antenna tracks the ground receiver. Several other cases are calculated including diffuse reflections that are orders of magnitude dimmer than specular beams from the same element. For purposes of comparison with starlight, the apparent stellar magnitude of the SPS antenna is -13.8, comparable to the full moon, and the diffuse reflection from the entire SPS is -3.6,comparable to Venus. These worst case irradiances may be reduced significantly and perhaps eliminated by appropriate modifications to the design and/or operation of the Space Power System spacecraft. By slight changes in vehicle orientation the specular reflections from the large solar panels can be directed away from the earth. Another modification is to introduce surface curvature or misalignment of flat panels that diverges the specular beams. Finally the specular surface quality of the SPS antenna, for example, can be eliminated (or sharply reduced) by etching or coating the material to create a diffuse reflecti on. The methodology for calculating sky brightness created by a diffuse source is also presented. For observations directed well away from the source, typical skylight is on the order of 1% of the direct irradiance. A complete set of equations and tabular entries is provided for evaluation of explicit cases.
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