Disadvantages With the PSPS system, there will be motion of the satellite relative to the rectenna as the satellite moves along its orbital path. This may be compensated by a slow spin of the satellite and mechanical and electronic steering of the microwave beam. The plane of the orbit should probably be kept orthogonal to the direction of the sun and the spin axis of the satellite should be parallel to the sun direction. (This flight configuration is illustrated in Fig. 3.) This has an additional advantage. The satellite will never be shadowed by the earth and therefore not subject to the thermal shock inherent in the geostationary SPS. Construction materials for the satellite structure with high coefficient of thermal expansion such as aluminium can now be used safely. An additional complication over the conventional SPS is the change in azimuth of the rectenna relative to the satellite as the rectenna undergoes circumpolar motion. This will preclude the use of a linearly polarized microwave beam. The feasibility of circular polarization will need to be investigated. Advantages and Summary We summarize the advantages of the PSPS as follows. The rectenna is removed from the populated areas of the globe, therefore, possible interactions between the microwave beam and the majority of the Earth's biological systems are eliminated. Radio frequency interference between the microwave beam and man-operated electronic systems such as computers and heart pacers, and communication systems including other geostationary satellites and radio telescopes is greatly reduced. Small proof-of-concept or modular growth satellites can be placed in orbits with apogee less than 6.6 Re and hence can transmit at lower than the nominal 5 GW and still provide a beam intensity at the rectenna capable of maximum conversion efficiency to electricity. This reduces the front-end cost and can lead to an earlier payback. In the case of modular growth satellites, as the power is increased by adding modules, apogee can be incrementally increased. These represent very important advantages over the geostationary SPS because they address objections to the SPS concept that involve financial, institutional and emotional factors. The obvious disadvantages to the PSPS are the motion of the satellite relative to the rectenna, the circumpolar motion of the rectenna, the need for more than one
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