Phase Control - The phase control system for the SPS must provide high accuracy beam focusing and pointing in the presence of a nonhomogeneous, time-varying ionosphere, thermal distortion of the transmit antenna array and subarrays, and phase variations in the phase reference distribution system, power amplifiers, conjugators, and other electronic components in the system. The subject has been investigated by Raytheon, JPL, Rockwell, Boeing, LinCom, MSFC and JSC. Raytheon studies in 1975 investigated both the command and adaptive (retrodirective) approaches. The command approach relies on (a) phase estimation measurements made over a matrix of sensors covering the received power beam, or (b) a "bit wiggle" examination of each transmit subarray performance by commanding a distinctive phase modulation on the output of each subarray. The retrodirective approach uses a reference pilot beam transmitted from the rectenna to each subarray or power module on the transmit array, where precise phase measurements are made and conjugation of the pilot signal occurs. To varying degrees, each organization mentioned studied the phase control problem and documented various concepts. In an effort to study the problem on a system level, end-to-end basis JSC awarded a contract to the LinCom Corporation in April 1977. Initial activity consisted of system analysis to evaluate potential techniques for accomplishing phase distribution and beam steering for such large arrays. Basic concepts previously discussed, only superficially, were compared and evaluated on an overall system engineering basis. The techniques considered for beam steering included (a) phase conjugation and open loop phase shift control, (b) ground network monitoring and uplink commanded phase adjustments. The techniques considered for phase distribution included (a) mutually coupled oscillators and (b) electronically compensated distribution system. This LinCom activity has resulted in a phase control concept which partitions the system into three levels and which has been incorporated into the microwave reference system. The first level consists of a reference phase distribution system or tree which electronically compensates for distribution path length variations to maintain a constant phase reference at each
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