In these relationships, i is the unit vector of the sun's incident light, h is the orbital altitude, and 6380 km is the earth's radius. Again n is the reflecting surface's unit normal vector and X is the unit vector from the reflecting surface to the earth's center. 3.1.3.2 Irradiance Levels Reflections toward earth produce ground irradiance that is described by the diffuse and specular expressions derived in Section 3.1.1 above. The formulas (Equations 1 and 2) are summarized here for convenience along with the definitions and units of the various factors and terms that enter the expressions. 2 The diffuse irradiance produced by SPS elements in W/m is given by o The specular irradiance in W/m produced by reflected light from an SPS subsystem is
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