1969 (6th) Vol. 1 Space, Technology, and The Space Congress® Proceedings Society

Apr 1st, 8:00 AM

Satellite Communications to Mobile Terminals -A Forecast of Needs and Techniques

Quintus C. Wilson Acton Laboratories, Inc.

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Scholarly Commons Citation Wilson, Quintus C., "Satellite Communications to Mobile Terminals -A Forecast of Needs and Techniques" (1969). The Space Congress® Proceedings. 4. https://commons.erau.edu/space-congress-proceedings/proceedings-1969-6th-v1/session-7/4

This Event is brought to you for free and open access by the Conferences at Scholarly Commons. It has been accepted for inclusion in The Space Congress® Proceedings by an authorized administrator of Scholarly Commons. For more information, please contact [email protected]. SATELLITE COMMUNICATIONS TO MOBILE TERMINALS A FORECAST OF NEEDS AND TECHNIQUES

By: Q. C. Wilson Acton Laboratories, Inc. Massachusetts

ABSTRACT intersystem and interorganizational relation­ ships. Several substantial studies have shown a Economic pressures and technical devel­ specific utility of satellite communications for opments lead to an international satellite com­ navigation and mapping, administrative traffic munications and surveillance system satisfying to air carriers, Atlantic air traffic control, and the needs of many mobile users, especially world-wide weather data systems. The plan for aircraft. The dominant need is to provide satisfying all these needs is a jig-saw puzzle not three-dimensional surveillance of air space yet assembled. However, air terminal congestion around airports. The large number of carrier takes precedence as the greatest need. Air and general aircraft requires the satellite traffic controllers have demonstrated in a system and its ground control facility to carry dramatic and responsible way that they are the burden of providing adequate link margins overloaded. The FAA has attempted to find suit­ and position computation. able relief in automation and display without making progress against the tremendous in­ crease in traffic. Proliferation of airports and systems of the present kind will push the FAA budget alone above one billion dollars and push a larger burden onto local communities. A INTRODUCTION better system seems to be promised by further automation. Economic pressures and recent technical advances will lead to an international satellite Contemporary radar defies simple automa­ communications system satisfying the needs of tion. An air traffic control computer should many mobile users, especially aircraft. It is track every target. But radar was designed to timely to encourage designing complete and track single targets with the aid of an operator competitive systems in order to provide guid­ who can discriminate a real target from clutter, ance to the ITU World Administrative Radio atmospheric reflections, multipath, etc. The Conference for Space Communications which basic limitation of radar is that the returned will convene by early 1971 to make treaty signal decreases by the 4th power of the dis - regulations for frequency allocations. tance. Transponders on aircraft cause jam­ ming because of the poor geometry of ground- Competitive designs will also provide based radar. Satellite-based radar can over­ guidance to the Department of Transportation come these faults. for satisfying air traffic control needs and navigation needs of many users. The previous Fortunately, the technical base for a sys­ DOT administration has already requested the tem which provides a surveillance function for aid of industry to this end. Vigorous design many users has resulted from military and efforts are needed now to prevent a drifting space efforts. Inexorable economic pressures policy detrimental to efficient and safe use of of air transportation will forge an effective national air space. System design is a vehicle union of organizations to use these techniques. for conducting intelligent dialog within the By attempting to forecast .the economic pressure technical community and is a prerequisite to and the most likely technical solutions, the sound budgetary estimates. A complete state­ author hopes to stimulate extensive dialog. ment of system performance (including an im­ plementation plan) can be matched against the HISTORICAL BACKGROUND most critical needs as well as against existing (TECHNOLOGY SEARCHING FOR A NEED) systems that may eventually be retired and proposed systems of limited utility. Many studies have explored the feasibility of using satellites to satisfy specific needs. Designing a system to meet these needs Some studies propose system designs. Their will be an exceedingly complex political, general shortcoming is parochialism. Satellite economic, and technical problem with many communications shows promise at considerable

