The calculation is actually simple to describe without the need for a diagram [I hope]. psi = solar depression angle (e.g. If Sun 12 deg below the horizon, psi=+12) theta = angle observer-Earth centre-satellite (R+h)^2 + R^2 -d^2 where cos(theta) = ------------------ 2.R.(R+h) h=satellite height R=Earth radius d=range of satellite Then the satellite elevation for reflection from a nadir-pointing surface is elev = psi - 2.theta and in the same azimuth as the Sun unless the phase angle (defined as new=180, full=0 deg) is less than 90 deg, in which case the azimuth is opposite the Sun. Cheers, Rob McNaught (rmn@aaocbn.aao.gov.au)