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WORKING PAPER SECTION 2 SPECIE IC CON SI DE RATION OF A UHF DBS-A SY STEM- SE RVICE Ultrahigh frequency could be chosen as a DBS-A system's operating frequency region. Ultrahigh rather than high frequency will be discussed first to reflect the committee's greater apparent interest in the former.~ Two recent papers speak to the use of VHF and FM for national and/or regional systems for the provision of DBS-A services. Several of the observations and judgements contained therein are of equal value when applied to UHF system-service considerations. Simply for illustrative purposes here, a relatively narrow band near 2.5 GHz, one that could be located within the 155 MHz-wide 2.500-2.655 GHz band, is chosen for study. This band is designated by international agreement for shared use between broadcasting satellite and fixed services. In general, the kinds of system parameters and their performance discussed here are representative of those designed specifically for operation in any very high VHF, or UHF, or very low SHE band. In the UHF system-service design outlined here, very large channel capacity and very small unit surface coverage areas are assumed; they are inherently required by any eventual global system-service. Their actual dimensions could be achieved but only by using technology not now expected to become available in much less than a decade. The cost of this technology and its financing, although undoubtedly acceptable (see Sections 4 and 8), would be relatively high. If an earlier commencement of service were desired, and/or if financial considerations suggested a lower initial cost, lO. Certain of the concepts described here for application at UHF have been considered earlier for application at high HE, and the professional reader therefore might now wish to note the references in Section 3. ll. "Sound broadcasting-satellite system for a national coverage in developing countries," O.P. Arora and K. Narayanan, Telecommunications Journal, Vol. 51-No.XIl/1984, pages 645-649. ~- 12. "Broadcasting [via VHF/FM] of Radio Programmes by Satel~ite Direct to Portable/Vehicle Receivers,' J. Chaplin, H.-H. Fromm, and C. Rosetti, E.S.A. Bulletin, February 1984; pages 77-~. See also "Satel~ite/sound broadcasting to portable and vehicle receivers" by these authors in Telecommunications Journal, Vol. 52-No. I/l 985. (This, the paper by Narayanan, and the studies they reference suggest somewhat different values for certain of the circuit parameters and somewhat different transmission methods than those used here. All should be carefully considered and compared in more detailed studies than thi s. ~ 17 WORKING PAPER

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WORKING PAPER the capacity could be scaled down and/or the surface coverage minimum unit area could be scaled up. Soace Repeater Seument A large, say 200-foot (60 meter) diameter, sophisticated parabolic reflector would be employed. It would be provided with many feeds that would allow a large number of independent, surface-directed radiation patterns or beams to be generated as they are driven by multiple subtransmitters. At the frequency that would be employed, the radio wavelength is 0.4 foot (0.12 meter) so that the antenna's dia~eter/waveJength is 500. With an illumination efficiency of 0.6, the gain re isotropic at the center of each beam Could be 62 db, and the l/2 power beam width would be about 0.14 degree.] Each such beam would illuminate a surface area and provide a broadcasting footprint roughly 100 miles (160 kilometers) in diameter. One beam's tote] surface area would be about 7,500 (circular) square miles (about 20,000 square kilometers) in the subsatellite region and larger than this at higher latitudes. Away from the subsatel~ite region, the footprints would become larger in area, oval in shape, and the Earth's geometry would elongate the patterns. In the farthest regions to be served the surface area/beam would be several tens of thousands of square miles. The basic unit of surface coverage (the standard coverage area) used here for illustrative purposes will be taken to be a footprint of 10,000 square miles (25,000 square kilometers). This is roughly the size of the U.S. states of Maryland, New Jersey, or Massachusetts; the countries of Belgium, Haiti, Israel or Albania; the island of Sicily in Italy; the Republic of Armenia in the Soviet Union, or the principality of Wales in the United Kingdom. Using such narrow beams would provide great flexibility to broadcasters and system operators in selecting audience locations, channels, and broadcasting times and durations. Using these beams, near rea1-time knowledge of the signal transmission and noise circumstances, and dynamic power control among the individual subtransmitters would allow a sophisticated statistical approach to minimizing space segment peak power in the face of large temporal variations in channel capacity demands; the influence of the ionosphere that can cause scintillation-type signal fading, physical structures, foliage and terrain roughness on radiowave path loss; and other variations in noise levels external to the surface receivers. In view of the narrowness of the beams and the need to assure that desired audience areas are properly illuminated, care would have to be taken in l3. NASA now has aspirations to develop a series of UHF space antennas, the most sophisticated of which would have radiation characteristics quite similar to these. They would be used in an R&D program that, in the l99Os, would study the provision of mobile communications in rural areas. Optical astronomers, using controlled parabolic reflector segmenting techniques, are now planning to construct and use lenses with gains of over 150 db re isotropic. ~ ~ _ WORKING PAPER

