1. A radar calibration apparatus comprising in combination: a radar unit with electronically steerable phased-array antenna to produce a radar beam, said radar unit being located on an airborne platform to observe terrain, terrain features and ground targets and to provide radar derived angle data; means for navigation mounted on said airborne platform to provide position information and distance measurements, and an oblique camera means located on said airborne platform to provide a first image of the field of view directly under said airborne platform and a second image of the field of view in the direction of said radar beam, said oblique camera means receiving a timing signal from said radar unit to synchronize the camera exposure to the same time as the radar angle measurement, said first and second images are scanned in said camera means to obtain angle and position measurements and of said terrain, terrain features and ground targets, said camera means receiving position information and distance measurements from said navigation means, said camera means providing an error correction signal to said radar unit to correct the difference between the radar beam angle and the camera optical angle.