Fig. 4. Effects of Antenna Tilt on Grating Lobe Magnitudes and Power Collection Efficiency, a) x Tilts with 104 m subarrays, b) = Tilts with 10.4 m subarrays. summary, the more distant lunar microwave array must have 100 times the surface area, and approximately 100 times as many phased subarrays as an equivalent geosynchronous SPS. To minimize electronic costs for these additional subarrays, full pilot beam re- trodirective phasing equipment might be limited to a restricted number of active control subarrays (e.g., a single 10.4 m x 10.4 m subarray located at the center of each 104 m cluster). The resultant phasing signal would be differentiated and distributed via a microprocessor to the other subarrays within the cluster. Wiring and hardware for such a scheme would cost approximately $300 for each passively phased subarray, thereby saving almost $5,000 each over the cost of fully retrodirec- tive subarrays. The 10 km lunar antenna then would incorporate 1,070,000 power conversion tubes at 10.4 m spacings, with only 7,220 fully retrodirective phase control receivers. Each of the 7,220 subarray clusters will require a two-dimensional mechanical pointing system. These active gimbal mounts are needed in order to track ground rectennas through their daily rotations and to compensate for lunar libration tilts. A third set of configurational handicaps for the lunar solar power concept arises from the intricate, non-geosynchronous orbital interaction of the Earth and moon. These problems include: the triple redundancy of ground rectennas with each inactive two thirds of the time due to the Earth's diurnal rotation; an approximate fourfold increase in required rectenna land area to capture the full power beam at
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