Shape and Doppler correction for a towed sonar array

Anti Submarine Warfare (ASW) is more and more focused toward shallow water environments. Many complications have come up for ASW sonar performance as a result of this. The problem that is tackled in this article is the performance loss due to the shape and motion of the sonar during a maneuver. Standard beamforming in towed sonar arrays is based on the assumption that all hydrophones are on a straight line and have equal 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. The first issue can be solved by insertion of the correct positions in the beamformer as was shown in earlier publications. In this paper it is shown under which circumstances the second problem causes loss in the beamforming. A theoretical derivation of the performance loss in a turn is given. To tackle the problem of performance degradation, the signal processing has been improved. Three different methods are developed to compensate the effects. All of them are described, implemented, tested and compared with the conventional sonar signal processing. This comparison is first tested on simulated data, generated with a validated sonar simulator that is equipped with Doppler in an exact way. The result proves that the methods work and also shows how the degrading shape and Doppler effects manifest in the sonar images. The article is concluded with an experimental data analysis of a sea trial with a state-of-the-art ASW sonar. The conducted experiment involved a surface vessel with a low-frequency active sonar and a submarine target. A sequence of high-speed sharp turns was sailed in order to show the sonar performance degradation and the benefit of the newly developed signal processing methods. A statistical analysis is given with the sonar performance versus two varied parameters: viz. the target beam and the radius of curvature of the sonar track.