equatorial scintillation Before the discovery of gigahertz scintillation, quantitative data on scintillation from the equatorial region were sparse. Early data in the range 100-136 MHz frequently suffered from low signal to noise ratios and the occurrence of strong scintillations. Enhanced scintillation was reported by Flood [1966]; Coates and Golden [1968] observed scintillations exceeding 35 dB at Santiago, Chile at 136 MHz. Fading at S-band was first reported by Christiansen [1971] with peak-to-peak fluctuation as large as 20 dB. More scintillations at 1-5, 4 and 6 GHz were reported by Skinner et al. [1971], Sessions [1972], Taur [1973a], and Paulson and Hopkins [1973]. It is now established that the amplitude of scintillations maximizes in the geomagnetic equatorial region. The fading period of VHF scintillation varies over quite a large range and can be as long as several minutes [Koster, 1971]. The fading period depends both upon the motion of the irregularities relative to the ray path, and in the case of strong scintillation, on its severity. For strong scintillation, the fading period is shorter than for weak scintillation with the same relative velocity. The fading period of gigahertz scintillation ranges between 2-15 seconds [Sessions, 1972; Paulson and Hopkins, 1973; Taur, 1973a]. The gigahertz measurements were in the weak scintillation limit and were made when both the transmitter and receiver were stationary; therefore the observed fading periods reflect the motion of the ionospheric irregularities. Equatorial ionospheric scintillation is primarily a night-time phenomenon with more severe fluctuations occurring in the early part of the night. Scintillation activity peaks near the equinoxes and apparently has a positive correlation with the sunspot variation [Taur, 1973a]. The boundary of equatorial activity lies between 20° to 30° geomagnetic latitudes [Aarons et al., 1971; Taur, 1973a]. Simultaneous measurements at VHF and L-band show that the 54 scintillation index [Briggs and Parkin, 1963] varies as J/x to X [Sessions, 1972]. Between 1-5 and 4 GHz, the scintillation index varies as X (Taur, 1973b). From 4 GHz upwards, the scintillation index is proportional to X2 [Craft and Westerlund, 1972]. The scale size of the amplitude scintillation pattern on the ground is a function of frequency and of the spatial wavenumber spectrum of the F-region density fluctuations. The scintillation patterns in the equatorial region are elongated in the north-south direction with axial ratios greater than 60:1 and generally move in the eastward direction [Koster, 1966]. For gigahertz scintillation, the east-west dimensions of the scintillation patterns are near 200 m [Taur, 1973b]. 2.3 Mid-latitude scintillations The diurnal variation of mid-latitude scintillation is well established with a maximum near midnight and a minimum near midday [Aarons, 1971]. However day-time scintillation is common during winter at sunspot maximum, produced by irregularities in the E region [Clark et al., 1970]. Fast regular scin tillation of VHF signals from satellites has been shown to correlate with the occurrence of sporadic E [Ireland and Preddey, 1967]. Since fading below system margin may destroy the intelligibility of certain types of modulation for the entire length of time the fading occurs, the percentage of occurrence of fading with various Rep. 263-3 — 226 — ranges in dB peak-to-peak excursions is listed in Table I for recordings of 136 MHz signals from a t s -3 at Hamilton, Mass., U.S.A. (invariant latitude of intersection of propagation path with level of the F2 peak, 53°). Table I Percentage of time for which fading of various levels took place for a station below the high-latitude irregularity region [Whitney et al., 1972] Fading (peak-to-peak) Percentage of occurrence (dB) (total time) (1968-1970) 3 -7 — 5-9 7-4 6 — 9-4 4-7 9-5 — 12-7 1-6 > 12-7 2-9 Elongation of the irregularities along the Earth’s magnetic field has been estimated to vary from 2:1 to 10:1, increasing with latitude [Preddey, 1969]. 2.4 High-latitude scintillations Scintillation of radio signals increases considerably when the ray path traverses the high-latitude irregularity region. In particular there is a marked increase with increasing latitude at a boundary, known as the scintillation boundary, located at the low-latitude side of the auroral oval. Scintillations increase in the auroral oval and the irregularities appear to form a closed boundary around the geomag netic pole [Fremouw and Rino, 1973]. Fading ranges at one station (Narsarssuak, Greenland) observing the 137 MHz beacon of a t s -3 through the auroral oval at night at an invariant latitude of 63° and an elevation angle of 18° are fisted for three years of data in Table II [Whitney et al., 1972]. Table II Fading (peak-to-peak) Percentage of occurrence (dB) 1968-1971 3 -7 — 5-9 150 6 — 9-4 17-9 9-5 — 12-7 12-9 > 12-7 29-5 Observations from Ottawa (a station near the scintillation boundary at night) have shown peak- to-peak fades at 230 MHz in excess of 2 dB for less than 1 % of the time and in excess of 10 dB for less than 0*1% of the time [Maynard, 1972 a and b]. In examining 40 days of data taken in Alaska at 137 MHz and 1695 MHz from essa 9 and itos 1, Pope and Fritz [1970] found scintillation amplitudes at 1695 MHz 10% that of the VHF scintillations, rarely exceeding a scintillation index (S4) of 0-2. I — 227 — Rep. 263-3 3. Absorption 3.1 Introduction The lower limit of the usable radio-frequency spectrum is determined by ionospheric reflection and absorption which, in general, increase with decreasing frequency [Davies, 1965]. For equatorial and temperate regions use of frequencies above 70 MHz will assure penetration of the ionosphere without significant absorption. In addition to the generally predictable seasonal and solar activity variations of the ionosphere, there are less predictable variations arising from short term changes in solar activity. These short term variations have their greatest effect near the auroral zones and over the polar caps and should be allowed for in planning a service for such regions. Enhanced absorption can occur due to polar cap and auroral events; these two phenomena occur at random intervals, last for different periods of time, and their effects are functions of the locations of the terminals and the elevation angle of the path. Therefore, for the most effective system design, these phenomena should be treated statistically bearing in mind that the durations for auroral absorption are of the order of hours and for polar cap absorption are of the order of days. Ionospheric absorption, which may last for periods of the order of 30 minutes, may also be caused by solar flares for which the greatest effect is near the sub-solar point. At VHF, the effects are small and decrease rapidly with increasing frequency. Absorption data which are obtained at vertical incidence at 30 MHz and which are subject to an inverse frequency squared law may be extrapolated to other elevations and frequencies by the use of a sec ijf2 relationship. For example, at mid-latitudes the absorption experienced by a one-way traversal of the ionosphere at vertical incidence may be expected, even during a solar flare, to be less than 5 dB. For the auroral zone and polar cap the values of Fig. 2 may be used. 3.2 Auroral absorption Auroral absorption is believed to be due to an enhancement of electron density in*the lower ionosphere (roughly between 45 and 90 km) by the entry of energetic electrons following a solar disturbance. This sporadic source of attenuation tends to maximize in the local morning hours and events are relatively short-lived, lasting up to a maximum of five or six hours, but with an average duration of about 30 minutes [Bellchambers et al., 1955-1959; Hargreaves and Cowley, 1967]. Most measurements have been made at frequencies near 30 MHz although some data are available at 65 MHz and 81 *5 MHz [Little, 1954; Little and Maxwell, 1952] with little information available at higher frequencies. Assuming that the absorption is inversely proportional to the square of the frequency, calculations based on 6000 hours of data from a typical auroral zone station (Kiruna) yielded the equivalent absorption at 127 MHz as shown in Table III. Thus, the effect appears to be of minor importance at VHF and will rapidly become negligible at the higher frequencies. T a b l e III Auroral absorption at 127 MHz (dB) Angle of elevation Percentage of the time 20° 5° 01 0-6 10 10 0-4 0-6 20 0-3 0-5 5-0 0-25 0-4 Rep. 263-3 — 228 — 3.3 Polar cap absorption At geomagnetic latitudes greater than 64° another source of attenuation is polar cap absorption which occurs on relatively rare occasions [Bailey and Harrington, 1962]. It usually occurs in discrete, though sometimes overlapping, events which are nearly always associated with discrete solar events. The absorption is long-lasting and is detectable over the sunlit polar cap [Davies, 1965; Ortner et al., 1962]. Polar cap absorption occurs most usually during the peak of the sunspot cycle, when there may be 10-12 events per year. Such an event may last up to a few days. This is in contrast to auroral absorption, which is frequently quite localized, with variations in periods of minutes. The absorption is due to bombardment of the upper atmosphere by solar protons, which are focused by the earth’s magnetic field to the polar regions. The absorption is produced by ionization at heights greater than about 30 km. Due to the high collision frequency between electrons and neutral molecules at heights between 30 km and 90 km, this additional ionization results in considerable attenuation principally in the HF and lower VHF range. A remarkable feature of a polar cap absorption event is the great reduction in the absorption during hours of darkness for a given rate of electron production. Fig. 2 is a hypothetical model of the diurnal variation of polar cap absorption following a major solar flare based upon riometer observations at various latitudes [C.C.I.R., 1970-1974a]. Table IV gives the number of principal polar cap absorption events (events with peak absorption greater than 20 dB) occurring during the last solar cycle. Included also is the smoothed Zurich sunspot number for each year, the average duration of each event, the average peak absorption at 32-2 MHz with respect to the reference path, and the maximum peak absorption with respect to the reference path [Bailey, 1964]. Table IV Polar cap absorption Year 1952-5 1956 1957 1958 1959 1960 1961 1962 1963 Smoothed Zurich sunspot number 6 to 38 142 190 188 160 114 55 38 28 Number o f principal polar cap events 0 4 13 9 5 12 3 0 2 Average duration (hours) — 104 60 76 149 54 69 — 72 Average peak absorption with respect to the reference path (dB) — 54 41 79 143 57 76 — 31 Maximum peak absorption with respect to the reference path (dB) ■ — 104 74 190 190 160 136 — 37 Polar cap absorption observed at 30 MHz at near vertical incidence may be extrapolated to other frequencies and angles by the sec i//2 relationship. This extrapolation is only approximate since — 229 — Rep. 263-3 the relationship with frequency obeys the inverse square law only for absorption generated above the level at which the angular wave frequency (to) equals the collision frequency (v). At 30 MHz, this level is about 55 km, while at 127 MHz the level is about 35 km. In events recorded at stations where the day/night ratio is near unity, the absorption (measured in decibels) normally decreases on the second day of the event to about half the maximum value of that on the first day and a further reduction by half is evident on the third day. Using this model and a 12 hours of daylight, Table V is constructed. The duration of the day within the polar cap can vary greatly depending on latitude and season. Table V Numbers of hours of polar cap absorption for various levels (1952 to 1963 inclusive) Absorption (dB) N o. of 127 MHz (10° elevation) hours 7-8 36 6-7 48 5-6 24 4-5 12 3-4 144 2-3 96 1-2 480 4. Variations in the direction of arrival Travelling Ionospheric Disturbances (TIDs) are large scale irregularities in electron density which often propagate over horizontal distances of thousands of kilometres. The spatial sizes of the disturbances are usually between 40 and 400 km [Friedman, 1966] but they have been reported as small as 20 km [Litva, 1971] and as large as 2000 km [Chan and Villard, 1962]. Although these disturbances usually are detected in wave trains containing about three wave cycles there are reports of events containing from 5 to 18 wave cycles [Wild et al., 1959; Litva, 1971; Reddi and Rao, 1971]. Some TIDs have been observed to travel at least 4000 km across the United States with little attenuation [Valverde, 1958]. Radio waves which propagate through regions of the ionosphere containing TIDs are observed to undergo quasi-periodic deviations in their direction of arrival. Measurements conducted at frequencies ranging from 45 to 490 MHz [Wild et al., 1959; Lawrence and Jespersen, 1961; Vitkevich, 1958;. Litva, 1971] indicate deviations around 20 minutes of arc, with the deviation in the angle being proportional to f~ 2. 