An alarm or signalling system between a transmitter station and a receiver station either or both of which may be in motion. A signal is selectively radiated from the transmitter station for detection at the receiver station. The receiver station includes processing means for responding to the received signal and producing a new signal indicative of selected physical relationships between the transmitter and receiver stations during the period of signal transmission and reception. The modified signal may indicate approaching or departing relative motion and/or magnitude of separation. One embodiment may incorporate synchronizing signals transmitted from a central control station. This embodiment may include receiver stations with and without the synchronizing equipment.