Figure 18 Subarray Size Considerations there will be a tradeoff of power loss vs controls cost. Figure 18 also shows other factors producing power loss: subarray tilt relative to the overall array nominal position, and subarray distortion, both of which will be strongly influenced by thermal effects. Efficiency and safety needs dictate that a closed Loop form of control be implemented for phase front or beam formation. Two approaches, adaptive and command, have been formulated and are illustrated in Figure 19. The command system uses a matrix of sensors at the ground antenna to determine the received power beam center and shape. A processor then develops commands which are routed to the subarrays over the telecommunications link. This approach has limited resolution, but nevertheless it is anticipated that antenna thermal distortions, a major source of error, can be accurately modeled and suitable command algorithms developed. In any event it will serve as a system monitor and as a safety override function. A potentially more accurate scheme calls for a reference beam to be launched from the center of the ground antenna. This is sensed at each subarray and at a reference subarray in the antenna center. The latter transmits the reference to the sub- array over a calibrated coaxial cable at which point it is compared with the incoming beam. A difference in phase between these signals is interpreted as a displacement of the subarrays from the nominal reference plane, due for example to thermal distortion of the structure, and a correction is applied to the phase of the transmitted beam at the subarray so that the required beam front is launched toward the ground antenna. A
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