Patent
1975-07-10
1977-01-18
Stewart, David L.
325 4, H04J 306
Patent
active
040040984
ABSTRACT:
A Space Division Multiple Access (SDMA) Communications satellite system which utilizes a pair of communications satellite relays having a communications link between them. Each satellite includes a plurality of receive spotbeam antennas which are selectively connected to a plurality of transmit spotbeam antennas by an on-board switching matrix under control of an on-board oscillator. Each spotbeam antenna communicates with a different geographical zone which includes one or more earth stations. Each station may communicate with all other stations in the system including those within the field of view of the spotbeam antennas aboard the other satellite. In order to maintain synchronization between the satellites, a designated reference station transmits a reference synchronization burst of four sequential unique words to the satellite within its field of view. The reference burst is related to the other satellite and back again to the first satellite where it is relayed back to the reference station. Relative phase drift of the satellites' respective oscillators is detected at the reference station as a failure to detect the first or fourth unique word. Improper propagation delay due to distance variation between the satellites is detected at the reference station as a failure to detect the second or third unique word during a predetermined time period. Oscillator drift is corrected by changing the frequency and/or phase of the oscillator in one of the satellites through telemetry control circuitry at a control facility which may be the reference station. Improper propagation delay is corrected by adding or subtracting delay in the satellite to satellite communications path by the use of variable delay pads aboard one of the satellites which are remotely controlled by telemetry signals from the control facility.
REFERENCES:
patent: 3757334 (1973-09-01), Raabe
patent: 3789142 (1974-01-01), Shimasaki
Communications Satellite Corporation (Comsat)
Kasper Alan J.
Maioli Jay H.
Stewart David L.
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