5. Variations in the propagation delay An increase in propagation time occurs in trans-ionospheric transmissions. This increase is proportional to the integrated electron density along the ray path and is inversely proportional to the square of the frequency. Numerical models of the Total Electron Content (T.E.C.) are available for the European and Mediterranean areas for selected time periods [Aarons, 1973; Klobuchar, 1973; Amayenc et al., 1971; C.C.I.R., 1970—1974b]. Similar models are being developed from Faraday rotation and polarization measurements of T.E.C. for additional geographical areas [C.C.I.R., 1970-1974c]. A continuing effort is being made to enlarge the T.E.C. observing network and data base with the view of producing world maps of T.E.C. for all levels of solar activity. An estimated upper limit of expected propagation delay at 100 MHz is 25 (xs. An example of consequential ranging system error is given in Fig. 3. Because of the linear dependence on integrated electron density, ranging system errors can be corrected by the use of multiple ranging frequencies. Rep. 263-3 — 230 — When trans-ionospheric signals occupy a significant bandwidth the propagation delay (being a function of frequency) introduces dispersion. The differential delay across the bandwidth is proportional to the integrated electron density along the ray path. For a fixed bandwidth the relative dispersion is inversely proportional to frequency cubed. Thus, systems involving wideband transmissions must take this effect into account at VHF and possibly UHF [Counter and Riedel, 1958; Elliot, 1957]. For example, for an integrated electron content of 5 X 1017 electron/m2 column and a 6 MHz bandwidth at 100 MHz the differential delay would amount to 0-625 p.s while at 500 MHz the corresponding figure is only 0-005 fxs. See Fig. 4 [Millman, 1967]. 6. Frequency change The effects of the ionosphere upon the apparent frequency of a radio signal from a satellite produces only a second-order effect, and its influence upon a communications circuit is generally unimportant. On the other hand, the dispersive Doppler effect and the related phase-modulation techniques are of considerable interest as a tool for measuring the electron content of the ionosphere and exosphere [Alpert, 1963; Rawer, 1964]. In the course of systematic observations of the Doppler effects at 48° N at 20 and 40 MHz, numerous ionospheric effects were found [Joint Satellite Studies Group, 1965]. Deformations of the curve of Doppler frequency as a function of time, due to refraction, were found to be strong in about 40 % of all cases, increasing considerably the inaccuracy of the determination of the time of closest approach. 7. Polarization The performance of Earth-space communication systems may be significantly affected by ionospheric Faraday rotation. This phenomenon seriously affects any VHF space communication system which uses linear polarization. It also provides the basis for an accurate and sensitive technique to measure the total electron content of the ionosphere. When a linearly polarized radio wave traverses the ionosphere, it separates into two independent components both elliptically polarized with opposite senses of rotation. The waves approach circular polarization as the frequency increases. Since the two waves progress inside the ionized medium with different velocities of propagation, their phase relationships are continuously being altered. On emerging from the ionosphere the waves recombine to form a linearly polarized wave whose plane of polarization has been rotated with respect to that of the incident wave. This phenomenon, which is commonly referred to as the Faraday effect, is brought about by the interaction of the electromagnetic wave with the electrons in the presence of the Earth’s magnetic field. The rotation is inversely proportional to the square of frequency and directly proportional to the integrated product of the electron density and the component of the Earth’s magnetic field along the propagation path. Millman [1967] has given calculations of typical angular rotations. The amount of rotation may be less by a factor of two to three depending upon solar activity and latitude [Garriott et al., 1970]. 8. Summary The magnitude and behaviour of ionospheric effects have been described. Each effect may be an important consideration in the design of different kinds of Earth-space finks. The effects decrease with increasing frequency; several can be mitigated even in frequency ranges where they would otherwise present severe problems. Table VI estimates maximum values for ionospheric effects at a frequency of 100 MHz. It is assumed that the total zenith electron content of the ionosphere is 1018 electrons/m2 column. An elevation angle of about 30° is also assumed. The values given are for the one-way traversal of the waves through the ionosphere. — 231 — Rep. 263-3 Table VI Estimated maximum ionospheric effects at 100 M Hz for elevation angles of about 30° one-way traversal Effect Magnitude Frequency dependence Faraday rotation 30 rotations l / / 2 Propagation delay 25 jxs 1 IP Refraction < 1° l / / 2 Variation in the 20 min I//2 direction of arrival of arc Absorption (polar cap absorption) 4 dB - 1 IP Absorption (auroral + polar cap absorption) 5 dB ~ 1 / / 2 Absorption (mid-latitude) < 1 dB l / / 2 Dispersion 0 4 ps/Hz 1 IP Scintillation See § 2 of the Report See § 2 of the Report R e f e r e n c e s A a r o n s , J. [1973] Total electron content and scintillation studies of the ionosphere. Agardograph 166. A a r o n s , J., W h it n e y , H.E. and A l l e n , R.S. [1971] Global morphology of ionospheric scintillations. Proc. IEEE, 5 9 , 159-172. A l p e r t , J.L. [1963] An investigation of the upper atmosphere on the basis of measurements of Doppler effect in satellite signals at coherent frequencies (in Russian). Geomagnetism i Aeronomiia, 3 . A m a y e n c , P., B e r t i n , F. and P a p e t -L e p in e , J. [1971] Sur 1’evolution latitudinale du contenu electronique de l’ionosphere. Ann. de geophys. 2 7 , 3, 345-357. B a il e y , D.K. [1964] Polar cap absoption. Planet, and Space Science, 1 2 , 495-541. B a il e y , D .K . and H a r r in g t o n , J.M. [1962] A survey of polar cap absorption events (solar proton events) in the period 1952 through 1960. J. Phys. Soc., Japan, 1 7 , Supplement A-II, 334. B ellchambers , W .H., P ig g o t t , W.R. and B a r k l e y , L.W. [1955-1959] Royal Society I.G.Y. Expedition, Hailey Bay, 2 , 209. B r i g g s , B.H. and P a r k i n , I.A. [1963] On the variation of radio star and satellite scintillations with zenith angle. J. Atmos. Terr. Phys., 2 5 , 334-365. C.C.I.R. [1970-1974a] Doc. 6/80, United States of America. C.C.I.R. [1970-1974b] Doc. 6/81, Germany (Federal Republic of). C.C.I.R. [1970-1974c] Doc. 6/182, United Kingdom. C h a n , K.L. and V il l a r d , O.G. [1962] Observations of large-scale travelling ionospheric disturbances by spaced-path high- frequency instantaneous-frequency measurements. J. Geophys. Res., 6 7 , 973-988. C h iv e r s , H.J.A. [1963] Radio star scintillations and spread-F. J. Atmos. Terr. Phys., 2 4 , 468-472. C hristiansen , R.M. [1971] Preliminary report of S-band propagation disturbance during ALSEP mission support. Goddard Space Flight Center, NASA X-861-71-239, Washington, D.C. C l a r k , D .H ., M a w d s l e y , J. and I r e l a n d , W. [1970] Mid-latitude radio-satellite scintillation: the height of the irregularities. Planet, and Space Science, 1 8 , 1785-1791. C o a t e s , R.J. and G o l d e n , T.S. [1968] Ionospheric effects on telemetry and tracking signals from orbiting space-craft. NASA- TM-TX 63152, X-520-68-76 (N68-20043), Washington, D.C. C o u n t e r , V.A. [1958] Calculations of ground-space propagation effects. Lockheed Aircraft Corp., Rept. LMSD-2461. Rep* 263-3 — 232 — C r a f t , Jr., H .D. and W e s t e r l u n d , L.H. [1972] Scintillation at 4 and 6 GHz caused by the ionosphere. AIAA Paper No. 72-179, AIAA Library, New York. C r a n e , R.K. [1974] Morphology of ionospheric scintillation. AIAA Paper No. 74-52. D a v ie s , K. [1965] Ionospheric radio propagation. NBS Monograph 80. (Also available from Dover Press.) D a v is , Jr., R.M . [1 9 7 3 ] Dependence of h ig h latitude spread-F occurrence on solar and magnetic activity. Report 7 3 -2 3 , Dept, of Commerce, Boulder, Colo. E l l io t t , R.S. [1957] Pulse waveform degradation due to dispersion in waveguide. Trans. IRE, MTT5, 254-257. F l o o d , W.A. [1966] Radio star fadeouts and radio propagation. Spread-F and its effects upon radiowave propagation and com munications. Edited by P. Newman, 267, Technivision, Maidenhead, England, 267-285. F r e m o u w , E.J. and Rino, C.L. [1973] An empirical model for average F-layer scintillation at VHF/UHF. Radio Science, 8, 213-222. F r i e d m a n , J.P. [1966] Propagation of internal gravity waves in a thermally stratified atmosphere. J. Geophys. Res., 71,1033-1054. G a r r io t t , O.K., da R o s a , A.V. and Ross, W.J. [1970] Electron content obtained from Faraday rotation and phase path length variations. J. Atmos, and. Terr. Phys., 32, 705-727. H a r g r e a v e s , J.K. and C o w l e y , F.C. [1967] Studies of auroral space absorption. Planet, and Space Science, 15, 1571-1585. I r e l a n d , W. and P r e d d e y , G.F. [1 9 6 7 ] Regular fading of satellite transmissions. J. Atmos. Terr. Phys., 29, 1 3 7 -1 4 8 . J o in t s a t e l l it e s t u d ie s g r o u p [1965] A synoptic study of scintillations of ionospheric origin in satellite signals. Planet, and Space Science, 13, 51-62. K l o b u c h a r , J.A. [1973] Total electron content studies of the ionosphere. Air Force Cambridge Research Labs. Report TR-73-0098. K o s t e r , J.R. [1966] Ionospheric studies using the tracking beacon of the Early Bird synchronous satellite. Ann. de geophys., 22, 435-439. L a w r e n c e , R.S. and J e s p e r s o n , J.L. [1961] Refraction effects of large-scale ionospheric irregularities observed at Boulder, Colorado. Space Research II, Proc. 2nd Intern. Space Sci. Symposium, 227. L it t l e , C.G. [1954] High latitude ionospheric observations using extra-terrestrial radio waves. Proc. IRE, 42, 1700-1. L it t l e , C.G. and M a x w e l l , A. [1952] Scintillation of radio stars during aurorae and magnetic storms. J. Atmos. Terr. Phys., 2, 356. L i t v a , J. [1971] Observations of very long duration Travelling Ionospheric Disturbances. Paper presented at the URSI Spring Meeting, Washington, D.C. M a y n a r d , L.A. [1972a] L-Band ionospheric fading measurements at Ottawa. Paper presented at I M C O Panel of Experts for Maritime Satellites. M a y n a r d , L.A. [1972b] Ionospheric fading margins at 1550 MHz at Churchill. Paper presented at IMCO Panel of Experts for Maritime Satellites. M il l m a n , G.H. [1967] A survey of tropospheric, ionospheric and extra-terrestrial effects on radio propagation between the earth and space vehicles; Propagation factors in space communications, 3-55; Ed. W.T. Blackband, Technivision, Maiden head, England (A68-23070). O r t n e r , J ., H u l t q v is t , B ., B r o w n , R.R., H a r t z , T.R., H o l t , O ., L a n d m a r k , B ., H o o k , J .L . and L e in b a c h , H . [1962] Cosmic noise absorption accompanying geomagnetic storm sudden commencements. J. Geophys. Res., 67, 4169-4186. P a u l s o n , M .R. and H o p k in s , R.U.F. [1973] Effects of equatorial scintillation fading on satcom signals. Naval Electronics Laboratory Center, TR-1875, San Diego, Calif. Pope, J.H. and F ritz, R.B. [1970] Observation of simultaneous scintillation on VHF and S-band satellite transmissions at high latitudes. NOAA, TR-ERL-207-OD 6. P r e d d e y , G.F. [1969] Mid-latitude radio-satellite scintillation: the variation with latitude. Planet, and Space Science, 17,1557-1561. R a w e r , K. [1964] Erforschung der Ionosphare mit Radiowellen von Satelliten und Raketen; Phasenverfahren (Investigation of the ionosphere with radio waves from satellites and rockets; phase methods). Space Science Review, 3, 380. R e d d i , C.R. and R a o , B.R. [1971] Characteristics of travelling ionospheric perturbations over Walfaire using phase path technique. / . Atmos. Terr. Phys., 33, 251-266. S e ss io n s , W.B. [1972] Amplitude fading of simultaneous transionospheric L-band and VHF signals received at the geomagnetic equator. NASA X-810-72-282, Goddard Space Flight Center, NASA. S k in n e r , T.S. and K e l l e h e r , R.F. [1971] Scintillation fading of signals within SHF band. Nature (Physical Science), 232, 19-21. T a u r , R.R. [1973a] Ionospheric scintillations at 4 and 6 GHz. COMSAT Technical Review, 3 (1). Taur, R.R. [1973b] Simultaneous measurement of ionospheric scintillation at 1-5 and 4 GHz. Paper presented at USNC/URSI Meeting, 21-24 August. — 233 — Rep. 263-3 V a l v e r d e , J.F. [1958] Motions of large-scale travelling disturbances determined from high-frequency backscatter and vertical incidence records. Sci. Rept. 1, Radio Propagation Laboratory, Stanford Electronic Laboratories, Stanford, California. V it k e v ic h , V.V. [1958] Investigation of ionospheric inhomogeneities by radioastronomical methods. Radioteknika i Elektronika 3, 478-486. W h it n e y , H.E., A a r o n s , J., A l l e n , R.S. and S e e m a n n , D.R. [1972] Estimation of the cumulative amplitude probability distri bution function of ionospheric scintillations. Radio Science, 7, 1095-1104. W h it n e y , H.E. and R i n g , W.F. [1971] Dependency of scintillation fading of oppositely polarized VHF signals. IEEE Trans. Ant. Prop., AP-19. W i l d , J.P., S h e r id a n , K.V. and N e y l a n , A.A. [1959] An investigation of the speed of the solar disturbances responsible for Type III radio bursts. Aust. J. Phys., 12, 369-398. Sunset Sunrise Midnight F ig u r e 1 Night-time picture o f scintillation occurrence. The density of matching is proportional to the occurrence o f deep fading Rep.263-3 Absorption (dB) lr Lcl time Local Flare Hypothetical model showing polar cap absorption following a following absorption cap polar showing model Hypothetical major solar flare as expected to be observed on riometers on observed be to expected as flare solar major B: High latitudes - equal period o f day and night zone and day f Auroral o C: period equal - latitudes High B: A: High latitudes - 24 hours o f daylight f o hours 24 - latitudes High A: tapoiaey3 z H M 30 approximately at 24 — 234 — F gure r u ig Day 2 Altitude (kilometres) oopei ag rosa 0 , n-a rnmsinpt [ila, 1967] [Millman, path transmission one-way z, H M 200 at errors range Ionospheric ag ro (metres) error Range E = elevation angle elevation = E Range error (feet) error Range 25 — 235 — F gure r u ig night-time daytime 3 Altitude (nautical miles) Rep.263-3 Rep.263-3 Frequency (MHz) Pulse shape distortion for propagation through the ionosphere, the through propagation for distortion shape Pulse n-a rnmsinpt [ila, 1967] [Millman, path transmission one-way J esr o us dgaain = degradation pulse of Measure N d = 5 x 1013 electrons/cm2 x 50 = dj t = pulse duration pulse = 26 — 236 — F gure r u ig 4 a — 237 — Rep. 342-2 REPORT 342-2 * RADIO NOISE WITHIN AND ABOVE THE IONOSPHERE ** (Question 7-1/6) (1966 - 1970 - 1974) 1. Introduction Radio noise levels, observable at a satellite in the ionosphere, depend on the properties of the medium surrounding the spacecraft in several different ways. The local ionosphere controls the antenna impedance and the propagation conditions for radio waves in the vicinity of the satellite, in such a way as to determine the effective angular aperture of the antenna for radiation from distant sources. Local ionospheric conditions also enter into the mechanisms for noise generation near the spacecraft. Accordingly, ionospheric influences are often of paramount importance in considerations of radio noise levels at satellite heights, and the present Report has been prepared with this fact in mind. In particular the reliability of some experimental results obtained with spacecraft is influenced by uncertainties about antenna impedance and the possible effects of shot noise from impinging charged particles. Thus the observable noise levels depend on the spacecraft location in the ionosphere or magnetosphere, and it is usually sufficient to use the geomagnetic L parameter [Mcllwain, 1961] for this, or the invariant magnetic latitude at the earth’s surface of the relevant L shell. This description of the radio noise bands will be in terms of magnetic field strength, electron number density and invariant latitude, and the observing frequency will be related to the characteristic frequencies of the local ionosphere. The different noise types, along with their properties and probable origins, can only be distinguished sensibly in this fashion. This Report covers mainly the frequency range of potential interest for radio services above the F-layer maximum for which ionospheric shielding from the Earth is desirable. Generally speaking, this includes the frequencies between about 10 kHz and 20 MHz. The factors discussed are also important for many research investigations using satellites in the ionosphere. Noise will also be a limiting factor in some services using frequencies which penetrate the ionosphere; in thi,s case, terrestrial noise and transmissions need to be considered as possible sources of interference. This Report does not discuss in any detail the sources of noise on frequencies much higher than the critical frequency, but a few comments on noise from terrestrial sources are included. 2. Cosmic noise Included here are galactic, and possibly extra-galactic, noise emissions which are observable above the limiting frequency for extraordinary wave propagation at the spacecraft. This limiting frequency is termed fxS and is defined by: f*S = [(/f/4)+/£]*+/J,/2 In the usual ionospheric terminology, this corresponds to the condition X — 1 — Y, where X — fftlf2, and Y = f Hlf\ / n (the plasma frequency) = (Ne2/4n2 e0m)1/2; f H(the electron gyro-frequency) = eBJlizm ; and / = the observing frequency. At frequencies above about 10 MHz, cosmic noise levels are fairly well established on the basis of measurements made with ground-based equipment [Andrew, 1966; Getmantsev et al., 1968]. Measurements have been made from a number of rockets and satellites at fixed frequencies below 10 MHz [Walsh et al., 1964; Huguenin et al., 1964; Alexander and Stone, 1965; Benediktov et al., 1965; * Adopted unanimously. ** Note. — The Director, C.C.I.R., is invited to thank the International Union for Radio Science (U.R.S.I.) for its recent contributions on this subject, and also to request further comments. Rep. 342-2 — 238 — Slysh, 1965 and 1966; Chapman, 1961], and fixed-frequency and sweep-frequency receiver measurements have been made from the Ariel, Alouette, isis, r a e , Zond and Luna satellites to determine the galactic noise spectrum [Smith, 1965; Hartz, 1964a; Hartz, 1969a; Alexander et al., 1969a; Hakura et al., 1969; Clark et al., 1970; Grigorieva and Slysh, 1970]. These results are consistent with those obtained by Ellis et al. [1962] with ground-based equipment during selected ionospheric conditions. Some variation of the noise level has been noted in the satellite and rocket results for different regions of the galaxy but, since the antenna aperture in most cases exceeded 2n steradians, the variations are not large and probably comparable with the experimental uncertainties. Some results of the measurements of cosmic noise in rockets and satellites are plotted in Fig. 1 in terms of effective noise temperatures, and some selected values from the smooth curve are listed in Table I. The more recent measurements from the r a e satellite [Alexander et al., 1969a] agree with the values shown in Fig. 1 within the limits of experimental error, as do the Imp 6 satellite measurements [Brown, 1973]. In the latter case results were obtained between 130 and 2600 kHz and the Table I values, for frequencies less than 0-6 MHz, are the minimum values for cosmic noise observed by that satellite. The maximum cosmic noise values measured were about 1-5 to 2-0 dB above those tabulated. These values can be converted to brightness by means of the relation brightness B = 2kTA/\2 W.m-2.Hz_1.sterad_1 where 1 is the wavelength (m) and k is Boltzmann’s constant (1-38 X 10-23 Joules/K). The flux density can also be derived from flux density S' = 4 tt k T J tf W.m-2.Hz-1. This relationship has been established on the assumption that typical antennae on spacecraft have effective apertures of 4n steradians, that they receive only one component of polarization, and that the noise is nearly randomly polarized. Flux densities are also listed in Table I. For comparison with values given in other reports in which the effective antenna noise factor is used, for example Report 322-1 on atmospheric noise, the results are listed also in this form in Table I. The values in the HF range agree with those given in Report 322-1 for cosmic noise within 2 dB. For the indicated frequency range and for a spacecraft antenna with a broad effective aperture [Hartz and Roger, 1964], the noise temperatures shown in Table I are the minimum galactic values that are observable within or above the ionosphere. In those cases where values significantly higher than this have been observed in this same frequency range, some additional source of noise or interference is to be sought. 3. Solar noise Radio noise emissions of solar origin have been observed sporadically at frequencies greater than fxS with various spacecraft [Hartz, 1964b;1 Hartz, 1969b; Hakura et al., 1969 and 1970; Alexander et al., 1969b; Slysh, 1967; Benediktov et al., 1968a; Grigorieva and Slysh, 1970; Haddock and Graedel, 1970]. Short-duration noise bursts have been observed consistently at frequencies as low as 200 kHz and, occasionally, down to 50 kHz or even lower, but some of the 20 kHz observations may be difficult to identify [Slysh, 1967; Dunkel et al., 1970; Mahtson et al., 1973; Fainberg et al., 1972]. At the higher frequencies, a clear association with ground observations of solar bursts can often be made. Long duration noise storms and type-IV events have also been observed with satellites; and the maximum noise level observed for solar events is approximately 10-16 W.m-2.Hz_1 [Hartz, 1969b]. 4. Ionospheric noise at VLF A number of measurements have been made with VLF receivers in rockets and satellites and the results show considerable variation both with time and with position of the spacecraft. In this section, consideration will be given to whistler-mode frequencies which are less than the lower of f H and f N at the spacecraft and greater than the local lower-hybrid resonancefi.H R - This latter frequency is defined by: — 239 — Rep. 342-2 f L H R ~ 1836 ( / y + / h ) m e f f where meff = (J^Aimp/nu)-1, in which Ai and m* are respectively the fractional abundance and mass of each ionic constituent, and m p is the proton mass. In general,/ l h r is less than 10 kHz for the main regions of the ionosphere. In the VLF band, the observed noise intensities vary by orders of magnitude and definitive trends are now beginning to emerge. Most of the reported values are for frequencies near 10 kHz, and range from peak values of about 10~n W.m_2.Hz_1 to less than 10-16W.m_2.Hz-1 [Oelberman, 1964; Leiphart et al., 1962; Gurnett, 1966; Jorgensen, 1968; Shawhan and Gurnett, 1968; Dunkel and Helliwell, 1969; Hughes et al., 1971; Gurnett and Frank, 1971]. Various authors have arrived at typical or average intensities of about 10~14 W.m_2.Hz_1, but this must be viewed with some caution because of the very large variance. It is now becoming clear that the highest values are found in a fairly restricted range of latitudes in association with the influx, into the upper atmosphere, of energetic charged particles from the solar wind and from the magnetosphere. The work of Gurnett [1966], Hughes and Kaiser [1971], Barrington et al. [1971], and others, indicates that, on the average, the most intense noise is found in the midday, afternoon and evening sectors of the Auroral Oval, where the average noise intensity is of the order 2 x l0~ 13 W.mr2.Hz_1. The noise is usually described as broad-band; it is often most intense at frequencies greater than f LHR and because of this has been termed ‘lower-hybrid-resonance noise’ by some authors [Brice and Smith, 1964; McEwen and Barrington, 1967]. However, there seems to be little reason to believe that this noise differs fundamentally from what some authors have termed ‘ VLF hiss’ [Gurnett, 1966], or what is now becoming known, at least at the higher latitudes, as ‘auroral hiss ’. The whistler mode noise is observed at frequencies well above 10 kHz, and appears to be continuous up to the lower limit off H and f N [Walsh et al., 1964; Harvey, 1968; Hartz, 1969a]. The noise intensity appears to drop steeply with increasing frequency [Jorgensen, 1968; Shawhan and Gurnett, 1968]. At frequencies greater than 100 kHz, the noise intensities have been found in the noon and evening portions of the Auroral Oval, where the average intensity is about 4 x 10~17 W.m_2.Hz_1 [Barrington et al., 1971; Laaspere et al., 1971]. At lower latitudes the values are of the order 10~19 W.m-2.Hz-1 or less. At 540 kHz a value of 1 -7 x 10-18 W.m-2.Hz-1 has been quoted as typical for auroral hiss [Laaspere et al., 1971]. However, at this frequency and at 200 kHz, peak values have been observed that exceed the typical auroral hiss levels by some three orders of magnitude. A one-to-one relationship appears to have been demonstrated between incoming electrons with energies of the order of 100 eV and the intensity of whistler-mode emissions [Hartz, 1971; Gurnett and Frank, 1971]. 5. Ionospheric noise at higher frequencies Walsh et al. [1964], observed unexpectedly high noise levels at 0-75, 1-225 and 2-0 MHz, during the course of a rocket flight, and were able to link these to certain properties of the ionosphere. In particular, they showed that intense noise enhancements occurred when the receiver frequency was between the local plasma frequency, /y , and the local upper-hybrid frequency, f T. This latter frequency is defined by the relation: f 2T= f 2N + f 2H Harvey [1965 and 1968] reported a simiiar observation for the Ariel II satellite, and showed that the intensity of the noise was related to the energetic particle flux in the inner Van Allen radiation belt. He reported, as well, that another anomalously intense noise band, also related to the energetic particle flux, was observable in the frequency range below the local plasma frequency and above the extraordinary wave cut-off frequency, fzS, at the satellite. fzS corresponds to the condition X = 1 + 7 in the usual ionospheric terminology; it is defined by the relation: fzS = [(/l/4 )+ /J]1/2_ /W2 Similar results have been reported by Hartz [1967 and 1969a] Burenin et al. [1968] Bauer and Stone [1968] and Gregory [1969 and 1971]. The more recent of these indicate that the greatest intensities occur at high latitudes, and that there is a relationship between the noise intensities and energetic particle influx. The anomalously high noise levels obtained by Benediktov et al. [1965] at 725 and 1525 kHz may also fall in one or both of these categories, and an association with the particle flux was noted in that Rep. 342-2 — 240 — case. The same may apply to the observations by Huguenin et al. [1964] at 700 kHz and by Slysh [1965, 1966 and 1967] at 20, 30 and 210 kHz. On the other hand, it is not clear that these observing frequencies occurred below the local f T and the emissions probably represent yet another noise type that may be more akin to ‘shot’ noise. Dunkel et al. [1970] report ‘broad-band’ and ‘highpass’ noise measure ments in the magnetosphere, at great distances from the earth, at frequencies well above the local f N and f T frequencies. Both noise types correlate with geomagnetic activity and may result from energetic charged particles. Benediktov et al. [1968a, 1968b and 1969] have reported similar results, at frequencies above 1 MHz, which show a clear correlation between noise intensities and the Kv index. Some observa tions from the Imp 6 satellite should be noted. Frankel [1973] has reported noise emissions, below about 300 kHz, that he identified as cyclotron-synchrotron noise generated by electrons in the Earth’s outer radiation belt. The intensity of such emissions would, in general, not be expected to exceed that of the cosmic noise at frequencies above about 2 MHz. The emissions reported by Frankel were variable with time and had an intensity proportional to /~ 2'8 between about 30 and 110 kHz; at 30 kHz the brightness exceeded 10-18 Wm-2 Hz-1 ster-1 during high Kp conditions. An additional noise band appears in a great many of the Alouette II and isis recordings at frequencies below fzS and above f H (although this latter limit is not always a firm one) in those situations where fzS is greater than f H [Hartz, 1967 and 1969a]. This frequency range will not support electro-magnetic wave propagation, and consequently the observations could perhaps be viewed as arising in a local source, for example, one involving the interaction of the satellite itself and the medium. It is difficult to arrive at estimates of the absolute intensities of the various types of noise included in this section because of uncertainties regarding the antenna impedance and the exact nature of the phenomenon that is being observed. Nevertheless, the reported values are several orders of magnitude above the expected cosmic noise level, and must be taken into account. 