7-9 Has Best Systems Capability but Appears expense. By attempting to satisfy limited needs, NASA the studies leave doubt that the effort is worth­ to be Withdrawing while. Thus, the characteristic flavor of the has conducted experiments and recent past has been technology searching for a NASA directed towards satellite need. studies of wide scope air traffic control. The Joint Navigation Satel­ provided comprehensive intra- However, needs are not lacking. In fact, lite Committee guidance towards system design . the air traffic control problem is so big that it governmental their limited budget has been directed appears to impede organization by intimidation. However, to the race to the moon. Futhermore, At last year's annual meeting of the AIAA primarily NASA deputy associate adminis­ several authorities suggested diverse political Mr. Harper, for aeronautics has said that system organizations that should or can do the system trator should be done by FAA, while NASA should design. New political unions are being explored design on hardware.Recent research fund­ in order to bring the effort to a workable ad­ concentrate cuts on air traffic control research and ministrative level. All of these activities are ing F and G satellites show that Congress is indicative of the pressures to form union that ATS sympathetic to allowing NASA to take a will implement a useful system. not more active role in air space problems. Even rather modest efforts to experiment with the Synoptic Weather Data System Requires 1540-1660 mHz band have been stopped. Industry Little Capacity has noted that weak fiscal position and is seek­ opportunities elsewhere. Many studies have considered imple­ ing menting a weather data system using satellites. An example of the type of system design Comparisons with aircraft interrogation that NASA has sponsored is given in the TRW systems show an economic advantage to satel­ NAVSTAR report . It recommends a passive lite systems. HF radio interrogation suffers navigation system coupled with position report­ from diurnal variability and irregular perfor­ ing by the aircraft back to the controller. The mance. Buoys designed for long life in the aircraft provides position computation. Mari­ ocean have been under development and testing time users are not excluded. for over five years. Missing elements appear to be a data relay network, a position-fixing History shows that NASA is endowed with capability, and reliable mooring. the management capability to attempt to do new and difficult things in full view of the nation. The benefits of long range weather pre­ This contrasts with the philosophy of an operat­ diction are really not known in fiscal terms. ing organization such as FAA which has such The efforts of DOT to present a substantial immediate difficult operational problems that case before Congressional committee have changes must be evolutionary. The solution to met questioning indicating that need priority is the air traffic problem is going to require inno­ assessed relatively low. Actually, data capacity vation. Although several organizations could do requirements are so low as to encourage the job, only NASA has recently demonstrated waiting to see what may develop in other satel­ this type of gut effort. lite communications experiments or system proposals. RTCA, ARINC,and COMSAT Proposed a System of Limited Capability Marine Navigation Users Show Interest The combination of the RTCA community, The Navy Navigation Satellite System ARINC, and COMSAT provides considerable called "Transit" (NNSS) has proven itself interest because it puts quantitative numbers useful for ships and research vehicles as well on certain needs. These three groups hoped to as for Navy requirements. Commercial receiv­ solve the problem of air traffic control over the are now available for about $50, 000. This ers oceans as well as to provide administrative passive system depends on the doppler shift of communications to aircraft. If FAA had joined signal from a low altitude polar satellite. a the union and accepted a limited capability of data is broadcasted to users. The Ephemeris ocean air traffic control (positions reported of only four satellites at low altitude limits use back from the aircraft over the satellite circuit) updating to every 108 minutes. The use position funding may have been under way by now. How­ UHF frequencies ultimately limits accu­ of low ever, the domestic airlines did not feel that the tenths of a mile. The measurement racy to capability of VHF satellite communications was time for the satellite to pass over time is the that great an improvement over present capa­ the user. bility to warrant picking up a 10 million dollar yearly bill. Thus the aviation community re­ ATS VHF satellites have communicated presented by RTCA has given some indication ship 1 ,. A NASA (ERG) study is with a merchant of the value of satellite communications to its the needs of the marine .community compiling operations and the FAA has indicated the value navigation* These experiments and for satellite of ocean air traffic control in light of its budget show the feasibility of satellite commu­ studies and political position on direct surveillance. nications for marine use,

