Previous studies of photovoltaic satellite power systems had concentrated on a low aspect ratio configuration using silicon solar cells with assembly taking place in low earth orbit. The fully assembled satellite was transferred from LEO to GEO using advanced technology electric propulsion systems. Early results from investigations conducted at Rockwell's Science Center were indicating a great potential for gallium-aluminum arsenide (GaAlAs) solar cells in the areas of thin-film cell development possibilities, higher theoretical efficiencies, and strong resistance to radiation damage. The high aspect ratio configuration alternative was selected for study primarily for two reasons. First, the long, thin structure allowed the design of a microwave (MW) antenna rotary joint which would not require microwave transmission through structural elements. Second, preliminary calculations showed that significant reductions could be achieved in the annual propellant requirements for attitude control. The SPS configuration used as a reference concept is shown in Figure 1.2-1. The selection of equatorial geosynchronous orbit as the reference satellite construction location using conventional chemical systems (e.g., LO2/LH2) appeared to be a good compromise between the advanced GaAlAs solar-cell technology assumption and the existing propulsion technology. Furthermore, the uncertainties surrounding the impact of day-night cycles every hour and a half and drag effects would not be encountered at GEO. Also the concerns of possible collisions, revenue losses incurred with long-duration transfers, and all degradation due to Van Allen belt radiation would be minimized. Table 1.2-1. SPS Concepts
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