1. Method for providing a known reference point for an airborne imaging system, the method comprising: (a) providing at measured earth co-ordinates a camera/mirror assembly having a camera and a mirror mounted in fixed mutual spatial relationship and capable of tilting about two mutually orthogonal axes; (b) using said camera to track the sun and produce at least two or more different measured times respective time tagged camera images; (c) using said time tagged camera images to obtain a transformation of mirror axes relative to axes of the earth at a mirror location on the earth where the mirror is mounted; (d) during a time window when the mirror is within a line of sight of the airborne imaging system and the sun, adjusting the azimuth and elevation of the mirror so that the sun is reflected by the mirror toward the airborne imaging system thereby capturing an image of the mirror in an aerial image produced by the airborne imaging system; and (e) determining a location in the aerial image corresponding to the mirror thus providing a known earth reference location in said aerial image.