7-10 COMSAT Over-Capitalization Seeks Relief DOD is Not Directly Involved from Charter Limitations on Opportunities U.S. military development has often The union is also interesting because it influenced navigation of the civilian and com­ illustrates complex organizational problems. mercial air transportation industry. DME went COMSAT by its Congressional charter cannot through extensive dialog before proving its sell services to individual users; it must be a suitability for general aviation. Now news carrier's carrier. An exception (obtained later) releases indicate that DOD is investigating satel­ is that it can sell directly to NASA or DOD. This lite communications under programs called and other expansions of service areunder fire TACSATCOM and ICNI. These public announce­ from INTELSAT. Thus COMSAT has national ments do not suggest that the proposed concepts and international constraints to look to in join­ or systems will be useful to civil aviation. ing a union. ARINC provides a carrier interface In fact, Dr. Finn Larson made clear at the between COMSAT and the individual users. The RTCA Fall Assembly that the DOD does not interface between COMSAT and FAA is presum­ consider this area to be within its jurisdiction. ably more difficult in that it may require Con­ It is probably safe to say that the tremendous gressional action when the concept expanded to resources of the military will not be applied provide control over the national air space. In directly to solving air traffic control problems. the 88th Congressional Senate Committee on Aeronautical Space Services, Mr. Jaffe of FAA and DOT Have Little Congressional Support NASA objected to COMSAT activity towards providing navigation and air traffic control to The FAA continues to fight its fiscal and ships and aircraft. COMSAT has surplus capital operational battles with courage and honesty if and an aggressive management, to help over­ not inspiration. Perhaps operational responsi­ come these inter organizational difficulties . bilities of operations vital to public safety are burden enough for one organization. Neverthe­ The implementation of a VHF communi­ less, FAA documentation and reports directed cations capability providing four voice channels to the FAA continue to give good account of the on three satellites was admittedly not very situation. Direction seems to be lacking because ambitious but would establish early precedence of conflicts in opinion within the FAA and between for a workable union. A j^reliminary study for FAA and NASA and industry. The question is COMSAT by Philco-Ford anticipated that the surveillance vs position reporting. eventual system could be in the 1540-1600 mHz band (so that it could provide position-fixing Several years ago, the growing load on air capability to 0. 2 mile using two satellites). traffic controllers led NASA and industry to say Oceanic needs were assumed so VHF provided that mpre responsibility should be given to the sufficient accuracy. Actually, a VHF imple­ pilots . Air collisions dramatized the need. mentation would probably have affected the The momentum carried several companies to WARC meeting which will be held in 1971 to cooperate with the FAA in a Collision Avoidance study allocations for space communications. System which would use a major portion of the COMSAT has filed with the FCC a request that 1540 to 1600 mHz band. These are now nearing the WARC allow the 1540-1660 mHz band to test. Meanwhile, in-route traffic control has include maritime mobile service. been sufficient to stop collisions between air carriers. So far the results of the CAS activity ICAO Takes a Long View are systems which will cost each aircraft over $50, 000 for a need which does not seem to be as ICAO is the international equivalent to urgent as terminal control. Furthermore, the RTCA. Last year it convened the ASTRA panel system competes for a major part of the most to study satellite communications technology. desirable frequency for position-fixing. During the RTCA Fall Assembly in 1968, comments were made by RTCA and ICAO mem­ Actually, surveillance which depends on bers on the relative positions of the two bodies . aircraft-derived position information is incon­ RTCA favored VHF without question, while ICAO sistent with positive control of the National Air felt that it was too early to make a recommenda­ Space and is not accepted by the FAA. CAS tion. (In fact, France has already recommended seems to be an independent safety system which an L-Band system). ICAO says that the problem cannot in the long run relieve terminal congesticn. is international and requires careful comprehen­ A surveillance system which depends on aircraft sive study so as to not commit the world to an reporting their positions obtains its strongest inadequate solution. The slow acceptance of proponents among those responsible for super­ VORTAC by smaller nations can be put in jeop­ sonic trans-oceanic safety. Some supersonic ardy by mention of a new system. aircraft will use triple-redundance inertial- guidance units costing about $300, 000. Cannot The RTCA discussions indicated the many this capability be coupled with a satellite subtle considerations that are required to obtain communications reporting circuit to provide general international acceptance of a system for the necessary air traffic control ? Those in the the world air fleets. FAA who disagree feel that terminal control

