A scanning spot-beam satellite system

We propose a satellite with a high gain, movable spot beam to communicate with individual earth stations time-sharing a single channel in the tdma (Time-Division Multiple Access) mode. It is estimated that this approach could readily save some 20 dB in the link budget while still providing full U.S. coverage. When this 20 dB is apportioned with the objectives of reducing the earth-station antenna size, increasing the satellite capacity, and reducing transmitter power, the effects are dramatic. This technique can be combined with a fixed-spot beam system serving major traffic areas. This combination can provide both full area coverage as well as multiple reuse of the frequency band. A tdma burst organization is proposed, and estimates of burst lengths, beam switching intervals, and buffer storage size are made for a 100-earth-station network operating on a 600 Mb/s channel. A phased array antenna with each element irradiating the entire U.S. is employed to form the movable spot-beam. This provides an attractive solution even though a closed-loop beam-forming algorithm may be required. It appears feasible to construct such an antenna with nearly 50-dB gain capable of forming a spot beam toward any position within the continental United States with a switching time of a few nanoseconds.