Potential improvements to current methods of recognizing single targets with a split-beam echo-sounder

The precision of in situ target-strength (TS) estimates generated by TS analysers is directly related to the ability of single-target detectors to reject multiple echoes in dense aggregations. The rejection performance of five single-target detection algorithms, utilizing phase, amplitude, and echo duration information from a split-beam echo-sounder as rejection criteria, were tested by simulation for tar et pairs randomly generated in a three-dimensional resolution volume. Each algorithm was also tested for selection bias. It was found that all five algorithms accept multiple echoes preferentially when they are in phase. Of the phase methods, that which used the standard deviation in phase rejected the multiples most effectively, while the echo duration method performed relatively poorly. A method which relies on the magnitude of amplitude differences between split-beam elements performed well, particularly for stronger targets. Another advantage of this method is that it also preferentially accepts our-of-phase multiples, thus partly compensating for its bias towards multiples which are in phase. We conclude that implementation of algorithms using either the standard phase deviation or the amplitude difference will lead to a significant improvement in single-target detection. The performance of the amplitude difference technique is particularly encouraging, considering the potential improvements possible by using averaging techniques to minimize the influence of noise. (C) 1996 International Council for the Exploration of the Sea