are 21 kW/m^2 and 23 mW/cm^2 , respectively, although future tests could indicate revisions. Since the variation of the two factors with antenna size is different, the limits can be used to calculate not only the maximum output, but also antenna and rectenna diameters. Using the microwave system parameters defined elsewhere in this report, maximum output on the ground is approximately 5 GW with an antenna diameter of 1 km and a rectenna diameter of 10 km (figure A-4). Construction - Construction of the satellite is discussed in Section F of this appendix. However, satellite configuration strongly influences ease of construction, which is probably necessary if the SPS is to be economically competitive. In general, a configuration that permits a high degree of automation in its construction, such as the reference photovoltaic system, can be built more easily. Conversely, a typical thermal cycle configuration requires a large number of different operations. Many of these are performed only a few dozen times. A large number of fluid connections must be made. Even the highly repetitive tasks, such as reflector facet installation, are largely discrete rather than continuous. All of these are comparatively difficult to automate, and as a result the thermal SPS is relatively difficult to construct. Power Output Variations - Ideally, SPS power output at the rectenna would remain uniform at all times. Actually, however, the power output will depend on the intensity of illumination of the solar col lector/converter, its efficiency, and the efficiency of the microwave transmission collection and conversion system. Figure A-5 illustrates the net effect of these variations for a solar photovoltaic solar collection/conversion system. In this case, the 180-day cyclic variation is caused by variation of the sun's declination (angle) with respect to the orbital plane. Superimposed on this variation is a daily cyclic fluctuation resulting from orbit eccentricity. Orbit eccentricity causes a variation in satel1ite-to-rectenna distance, affecting transmission/collection efficiency. The power output of a solar array depends on the intensity of illumination at the cells and the temperature of the cells, the power output of cells diminishing as the cells become hotter. In geosynchronous orbit the temperature of the solar cell is related to the intensity of sunlight for any given panel configuration.
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