Search

—For a low-frequency active sonar (LFAS) with a triplet receiver array, it is not clear in advance which signal processing techniques optimize its performance. Here, several advanced beamformers are analyzed theoretically, and the results are compared to experimental data obtained in sea trials. Triplet arrays are single line arrays with three hydrophones on a circular section of the array. The triplet structure provides the ability to solve the notorious port–starboard (PS) ambiguity problem of ordinary single-array receivers. More importantly, the PS rejection can be so strong that it allows to unmask targets in the presence of strong coastal reverberation or traffic noise. The theoretical and experimental performance of triplet array beamformers is determined in terms of two performance indicators: array gain and PS rejection. Results are obtained under several typical acoustic environments: sea noise, flow noise, coastal reverberation, and mixtures of these. A new algorithm for (beam space) adaptive triplet beamforming is implemented and tuned. Its results are compared to those of other triplet beamforming techniques (optimum and cardioid beamforming). These beamformers optimize for only one performance indicator, whereas in theory, the adaptive beamformer gives the best overall performance (in any given environment). The different beamformers are applied to data obtained with an LFAS at sea. Analysis shows that adaptive triplet beamforming outperforms conventional beamforming algorithms. Adaptive triplet beamforming provides strong PS rejection, allowing the unmasking of targets in the presence of strong directional reverberation (e.g., from a coast) and at the same time provides positive array gain in most environments

ASW operations have shifted toward shallow water environments, where water space is limited and manoeuvring an essential part of operations. The problem that is tackled in this article is the performance loss due to the shape and motion of a manoeuvring sonar. Standard beamforming in towed sonar arrays is based on the assumption that all hydrophones move on a straight line with constant velocity. However, when the tow ship turns the array follows and gets bent. The two damaging effects are deviation from the straight line and variation in Doppler. In 2002 and 2003 dedicated operational high-speed experiments with tow ship manoeuvres were conducted to illustrate and tackle the problem. The solution starts with estimation of the dynamic sonar element positions. Subsequently these estimates are incorporated in the beamformer and/or matched filter. A new method for shape and Doppler corrected processing are developed and validated on simulated and experimental data.

Low Frequency Active Sonar (LFAS) systems are originally designed for ASW purposes. Although their main purpose is detection of submarines, they can, if adjusted waveforms are transmitted, also be used for detection of small fast moving targets (torpedoes). In this study the Detection, Classification and Localisation performance of classical HFM and CW pulses is compared to that of three new wideband Doppler sensitive pulses. The PTFM (Pulse Train of Frequency Modulated signals) has already proven effective in suppression of reverberation in ASW trials. Two mutants, the DTFM and TTFM (a double train and a triple train) combine this property with high range resolution, at the cost of little lower Doppler resolution. This improves localisation and even allows for hard-kill solutions. Pulse properties of the 5 candidate pulses have been analysed theoretically and experimentally during a sea trial. The first analysis results of this sea trial are compliant with theory.

For littoral ASW, reverberation is a big problem and rejection of reverberation is of utmost importance. The influence of the transmitted signal on the signal to reverberation ratio has been presented in three preceding papers. In this paper, the influence of improved signal processing on the receiver side is investigated. It is known that low sidelobes of the beampattern reduce the received reverberation. Two different types of port-starboard (PS) beamforming are compared, viz. cardioid and adaptive PS beamforming. Also three types of waveforms are considered: FM pulses, CW pulses and a train of linear FM pulses (PTFM). Experimental results on signal to reverberation ratios are presented and compared to theory. It appears that adaptive PS beamforming leads to very low sidelobes, which improves the reverberation rejection with 10 dB or more.

The influence of the waveform of the transmitted signal on the signal to reverberation ratio of a moving sonar array is analysed on an experimental basis. Experiments have been conducted during sea-trials in shallow water environments in September 98 and October 99. The aim of the trials was to compare the performance of three waveforms: frequency modulated (FM) signals, continuous wave (CW) signals and a train of linear FM pulses (FM Train). Transmissions were combined (CW/FM and CW/FM Train) in order to compare the performance of the three signals on the same basis. Experimental results on signal to reverberation ratios are presented and compared to theory.

Triplet arrays are single line arrays with three hydrophones on a circular section of the array. The triplet structure provides immediate port-starboard (PS) discrimination. This paper discusses the theoretical and experimental performance of triplet arrays. Results are obtained on detection gain and PS discrimination in several environmental conditions: sea noise, flow noise, coastal reverberation, jammers. New algorithms on adaptive triplet beamforming are applied to data of a sea-trial with a low frequency active sonar. The adaptive beamforming results are compared to other triplet beamforming techniques (optimum gain and cardioid beamforming). It shows that adaptive triplet beamforming outperforms the other beamforming algorithms, either in PS discrimination or in detection. In conclusion, adaptive beamforming appears to be the ultimate solution for triplet beamforming.

In the design of towed array sonar the reduction of flow noise is an important issue. In this paper flow noise theory and several methods to analyse flow noise are presented. These methods are applied to sea trial data of an experimental triplet array. The obtained results agree very well with accepted theory. It appears that the methods in combination provide not only the levels, but also the causes of flow noise.