Abstract: A counter-terrorism underwater surveillance system for detecting swimming intruders includes a sonar array 20 comprising a plurality of sensor elements which both transmit and receive acoustic signals. Power amplifiers 22 generate electric transmit signals which are converted to acoustic form for transmission by the sonar array 20. Incoming acoustic signals, including echo data from any intruder in the water, are received by the sonar array 20 and passed to a data acquisition subsystem 24, which digitises the data for processing. The digitised data is passed to a beamforming subsystem 26 which forms defined beams from omni-element data. A detection processing subsystem 28 then extracts signals from noise and reverberation and passes these to a display processing subsystem 30 which defines tracks from the intruder echo data. Finally, intruder images and tracks are displayed on a display subsystem 32.