Abstract: An intrusion detector operating with electromagnetic sensors to detect intruder motion within large, defined areas is disclosed. The system provides an acceptably low false alarm rate by means of range discrimination whereby moving objects outside the protected area do not affect the system and by improving sensitivity to slow-moving targets even when located very close to the sensor location. The system incorporates a transmitter which is frequency modulated by a triangular or sawtooth wave and a receiver which produces an output signal that varies in accordance with the instantaneous difference between the transmitted and received frequencies. The frequency of this difference signal is the beat frequency, and is proportional to the range of the target. Storage means are provided to permit comparison of successively received signals over a predetermined period of time, and this comparison serves to reveal slowly varying changes in the return signal pattern.