6. Interference of terrestrial origin Interference from ground-based HF transmitters can be received at a spacecraft at frequencies above the ionospheric penetration frequency, or critical frequency as it is more conventionally termed. This critical frequency varies with geographic region, time of day, season, and sunspot epoch; it may be as low as 0-5 MHz at night in certain latitudes, or greater than 15 MHz in equatorial regions by day. As a consequence, a receiver in a high altitude satellite can be subjected to strong interference from the ground within and above this frequency range. Measurements on such signals have shown them to exceed the cosmic noise level by more than 50 dB [Huguenin and Papagiannis, 1965]. Even in relatively narrow frequency gaps between terrestrial transmitters, the peak values of interference from atmos pherics may exceed the galactic level by significant amounts over major thunderstorm areas of the globe [Huguenin and Papagiannis, 1965; Horner, 1965; Aksenov et al., 1970; Herman et al., 1973]. A previous survey of noise from terrestrial sources has been made by Rawer [1967]. The Appleton-Hartree magneto-ionic theory predicts that interference from ground-based trans mitters can be received at a spacecraft orbiting above the F-layer peak when foF2 < j/2 f HF2 at frequencies between fzF2 and fzl if fzl > fzF2 where foF2 is the ordinary wave critical frequency of the F layer, f HF2 is the gyro-frequency at the peak of the F layer, fzF2 is the extraordinary wave critical frequency of the F layer and is given by: 1/2 fH F 2 fzF2 = p^ + (foF2)2 and where fzl is the z wave infinity at the spacecraft, corresponding to the condition where the index of refraction for the vertical incidence z mode of propagation becomes infinite and is given by fz i= f cos29 ]/!] — 241 — Rep. 342-2 where z refers to the lower branch extraordinary wave as described by the Appleton-Hartree equation [Budden, 1961] and: f N is the plasma frequency at the spacecraft, f H is the gyro-frequency at the spacecraft, fT=(PN+fi)% 0 is the angle between the vertical and the direction of the earth’s magnetic field. For the condition foF2 < ]/2 /# f 2> the main source of interference is the standard MF broadcast transmissions from 0-54 up to about 1-3 MHz [Hagg et al., 1969]. Such interference is fairly common in the winter months at night, over the region of the main F-layer trough [Muldrew, 1965]. The Appleton-Hartree theory also predicts interference from ground-based transmitters at frequencies less than fwl, where fwl is the whistler wave infinity at the spacecraft, corresponding to the condition where the vertical incidence whistler mode of propagation becomes infinite and is given by: Vi f 2 f 2 % fwl = T r- / ^ cos20 where w refers to the lower branch ordinary wave and / jv,/# ,/ r and 0 are defined previously. The highest frequency at which whistler mode breakthrough can be detected at the spacecraft is equal to : / n i f / n ^ fii and / h i f / h < / j v Interference propagating through the peak of the F layer via the whistler mode is the result of coupling of signals from the z mode to the whistler mode in the D and E regions of the ionosphere. Such coupling may also occur in the F region when foE < fHF2 (foEis the upper branch ordinary wave critical frequency of the E layer) but such coupling will be inefficient due to low collision frequency in the F region. The main sources of whistler mode interference on the ionograms obtained by the Alouette II and isis satellites are the Loran C pulse transmissions at 0 -1 MHz, long wave broadcast transmissions from 0 1 5 to 0-3 MHz and the standard MF broadcast transmissions from 0 -5 4 to about 1-3 MHz. A second type of man-made interference for satellite reception is now recognized. Because of the non-linear behaviour of the ionospheric plasma surrounding the spacecraft, discrete radio signals can give rise to beat frequencies or intermodulation products. The beat mechanism may involve various combinations of the frequencies of these discrete signals which may be radiated by the satellite itself and at frequencies harmonically related to such radiations, or which may occur at ionospheric resonance frequencies that are excited in some (non-linear) fashion by satellite emissions; the signals may even come from ground-based transmitters. Numerous examples of this form of interference appear in the Alouette and isis records at frequencies well below the ionospheric critical frequency. Some of these effects can perhaps be attributed to very oblique propagation from the ground at the observed frequency, or to beats produced in the receiving equipment itself. However, in many cases such explanations are implausible and it has been concluded that a significant portion of the observed beat frequencies is produced in the ionospheric plasma near the spacecraft [Barrington and Hartz, 1 9 6 8 and 1 9 6 9 ; Hagg, 1 9 6 7 ; Hartz and Barrington, 1 9 6 9 ]. Another subject that requires further consideration in this regard involves parametric processes in the ionospheric plasma. Considerable current interest is being devoted to this topic, and a separate study in this area might be appropriate [Tzoar, 19 6 9 ]. A third type of interference involves VLF emissions from the ground that can propagate to the satellite by way of the whistler mode, mainly at night [Bullough et al., 1971]. The observed signal intensities appear to depend on latitude, D-region absorption and F-region structure, but further studies should be made. Rep. 342-2 — 242 — 7. Resonances observed with ionospheric sounders Ionospheric sounders aboard rockets and satellites transmit short radio-wave pulses and record the subsequent response of the ionosphere with a radio receiver on the spacecraft [Knecht et al., 1961; Warren, 1962]. Discrete signals or resonances are observed which can persist from a fraction of a millisecond to many milliseconds after the termination of the transmitted pulse. The most frequent and predictable of these resonances occur when the operating frequency is at about the local plasma frequency, fa , the local upper-hybrid frequency, f T, and its harmonic 2f T, and harmonics of the local electron gyro-frequency, nfa [Lockwood, 1963; Calvert and Goe, 1963]. Other resonance responses can be found at lower frequencies that bear a fractional relationship to one or more of the principal resonances of the ambient plasma [Hartz and Barrington, 1969], at frequencies corresponding to the electrostatic or Bernstein resonances of the local medium [Warren and Hagg, 1968], and at a frequency that corresponds to 2fa at a location in the ionosphere remote from the spacecraft [Muldrew and Hagg, 1966]. At times, and notably during disturbed ionospheric conditions, a number of these resonances may be observed in the receiver recordings when the sounder transmitter is not operating [Hartz, 1969a; Mosier et al., 1973]. Resonances have also been observed when the pulse transmitter is fixed and the receiver is swept in frequency [Lockwood, 1973]. These various resonant responses may have to be considered in the design of electronic circuitry for spacecraft. Matuura and Nishizaki [1969] found pulse echoes, named proton cyclotron echoes, on the Alouette II ionograms which may be attributed to the interaction of repeated pulse signals emitted by the topside sounder with the ionospheric plasma surrounding the spacecraft. The delay time between the pulse signal emission and the reception of the pulse echoes is equal to the gyration time of protons around the magnetic field at the satellite position and independent of the frequency of the emitted waves. Many of the characteristics of the principal resonances can be explained by the theory of the propagation of electrostatic waves [McAfee, 1968, 1969a, 1969b, 1970; Muldrew, 1972a, 1972b]. The electrostatic waves launched obliquely at frequencies within several kilohertz of the plasma frequency or upper hybrid frequency propagate short distances from the spacecraft, are reflected by the ionized medium and returned to the satellite, producing a signal with a given delay. The delay of this signal is so extremely sensitive to frequency that the frequency spectrum of a transmitted pulse can produce echoes over a wide range of delays thus producing a long receiver response. Other resonances may be related to non-linear effects in the local plasma [Hartz and Barrington, 1969], to the radiation of sidebands of the transmitter pulse [Lockwood, 1973], and to the generation of electrostatic waves with possibly a subsequent conversion to and propagation of electromagnetic radiation in the case of some remote resonances. 8. Conclusion This Report deals mainly with noise at frequencies below the critical frequency of the ionosphere, and, although cosmic noise values can be extrapolated to higher frequencies [see C.C.I.R. Report 322-1 and Andrew, 1966], noise from lightning discharges and man-made sources on the ground will need to be taken into account, probably up to a frequency of approximately 500 MHz, depending on the antenna characteristics. Noise arising from the non-linear properties of the plasma needs much more study. Measurements of noise being made at the present time in various satellites should provide useful additional data. Consideration should be given to turbulence generated in the ionosphere plasma by the vehicles moving through it. In general, the subject of kinetics of plasmas and the generation of noise should be investigated more extensively and more carefully. Note. — Contributions from U.R.S.I. have been used in the preparation of this Report. — 243 — Rep. 342-2 R eferences A k s e n o v , V.I., Dubinskii, B.A., Z h e k u lin , L.A., K is e le v a , Z.Y., L ish in , I.V., M a k a r o v , V.A. and M ikhailov, L.N. [1970] Investigation of transmission of ultralong radio waves through the Earth’s ionosphere: preliminary results from the Cosmos 142 satellite. J. Cosmic Research (U.S.S.R.), 8, 571 (in Russian). Alexander, J.K. and S to n e , R.G. 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B.M. McCormac), 336, D. Reidel Publ. Co., Dordrecht, Holland. H u g h e s , A.R.W., K a is e r , T.R. and Bullough, K. [1971] The frequency of occurrence of V L F radio emissions at high latitudes. Space Research XI, 1323. H u g u e n in , G.R., L i ll e y , A.E., McDonough, W.H. and Papagiannis, M.D. [1964] Measurements of radio noise at 0-7 and 2 0 Mc/s from a high-altitude rocket probe. Planet, and Space Science, 12, 1157. H u g u e n in , G.R. and Papagiannis, M.D. [1965] Spaceborne observations of radio noise from 0 -7 to 7 0 MHz and their dependence on the terrestrial environment. Ann. Astrophys., 28, 239. J o r g e n s e n , T.S. [1968] Interpretation of auroral hiss measured on OGO 2 and at Byrd Station in terms of incoherent Cerenkov radiation. J. Geophys. Res., 73, 1055. K n e c h t , R.W., V a n z a n d t , T.E. and R u s s e l, S. [1961] First pulsed radio soundings of the top side of the ionosphere. J. Geophys. Res., 66, 3078-3081. 