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SATELLITE COMMUNICATIONS The outlook for satellite communications is different than that for other space activities in that this represents the one space applications area where there is a healthy and rapidly growing industry. The momentum that already exists in this industry indicates that considerable progress may be expected during the balance of this decade. In response to the question of what practical applications of space systems may be expected in the l990s, the Panel focused the majority of its attention on potential new services that have promise for this period. This chapter is an examination of the field of satellite communications, portraying the current state of affairs, projecting what change might occur in the future, and defining what impact future needs might have on the development of future space programs. The Panel noted that the l979 report of the Committee on Satellite Communications of the Space Applications Board explored many of the issues related to practical applications and technology development in the area of satellite communications (Committee on Satellite Communications, l977). This report treats many of the same issues and focuses attention on the space station. MAJOR DOMESTIC SATELLITE COMMUNICATIONS SERVICES Today communications satellites are in wide use for commercial and military purposes. The growth of the domestic satellite (domsat) communications industry has followed the pattern of many successful new telecommunications services during their early years. It is now characterized by a fast-growing demand for services that may outstrip the capacity of available facilities in 60
6l the foreseeable future. While a growing number of satellite suppliers and carriers are providing the satellite capacity to meet present and future demands, advances in technology and the prospect of new and innovative services continue to stimulate the growth of the market. The market for services and the public benefits provided are expected to grow dramatically over the next several decades. Currently, more than a dozen domestic satellites provide over two hundred transponders located in geostationary orbits over the United States. These communications satellites are spaced approximately 4Â° apart in the geostationary orbital arc in order to avoid mutual radio-frequency interference. The Federal Communications Commission's (FCC) authorization of domsat expansion of 4 December l980 would permit the inclusion of five additional satellites. Other authorizations are expected, along with a change to 2Â° spacing at C-band, as well as at l^-band (l4/l2 GHz). U.S. domsats have been allocated 500 MHz of bandwidth at both the C- and Ku-bands. Satellites using separated frequency bands can be collocated at the same longitude because the probability of physical collision is negligible. An additional 300 MHz has been allocated by the International Telecommunications Union at C-band, but is not currently planned for use in U.S. domsats because of the military use of this band. In the Ka-band (30/20 GHz), a bandwidth of 2500 MHz has been allocated for satellite service. This band currently is subject to outages in rainstorms, but further advances in technology may relieve most of this limitation. Spot beams formed by large spacecraft antennas can illuminate small areas called "footprints" on earth. If these footprints are sufficiently isolated from one another, any given frequency band can be reused in separate footprints. This results in multiple reuse of the allocated spectrum. Frequency reuse at C-band is limited, however, because of adjacent beam side lobe interference and geographically skewed traffic distribution in the most developed portions of the country. Triple-frequency reuse at Ky-band seems achievable with current technology. Currently, the allocated bands are divided into 40-MHz segments. Allowing for guard bands, each segment supports a single transponder with a radiofrequency bandwidth of approximately 36 MHz. Eventually, improvements in modulation techniques could double the traffic-carrying capacity.
62 Advantages would accrue from interconnecting domsats. If intersatellite links were developed so that signals could be switched between domsats, then a larger number of earth stations could be interconnected. These intersatellite links could be radiofrequency links between separate domsats, or in the case of platforms carrying multiple communications payloads the interconnecting links could be hard-wired. In either case, the interconnections could enhance the network available to each earth station. This section concentrates on three major domsat services that are now being provided by carriers or that appear to have high potential in the commercial marketplace and the public sector. They are fixed satellite service (FSS), direct broadcast service (DBS), and mobile satellite (MSAT) service. Fixed Satellite Service Included in the fixed-service category are point-to-point voice, video, and data services delivered either through commercial trunking networks or private networks, as well as distribution (point-to-multipoint) services, of which the most familiar is the distribution of television programming. In two parallel studies sponsored by the National Aeronautics and Space Administration (NASA) in l979, forecasts of the fixed-service market through the year 2000 were offered by Western Union and U.S. Telephone and Telegraph Corporation (Kratochvil et al., l983). These forecasts were updated recently by a NASA internal analysis (Gamble et al., l979). The forecast is in terms of equivalent 36-MHz transponders; a 36-MHz transponder is a convenient measure of capacity, since it will support 2,400 one-way voice circuits or two one-way color television circuits. The demand forecast in the original l979 NASA analysis for the year l982 was l40 transponders (there are now more than 250 in orbit); for l985, 350; for l990, 650; and for the year 2000, 2,580 transponders. The kinds of service predicted for l990 are l0 percent data, 40 percent video, and 50 percent voice; for the year 2000, service use is predicted to be l5 percent data, 25 percent video, and 60 percent voice. These percentages contrast with current demand, which is dominated by the 60 to 70 percent use for video services.
