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False alarm reduction by simultaneous transmission of two wideband sonar pulses

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Author: Beerens, S.P. · Spek, E. van der
Publisher: Nexus Media, Ltd
Place: Swanley, Kent
Institution: TNO Defensie en Veiligheid
Source:UDT Europe 2009 - Conference Proceedings Undersea Defence Technology, 9 - 11 June, 2009, Cannes, France
Identifier: 431556
Keywords: Physics


Thanks to technological advance, the bandwidth of sonar transducers has increased considerably the last decades. For signal processing engineers this means that more possibilities are available to improve the sonar output. One of the problems of active sonar is the excessive false alarm rate in shallow water operations. In this field some improvements still can be made. The idea in this paper is to use the abundant bandwidth by transmitting two band separated (so non-interfering) wideband sonar pulses at the same time. Typically both pulses differ about one octave in centre frequency, but have similar bandwidth. To mininise interference even further one pulse can be an upsweep, whereas the other is a downsweep. This pulse was implemented in The Netherlands IRLFAS system. The source of this system has a free-flooded ring transducer with one octave of bandwidth centered at 1500 Hz. During an experiment at sea 60 pings were transmitted. Per ping two sonar pictures were compiled and contacts were abstracted. Next false alarm reduction may be achieved by data fusion, for instance by application of an “AND” or “OR” rule. In this paper we have evaluated the false alarm reduction that can be achieved by fusion of two simultaneous sonar pictures using real data. This evaluation is based on ROC curves, which apart from probability of false alarm also take probability of detection into account. The conclusion is that there is potential for the use of simultaneous transmissions of two band separated sonar pulses. The resulting sonar pictures at different frequencies look quite different. These differences can be exploited by clever fusion techniques with the aim to achieve false alarm reduction. The results of the very simple “AND rule” are maybe not convincing (about 1 dB fusion gain, but similar processing loss due to band separation), but this fusion technique is the simplest, and can probably be improved.