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M u ld r e w , D.B. [1965] F layer ionization troughs deduced from Alouette data. J. Geophys. Res., 70, 2635. M u ld r e w , D.B. [1972a] The electrostatic resonances associated with the maximum frequencies of cyclotron harmonic waves. / . Geophys. Res., 77, 1794-1801. M u ld r e w , D.B. [1972b] Electron resonances observed with top side sounders. Radio Science, 7, 779-789. M u ld r e w , D.B. and H a g g , E.L. [1966] A novel ionospheric cyclotron resonance phenomenon observed on Alouette I data. Can. J. Phys., 44, 925. Oelberman, EJ. [1964] Expected radio noise levels in the upper atmosphere from 100 cps to 100 kMc/s. IEEE International Convention Record, 12, Part 6, 331. R a w e r , K. [1967] Noise produced by terrestrial sources in the near-earth space, in “ Propagation factors in space communications ”, ed. W.T. Blackband, 383-408 — McKay and Co., London. S h a w h a n , S.D. and G u r n e t t , D.A. [1968] VLF electric and magnetic fields observed with the Javelin 8-45 rocket experiment. J. Geophys. Res., 73, 5649. S ly s h , V.E. [1965] The measurement of cosmic radiation on frequencies 210 and 2200 kHz at heights to 8 earth radii on Zond 2 (in Russian). J. Cosmic Research (U.S.S.R.), 3, 760. S ly s h , V.E. [1966] Measurements of kilometre radio-emission in inter-planetary space (in Russian). J. Cosmic Research (U.S.S.R.), 6, 923. S ly s h , V.E. [1967] Solar radio emission at 2 and 0-2 MHz (in Russian). Astronomical J., 44, 94. S m ith , F.G. [1965] Cosmic radio noise as measured in the satellite Ariel II; Part II, Analysis of the observed sky brightness. Mon. Not. R. Astr. Soc., 131, 145. T z o a r , N. [1969] Parametric excitations in plasma in a magnetic field. Phys. Rev., 178, 356. W a ls h , D., H a d d o c k , F.T. and S h u lt e , H.F. [1964] Cosmic radio intensities at 1-225 and 2-0 Mc/s measured up to an altitude of 1700 km. Space Research IV (Ed. P. Muller), North-Holland Publ. Co., Amsterdam, 935. W a r r e n , E.S. [1962] Sweep frequency radiation sounding of the top side of the ionosphere. Can. J. Phys., 40, 1692. W a r r e n , E.S. and H a g g , E.L. [1968] Observation of electrostatic resonances of the ionospheric plasma. Nature, 220, 466. T a b le I Cosmic background noise observed with receivers in spacecraft / T Power flux-density in one Effective antenna (MHz) (10« K) polarization component noise factor 0 (10_21W.m_2.Hz_1) (dB) 0-2 2-4 018 39 0-4 14 4-3 47 0-6 21 15 49 0-8 21 26 49 1-0 19 37 48 2-0 9 69 45 3-0 5 0 86 42 50 1-8 86 38 100 0-42 80 31 C1) Reference temperature 288 K. Rep.342-2 Temperature (K) f i empeat e of enna rcii gal i ise o n tic c la a g g receivin a n n te n a n a f o re tu era p m te e tiv c ffe E Walsh h s l a W * Hugueni in n e u g u H A S v to ik d e n e B * ■ 0 C + t 1 A o * B s i l l E S exander r e d n a x le A A th it m h s y l n a m p a h ouette lo louette n w o r 26 — 246 — , , , , al., a t e F 1965 1965 1965 al., a t e gure r u ig 1973 , , , II I 1961 al., a t e d n a 1962 d n a al., a t e 1 1964 1965 , e n o t S 1966 1964 1965 — 247 — Q. 2-2/6, S.P. 2A-2/6 QUESTIONS AND STUDY PROGRAMMES, DECISIONS, RESOLUTIONS AND OPINIONS QUESTION 2-2/6 GEOGRAPHIC DISTRIBUTION AND PROGRAMME OF REGULAR IONOSPHERIC OBSERVATIONS (1965 - 1970 - 1974) The C.C.I.R., CONSIDERING that ionospheric propagation predictions can be improved by additional information to the observational data base for numerical mapping and short-term synoptic studies; unanimously decides that the following question should be studied: what are the optimum ionospheric observing programmes for: — long-term predictions; — short-term predictions (from a few hours to a few days in advance) having in mind ground-based ionosondes, back-scatter sounders, specialized satellites and other appro priate techniques ? STUDY PROGRAMME 2A-2/6 IMPROVEMENT IN THE WORLD-WIDE IONOSPHERIC OBSERVING PROGRAMME FOR NUMERICAL MAPPING PURPOSES (1965 - 1966 - 1970 - 1974) The C.C.I.R., . considering (a) that the present world-wide network of ground-based ionosondes, operating regularly and participating in the world-wide interchange of data, is far from ideal from the viewpoint of ionospheric propagation predictions; (b) that regular satellite observations could be used in ionospheric numerical mapping, but: — at present, satellite soundings are not conducted often enough in certain regions to permit the deduc tions of values of foF2 and hmax with a sufficient degree of accuracy; — at present, the number of telemetry stations for satellites is seriously limited; (c) that other regular ionospheric observations could be used in ionospheric numerical mapping; unanimously decides that the following studies should be carried out: 1. the identification of geographical areas within which, for both long- and short-term predictions: S.P. 2A-2/6, Q. 3-2/6, 4-1/6 — 248 — — the installation of at least one fixed ionosonde would provide a significant and necessary improvement; — the actual density of fixed ionosondes working on a routine basis is more than adequate; 2. the practicability of incorporating, in the work of numerical mapping, ionospheric observations from satellites, when they are made on a regular basis: — by comparing soundings from above, within and below the ionosphere, to determine whether any systematic errors are present due to interference, horizontal electron-density gradients, or non-vertical paths; — by identifying regions where additional telemetry stations would make improved coverage possible; 3. the possibility of improvement in the work of numerical mapping by incorporating ionospheric data derived from observations of airglow, observations from oblique incidence sounders, (including highly directional back-scatter sounders), and other techniques when they are made on a regular basis. Note. — The Director, C.C.I.R., is requested to transmit this text to the International Union of Radio Science (U.R.S.I.) and the Special Committee for Solar-terrestrial Physics (SCOSTEP) for comment. QUESTION 3-2/6 SIDE-SCATTER RESULTING FROM GROUND OR IONOSPHERIC IRREGULARITIES (1965 - 1966 - 1970 - 1974) The C.C.I.R. unanimously decides that the following question should be studied: what is the effect in operational situations of side-scatter resulting from ground or ionospheric irregularities ? QUESTION 4-1/6 PROPAGATION BY WAY OF SPORADIC E AND OTHER ANOMALOUS IONIZATION (1965 - 1974) The C.C.I.R., CONSIDERING that propagation by way of sporadic E and other anomalous ionization is not taken into account in regular ionospheric predictions and gives rise at times to serious interference to HF and VHF radiocommunication services; — 249 — Q. 4-1/6, S.P. 4A-2/6, 4B-2/6 unanimously decides that the following question should be studied: 1. what are the properties of sporadic E and other anomalous ionization in the ionosphere which affect propagation in ways that are important to radiocommunication services; 1.1 can useful diurnal, seasonal and annual variations of these properties be determined; 1.2 can useful short-term forecasts of these properties be made ? STUDY PROGRAMME 4A-2/6 PREDICTION OF SPORADIC E (1965 - 1970 - 1974) ♦ The C.C.I.R., CONSIDERING (a) that a major factor in the prediction of ionospheric propagation conditions is the occurrence of sporadic E; (b) that the routine scaling and statistical analysis of ionosonde observations of sporadic E, while extensive and useful from certain viewpoints, have not led to the development of techniques for reliable prediction of sporadic E; (c) that fruitful approaches to the short-term prediction of sporadic E are likely to come through improved understanding of the physical mechanisms responsible for sporadic E; unanimously decides that the following studies should be carried out: determination of methods of predicting the occurrence of the various kinds of sporadic E and the relative importance of each to radiocommunications. Note. — The Director, C.C.I.R., is invited to thank the International Union for Radio Science (U.R.S.I.) for its recent contribution on this subject and also to request further comments. STUDY PROGRAMME 4B-2/6 PROPAGATION BY WAY OF SPORADIC E AND OTHER ANOMALOUS IONIZATION (1959 - 1963 - 1966 - 1970 - 1974) The C.C.I.R., CONSIDERING (a) that the available information on propagation by sporadic E and other abnormal ionization is insufficient to provide statistical data of the type needed by telecommunication engineers; S.P. 4B-2/6, Q. 5-2/6 — 250 — (b) that the application of vertical incidence Es data, obtained from ionosondes, to oblique incidence field strengths is difficult to accomplish at present; unanimously decides that the following studies should be carried out: 1. the dependence of the field strength or transmission loss of waves propagated by sporadic E and anomalous modes in the ionosphere upon: — characteristics of the terminal equipment; — modes of propagation; — geographic region; — length of the propagation path; — operating frequency; — time of day, season and solar activity; 2. a comparison, where possible, of the results obtained in § 1 with data from ionosondes (for example, fbEs, foEs and the occurrence of spread echoes) and from satellite beacons; 3. the determination of the elevation and azimuthal angles of arrival of the signalspropagated by the various anomalous modes; 4. the preparation of simple world-wide and regional charts, both numerical and graphical, of received signal level relative to free space, or of transmission loss, at suitable frequencies for those anomalous modes which are found to be significant. QUESTION 5-2/6 PROPAGATION BETWEEN STATIONS BELOW THE IONOSPHERE BY DUCTING ABOVE THE IONIZATION MAXIMUM OF THE F REGION (1965 - 1966 - 1970 - 1974) The C.C.I.R., CONSIDERING that interference to HF and VHF radiocommunications can result from ducting by field-aligned ionospheric irregularities, especially at the lower magnetic latitudes; unanimously decides that the following question should be studied: how and under what circumstances can such propagation be important to radiocommunication? — 251 — Q. 6-2/6, S.P. 6A/6 QUESTION 6-2/6 SPECIAL PROBLEMS OF HF RADIOCOMMUNICATIONS ASSOCIATED WITH THE EQUATORIAL IONOSPHERE (1963 - 1966 - 1970 - 1974) The C.C.I.R., CONSIDERING that HF radiocommunications are known to be influenced by special propagation conditions in the equatorial ionosphere; unanimously decides that the following question should be studied: what are the properties of the ionosphere at or near the magnetic equator which have a marked influence on HF radiocommunications ? STUDY PROGRAMME 6A/6 SPECIAL PROBLEMS OF HF RADIOCOMMUNICATIONS ASSOCIATED WITH THE EQUATORIAL IONOSPHERE (1974) The C.C.I.R. unanimously decides that the following studies should be carried out: 1, the variations of the physical properties and structure of the ionosphere at or near the magnetic equator which have an influence on HF radiocommunications and the dependence of such variations on: — geographic location, — time of day, — season of the year, — solar and magnetic activity, — thunderstorm activity; 2. the influence on HF radiocommunications at or near the magnetic equator by the choice of: — operating frequency, — class and bandwidth of emission, — antenna characteristics, — orientation and length of propagation path. Q. 7-1/6, S.P. 7A/6 — 252 — QUESTION 7-1/6 RADIO NOISE (1965 - 1974) The C.C.I.R, CONSIDERING (a) that radio noise of natural or man-made origin often determines the practical limit of performance for radio systems and thus is an important factor in planning efficient use of the spectrum; (b) that much has been learned about the origin, character and general levels of both natural and man-made radio noise, but that more information is needed for planning of telecommunication systems, particularly about “wide-band” characteristics of man-made noise, directions of arrival of atmospheric noise, other noise originating in the troposphere and improved predictions for situations where direct measurements are unavailable; (c) that uniform methods of measurement, of presentation of results of measurement and of prediction, are needed to realize the greatest advantage from such information; (d) that, for the determination of system performance and spectrum utilization factors, it is essential to specify the noise parameters described in Reports 322-1 and 413, namely, power spectral density, amplitude probability distribution, duration and spacing of pulses; (e) that effective techniques, as described in U.R.S.I. Special Report No. 7, § 8 of Report 413, and references therein, exist for measuring statistically those various parameters; unanimously decides that the following question should be studied: what are the characteristic levels, temporal and geographical variations, and directions of arrival of atmospheric radio noise or of noise of galactic or ionospheric origin, or from man-made sources, at frequencies below about 50 MHz ? Note 1. — Other radio noise studies are within the competence of Study Groups 1 and 5 (see Question 46/1). Note 2. — The text of this Question should be communicated to Study Group 1 for information. STUDY PROGRAMME 7A/6 RADIO NOISE WITHIN AND ABOVE THE IONOSPHERE (1974) The C.C.I.R. unanimously decides that the following studies should be carried out: 1. the characteristics of radio noise at frequencies below about 50 MHz received in a spacecraft, within or above the ionosphere, such as: — galactic and extragalactic noise (cosmic noise), — 253 — S.P. 7A/6, 7B/6 — solar noise, — planetary and interplanetary space noise, — ionospheric noise, generated within the ionosphere and magnetosphere, — atmospheric and man-made noise of terrestrial origin; 2. techniques and methods that are available or may be developed for prediction of levels of radio noise received in spacecraft. Note. — The Director, C.C.I.R., is invited to thank the International Union for Radio Science (U.R.S.I.) for its recent contribution on this subject and also to request further comments. STUDY PROGRAMME 7B/6 MEASUREMENT OF ATMOSPHERIC RADIO NOISE CAUSED BY LIGHTNING (1951 - 1953 - 1956 - 1959 - 1963 - 1970 - 1974) The C.C.I.R., CONSIDERING (a) that information on atmospheric noise in Report 322-1 (published separately) is available for provisional use; (b) that this Report now contains information on the short-term amplitude probability distribution of noise and on prediction uncertainties, in addition to improved information on the noise-power distribution over the world, as received on a short vertical grounded antenna; (c) that these and other characteristics of the noise are known to be important in determining the interference to radio services; , (d) that additional measurements are required for further revision of this Report and to extend its scope; (e) that a knowledge of the distribution of lightning discharges, the power radiated by them and the influence of propagation is valuable in estimating the intensity and properties of radio noise; unanimously decides that the following studies should be carried out: 1. the world distribution of noise power, either directly or by deduction from other characteristics, with the existing or an augmented network of stations; 2. the other characteristics of the noise already described in Report 322-1; 3. the