A technique for deriving correction signals for inertial instruments on a missile, such as an intercontinental ballistic or a cruise missile, is shown to include: a radar operative in a synthetic aperture mode when the missile approaches a predetermined area containing at least three separate fixed terrain features whose radar signatures (as observed from a preselected position) have been predetermined and stored on the missile; successively illuminating each one of such features and comparing the actual radar signals with the stored radar signatures; and deriving correction signals for the inertial instruments in accordance with the difference between the actual and stored radar signals.