B. SPECULAR REFLECTION OF SUNLIGHT Both the solar cell blanket and the transmitting antenna array have large, flat specularly-reflecting surfaces. Although the antenna array is much 2 2 smaller, 0.8 km versus 54 km , its expected reflectance is much greater, leading to comparable estimated illumination levels at the Earth's surface for reflections from each surface. The antenna's aluminum front surface is expected to have a specular reflectance above 0.9. Of the various possible reflecting interfaces in the solar blanket, the boundary between vacuum and front cover sheet is easily analyzed and can be used to set a lower bound to array reflectance. The silicon cell option employs a borosilicate glass cover sheet with a reflectance of 0.04. The GaAlAs option employs synthetic sapphire with a reflectance of 0.063, but at a concentration ratio of two, so that only half of the blanket area is cover sheet. The effective reflectance for the GaAlAs option is therefore 0.032. We adopt a mean value of 0.036. The reflected spot of light on the surface of the Earth may be regarded as a pinhole camera image of the Sun approximately 330 km in diameter. It will be reduced in brightness from that of noon sunlight by the product of the specular reflectance of the surface and the ratio of solid angle subtended by the satellite to that subtended by the Sun's disk. For the solar blanket, the _5 illumination as a fraction of noon sunlight is 2.3 x 10 , or approximately ten times that of brightest moonlight. For the antenna array, the fractional _ 6 illumination is 8 x 10 , roughly four times that of brightest moonlight. If the solar blanket were held precisely facing the Sun, then specular reflection from it could fall on the Earth only for the brief period when the satellite was in the Earth's penumbra, and would be visible only at local sunset or sunrise. If the satellite's attitude is controlled only well enough to avoid significant power loss, the reflected spot could fall on a much larger portion of the Earth's night side. The transmitting antenna is constrained by beam-forming requirements to point with high precision directly toward its rectenna array. The path of the reflected spot across the surface of the Earth is, therefore, completely determined once the longitude of the satellite and location of the rectenna are specified. Livingston (L. E. Livingston, Visibility of Solar Power Satellites from the Earth, Document JSC-14715, L. B. Johnson Space Center,
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