measurement, at stations with suitable facilities, of atmospheric noise on typical telecommunication antennae having various directional and polarization characteristics, and the correlation of the results with information on the distribution of thunderstorms; 4. the frequency of occurrence of lightning discharges throughout the world: — by the use of networks of counters designed to record local discharges; — by the use of direction-finding networks designed to locate thunderstorms at a distance; — by the analysis of other related ground-level measurements, e.g. those involving the use of radar equipment; — by the use of satellite sensors of the electromagnetic radiation from lightning; — by the examination of satellite cloud photographs; S.P. 7B/6, 7C/6 — 254 — 5. the intensity and nature of the nois e from individual lightning discharges and the influence of propagation; 6. the distribution, over short periods of time for which the statistics are stationary, of the duration of noise impulses and of the intervals between them; these distributions should be measured as a function of threshold; 7. the power spectrum, or the corresponding autocorrelation function, of the waveform of the noise envelope, and the higher order probability distributions, for example those describing the probability that specified envelope voltages will be present on two successive occasions separated by a known small time interval; 8. the further development of methods of using statistical data on the characteristics of noise, in addition to its mean power, for use in assessing the interference to radio services; 9. the relative importance of atmospheric noise, as compared with other types of interference, as a limiting factor in radiocommunication. Note. — The Director, C.C.I.R., is invited to thank the International Union for Radio Science (U.R.S.I.) for its recent contribution on this subject and also to request further comments. STUDY PROGRAMME 7C/6 MAN-MADE RADIO NOISE (1959 - 1970 - 1974) The C.C.I.R., CONSIDERING (a) that knowledge of the properties of additive, undesired radiation from man-made sources is useful for efficient spectrum utilization; (b) that the majority of previous man-made noise measurements have pertained to individual sources, the principal objective being the reduction in noise rather than a determination of the composite effect; (c) that the results of previous work have been difficult to compare, in part due to technological restraints and to the differing objectives of the individual service Study Groups; (d) that there is evidence that man-mdde radio noise may be the limiting factor in the reception of radio signals over a wide frequency range; unanimously decides that the following studies should be carried out: 1. the measurement of the statistical characteristics of composite man-made radio noise at frequencies below about 50 MHz; 2. the investigation of the difference in these characteristics when observed on a variety of antennae; 3. the investigation of methods for inferring the characteristics of man-made noise that would be received on a variety of antennae when data are obtained by the use of only one particular type of antenna; 4. the investigation of the correlation of man-made noise characteristics with urbanization (i.e., population density, industrial activity, electrical power distribution and consumption, vehicular traffic, etc.); 5. the investigation of the variability of such characteristics as a function of geographical location, time and frequency. Note. — The Director, C.C.I.R., is requested to transmit this text to the International Union for Radio Science (U.R.S.I.) for comment and to Study Group 1 for information. — 255 — Q. 8/6, S.P. 8A-1/6 QUESTION 8/6 * CHOICE AND PREDICTION OF LONG-TERM BASIC INDICES FOR IONOSPHERIC PROPAGATION (1974) The C.C.I.R., CONSIDERING (a) that accurate, quantitative predictions of ionospheric variations will permit more efficient utilization of radio frequencies and increase the reliability of radiocommunication services; (b) that recommendations of certain basic indices for use in long-term ionospheric predictions are given in Recommendation 371-2; unanimously decides that the following question should be studied: 1. what solar and ionospheric indices can be devised for long-term ionospheric predictions that would be preferable to those indicated in Recommendation 371-2; 2. what are the best methods and means for predicting these indices ? STUDY PROGRAMME 8A-1/6 LONG-TERM PREDICTIONS OF SOLAR INDICES (1956 - 1963 - 1974) The C.C.I.R., CONSIDERING that long-term predictions of indices of solar activity are essential for practical use of ionospheric predictions; unanimously decides that the following studies should be carried out: 1. comparison of recent solar index predictions, as made by all published autocorrelation or quasi autocorrelation methods, with one another and with subsequent observations; these comparisons should be kept up to date; 2. detailed examination of those combinations of autocorrelation, empirical, and other methods which may possibly yield more accurate predictions. * This Question replaces Question 1-1/6. S.P. 9A-1/6, Q. 10/6 — 256 — STUDY PROGRAMME 9A-1/6 * BASIC PREDICTION INFORMATION FOR IONOSPHERIC PROPAGATION (1953 - 1959 - 1963 - 1966 - 1974) The C.C.I.R., CONSIDERING that basic ionospheric predictions are never more than approximations to the state of the ionosphere, and, therefore, substantial extension and improvement are desirable; unanimously decides that the following studies should be carried out to determine: 1. the statistics of the day-to-day variation of both the operational MUF (MUF) and the standard MUF (EJF) in terms of season, solar cycle, and location, for introduction of appropriate adjustments in monthly predictions, and for analysis of the differences between MUF and EJF from hour-to-hour and from day-to-day; 2. the possibility of improving the present methods for predicting vertical incidence data and for calculating EJF from predicted vertical-incidence data; 3. the importance of the various ionospheric layers and modes of propagation for radiocommunications; 4. the possible usefulness of ray-tracing techniques and improved models for the ionospheric layers in obtaining better oblique-incidence predictions from vertical-incidence predictions. QUESTION 10/6** SHORT-TERM PREDICTIONS OF OPERATIONAL PARAMETERS FOR IONOSPHERIC RADIOCOMMUNICATIONS (1974) The C.C.I.R., CONSIDERING (a) that accurate, quantitative short-term predictions of ionospheric variations a few hours or days in advance would permit more efficient utilization of radio frequencies and increase the reliability of radiocommunication services; (b) that, in addition to the widespread disturbances associated with major geophysical or solar events, there are other hour-to-hour and day-to-day ionospheric variations (which may be local in influence) whose effects on operational characteristics, such as MUF and those associated with attenuation, fading, multipath interference and atmospheric noise, cannot be predicted by well established techniques; unanimously decides that the following question should be studied: what are the needs and techniques for the short-term prediction (a few hours or days in advance) of operational parameters for ionospheric radiocommunications? * This Study Programme does not arise from any Question under study. ** This Question replaces Question 22/6. — 257 — S.P. 10A-1/6, Q. 11-1/6 STUDY PROGRAMME 10A-1/6 IDENTIFICATION OF INDICATORS AND PRECURSORS FOR SHORT-TERM VARIATIONS AND DISTURBANCES OF THE IONOSPHERE AND RADIO CIRCUITS (1953 - 1956 - 1963 - 1966 - 1974) The C.C.I.R., CONSIDERING that the performance of radio circuits can be affected by short-term disturbances in the ionosphere; unanimously decides that the following studies should be carried out: 1. investigation of the practicability of: — selecting particular kinds of solar observations or observations of geophysical phenomena which can be made objectively and which may be usefully employed for short-term predictions of ionospheric propagation conditions; — defining an objective scale of the importance of sudden ionospheric disturbances; • — describing ionospheric disturbances in terms comparable with the forecasts; — establishing a common method for an adequate dynamic description of ionospheric disturbances, for use in forecasting and verification, and including indices, for describing the intensity of ionospheric disturbances as a whole, or in each of a series of equal short periods; 2. investigation of the relationship between the characteristics of a disturbance, as defined and described above, and the expected performance of radio circuits of various kinds. QUESTION 11-1/6 SKY-WAVE FIELD STRENGTH AND TRANSMISSION LOSS AT FREQUENCIES ABOVE 1 5 MHz (1970 - 1974) The C.C.I.R. ft unanimously decides that the following question should be studied: what is the best method of calculating the sky-wave field strength and transmission loss at frequencies above 1*5 MHz? S.P. 11 A-2/6 — 258 — STUDY PROGRAMME 11 A-2/6 ESTIMATION OF SKY-WAYE FIELD STRENGTH AND TRANSMISSION LOSS AT FREQUENCIES ABOVE 15 MHz (1951 - 1953 - 1956 - 1959 - 1963 - 1966 - 1970- 1974) The C.C.I.R., CONSIDERING that the interim method for estimating sky-wave field strength or transmission loss as developed by Interim Working Party 6/1 (Report 252-2), “ C.C.I.R. interim method for estimating sky-wave field strength and transmission loss at frequencies between the approximate limits of 2 and 30 MHz”, may not always be sufficiently complete or accurate; unanimously decides that the following studies should be carried out: 1. ionospheric absorption to obtain quantitative information on the geographic, temporal and frequency variations, its day-to-day variability and its relation to solar-activity indices; 2. transmission loss associated with operating frequencies near and above the standard M UF; 3. ray-path geometry, including the role of ionospheric gradients, off-great-circle propagation, focusing and defocusing, differences in angle of arrival and departure of the radio wave and long-distance propa gation without intermediate ground reflection; 4. improved ionospheric models to assist in better determination of the active modes; 5. the effects of anomalous ionization, like sporadic E and spread F, on the ray-path geometry, absorption and scattering; 6. earth reflection losses in the course of multihop ionospheric propagation, with emphasis on: — the relative importance of the scattering mechanism and Fresnel reflections under various conditions; — the effects of extreme types of terrain, or terrain cover, such as rain forest, mountains, ice and tundra; 7. day-to-day variations in hourly median signal levels and standard MUF for various modes, frequencies and geographical location of circuits, and for normal and disturbed conditions; 8. the role of polarization of the transmitted and received signals taking account of the magneto-ionic properties of the ionosphere; 9. system characteristics, such as the antenna and diversity systems to improve the quality of received signals; 10. the reliability of the interim method, particularly at long distances. o — 259 — S.P. 12A-1/6,14A-1/6 STUDY PROGRAMME 12A-1/6 * IONOSPHERIC SOUNDING AT OBLIQUE INCIDENCE (1965 - 1966 - 1974) The C.C.I.R., CONSIDERING that the use of oblique-incidence transmissions as, for example, with oblique-incidence sounders, would be of great assistance in the study of many problems of concern to the C.C.I.R., especially when supported by associated field strength, direction-of-arrival and delay time measurements; unanimously decides that the following studies should be carried out: 1. various modes of propagation over oblique paths and their frequency limits in relation to: — the standard MUF (EJF) calculated for the various modes; — the MUF for circuits operating simultaneously on the same path; 2. measurement of transmission loss and its statistical distribution compared to free space, or transmission loss referring to each mode, for comparison with observed loss on operating circuits; 3. off-great-circle modes of propagation, as indicated by direction-of-arrival measurements for individual modes; 4. single hop Pedersen-ray propagation at various distances beyond 4000 km and in various parts of the world; 5. the influence of sporadic-E reflection, F-region spread echoes and abnormal absorption on oblique soundings in polar, temperate and equatorial latitudes; 6. measurements of ionospheric and radio propagation parameters as required to obtain data needed by Interim Working Party 6/1 for its high-frequency sky-wave field strength and transmission-loss studies; 7. the relation of oblique soundings to vertical-incidence soundings at appropriate points along the trans mission path. STUDY PROGRAMME 14A-1/6 * BACK-SCATTERING (1951 - 1953 - 1956 - 1959 - 1963 - 1966 - 1974) The C.C.I.R., CONSIDERING (a) that ionospherically propagated back-scatter echoes pan be used by radar for such purposes as observing sea state conditions and aspects of rocket launching; * This Study Programme does not arise from any Question under study. S.P. 14A-1/6, Q. 16-1/6 — 260 — (b) that in addition this technique offers a method of observing and using the propagation capabilities of the ionosphere over a wide area from a single site; unanimously decides that the following studies should be carried out: 1. identification of various back-scatter sources on the ground or in the ionosphere; 2. investigation, by back-scatter technique, of the formation and movement of localized areas of sporadic E ; 3. application of back-scatter techniques for both fixed and swept frequency operation, to supplement the information obtained’by oblique-incidence soundings; 4. determination by field-strength