7-11 remains the significant problem, especially for Congress will Limit U. S. Government Partic­ the many aircraft without accurate navigation ipation aids. However, control of the air space is a The FAA is commendable in its reporting function which cannot be passed on to private of experiments . Both the FAA and its many organizations without rending the fabric of the critics give a clear indication of the limitations governmental authority since regulation is of radar as the prime sensor of the controller. implied. There are proponents for either NASA, Critics emphasize the poor reliability of radar intra-governmental unions, or not-for-profits while the FAA reports a struggle with the dis­ to do the system design for FAA. So far the most play interface as well as with automation of the interesting system design proposals have come tracking function. The FAA has found to its through NASA efforts. Congress, however, has chagrin that transponders introduce interference. cut NASA funds towards this end. The R & D Angels and bad weather also confuse the display. funds of the FAA have steadily dropped. Thus, it Thus automation of the tracking function is not appears that Congress repudiates the responsi­ going to be complete or comfortably reliable. ble government agencies.

Meanwhile, the air traffic controller The international aspect of the system will continues to be overworked and even irritated lead to the development of regulatory laws by attempts to help ease his burden since any through treaty agreements of ITU and ICAO system test imposes further strains on an members, and to system development through an already tense operation situation. international consortium. International coopera­ tion would broaden the fiscal base while permit­ The FAA clearly points out that automa­ ting the least expensive satellite configuration . tion is the least expensive way to relieve ter­ Congress will enjoy release from the responsi­ minal congestion . Proliferation of airports bility of difficult management decisions while can provide the greatest potential improvement retaining a maximum of funds for defense but at tremendous expense to the community spending. COMSAT capitalization will start served . The impact of this observation is development. weakened by the credibility of the promise of automation. Considering the limitations of the NEEDS present billion dollar radar system, automation PRIMARILY AIR TRAFFIC SURVEILLANCE is only a partial aid. Automation is the least expensive way to ASSIGNMENT OF RESPONSIBILITY LACKING minimize termial congestion . High speed ground transportation will be limited to very The DOT, through the FAA, obtensibly dense centers such as between New York and has responsibility for our future air space,Fiscal Boston and Washington. Since the U.S. is control is in the hands of the President and committed to air transporation for long distance Congress. The FAA has been criticized for not travel, congestion is a fundamental limitation vigorously fighting for funds to do its job. No on our capability to do business. Therefore, the comprehensive and detailed long-range plan is bottleneck will be attacked directly. Fortunately, visible in the documents of the FAA or in its the FAA has been giving careful attention to the record before the BOB or Congress. Since the causes and possible remedies to terminal delay. necessary programs are costly, funding really No compromise with safety is apparent in their cannot be expected without showing the Admi­ deliberations. Organizations such as RTCA 10 nistration and Congress that an improvement is and MIT also give insight into our domestic needed and is possible at reasonable cost. A air traffic problem. spokesman with a detailed plan is wanting. What then prevents automation ? The many Q Dr. Seiffert of MIT , Dr. Cherrington of studies of the FAA lead to the conclusion that Harvard, and others who despair of the lack of the judgment of air traffic controllers is neces­ directed activity suggest that a super agency of sary to overcome the inherent deficiencies of private form will become the chosen instrument the radar system which is the primary sensor chartered by Congress to handle air traffic of the controller. No significant improvement control. COMSAT itself has proposed to build a can come from improving the control center or limited system. COMSAT is over-capitalized the display system. Radar transponders do not and also limited by charter to few investment increase reliability as expected because they opportunities. Personnel within the FAA are jam the display system. It is necessary to look sympathetic to COMSAT. Members of the at this limitation in more detail. aviation community will give support. Therefore, the possibility of another COMSAT activity is a Air Traffic Control Limited by Radar distinct possibility if the Congress does not give the terminal control­ choose to solve the problem. Radar does not ler sufficiently reliable position-fixing data. The terminal area may include over 100 targets. Radar depends on repeated reflections to

