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An FM-CW radar is described which simultaneously furnishes three-dimensional velocity and absolute altitude information. The velocity is measured by means of a low frequency FM-CW Doppler technique and the altitude information by means of phase comparison on one of the sidebands of the received spectrum. The optimum radar parameters required for the two functions are completely compatible. Use of the first order sideband permits operation of the system down to zero feet altitude. Offset reference frequency techniques and narrow band leakage elimination techniques permit system operation through zero speed and to negative speeds. Contrary to conventional FM-CW altimeters, the altitude measurement technique is unaffected by amplitude noise and by frequency (Doppler) noise due to vehicle motion. The radar technique is particularly applicable to helicopters, VTOL aircraft and lunar landing vehicles, and possibly also to satellite rendezvous. Actual flight test results have demonstrated the feasibility of the radar technique.