measurements of the back-scattering coefficient as a function of frequency, the nature of the scattering source, and the angle of incidence at the scattering source; 5. determination, by back-scatter measurements of actual propagation conditions, of unusual types of propagation such as the persistency of long range echoes, and of focusing effects and the characteristics of ionospheric irregularities, including the relationship to acoustic and gravity waves in the ionosphere; 6. determination of antenna characteristics appropriate for back-scatter measurements. QUESTION 16-1/6 IONOSPHERIC PROPAGATION CHARACTERISTICS PERTINENT TO RADIOCOMMUNICATION SYSTEMS DESIGN (1970 - 1974) The C.C.I.R., CONSIDERING that the telecommunications engineer, concerned with the design and operation of systems, has a need to interpret ionospheric propagation information in radiocommunication terms; unanimously decides that the following question should be studied: what are the values of the ionospheric propagation parameters, such as: — fading, — the channel transfer function, — diversity (space, time, frequency and polarization), that are pertinent to the design of radiocommunication systems? \ — 261 — S.P. 16A-2/6 STUDY PROGRAMME 16A-2/6 IONOSPHERIC PROPAGATION CHARACTERISTICS PERTINENT TO RADIOCOMMUNICATION SYSTEMS DESIGN (1966 - 1970 - 1974) The C.C.I.R., CONSIDERING (a) that the practical requirements of radiocommunication necessitate information, not only on the median value of the received signal strength, but also on: — the amplitude distribution, — the rapidity of variations, — the differential fading with diversity system antennae of the space, frequency or polarization type; (b) that it has become common practice to distinguish: — rapid variations; — much slower variations (i.e. hour-to-hour or day-to-day changes); — regular variations with time of day, season and solar activity; (c) that the presence of various modes and paths, which provide the component signals at the receiving antenna, significantly affects the time characteristics (pulse response), as well as the space and frequency characteristics; unanimously decides that the following studies should be carried out: 1. the distributions of signal strength values for short period variations, and the day-to-day variations of hourly median signal strengths, including the fading characteristics under conditions of combined rapid'and slow variations; 2. the distribution of the rapidity and variation of fades; 3. the correlation functions pertinent to diversity operation; 4. the extent to which the above statistical results are affected by such factors as: — multipath effects including phase shift for each mode; — Doppler effect; — frequency, especially in relation to the standard MUF of the path; — time of day, season, and solar activity; — the path length, and the relation of the path to the direction of the earth’s magnetic field; 5. the nature and relative significance of the physical causes of signal strength variations, such as: — regional effects resulting from the geographical and geomagnetic characteristics of different regions, with particular reference to the polar, auroral, middle and equatorial latitudes, bearing in mind that, in some cases, one factor may have an important control over the effect of another factor; for example, the effects of season and of solar activity will probably require separate examination for each of the main geographical and geomagnetic .regions. Note. — The Director, C.C.I.R., is invited to thank the International Union for Radio Science (U.R.S.I.) for its recent contribution on this subject and also to request further comments. Q. 17-1/6, S.P. 17A-2/6 — 262 — QUESTION 17-1/6 PROPAGATION AT FREQUENCIES BELOW 1600 kHz WITH PARTICULAR EMPHASIS ON IONOSPHERIC EFFECTS (1970 - 1974) The C.C.I.R. unanimously decides that the following question should be studied: what factors affect sky-wave propagation at frequencies below 1600 kHz and how can the relevant propagation curves be obtained? STUDY PROGRAMME 17A-2/6 SKY-WAVE PROPAGATION AT FREQUENCIES BETWEEN 150 AND 1600 kHz (1951 - 1953 - 1959 - 1963 - 1966 - 1970 - 1974) The C.C.I.R., CONSIDERING that new night-time propagation curves and formulae for use at frequencies between 150 and 1600 kHz for world-wide application are required by: — Administrations in connection with their broadcasting and other services, — the I.F.R.B., — the I.T.U., in connection with future regional broadcasting conferences; unanimously decides that the following studies should be carried out: 1. the continuation of measurements at vertical and oblique incidence at frequencies between 150 and 1600 kHz; 2. the effects of the physical nature and of the structure of the ionosphere upon vertical and oblique propagation at frequencies in this range; 3. the variation of field strength and phase with frequency and distance considering: — the geographical location and orientation of the propagation path in relation to the day-night line and the Earth’s magnetic field; — the electrical properties of the ground; — the time of day, season and sunspot cycle; — meteorological factors; — effects of ionospheric disturbances, including polar cap and auroral absorption; 4. development of a mathematical model for long-distance propagation, which will include variable environmental and propagation conditions along the path and for the purposes of navigational systems using the frequencies in this range, will permit the calculation of wave phase as well as amplitude; — 263 — S.P. 17A-2/6,17B-1/6, Q. 18-1/6 5. revision of the night-time propagation curves in Report 264-3 and their extension to both shorter and longer distances and to other areas at low and high latitudes; 6. the possibility of establishing curves and formulae for daytime propagation. STUDY PROGRAMME 17B-1/6 PROPAGATION AT FREQUENCIES BELOW 150 kHz WITH PARTICULAR EMPHASIS ON IONOSPHERIC EFFECTS (1970 - 1974) The C.C.I.R. unanimously decides that the following studies should be carried out: 1. diurnal and seasonal changes in LF and VLF propagation at various frequencies, distances, latitudes and epochs of the solar cycle in relation to the calculation of field strengths and phases for frequencies below 150 kHz; 2. LF and VLF propagation of waves between the ionosphere and the Earth using an appropriate wave solution and the best available profiles of electron density and collision frequency; 3. the effect of variable propagation conditions along the path upon the phase and field strength of LF and VLF signals, using pulse and CW techniques where possible, with special reference to standard frequency and time services as well as to navigation and communication systems; 4. phenomena associated with interference between waveguide modes at VLF, by experiments at one or more frequencies over fixed paths, or over varying distances, using airborne observations. Note. — The Director, C.C.I.R., is requested to transmit this text to the International Union for Radio Science (U.R.S.I.) for comments. QUESTION 18-1/6 IONOSPHERIC FACTORS INFLUENCING COMMUNICATION AND NAVIGATION SYSTEMS INVOLVING SPACECRAFT (1970- 1974) The C.C.I.R., CONSIDERING that the ionosphere affects the propagation of radio waves at all frequencies transmitted through it; Q. 18-1/6, S.P. 18A-2/6,18B-1/6 — 264 — unanimously decides that the following question should be studied: what are the factors in the propagation of radio waves in and through the ionosphere which: — influence their use for telecommunication and navigation; — affect the practicability of frequency sharing between terrestrial systems and space systems and among space systems; — enable radio waves to be used to derive information on the physical properties of the ionosphere particularly for application to space telecommunications ? STUDY PROGRAMME 18A-2/6 IONOSPHERIC INFLUENCES ON SPACE COMMUNICATIONS AT FREQUENCIES BELOW ABOUT 15 MHz (1*959 - 1963 - 1966 - 1970 - 1974) The C.C.I.R. unanimously decides that the following studies should be carried out: 1. the potential usefulness of waves at frequencies below about 1-5 MHz, guided along the magnetic field lines of the Earth, as a means of communication between the Earth and space, particularly at YLF; 2. the interference potentialities of waves propagated in and through the ionosphere, in this frequency range; 3. the calculation of field strength or transmission loss for various terminal points of the path; 4. the impedance and directional patterns of antennae situated in the ionosphere. STUDY PROGRAMME 18B-1/6 IONOSPHERIC INFLUENCES ON SPACE COMMUNICATIONS AT FREQUENCIES ABOVE ABOUT 15 MHz (1963 - 1966 - 1970 - 1974) The C.C.I.R., CONSIDERING that, in the case of some high-performance space systems involving satellites in the geostationary- satellite orbit, ionospheric effects should be considered up to frequencies as high as 30 GHz; unanimously decides that the following studies should be carried out: 1. methods for measuring and predicting: — refraction, affecting in particular the direction of arrival and also the phase and group delays; — 265 — S.P. 18B-1/6, Q. 23-1/6, S.P. 23A/6 3. — attenuation effects; — Doppler effect; — Faraday effect, particularly with regard to polarization discrimination; — scintillation effects on phase, angle of arrival, amplitude and polarization; — the degree of isolation afforded by the ionosphere; 2. world-wide data collection and mapping of: — total electron content and magnetic field in the ionosphere; — other parameters, such as the size spectrum of irregularities and the corresponding velocities, needed for § 1 above; the possibility of guided propagation modes between spacecraft. QUESTION 23-1/6 IONOSPHERIC CROSS-MODULATION (1970 - 1974) The C.C.I.R., CONSIDERING that the use of very high power transmitters in the MF and LF bands may cause interference by cross-modulation in the ionosphere; unanimously decides that the following question should be studied: under what conditions does ionospheric cross-modulation occur and how does the percentage of cross-modulation depend upon the various factors, such as, the power, the modulation depth, the frequency, and the location, with respect to the receiving point, of the wanted and unwanted transmitters ? Note 1. — Report 574 constitutes a partial reply to this Question. Note 2. — The Director, C.C.I.R., is requested to transmit this text to the International Union for Radio Science (U.R.S.I.) for comments. STUDY PROGRAMME 23A/6 IONOSPHERIC CROSS-MODULATION (1974) The C.C.I.R, CONSIDERING that the increasing use of high-power radio transmissions may increase interference problems due to ionospheric cross-modulation; S.P. 23A/6, Dec. 6 — 266 — unanimously decides that the following studies should be carried out: 1. the ionospheric conditions (electron density, temperature, collision frequency, gyro-frequency, etc.) which tend to favour production of ionospheric cross-modulation; 2. the dependence of cross-modulation on: — carrier frequencies; — modulation frequency; — modulation depth; — effective isotropic radiated power; — polarization; — geographical location; 3. the methods of minimizing cross-modulation. Note. — This Study Programme should be drawn to the attention of Study Group 10. DECISION 6 (resolution 7-2) SKY-WAVE FIELD STRENGTH AND TRANSMISSION LOSS AT FREQUENCIES ABOVE 15 MHz (Study Programme 11 A-2/6) (1956 - 1959 - 1963 - 1966 - 1970 - 1974) The C.C.I.R, CONSIDERING (a) that Interim Working Party 6/1 has produced a provisional method of estimating sky-wave field strengths adapted for use by computers and is planning to prepare tabulations or charts for manual usage; (b) that a more accurate method of estimating sky-wave field strengths is desirable; UNANIMOUSLY DECIDES 1. that Interim Working Party 6/1 should continue to work on this subject; 2. that in addition to making improvements in the C.C.I.R. interim method (Report 252-2), Interim Working Party 6/1 should continue to develop a more accurate method of estimating sky-wave field strength and transmission loss; 3. that Interim Working Party 6/1 should continue to cooperate with Administrations and international organizations by assisting in making predictions of field strengths for circuits under study by the methods of the C.C.I.R. or by any other method; 4. that the C.C.I.R. Secretariat should continue to assist the Interim Working Party, for example, by collecting and collating material submitted by Administrations and organizations, preparing summaries for the Interim Working Party and especially by preparing tabulations or charts needed for the estimation of sky-wave field strengths for manual usage. — 267 — Dec. 7, 8 DECISION 7 (resolu tion 10-2) BASIC LONG-TERM IONOSPHERIC PREDICTIONS (Study Programme 9A-1/6) (1963 - 1966 - 1970 - 1974) The C.C.I.R, CONSIDERING that Interim Working Party 6/3 has produced a provisional C.C.I.R. Atlas (see Recommendation 434-2): “ C.C.I.R. Atlas of Ionospheric Characteristics ”, Report 340-2, to assist users of radio wave propagation either in or through the ionosphere; UNANIMOUSLY DECIDES that Interim Working Party 6/3 should: 1. continue to develop further improvements in the representation of world-wide ionospheric characteristics; 2. determine whenever possible, in consultation with users, how the later editions of the Atlas may be improved and what additional material should be added; 3. devise a practical method for including the statistical fluctuations about the medians in the monthly predictions; 4. determine the feasibility of producing numerical maps of total electron content. DECISION 8 (resolu tion 12-2) SKY-WAVE PROPAGATION AT FREQUENCIES BETWEEN 150 AND 1600 kHz (Study Programme 17A-2/6) (1963 - 1966 - 1970 - 1974) The C.C.I.R, CONSIDERING (a) that new night-time propagation curves and formulae for use at frequencies between 150 