7-12 display these targets. The display often shows telephone communications have led to facsimile clutter and angels from clouds, birds, and and picturephone experiments. New airports are multipath reflections. Omnidirectional aircraft being planned. No doubt all will be needed;never- antenna patterns have deep nulls. Often MTI theless, our nation's commitment to air for long circuits and aircraft maneuvers cause the distance transportation will require in addition a target to disappear. The controller must com­ workable air traffic control system. pensate these deficiencies by mentally tracking the aircraft during holding or landing. The New Airports are not the Whole Answer strain during poor conditions is great. Futher- more, the shortage of controllers leads to extra New airports cause problems. Land duty. Automation of the radar tracking function acquisition is often very difficult because of the is expected to help. political strength of the large number of affected people. Furthermore, if the new airport is close The ARTS program for providing.this to the old, interference between air traffic tracking function, while also tagging the dis­ compounds the controller's job or limits the in­ play with identification and altitude information, creased capability. Finally, the cost of one is now being installed in several locations. The large airport is about the same as a world-wide addition of the third dimension, altitude, comes satellite facility. The FAA has indicated that from the aircraft altimeter. The acquisition of new airports are going to be the most costly track still depends on operator selection. The solution. The major burden of that solution will • transponders have introduced a jamming pro­ fall on the local community. Therefore airport blem since the transponded signal is strong development leaders should be very interested enough to hide a passive target. It can even be in air traffic control development. reflected into the backlobes of the radar antenna. Automation has not relieved the operator of Geometry is Important for Good Radar concern for the correctness of his display. Performance

There are many minor airports which do Why does radar perform so badly ? Two not have a radar facility at all. Several acci­ geometric factors (the signal strength decreas­ dents have occurred to aircraft off-course ing with the 4th power of range, and the multi- while landing. The possibility of altitude error path and atmospheric effects on a signal which has been often indicated. A need for extending propagates nearly parallel with the earth) cause two dimensional radar coverage to these areas the most trouble. A radar on a satellite avoids has lead to FAA and industrial development of these problems for elevation angles greater than low cost systems. Three dimensional radar is 20 degrees. Of course, this type of radar needed. requires several satellites to be visible simul­ taneously to transponder-equipped aircraft. The number of voice transmission between Angle sensing can no longer be done by steering aircraft and ground overload the available a dish. In fact, this radar looks more like a channels and require frequent channel switching. navigation system working in reverse. The RTCA has proposed a digital message entry advantages are : a three-dimensional radar device for automating and speeding up the com­ which has nearly constant range (signal strength) munications process by at least cutting down to all aircraft, several orders of magnitude more identification time surveillance area, reduced clutter and inter­ ference from multipath from a transponded sig­ The mechanism for assuring safety is nal, and no signal returns from weather targets spatial separation. Separation distances are such as clouds. based on radar accuracies and aircraft closing speeds. If anomalous conditions such as bad Costs of Related or Comparitive Systems weather interfere with operations, increased separation is scheduled. Unfortunately, the peak Some quantitative indication of the need can traffic loads at several airports are overloading be found by looking at the present air traffic the traffic control system even in good weather. control plant, the aviation industry, proposed Bad conditions cause the controllers to stack system costs and other large programs. Table I aircraft in holding patterns. Thus the slowdown lists these cost estimates. The table indicates increases the load on the controller and an that proposed systems are appropriate for the excessive delay builds up. The present solution expected benefits and are similar to present is a FAA limitation on the number of flights air traffic control expenditures. In fact, the sizs during peak periods. Adequate safety is trans­ of the industry will be a potent pressure towards lated into a limitation on the revenue of carriers. a usable system.