63 The number of FSS satellites that can be accommodated in orbit and the number of equivalent 36-MHz transponders that can be carried by each satellite depends on how much information can be handled by each transponder, and this varies with the modulation and signal-compression techniques in use. Nominal advances in such techniques have been accounted for in the FSS forecasts previously discussed, and hence capacity in terms of transponders is the proper measure for comparisons with the forecast. Currently, 500 MHz at C-band can be used twice on each satellite through the use of orthogonal polarization of the signal. This provides 24 equivalent transponders on each C-band satellite. Similar polarization reuse can be achieved on Ky-band satellites, and an additional frequency reuse can be achieved using a spot beam antenna. Current methods should enable the formation of lÂ° to l.5Â° spot beams (probably aimed toward the east and west coasts of the continental United States), which, when combined with polarization isolation, would increase the frequency reuse to three times. Assuming l2 actual transponders per satellite, this would provide the equivalent of 36 transponders. The use of 24-equivalent transponder C-band satellites and of 36-equivalent transponder Ku-band satellites both spaced at 2.5Â° would result in total U.S. domsat capacity of l,5l2 equivalent 36-MHz transponders (648 of C-band and 864 of Ku-band), which is more than l000 transponders short of the year 2000 demand forecast earlier (indicated to be 2,580 transponders). At Ku-band, wide-area distribution services could be addressed in l995 by 0.75Â° spot beams covering the continental United States and providing four-time frequency reuse in a time division multiple access (TDMA) mode. The use of Kfl-band (for which technology is being developed but is not yet available) would provide for eight new satellites. As the radiofrequency wavelength decreases (higher frequency), the effect of atmospheric attenuation on the signals is more pronounced at ground station antennas with low elevation angles, resulting in a decrease in the number of usable satellites. The 2500 MHz of Kfl-band allocation would allow for 60 transponders per satellite without frequency reuse, but it is unlikely that Kfl-band satellites would be implemented without spot beam reuse. Link power requirements at Kg-band would dictate the use of a high-gain spot beam antenna, and thus frequency reuse would be inherent. A frequency
64 reuse of three would permit a U.S. domsat capacity of l,440 transponders at Kg-band. This capacity, when combined with the C-band and Ku-band capacities calculated above, will meet the forecasted demands for the year 2000. At Ka-band, it should be possible by l990 to produce 20 to 40 fixed beams of 0.3Â° beam width and 35-decibel side lobe isolation for trunking applications. During the l980s, the Ka-band beam-forming circuitry for a multiple-beam antenna will be fabricated largely from discrete components. By the year 2000, the discrete components should give way to monolithic technology, providing weight and size reductions and reliability improvements. Direct Broadcast Satellite Service A new direct-broadcast satellite (DBS) service is expected to be offered by several suppliers by late l984. Initially it will deliver television programming direct from the satellite to the viewer's premises through small receiving antennas. Because DBS is a new service, forecasts of its market demand are more uncertain than are those for fixed services. However, a comprehensive forecast was presented by the Federal Communications Commission's Regional Administrative Radio Conference Advisory Committee (RARC, l983) in its final report. The committee's forecast, made in terms of standard TV channels, was for 36 to 46 channels in the year l986, and 68 to 2l5 channels in the year 2000. Since the United States achieved the goals sought by the l983 RARC, it appears that the lower market demand in the year 2000 can be accommodated with current or near-term methods. The emphasis in DBS between l990 and 2000 will be on developing l00- to 200-W power amplifiers that are reliable and efficient, and on mass production of low-cost home terminals. Mobile Satellite Service Mobile satellite (MSAT) service, another new service under study, involves the direct delivery of voice and low-data-rate communications via satellite between mobile radio units and the U.S. switched telephone network. The system providing this service would augment the planned
65 cellular mobile radio telephone systems* in urban areas by providing satellite coverage in rural and low-density population areas. Such a system could also provide commercial mobile radio (dispatch) services in rural areas. If a satellite system was implemented in conjunction with existing or planned terrestrial mobile radio systems, it would provide for nationwide mobile radio service. . Through the next two decades, the terrestrial cellular system is targeted to large urban Standard Metropolitan Statistical Areas (SMSAs), and it is unlikely that it will ever be implemented in small towns or in rural areas. This means that in the year 2000 there will be an unserved population of about 77 million people, and about 50 million of these people will have little hope of ever obtaining service from the cellular system. Thus, if one applies typical market-potential factors (about one percent is normal) to the unserved cellular system market, a probable market for MSAT should be between 500,000 and 700,000 mobile radio telephone users. To capture this market, it is felt that the MSAT service must not only be comparable in cost to cellular service, but that the MSAT service and system characteristics must be compatible with cellular service and system characteristics. In addition, there are approximately l million current commercial mobile radio units in use in the non-SMSA counties of the United States, and it is estimated that 20 percent, or 200,000, of these users are receiving unsatisfactory service from the present system because of range and/or coverage problems. The Panel believes that satisfactory service could be provided by a MSAT system if it can be cost competitive to that of current services. While there are other potential users of a MSAT serviceâfor example, the oil and gas industry or emergency medical servicesâthe sum of mobile radio telephone and private mobile radio users discussed above represents at least 700,000 users who need better service. To achieve MSAT system capacities approaching the potential user needs, frequency reuse must be employed. This is possible with spot beams, but because of the low *In cellular systems, the geographical areas are divided into cells, each served by a terrestrial multichannel repeater station. Frequency is reused in nonadjacent cells.