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WORKING PAPER designing the space segment to insure that satisfactory methods and means of beam steering and stabilization were employed, and that the illumination performance could be easily verified in near real time. This would have to be a truly sophisticated multiple subtransmitter, multiple-feed, space ensemble. Its satisfactory development calls for engineering imagination and skill. Assuming that there would be four active geostationary satellites to provide adequate surface coverage (two each serving the Eastern and Western hemispheres) and with a total of some 30 million square surface miles (80 million square kilometers) to be served on a worldwide basis, on average each would have to deliver an adequate radiowave flux density to the surface over an area of some 7.5 million square miles (20 million square kilometers). Most of today's HE shortwave audio listening population listens during an average 2-hour period during the local dinner and breakfast time hours. Because their local times widely separate them from each other, these audiences around the world would be served simultaneously by separate satellite transmitters. Each satellite transmitter could serve an area as large as 6,000-miles long (Arctic Circle to Antarctic Circle) by lo, 000-kilometer wide, measured at the Earth's equator. With the rotation of the Earth, this area corresponds to a total daily time duration of 6 hours in the A.M. and another in the P.M. It would serve three separate 2-hour (2,000-mile-wide, 3,200-kilometer-wide) areas one-at-a-time by switching the beams, as a group to each of three azimuthal positions in sequence as local times suggest. The number of l0,000-square-mile footprints, and consequently the number of beams required per space transmitter, would thus be (7.5-million/3~/10,000 = 250. Conservatively estimated about 300 subtransmitters would be required to accommodate the actual size and distribution of specific surface areas to be served in any region. Each of the beams, and its associated subtransmitter, would be capable of carrying at least a single audio broadcasting channel, and two or more beams, usually adjacent, could be employed to serve areas larger than 10,000 square miles with the same program (channel) at the same time. Each beam could simultaneously carry, in the limit, as many channels as the system's full capacity, or 300 channels. Those beams serving low population areas would probably carry only a few channels while large metropolitan areas would be served by many. (This subject is discussed further in Section 3.) The reference here to 300 audio channels is a rough initial estimate of the capacity required to serve a single large region by 100 broadcasting governments. Larger countries could well employ lO or more channels simultaneously in a region, while smaller countries perhaps could meet their needs wi th a si ng ~ e channel . To avoid interference, adjacent beams would radiate signals on different carrier frequencies (and perhaps orthogonal polarizations) when their channel s carri eci different programs. The sharpness of the beams and the use of frequency modulation (and perhaps polarization orthogonality) would allow the maximum frequency re-use and thus minimize the number of channels and _ ,9 WORKING PAPER