Other Solutions will also be Needed

Several alternate solutions are being studied. Ground transportation such as high speed trains in the Northeast are being encour­ aged by DOT. Alternates to travel such as

7-13 matically is reduced to a position-fix after Table I -- Costs in Billions of Dollars passing simultaneously through several satel­ lites to a ground-based computer facility. The answer contains the position-fix. Satellite System with Control 0. 6 return Facility 12 . (5 years) The above need is similar or even iden­ to what is needed for an oceanic buoy Automated Air Traffic Control (ARTS) 1.0 tical system. A position-fixing capability helps solve one of the most difficult mooring problems of a National Air Space Program 0. 3 buoy field. Buoys need constant attention because they loose their moorings in a few months. Even FAA Radar System " 0.6 untethered buoys become possible if they can be tracked. FAA Yearly Operational Budget 0. 6 The cost benefits of a satellite system for New Airport for Boston 0. 5 oceanic air traffic control is based on the nar­ row flight path variations for economical super­ New Carrier Aircraft Sales per Year 2. 5 sonic flight. The forecasts show that congestion is likely by 1979. These aircrafts are scheduled Sentinel System Authorized to have three inertial navigation platforms. Expenditures 1.8 Using this reliable system to obtain the air­ craft position for transmission to the control facility over a dependable communications system is the heart of several proposed systems. However, the cost of this navigation surveil­ Subsidiary Needs lance package eliminates the less sophisticated user since the navigation system alone costs Several important subsidiary needs are about $300, 000. Implementation would provide marine navigation, buoy weather systems, good communications to many users but would oceanic air traffic control, and collision avoid­ not satisfy the basic needs of air traffic control ance aids. These are fortunately not as de­ in the USA. On the other hand, the implementa­ manding on satellite capacity as a system for tion of an air traffic control system for general control of air space. In fact, oceanic control is use would satisfy both needs for a community of assured by a domestic system. These needs users at least two orders of magnitude larger. require a small fraction of the prime power requirements of the satellite system because of WATERSHED IDEAS their low data requirement. Initial assumptions have controlled the Long range marine navigation is aided by style of proposed systems. This section states Loran, Omega, and Transit systems. All of several ideas which seem pivotal if all needs these aids have especially useful characteristics are to be met. Since needs exert pressure on to different users. On what basis is a competi­ the system design, this section is a forecast of tive system needed ? The need could be based how satellites will be used to provide air traffic upon the long range retirment of Loran stations control and other functions for the civilian popu­ (but probably not Omega stations which use very lation. low frequencies in order to obtain long range even to submerged vehicles). The Transit satel­ Surveillance and Communications Service lite system uses a relatively inexpensive polar Not Passive Navigation satellite system and puts the burden of naviga­ on the user. The basic reason tion computation While past proposals have been based on considering a new system is to get wider for passive navigation, the future system will use these systems by making the receiver use from active radar surveillance with transponding and doing the computation at a central simple targets. The transponders also will provide facility. communications. All computations and message- routing will be done by a satellite control facility. toll-type communication system, A These services will be offered to private users to users for emergencies and business available through organizations such as ARINC, to mili­ as for requesting a position-fix, satis - as well tary users through DCA, and directrly to gov - fies the need. Present navigation aids are simi­ ernmental departments. The service organiza­ to position-fixing by sextant in that it takes lar tion will be capable of dealing with national and to obtain the measurements and reduce time international relationships. The service orga­ to a position. Sometimes bad conditions them nization will be (or be similar to ) COMSAT. with Loran and Omega and occasionally interfere There will be no direct military participation The availability of Transit for up­ Transit. requiring passive navigation. dating is limited by the few number of satellites in low polar orbit. What is needed is a system in which a user requests, a position-fix by satel­ lite communications and the request auto­

