Ultimate communications capacity of the geostationary-satellite orbit

The geostationary-satellite orbit is uniquely useful for communication satellites. There is, however, an ultimate limit to the number of communications channels which the orbit can support in a given bandwidth owing to the effects of interference between adjacent satellites. By calculating the minimum separation angles between satellites, the capacity of a 10° arc of orbit and a 500 MHz bandwidth is determined as a function of earth-station-aerial diameter and satellite-transmitter power. Results are given for frequency modulation and pulse-code modulation for telephony, and for frequency modulation only for television. The effect on the results of variations in some of the basic assumptions made are discussed in some detail. These include the interference noise permitted, the interleaving of channel carrier frequencies, the use of narrow-beam satellite aerials, and the use of frequency bands higher than those at 4GHz and 6GHz which are used at present. It is shown that capacities of 2000 telephone circuits per degree of orbit might be expected using earth-station aerials 20¿30m in diameter. It is further concluded that one of the most significant ways to increase the efficient use of a given arc of orbit is to carefully co-ordinate the characteristics and methods of operation of systems using adjacent satellites.