Abstract: 3. An underwater moving-target detection system comprising, in combination, eans for generating and projecting acoustic energy at an adjustable carrier frequency as generated, means for receiving resultant reflected acoustic energy including water reverberation, false-target echoes at substantially reverberation frequency, and moving-target echoes at differing frequencies, means responsive to said received reverberation and adapted to readjust said generated carrier frequency to such value that the frequency of resultant reverberation, as received, is substantially of predetermined fixed value, and coincidence circuit means for enabling utilization of only such said echoes which as received exhibit frequencies differing from said predetermined fixed value of frequency by more than a target-doppler threshold value.