The SPS, in synchronous equatorial orbit, will be eclipsed by the Earth daily for about 43 days at the spring equinox and 44 days at the fall equinox (fig. IV-5). The maximum duration is about 75 minutes. The eclipse is total and occurs at about local midnight. Because the maximum dimension of a typical SPS is 6 to 7 percent of the width of the penumbra, the illumination gradient is slight. Total power loss is slightly less than 1 percent of total annual output. The close spacing (about 0.5° of longitude) that results from a large number of satellites (112 located to serve the United States) will cause the satellites to eclipse each other twice a day, at about 6 a.m. and 6 p.m., for about 2 weeks, at the equinoxes. This eclipse is shorter and is not total, but will cause almost complete microwave power loss for as long as 15 minutes. The penumbra is much narrower than the satellite dimensions, so that illumination gradients are steep. Differential thermal expansion must therefore be accounted for in the system design. Figure IV-5.- Eclipse geometry.
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