While the RF-generation and beam propagation are superficially similar to those used in SPS, several distinctions should be noted. 1. An LPS array with adequate projected diameters can be beam shaped by phasing to illuminate a fixed Earth rectenna site as the lunar elevation changes. 2. The LPS can be electrically steered without mechanical motion (except for feed sources or virtual images). 3. Many separate beams can be operated by common reflector elements. 4. Grating lobe and misalignment error problems may be eliminated by aperiodic or quasi-random subarray distributions. RF-DC Conversion The duty cycle of rectennas will depend in a complex manner on latitude, minimum lunar elevation, time dependence of beam intercept profile and other factors. In the absence of Earth orbit reflectors the “effective duty cycle” of LPS would usually be in the range of 27-33%. This may be increased to close to 100% using microwave reflectors in Earth orbit. As previously noted the large aperture of the LPS and use of orbital microwave mirrors allows far more uniform illumination of the terrestrial rectenna. Assuming uniform illumination the LPS system will put 2 to 3 times more energy into a rectenna than can be done with SPS. SPS is essentially restricted to providing base load power. A given SPS can most efficiently service equatorial rectennas situated along its meridian location. There is considerable doubt that SPS could economically service receivers in Scandinavia, for example. Conversely, LPS and its system of orbital reflectors could balance out the diurnal and seasonal power needs of all regions of the world. Operation at higher frequencies with elevated surface and ionospheric power densities can result in considerable downward cost leverage for LPS as compared with SPS. Space Operational Considerations Space operational factors will play an important role in the overall costs of either LPS or SPS. The three primary features of LPS impacting on operations are: 1. Virtual total elimination of structural frames and all interconnecting operations needed for SPS structures. 2. Elimination of more than 90% and perhaps as high at 99% of Earth-to-Space transportation system capacity (mass/year). 3. In-situ deposition of solar converters. Other factors include differences in human and machine productivity due to nature of tasks, residual gravity, ease of providing pressurized work spaces, shielded habitat, absence of interim and final station keeping requirements, etc. Since the time periods of intensive study of SPS and the Convair study of lunar supported SPS, significant advances in automation art have occurred and it is our opinion that such advances will more favorably impact LPS than SPS operations. Access to power and materials on the moon will enable new ways to decrease the costs and increase the safety of space flight and create new industries off Earth.
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