and 1600 kHz for world-wide application are required by: — Administrations in connection with their broadcasting and other services; — the I.F.R.B.; — the I.T.U. in connection with future regional broadcasting conferences; (b) that the results of work done in the European Broadcasting Area (see Report 264-3) in response to Study Programme 17A-2/6 do not necessarily apply to low latitudes or to middle latitudes in the southern hemisphere or even in other parts of the northern hemisphere; Dec. 8, 9 — 268 — (c) that, for the determination of the interference zone between the sky wave and the ground wave, it is desirable to have sky-wave propagation curves for distances less than 300 km; (d) that there is a lack of systematic observations of the critical frequency of the nocturnal E-layer; UNANIMOUSLY DECIDES 1. that Interim Working Party 6/4 should continue its work and should invite assistance from, other Administrations and organizations who are able to undertake measurements in the areas referred to in § (b), and to cooperate in tests made over long distances; 2. that the Interim Working Party should be asked to report as early as possible to Study Group 6 on: — the results of studies on LF and MF propagation in various geographical areas, including the field strength of signals propagated over paths less than 300 km and paths greater than 3500 km; — the usefulness and limitations of presently available propagation data as to their suitability for the production of propagation curves for frequencies between 150 kHz and 1600 kHz; — the nocturnal values of foE as a function of geographical location, time and season; — the effects of polarization coupling loss, as well as absorption loss, with particular emphasis on the equatorial region, including multi-hop transmissions; 3. that the Interim Working Party should be asked to report further on: — the accuracy with which LF and MF field strengths can be calculated for the various areas of the world and particularly for low latitudes, as a matter of urgency; — the information needed to achieve such accuracy in the provision of propagation curves of sky-wave field strengths for frequencies between 150 and 1600 kHz. Note. — Attention is drawn to Reports 264-3, 432, and new Report 575. DECISION 9 (resolution 13-2) PROPAGATION AT FREQUENCIES BELOW 150 kHz WITH PARTICULAR EMPHASIS ON IONOSPHERIC EFFECTS (Study Programme 17B-1/6) (1963 - 1966 - 1970 - 1974) The C.C.I.R, CONSIDERING (a) that there is a need for long-distance sky-wave propagation curves and formulae at frequencies below 150 kHz, especially at distances for which the sky wave predominates, for use in connection with: — radiocommunications; — standard frequency and time services; — navigational systems; — 269 — Dec. 9,10 (b) that the information so far available in Report 265-3 is insufficient to serve as a basis for establishing such curves; UNANIMOUSLY DECIDES 1. that Interim Working Party 6/5 under the Chairman provided by the Administration of Canada should continue its work by correspondence, towards the goal of producing propagation curves and formulae for world-wide use for frequencies below 150 kHz; 2. that Interim Working Party 6/5 should invite assistance from other Administrations or organizations, favourably placed geographically to extend the available coverage of field-strength data, and should also invite participation from those countries which have contributed to this study or are interested in it. DECISION 10 (resolution 47) SHORT-TERM PREDICTIONS OF OPERATIONAL PARAMETERS FOR IONOSPHERIC RADIOCOMMUNICATIONS (Question 10/6) (1970 - 1974) The C.C.I.R, CONSIDERING (a) that there is need for information on possibilities for short-term predictions of operational characteristics for ionospheric radiocommunications; (b) that special cooperation needs to be maintained between Study Group 6 and the relevant Study Groups concerned with systems, in particular the fixed, broadcasting, tropical broadcasting and mobile services, to learn the specific requirements of different radiocommunication services for short-term predictions; UNANIMOUSLY DECIDES 1. that Interim Working Party 6/7 should be continued for the study of Question 10/6 in general and Study Programme 10A-1/6; the coordination of its work will continue to be assured by a representative of the Administration of Australia, whose address is: Ionospheric Prediction Service, P.O. Box 702, Darlinghurst, N.S.W. 2010, Australia; 2. that any other Administration or organization participating in the work of Study Group 6 which would like to participate in the work of the Interim Working Party should make known to the Director, C.C.I.R, the name and address of their expert member preferably not later than 1 September, 1974. Dec. 11, Res. 4-2 — 270 — DECISION 11 (resolution 50) VHF PROPAGATION BY SPORADIC E (1970 - 1974) The C.C.I.R., CONSIDERING that VHF propagation by sporadic E is often responsible for serious interference to sound and television broadcasting services, mobile services, and to systems employing tropospheric and ionospheric scattering, and, if ignored, can sometimes seriously affect estimates of system reliability; UNANIMOUSLY DECIDES 1. that Interim Working Party 6/8, established under Resolution 50, New Delhi, 1970, shall continue studying the VHF propagation aspects of sporadic E, to develop methods for estimating the field strength or transmission loss at VHF resulting from propagation by sporadic-E reflections; 2. that this Interim Working Party composed of members nominated by the E.B.U. and the Administrations of the United States of America, India, Japan, Federal Republic of Germany, the United Kingdom, Sweden and the U.S.S.R. should continue under the Chairmanship of Dr. K. Miya (Japan) assisted by Dr. K. Tao (Japan) whose address is: Radio Research Laboratories, 4-2-1, Nukuikitamachi, Koganei- shi, Tokyo, Japan; 3. that any Administration or organization participating in the work of Study Group 6 which would like to join in the work of this Interim Working Party should make known to the Director, C.C.I.R, the name and address of their expert member preferably not later than 1 September, 1974. RESOLUTION 4-2 DISSEMINATION OF BASIC INDICES FOR IONOSPHERIC PROPAGATION (1963 - 1966 - 1974) The C.C.I.R, CONSIDERING (a) that R12, 7f2 and UNANIMOUSLY DECIDES 1. that the Director, C.C.I.R, should be requested: — 271 — Res. 4-2, 48-1 1.1 to make arrangements to obtain the monthly mean value of O, and the necessary solar and ionospheric data needed for calculating monthly values of the indices i?12 and 1>2; 1.2 to have these indices published in the Telecommunication Journal together with any predictions of the indices which can be made available by organizations and Administrations; 2. that organizations which are at present obtaining basic solar and ionospheric data useful for the pro duction of these indices should be urged to continue to make the necessary observations and to forward them to the Director, C.C.I.R. RESOLUTION 48-1 DEVELOPMENT OF SHORT-TERM INDICES FOR IONOSPHERIC PROPAGATION (1970 - 1974) The C.C.I.R., CONSIDERING (a) that the C.C.I.R. Specialized Secretariat has prepared a computer programme which provides the correlation between the values of solar index, (b) that some of these techniques may be applicable on a daily or even shorter time scale; UNANIMOUSLY DECIDES 1. that the Director of the C.C.I.R. should conduct studies on the development of automatic correlation schemes involving 2. that, if it appears to be necessary to the development of the scheme , and to be otherwise feasible, the Director, C.C.I.R., include in the development a simple observational arrangement for continuous measurement of solar radio noise flux at I.T.U. Headquarters, using insofar as possible the material which is already available to the Secretariat; 3. that the Director, C.C.I.R., report to Administrations any relevant conclusions from this work for possible adoption in national programmes for propagation predictions or forecasts. Op. 22-2, 23-2 — 272 — OPINION 22-2 ROUTINE IONOSPHERIC SOUNDING (1966 - 1970 - 1974) The C.C.I.R., CONSIDERING (a) that the routine observations from the existing ground-based ionosonde network together with satellite and oblique sounding programmes provide the bases for continuing improvements in both long- and short-term ionospheric predictions; (b) that the increasing importance of space research and Earth-space communications will require continued collection of such information, derived as a matter of routine, together with possible increases and changes in the quantity and nature of the information; (c) that U.R.S.I. Commission 3 has formed an Ionospheric Network Advisory Group (I.N.A.G.) which is responsible for advising ionospheric sounding stations on scientific questions and for advising U.R.S.I. on questions concerning the network as a whole; is unanimously of the opinion that Administrations should make every effort: 1. to continue the operation of the existing ionosonde network, and the interchange of basic data through World Data Centres; 2. to establish new ionosondes at, or transfer existing ionosondes to, places recommended by the C.C.I.R. in fulfilment of Study Programme 2A-2/6 or to support the organizations responsible for new and relocated ionosondes; 3. to consult U.R.S.I. (I.N.A.G.) on all questions relating to the establishment or closure of stations in the ionosonde network and proposed changes in the programme of operation or analysis of the iono grams ; 4. to support the work under Study Programme 2A-2/6 concerning the use of ionospheric data from satellite programmes and to explore the use of such data as are now available at the World Data Centres, for ionospheric predictions. Note. — The Director, C.C.I.R., is requested to transmit the amended text to the International Union for Radio Science (U.R.S.I.), the International Union for Geodesy and Geophysics (I.U.G.G.), the Special Committee for Solar-Terrestrial Physics (SCOSTEP), the Special Committee for Antarctic Research (S.C.A.R.) and the Committee for Space Research (COSPAR) for comments. ' OPINION 23-2 OBSERVATIONS NEEDED TO PROVIDE BASIC INDICES FOR IONOSPHERIC PROPAGATION (1966 - 1970 - 1974) The C.C.I.R., CONSIDERING (a) that 7f2 is recommended as the index to be used for predicting monthly median values of foF2 for dates, certainly up to 6 months, and perhaps up to 12 months ahead of the date of the last observed value of i>2; — 273 — Op. 23-2, 45 (b) that IS UNANIMOUSLY OF THE OPINION 1. that the following thirteen long-established ionospheric observing stations (or suitable replacements) be encouraged to continue in operation for the production of the index / f2: Canberra College Johannesburg Port Stanley Christchurch Delhi Moscow Slough Churchill Huancayo Mundaring Tokyo Washington (Wallops Island) 2. that the National Research Council, Ottawa, Canada, should be encouraged to continue the 10-7 cm solar radio-noise flux measurements necessary for determination of the index d>. OPINION 45* EVALUATION OF THE C.C.I.R. INTERIM METHOD FOR ESTIMATING SKY-WAVE FIELD STRENGTH AND TRANSMISSION LOSS AT FREQUENCIES BETWEEN 2 AND 30 MHz (Study Programme 11 A-2/6) (1974) The C.C.I.R., CONSIDERING that the method (Report 252-2) for estimating sky-wave field strength and transmission loss as developed by Interim Working Party 6/1 in accordance with Decision 8 is considered an interim method; IS UNANIMOUSLY OF THE OPINION 1. that Administrations and organizations should evaluate this method according to their different needs, e.g. by comparison with their methods and experimental observations; 2. that Administrations and organizations should report their evaluations to the Director, C.C.I.R., for the use of Interim Working Party 6/1. * This Opinion replaces Resolution 49, which is thereby cancelled. Op. 46 — 274 — OPINION 46 ESTABLISHMENT OF A CENTRAL CARD INDEX OF RESULTS OF FIELD-STRENGTH MEASUREMENTS ON MEDIUM AND LOW-FREQUENCY WAVES (1974) The C.C.I.R., CONSIDERING (a) that numerous measurements of the field strength resulting from the radiation by LF and MF broadcast transmitters and propagated by the intermediary of the ionosphere are being made by various receiving stations throughout the world; (b) that the study of those results and in particular of the differences between the observed field strengths and the field strengths calculated by the methods recommended by the C.C.I.R. or by other methods, would be most useful in the improvement of the methods of forecasting; (c) that such studies are possible only if the measurements are made using comparable methods; (d) that furthermore these studies would be greatly facilitated if the results of measurements were presented in a standardized form; IS UNANIMOUSLY OF THE OPINION 1. that the Administrations and other organizations that make field-strength measurements on the sky waves of LF and MF broadcast transmitters should be invited to communicate to the C.C.I.R., in a standardized form to be specified by the C.C.I.R., the measurement results already in their possession and that they should also be invited to communicate regularly the results of measurements made by them in the future; 2. that the C.C.I.R. should make the necessary arrangements for collecting those measurement results, for using them in studies of methods of forecasting field strengths, and for maintaining them at the disposal of Administrations or other organizations wishing to undertake similar studies; 3. that the C.C.I.R. should specify the conditions under which such measurements should be made; 4. that the C.C.I.R. should standardize a specimen form to contain all the information necessary, in particular for studies relating to the differences between measured and forecast field strengths. 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