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The Technical Note presents the possibility of applying the classical onboard aviation radar altimeter in the function of a simple anticollision system. Currently, classical radar altimeter offers two important pieces of information for pilots: actual flight altitude and aircraft descent at the level of decision making. The goal here is to draw attention to the innovative technique of using the radar altimeter to generate further crucial information for the pilot of a helicopter or small aircraft for predicting the threat of collision with the terrain. This new technique is based on the evaluation of the Doppler frequency in the differential signal of the radar altimeter. The theoretical assumptions have been verified by simulation and experiment.Nomenclature c speed of light, m/s F D differential signal frequency of radar altimeter corresponding to Doppler effect, Hz F M frequency modulation, Hz F r differential signal frequency of radar altimeter corresponding to helicopter's altitude, Hz F rD resulting differential signal frequency shaped by radar altimeter, Hz f 0 center frequency sweep of radar altimeter, Hz H helicopter altitude, m s distance flown, m t c time remaining to the collision with terrain, s t cr critical time, s U D voltage proportional to difference of the measured frequencies at each half-period of the modulation frequency, V U H voltage proportional to altitude, V v speed of aircraft flight, m/s v v vertical component of speed, m/s α terrain inclination angle, deg β hill inclination, deg f frequency deviation, Hz

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