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WORKING PAPER the amount of radiowave spectrum that would have to be allocated for a DBS-A service. Of course each of the four separate space segments also could re-use the same allocated spectrum. As a first estimate, the total worldwide DBS-A service allocation would require a few MHz of bandwidth. A multibeam switch would be required in the space segment--one that would allow subtransmitters, beams, channels, and frequencies to be appropriately matched in herons of the areas to be served with various programs. The switching would be accomplished reliably and quickly under con~nand from the surface feeder station. The feeder station would receive the information it needed to command the switch from both the individual organizations supplying the programs to be broadcast and from a network of surface signal monitoring sites. The RF power delivered to each beam also would be controlled by the surface feeder station in order to provide a higher or lower than nominal flux density to some areas at some times. This control capability would preserve an appropriate quality of service in the face of changing program-channel-service quality demands and accommodate excess radiowave path loss associated with different areas served, season, weather, and ionospheric conditions. All of this activity would be accomplished in a manner that would keep the required space segment DC peak power demand as low as possible. The space segment maximum DC power required would be some 5,000 watts (see the space segment-surface segment power budget estimation in Section 3~. The system design should anticipate, in a statistical sense, all of the various likely demands for space segment electrical power in order to minimize the peak demand and to minimize the cost of meeting it in space. Only one surface feeder station would probably be needed, since the programs could be routed space segment-to-space segment, as necessary, vi a wideband line-of-sight optical or millimeter wave distribution circuits. If the present aspirations of the leaders of the U.S. Government's civilian space program are realized, a low-Earth-orbit (LEO) Space Station, one or more geostationary service platforms, and an economic LEO-geostationary orbit spacecraft servicing capability should be in place within a decade. Some space industry satellite communications engineers are not as close to these potential developments as are leaders of the U.S. public space program at NASA. They, too, must by necessity plan their costly and financially risky space activities in a conservative, paced, fashion. Some are therefore understandably reluctant at this early date to accept these aspirations as a basis for sound planning. But as such aspi rat i ons are tran s ~ ate d i nto f i rm programs and schedu ~ e s, the space segments could be designed to share basic support services with other assets used in nearby geostationary orbit. The space segments could also be designed to take full advantage of Space Station-based, externally provided, in-space, short-response-time maintenance services that would restore - 20 - WORKI NO PAP ER

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WORKING PAPER performance upon its degradation, and be able to accommodate, promptly, the large changes in broadcasting service requirements. Surface Feeder Seqment The surface feeder site could be served by Intelsat circuits that would allow programs to originate essentially anywhere in the world and be carried to the site for transmission upward via a line-of-sight microwave circuit to the nearest geostationary space segment. It could also be served by Inte~sat circuits that could carry information to the site from surface mon i to ri ng network ~ ocat i on s. The design o, the surface feeder system segment should be straightforward. The maximum base bandwidth required to be transmitted would be 300 multiplexed audio channels each at least 5 kHz wide (as many as lO percent of the channels could be 15 kHz wide) to be delivered to each of four space segments simultaneously, i.e., the rough equivalent of two standard television channels to be transmitted upward over a 23,000-mile, ire-of-sight path. A relatively less valuable portion of the spectrum could be employed, probably in the high SHE region or higher. Surface Receiver Segment The spacewave receivers should be designed to be small, rugged, long-lived, easily and precisely tunable by electronic (not mechanical) means, to have low self-generated input noise; and to have a small antenna, easily pointed toward the fixed, in-space broadcasting transmitter. Different receiver models could (~) be powered by local sources of electricity, batteries, or solar cells, (2) have various degrees of antenna directivity-gain, (3) produce various levels of S/N, dynamic range, and audio power output in various audio bandwidths, and (4) provide other electronic services such as reception of local AM and FM broadcasts, stereo, and aud i o t ape reco rd i ng an d p ] aybac k. A retail price of a very few tens of dollars should be the objective for the Basic and Standard service, handheld, and "kitchen table," fixed and transportable, mass receiver markets. Extra attention would have to be given to the design of truly mobile receivers. Two classes of antennas would be employed: l. For Basic and Standard Service signal quality service reception, perhaps a single half-wave folded dipole with a reflector and 8-10 director elements arrayed in end-fire, Yagi, fashion. Such an antenna's outside dimensions (keeping in mind that the radio wavelength is less than 5 inches) would be about 2" x l/2" x 8". It would have a wide half-power azimuthal beamwidth of about 40, and a gain of some 13 db re isotropic, inclusive of a small cable loss. Use of circular polarization should be considered, and this would suggest a helix rather than a linear Yagi. 21 WORKING PAPER

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WORKING PAPER 2. A high-efficiency parabolic antenna, perhaps l-~/4 feet in diameter would be used for Superior quality service reception and for reception in particularly high latitude or otherwise difficult reception areas. Such an antenna would have a ha~f-power beamwidth of some 25 and a gain of some lS db re isotropic, inclusive of a modest cable loss. In particularly difUcult terrain or in the presence of urban building signal "shadowing" where there could be significant additional diffraction field propagation loss, reception would be improved by elevating the antenna. The spacewave receivers should have noncooJed, low-noise front ends, and exhibit a low noise figure under all usual operating conditions. - 22 - WORKI NG PAPER