66 frequencies involved to provide compatibility with terrestrial service, large satellite antennas are required. An antenna on the order of 50 ft in diameter would allow for a frequency reuse of two and a capacity of 40,000 to 240,000 users. But uncertainties in FCC frequency and orbital location allocations, similar to those affecting DBS, make mobile satellite capacity uncertain. DEVELOPMENTS EXPECTED THROUGH THE l980s Growth of satellite telephony services is expected to occur in domestic, regional, and international areas, as advanced countries continue to augment existing terrestrial systems. Continued use of the 6/4 GHz band will be supplemented by growth in the use of the l4/ll GHz band. It is also reasonable to expect some operational use of the 980/450 MHz band for mobile applications, augmenting the cellular system, as well as experimental and some operational use of the 30/20 GHz band. Developments that would assist growth of direct broadcast services are high-power broadcast satellites (in the range of one-half to one million effective radiated watts), and high-efficiency shaped-beam satellite antennas that confine the broadcast signal to the desired region and permit low-cost receiving earth stations. Digital television transmission will become attractive in the foreseeable future. Use of digital techniques will require developments in source coding and error control, low-cost devices to convert the signal from analog to digital form, and inexpensive video receivers that operate directly from a digital signal. Standard format converters will be required for international transmission. Wideband services include transmission of digital data, computer networking, and point-to-point video transmission. Wideband services can be expected to make extensive use of both the 6/4- and l4/ll-GHz bands in the l980s. The 30/20-GHz band may be used for certain high-data-rate applications such as electronic mail and digital trunking, and some operational 30/20-GHz systems will emerge by l990. Communications services that use small transportable terminals with small antennas are now available to some users. Existing systems provide limited land, sea, and air services, but are expected to grow throughout the
67 l980s. The Panel estimates that by l990 more than 6,000 maritime installations will be in use worldwide, requiring almost l00 voice channels in the Atlantic and 50 each in the Indian and Pacific oceans. If satellites were equipped with large antennas and high power, a much larger market for systems using smaller antennas would exist. POTENTIAL NEW SERVICESâl990-2000 The Panel identified potential new services that could become available in the decade l990-2000: personal and business paging service; broadcast FM service to mobile users (clear-channel broadcast-type service); air and ship traffic control and safety services; vehicle emergency alerting and location; disaster warning to individual homes; and multimode land and maritime remote-area service. Personal and Business Paging Service Today mobile radio is used mainly for telephone extension service and dispatching (police, fire) in which a one-way broadcast signal is transmitted from a central location in response to an incoming request. The transmission is ordered to trigger visual or auditory response in a small portable receiver, often called a "beeper." Since the system is one-way, the called party uses another communications medium for response. The growing popularity of this service is now confined to urban areas in which a concentration of users coincides with the available radio range of a few miles. A satellite-based paging service could permit area coverage to the vast majority of the nation's nonurban areas and augment the market for urban areas as well. Because of easy and inexpensive compatibility with existing receivers, such a paging service should operate in a frequency band designated for mobile use, if band allocations permit. Service augmentations, such as display of the phone number to call back or short messages, could be included. Only modest satellite power is required, because a short simple message and address requires only a narrow bandwidth.
