SATELLITE OPTICAL EFFECTS A. DIFFUSE REFLECTION Each satellite is oriented so that the solar cell array approximately faces an observer at the subsatellite point once each day, at local midnight. This condition combines the maximum projected area visible to the observer with the darkest sky and represents a worst case. For this worst-case situation, an attenuation factor for scattered sunlight can be obtained by assuming that sunlight intercepted by the solar array is scattered in a Lambertian (cosine) pattern with a diffuse albedo a. If the solid angle subtended by the satellite is ft , the expected illuminance at the Eartn's surface, expressed as a fraction of noon sunlight, is aft /II = -8 1.38 x 10 a. For a 4% albedo, approximately 1/4 that of lunar material, the satellite will appear as bright as Venus at its most brilliant. The actual albedo is a combination of the diffuse reflectance of the solar cell surface, whicn will be small for any efficient cell, and the combined specular reflections from variously oriented pieces of satellite structure. Discussions with a solar cell manufacturer indicate that current Si cells absorb 93 to 96% of all incident visible light, leaving no more than 7% total reflectance, most of which is probably specular. However, the end- of-life degradation in cell and concentrator efficiency is presumably a result of increased surface roughness and other damage. This would increase diffuse reflectivity. The uncertainties encountered here are typical of those to be found tnroughout this effort to assess SPS effects. Pending the availability of better data, the value a = 0.04 mentioned above can be recommended as a reference value, and the satellite taken as approximately as bright as Venus ever is. The combined light from 60 satellites will then be approximately as bright as that of the moon halfway between new and quarter phase. The solar blanket subtends an apparent angle of 1/2 x 1 arc minutes, and will appear to the naked eye as a point source under all but ideal conditions.
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