Abstract: The present invention is related to intrusion sensors for alarm systems which employ Doppler detection. The transmitter of this invention transmits pulse modulated signals having a pulse repetition frequency greater than twice the highest Doppler frequency to be detected. In a manner similar to the well known sampling theorem, the received reflected pulses contain all the Doppler information contained in a similar continuous oscillation Doppler detection system because the highest Doppler frequency to be detected can be reconstituted from the received reflected signals. The duty cycle of the transmitted pulses is limited by the above described pulse repetition frequency limitation and a further requirement that the pulse length be sufficient to enable return of the leading edge of the pulse prior to the end of the pulse.