7-14 Between 10, 000 and 100, 000 Users SUPPORTING TECHNOLOGY

The large number of users radically Today's technology supports the type of drives the system design to minimum-cost system which will satisfy the needs listed above. user terminals. This single consideration will Several companies are already entering the select the type of modulation , the frequency for breadboard testing phase on company-sponsored transmission, and the radiated power of the activities anticipating satellite communications satellite system. It will cause the position com­ and navigation systems. The type of system that putations to be done by the service organization. is satisfactory for the larger community and for The large number of users eliminates the possi­ quick position-fixing will use frequencies around bility of free access : toll service will be avail­ one gHz. Furthermore, the user terminal will able to subscribers on demand. Each message use a rudimentary fractional-hemispherical an­ unit will be quickly handled to avoid system tenna which does not introduce a significant drag saturation; therefore, the type of operation will penalty to supersonic planes. Consequently, the not resemble the processes of Omega, Loran, dominant problem is how to obtain the required and Transit which require human judgment or satellite radiated power to satisfy the link manipulation. The reliability of the service will equations. Phased array antennas on satellites be like that of the telephone. having about 1000 watts prime power are within the weight capability of several of our opera­ Many Satellites at all Latitudes tional launch vehicles. The present activities are to determine the best type of array for this System reliability will come from having type of system. The question is really a system many satellites. Position-fixing depends on synthesis trade-off which also includes the having several satellites available to every user selection of modulation and orbit configuration. at angles greater than 20 degrees elevation but Finally, input/output technology will be utilized distributed around the user's horizon. This by user terminals to assure that the system does minimum elevation angle minimizes the effect not saturate. of atmospheric and ionospheric conditions which lay parallel to the surface of the earth and plagie TDM for the Satellite Phase Array present radar and communication signals. Furthermore, the user's omnidirectional an­ The phased array competes directly with tenna can be designed to have relatively lobe- solar arrays in providing high radiated powers. free performance over this fraction of a hemi­ The phased array can also be used by the satel­ sphere. The large number of users makes the lite in its receiving mode so as to minimize satellite system cost a small fraction of the user transmitter power. If the organization of total system cost. In fact, the estimated satel­ the phased array depends on linear power lite system costs are comparable to costs of our amplifiers in order to generate simultaneous present-unsatisfactory system. In any case, the beams , a 3 dB amplifier penalty effectively satellite costs will not be provided directly by doubles the satellite weight over a system which U. S. funding. steers a single beam. Time Division Multi­ plexing (TDM) permits steering a single beam The Air Traffic Surveillance will be Automatic to many users. Experiments have been conducted by COMSAT demonstrating TDM modulation . An automatic surveillance system depends The implied wide bandwidth signals do not need on air tubes with fixed schedules, like a railway, to use high accuracy oscillators in the user to assure safe separation. The satellite system equipments. These arguments suggest that the will interrogate aircraft transponders as needed final system will use burst signals much like to assure separation and will advise air traffic the response of a IFF transponder to radar control and the effected aircraft of deviations. interrogation. This service would permit positive control of the air space for carriers and others equipped High-Altitude Inclined-Circular Orbits with with sufficient on-board navigation to fly area Phase-Phase Array navigation routes. General aviation, at lower altitudes (as well as marine users), would use A few satellites in synchronous orbit do the system as navigation aid available on request. not satisfy navigational accuracy or coverage The air traffic controller in the terminal area requirements. Such satellites could use a high would be relieved of the task of using a radar gain frequency-scanned array. Instead the satel­ scope as the prime sensor for watching carrier lites must have earth tracks which reach at least and business plane s. Furthermore much of the middle latitudes. However, the moving satellite controller's present communications traffic cannot easily use a frequency scanned antenna will be accomplished automatically by the radar because the user would have to change fre­ interrogation system. quencies according to the position of the satel­ lite. Investigations of inclined synchronous satel­ lite orbits are available . These orbits and polar orbits have earth tracks which cross. Two satellites in close proximity only provide the position-fixing triangulation of one satellite.