68 Clear-Channel FM Broadcast Service to Users Today commercial FM broadcasts are limited to line-of-sight coverage, and serve urban, suburban, and some portion of rural areas. The effects of terrain and building blockage, multipath fading, and distortion inhibit mobile use. An FM broadcast satellite for rural and mobile use could be attractive; it could also have utility as a universal warning and alerting system employing inexpensive receivers. Some changes in band allocation or licensing rules would be necessary in order to provide a few clear channels, but the improvement in quality of service would be in the public interest. Air and Ship Traffic Control and Safety Services Few existing or planned air traffic control and vessel traffic service systems are adequate. A combination of satellite observation, position location, and communication services could help justify an improved future system. This complex group of multifrequency communication and navigation systems would be improved and its cost lowered by use of a satellite-based central observation and control facility. Vehicle Emergency Alerting and Location System The growing use of simple beacons that are triggered by impact or by seawater to provide an alerting and homing signal for rescuers is being improved by an international satellite-based project called COSPAS-SARSAT. However, the beacons are to be used only for certain aircraft and ships. The extension of use of these beacons to ground vehicles and individuals for alerting and location purposes should broaden the envisioned benefits of quick rescue. An expanded system for vehicles and individuals could use an integrated set of satellites at geosynchronous orbit (for continuous detection) or at low earth orbit (for simplified Doppler positioning), with rescuer communications included if necessary.
69 Disaster Warning to Individual Homes The broad coverage of satellite broadcasts could be used for alerting individual homes and businesses that currently have inadequate reception to pending natural disasters. Current warning systems, integrated with weather broadcast services, are intended to cover all areas of the United States, but terrain blockage, multipath distortion and fading, and building attenuation cause the effectiveness to be marginal in many locations. The use of a satellite system to augment or replace the existing terrestrial VHF-FM broadcast system could improve service, reduce operating costs, and overcome user apathy. Additional utility might be achieved by providing information (in addition to the alert) that would help people prepare for natural disasters, and by providing postdisaster communications for assessment and rescue control. Multimode Land and Maritime Remote-Area Service Radio is the practical medium in the vast thinly populated areas of the earth's surface for communications related to safety, survival, business, and education. A satellite system could provide the most effective coverage for rural and remote areas, but the technology for such a service at affordable costs has yet to emerge. CONCLUSIONS AND RECOMMENDATIONS The Panel, in examining the technical and operational requirements for the conceptual design of a space platform or a space station, makes the following conclusions: Almost all communication services projected for the period l990-2000 could be provided at geostationary earth orbit, and all satellites and their deployment could be accommodated by Space Shuttle launch capabilities. This assessment also applies to presently contemplated direct-broadcast satellite national television services (DBS-TV), although an advanced DBS-TV satellite might have characteristics resembling a platform.
70 Nearly all telecommunication modes that could be performed at low earth orbit are better accomplished at geosynchronous orbit. A possible exception is a hypothetical medium-frequency broadcast service. A space station or platform could be useful in two major ways: (l) as a fuel depot, to reduce launch costs, or to allow greater payloads, and (2) as a means for allowing assembly of, modification to, or refurbishment of in-orbit satellites. Also, some advantages would accrue from interconnecting domestic satellites carried on a space platform. An unmanned geosynchronous-orbit operational platform could serve as the support platform for varied communications missions. The desired characteristics of orbit, power, attitude control, and thermal control for such a platform are: Orbitâ0Â° angle inclination with 24-hour synchronous orbit Powerâ25-kW power level initially with an eventual l00-kW power level Attitude controlâlÂ° to 3Â° inclination about three axes with finer control by individual payload subsystems Thermal controlâcentralized thermal control-heat rejection system sized to approximately 75 percent of the power available If a low-earth-orbit service station is planned, the Panel recommends the following characteristics in relation to orbit, power, attitude control, and fuel storage: Orbitâaltitude consistent with long life and inclination in the 28Â° to 0Â° range consistent with requirements for raising and lowering orbits of spacecraft Powerâsufficient to operate station/platform housekeeping services, estimated to be in the l5- to 25-kW range Attitude controlâflexible requirements (few degrees acceptable) for long periods; for short periods, vernier control necessary to permit acquisition and fueling of spacecraft Fuel storageâadequate for boost to geosynchronous orbit (approximately l5,000 Ibs)
7l The role of man in space with respect to satellite communications appears to be limited. The functions of fueling and assembly of spacecraft, which require human presence, have been noted. REFERENCES Committee on Satellite Communications. l977. Federal Research and Development for Satellite Communications. Washington, D.C.: National Academy of Sciences. Gamble, R. B., et al. l979. 30/20 GHz Fixed Communications Systems Service Demand Assessment. ITT-U.S. Telephone and Telegraph Corporation, Final Project Review at NASA Lewis Research Center, Cleveland, Ohio. Kratochvil, D., et al. l983. Satellite Provided Fixed Communication Services: A Forcast of Potential Domestic Demand Through the Year 2000. Western Union, Final Project Review at NASA Lewis Research Center, Cleveland, Ohio. Regional Administrative Radio Conference Advisory Committee. l983. Final Report. Washington, D.C.: Federal Communications Commission.