1. Apparatus for determining the ground velocity of an aircraft comprising: means for detecting from the aircraft a first radiant energy contour of a first area of the earth's surface and, after a time delay, a second radiant energy contour of a second area of the earth's surface, which second area overlaps in part with the first area; means for comparing said first contour with said second contour to produce a position shift signal which represents the relative contour position shift necessary to obtain the maximum degree of correlation between said first contour and said second contour; means for producing an altitude correction signal; means for multiplying said position shift signal by said altitude correction signal so as to produce a distance signal; and means for dividing said distance signal by said time delay to obtain a ground velocity signal of the aircraft.