Passive Synthetic Aperture Sonar techniques in combination with tow ship noise canceling: application to a triplet towed array

An important issue in research on passive ASW operations is improvement in signal-to-noise ratio (SNR) and bearing resolution for targets emitting low frequency signals. One of the techniques believed to improve these characteristics is Synthetic Aperture Sonar (SAS). The method is based on the artificial enlargement of a sonar array by coherently integrating acoustic snapshots at different antenna positions. This paper reports on the application of passive SAS in combination with other signal-processing algorithms. The used passive SAS algorithm is the method developed by S. Stergiopoulos, known as Extended Towed Array Measurement (ETAM) or the overlap correlator. It is based on the correlation of data snapshots on overlapping hydrophones. Correlation is a key issue in this method and since it is affected by noisy targets, some gain can be expected from noise cancellation. The influence on the performance of ETAM of a method of tow ship noise canceling at hydrophone level (Inverse Beam Forming, IBF) is analyzed. This approach increases ETAM performance by removing a loud and highly correlated noise source, the tow ship, and thus enhancing the other targets in the beam pattern. Eventually the synthetic aperture is processed by means of subaperture beamforming and port/starboard beamforming. The details of the processing chain are described in this paper. The methods are analyzed using data recorded during ASW trials at sea with the CAPTAS (Combined Active and Passive Towed Array System) triplet array. The TNO-FEL towed source (Socrates), transmitting controlled broadband noise and tonals, as well as passing civilian ships are used as targets. The performance of the processing chain and the interactions of its different elements are analyzed through the monitoring of SNR and beamwidth. The results of the algorithms applied to these experimental data show that they bring an improvement of a factor 4 on both SNR and beamwidth in comparison with a conventional beamformer. Port/starboard discrimination and the successful combination IBF-ETAM make this approach innovative.