Fig. 1. Reference SPS configurations. within about three minutes of arc (1). Since this is much less than the apparent diameter of the sun (about 32 arc minutes), there will be no appreciable additional spreading of the reflected beam. The antenna could, therefore, be regarded as an excellent mirror. The solar cells contribute most of the reflectance of the solar array. For the CR1 configuration, cell area is about 50.1 km2. Cell reflectance depends on the type of cell used. The silicon cell now planned (2) would incorporate a reflective underlayer for improved efficiency and heat rejection, and is expected to have a total reflectance of about 0.25 in the spectral range from 0.35 to 2.0 /xm. The relative magnitudes of the specular and diffuse components are not well established; in this analysis, they are assumed to be equal, i.e., 0.125 each. Other types of cells may have lower reflectance, e.g., 0.04 diffuse and 0.035 specular. Because the difference could be significant, both values are used. The Kapton substrate on which the individual solar cells are mounted has about 1.8 km2 of exposed area between the cells. With a diffuse reflectance (2) of 0.67, this can make an appreciable contribution to the total luminosity of the array. The exposed structure on the front surface of the array has an effective area of about 0.24 km2 (estimate by the author). A reflectance of 0.85 has been assumed. Both diffuse and specular reflections must be considered. Because the analyses are entirely different, they will be examined separately. 3. DIFFUSE REFLECTION If the reflected energy were distributed uniformly over the hemisphere of radius R visible from the array surface, the apparent luminosity would be that of the sun
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