Tracking a moving acoustic source in a three-dimensional space

One of the most challenging tracking applications is to monitor the precise underwater movements of echo-locating cetaceans (dolphins, porpoises and whales) in the vicinity of a large pelagic trawl, i.e. in a moving frame of reference. The aim of the research is to provide information that may lead to a reduction in the by-catch of these animals. For example, by observing a dolphin as it approaches the trawl it may be possible to determine if or at what range it becomes aware of various deterrents, such as the trawl itself, or attached passive sonar reflectors and active acoustic pingers or transponders. To produce a valid estimate of a dolphin´s instantaneous position, the tracking algorithm must determine the arrival times of the same echo-location click at all the hydrophones. The maximum spread of these arrival times is dependent only on the positioning of the hydrophones and the sound speed in water. The practical arrangement on the net comprises two parallel streamers, each with two hydrophones, all four therefore being in the same plane, and a fifth hydrophone above the trawl headline out of this plane