Fig. 2. Arrangement of a Lunar Solar Collection Mound (triangular cross-section). as compared to SPS construction. These low lunar costs per unit area versus SPS costs were assumed in order to present a very conservative comparison, weighed in favor of the lunar case. The lunar microwave system has the same configuration as that proposed for the geosynchronous sytem; i.e., a phased array antenna with either klystron or magnetron power conversion tubes operating at 2.45 GHz. The phase control system uses a pilot beam signal from the ground rectenna to provide phase conjugation information for retrodirective phasing. This type of antenna configuration could support either single or multi-beams. GEOSYNCHRONOUS SPS MICROWAVE SYSTEM The reference SPS configuration had a 1 km antenna with a 10 dB gaussian taper transmitting 6.7 GW of power to a 10 x 13 km rectenna (2). This single beam system met environmental constraints of 23 kW/m2 on the transmitting antenna (a thermal limit) and a peak power density of 23 mW/cm2 on the ground (to prevent possible nonlinear heating effects within the ionosphere). One negative aspect of this system was that the first side-lobe level had a peak of .08 mW/cm2 which, by some microwave standards, was excessive. An optimized SPS configuration was subsequently developed which had a 1.4 km diameter antenna transmitting four microwave beams with a total of 11.7 GW to four ground rectennas (3). These beams were optimized to meet the ionospheric and thermal environmental guidelines and also a first side-lobe peak of .01 mW/cm2 (the most stringent microwave standard). The electricity costs were reduced to 45.5 mills per kWH. For the present study, the electricity costs for the lunar based system will be compared only with the initial reference SPS configuration with a cost of 46.8 mills per kWH (in 1977 dollars).
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