A method and a device for measuring altitude of an aircraft relative to a point on the ground, said aircraft carrying a radar system comprising a directional antenna to transmit a radio frequency signal along a aiming axis, including. controlling the transmission of a radiofrequency signal along the axis, calculating received powers as a function of radial distance on a sum channel and an elevation deviation channel, calculating tilt angular deviation values, determining an estimator of the radial distance of the aircraft relative to the point on the ground intercepted by the aiming axis as a function of at least one zero crossing of the angular deviation measurement in a selected area of the angular deviation measurement curve, calculating an aircraft altitude relative to said point on the ground as a function of the estimator of the radial distance and the elevation angle of the aiming axis.