The detection and measurement of the distances to obstacles from an aircraft equipped with detection apparatus according to my invention includes means for transmitting recurring impulses and means for detecting echo pulses produced by the obstacles in response to the transmitted impulses. The apparatus also includes a circuit in which the flow of current is controlled by means responsive to the transmission of impulses on the one hand and the echo pulses on the other hand. This control is effected by producing a first series of control pulses in response to the transmission of impulses and a second series of control pulses in response to echo pulses received from the nearest or some other more distant obstacle. I