44 in the fall. Maximum duration is about 75 minutes for an eccentricity of 0.04 with perigee toward the sun. Other perigee positions reduce the time slightly; higher eccentricities increase it. Duration varies from day to day as the earth's shadow moves across the orbit (figure IV-A-3-12). With the exception of the first and last day or two of each series, the eclipse is total. Minimum time in the penumbra is about 2.3 minutes and occurs at the beginning and end of the maximum duration eclipse (see figure IV-A-3-12). Since the width of the penumbra is roughly 400 km and the array is less than 30 km in its longest dimension, the illumination gradient over the array is slight at any instant of time. Power loss due to eclipse by the earth is 0.97% of the total output averaged over a year if the SPS can begin transmitting at full power as soon as it emerges from the eclipse. This figure must be increased to account for any start-up time that may be required. The impact on the ground distribution system is lessened by the fact that the eclipse occurs at about local midnight, when demand is relatively low. However, electrical energy storage must be provided to operate spacecraft systems during the eclipse. For the high-performance reaction control systems, with average power requirements in the multi-megawatt range, this can be a significant item (see section IV-B-4). At an average longitude separation of 0.5° (see following section), as many as 38 satellites will be in the earth's shadow at one time. If each SPS is located as close as possible to its rectennas to minimize losses, eclipse by the earth will tend to cause simultaneous loss of all satellite-produced power to large sections of the country. Some losses due to longitude offset may be preferable to such concentrated outages. Eclipse by Other Satellites The number of satellites contemplated by the SPS program will require close spacing between satellites. As noted in section IV-A-3-b, 112 satellites between the longitudes of Maine and Oregon would be separated by about 0.5° of longitude, or 368 km at synchronous altitude. At this distance, an SPS can be significantly eclipsed by its neighbors when the sun is in or near the orbit plane. The eclipse occurs where the solar vector is tangent to the orbit or, roughly, at 6:00 a.m. and 6:00 p.m. rectenna time. In an eccentric orbit, the morning eclipse is later and the evening eclipse earlier. The easternmost satellite, of course, will not be eclipsed in the morning (figure IV-A-3-13), nor the westernmost in the evening. The duration and overall intensity of the eclipse are illustrated in figure IV-A-3-14 for several dates before or after the equinox. The
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