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Orbits
By far the most useful orbit for communication satellites is the geostationary satellite orbit. This is a direct equatorial orbit about 35800km above the ground, the period of which is the same as the length of the sidereal day, about 23 hours 56 minutes. A satellite in this orbit, moving in the same direction as the Earth's rotation, remains stationary as seen from points on the Earth's surface. A geostationary satellite has line of sight coverage of a great area of the Earth and, as Clarke noted in 1945, three of them suitably located around the Equator could cover almost all the Earth's surface. Figure 51.1 shows, for example, the coverage provided by satellites located at 30°W, 150°W and 90°E longitude.
There are, however, other orbits of interest for satellite communication. The USSR, with a territory which has an exceptionally wide span in longitude at high latitudes, found 12-hour elliptical orbits inclined at about 63° to the equatorial plane to be preferable for its domestic ORBITA network. These satellites are operated for periods of about eight hours when they are close to their apogee, about 40(X)0km high above Siberia, but three satellites are required to provide continuous coverage.
Satellites in geostationary and 12-hour elliptical orbits are at great distances from the surface of the Earth and the transmission loss is very high; at 1.6GHz, for example, the free space loss is about 188dB. This loss has particularly serious consequences for satellite communication with mobile earth stations and in particular for hand portable and vehicle mounted stations, which typically have little antenna gain. Links with satellites in low orbits would have significantly less loss; for example, for an orbit only 1500km above the ground the loss would be about 28dB less, which would make a great difference to the weight of batteries required in a pocket radio telephone. For reasons such as these, low orbits are being seriously considered for this application.
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