"Sonar will remain the prime surface ship sensor for surveillance, detection and tracking submarines for the foreseeable future. But with the shift to the littorals and the proliferation of quiet non-nuclear submarines there is today a drive towards developing new technologies and techniques aimed at optimising anti-submarine warfare (ASW) performance in the acoustically poor conditions found in coastal zones. The littoral is a complex and challenging environment for ASW. High ambient noise levels and reverberation levels hamper the operation of both passive and active sonars. Another problem is the mixing of estuarine fresh water with the sea, creating marked variations in temperature and salinity. It follows that threats that are difficult to detect and the clutter found close to the shore often cause a high rate of false alarms, which distract the sonar operating team."