 In these systems a "carrier" radar signal is frequency modulated in a predictable way, typically varying up and down with a sine wave or sawtooth pattern at audio frequencies.The signal is then sent out from one antenna and received on another, typically located on the bottom of the aircraft, and the signal can be continuously compared using a simple "beat frequency" modulator that produces an audio frequency tone from the returned signal and a portion of the transmitted signal.Since the signal frequency is changing, by the time the signal returns to the aircraft the transmit frequency has changed.