7-15 Other orbits will overcome this doubling and 3. David D. Otten, "Study of a Navigation and also reduce the crowding expected at synchro­ Traffic Control Technique Employing nous altitudes which shall be reserved by the Satellites - Volumn I, Summary". IT for fixed ground stations. As an example, N68-31277, Contract NAS 12-539, prepared a subsynchronous orbit causes the figure-eight for NASA (ERC) by TRW Systems Group, earth track to open up into a sinusoidal track Redondo Beach, Calif. , December 1967. having no cross-overs. Four or five cycles around the earth at 45 degrees inclination re­ 4. R.E. Horasch and C. L. Murphy, Satel­ quires less ETR launch vehicle capability than "Synthesis of an Aeronautical Service a stationary satellite .Station keeping is only lite System", A1AA 2nd Communications necessary relative to the other satellites and Satellite Systems Conference, San Francisco, does not depend on geospheric irregularities. Calif. April 8-10, 1968. Futhermore, position-fixing implies simul­ Air Traffic; 1 taneous paths to several satellites. Therefore, 5. Paul G. Thomas, "Controlling 1968 a phase-phase antenna array is indicated. SPACE AND AERONAUTICS, May pp 76-87. Digital Input/Output Devices 6. "Goals, Achievements, Trends", AD 674412, 1968. Simple automatic digital input/output FAA, Washington, D. C. devices will be used to eliminate system "Alternate Approaches for saturation. A request for a position-fix 7. Milton Meisner, Terminal Areas", should only require a push of a button. The Reducing Delays in answer should go only to the requester as a FAA, 1968. printout, alpha-numeric display, or the pre­ recorded voice from an enunciator. Trans­ 8. William W. Seifert, "Transportation-1993',' 1968, P. 56. mission of a message should occur after the SPECTRUM, October message has been composed. These digital for the Air Traffic devices eliminate the time required for human 9. Long Range Planning Washington D. C. , communications which now burdens the aircrew System, DO-135, RTCA, and the airtraffic controller. Fortunately, March 17, 1967. several types of equipments are now being A. Schriever and William W. investigated by the airlines, RTCA , the 10. Bernard Cochairmen, Air Transportation police, and computer terminal vendors. Seifert, 1975 and Beyond : A Systems Approach, of the transportation Workshop, SUMMARY Report 1967, the MIT Press, 1968. Automation of air traffic control will 11. Universal Air-Ground Digital Communi­ require a satellite system to provide position- cations System Standards,DO 136 RTCA, fixing and communications. Navigation, com­ Washington, D. C. , March 7, 1968. munications, and telemetry needs of ships,, air­ craft, and buoy networks will also be satisfied. 12. Q.C.Wilson, W. T. Brandon, and Alan G. The system will result from a bold application Cameron, "International Satellite System of space technology; it will not be an evolu­ for Aircraft Communications and Air tionary development. The design will minimize Traffic Control", EASCON 68, IEEE subscriber equipment costs. Nevertheless, the New York, N. Y. 1968. system costs will be less than the present system for air traffic control and will 13. Tadahiro Sekinoto and John G. Puente, provide for forecasted air transportation A Satellite Time-Division Multiple-Access growth. Experiment, IEEE TRANSACTIONS ON COMMUNICATIONS TECHNOLOGY, Vol PP581-588. BIBLIOGRAPHY COM-16, No. 4, August 1968,

"Efficient "Maritime Mobile Satellite Communica­ 14. H.E. RoweandA.A. Penzids, tions Tests Performed on S. S. Santa Spacing of Synchronous Communication 10, Lucia", PB 179 298, prepared for U. S. Satellites", BSTJ, Vol 47, No. Dept. of Commerce Maritime Adminis­ December 1968 PP 2379-2433. tration by Westinghouse Electric, Balti­ more, 1968.

Eugene Ehrlich, "The General Utility and Character of Prospective Navigation Ser­ vices Satellite Systems", United Nations Conference on the Exploration and Peace­ ful Uses of Outer Space, A/Conf. 34/111 3, 3 June 1968.

7-16