Abstract: A receiver for detecting the presence of FM-CW signals emanating from a radio altimeter, and for determining the identity of the source. The received signals are amplified and applied both directly and through a delay circuit to a mixer. The delay is such that the resulting difference signal from the mixer is a low frequency signal in the audio frequency range. This signal is filtered, converted to digital values, and processed by fast Fourier transform techniques. Once processed, the spectral content of the difference signal is analyzed to determine if the signal originated from an airborne altimeter. Characteristics of the original signal modulation are also analyzed to ascertain the identity of the altimeter and its transporting aircraft.