Abstract: A bistatic, Doppler sonar that having a plurality of receiver channels det intrusion in an insonified water zone processes return signals in a plurality of discrete up Doppler and down Doppler frequencies to effect short and long term averaging of return energy and short and long term averaging of return energy slope prior to sampling and comparison to generate a multiple bit binary word that is periodically updated and read out in parallel form to detection logic. The detection logic utilizes parallel sets of shift register memories and comparators to detect when rates of occurrence of energy, slope, and stroke conditions exceed predetermined rates, and utilizes a plurality of levels of coincidence gates to detect predetermined combinations of such conditions that will produce an alarm initiating signal.