Sonar Signal Processing Methods for the Detection and Localization of Fast Surface Watercraft and Underwater Swimmers in a Harbour Environment

Cavitation inception behind each blade of a rapidly rotating propeller generates intense continuous broadband sound underwater. This sound can be considered as an acoustic signal propagating in the underwater medium where it can be received by a single hydrophone located above the sea floor. Reflections of the sound from the boundaries of the shallow water medium result in multipath arrivals at the hydrophone. Interference between the direct path and multipath arrivals produces a Lloyd’s mirror pattern on the timefrequency display of the hydrophone’s output. It is shown using real data that cepstral analysis of the output of a single hydrophone enables the multipath time delay to be measured, which in turn, enables the variation with time of the instantaneous range of a fast surface watercraft to be estimated during its transit past the sensor. Also, a nonlinear least squares approach is adopted to estimate the three motion parameters of the surface watercraft using the sequence of multipath delay measurements obtained during the watercraft’s transit. This type of approach is extended to the estimation of a complete set of five motion parameters that describe fully the trajectory of the watercraft’s transit, using the sequence of differential time-of-arrival measurements of the direct path signal at each pair of sensors that comprise a four-element line array. The rotating propeller of a fast surface watercraft also generates a wake of bubbles that persists for minutes. When the wake is insonified by high-frequency (HF) active sonar transmissions, wake echoes are observed which are composed of a multitude of reflections originating throughout the entire wake. Echolocation of the returned sonar signals provides a trace of the wake and hence the trajectory of the watercraft. The use of a monostatic HF high-resolution active sonar for the automatic detection, localization and tracking of a fast surface craft is reviewed. The HF sonar is then used to monitor the movements of underwater swimmers and dive boats during a night navigation training exercise. Sonar imagery capturing the dive boat wakes and the egress of two underwater swimmers in shallow waters adjacent to a naval base are presented.