Cross-pattern coherence spatial filtering with hydrophone arrays for passive-sonar applications

This paper presents a spatial post-filtering algorithm for passive-sonar systems deploying linear hydrophone arrays. The algorithm provides an attenuation parameter in the time-frequency domain based on the normalised cross-spectral density between two signals, which originate from two coincidentally steered conventional beamformers. The computed parameter can be applied as a single-channel spectral post-filter to the output of another beamformer to suppress background noise and interference. The technique is evaluated with simulated data of two 96-channel flank arrays, as well as with real-world data derived from smaller towed linear arrays. The evaluation includes two array configurations: a) a single-line configuration using two non-overlapping subdivisions of a linear array for the computation of the parameter, and b) a dual-line configuration using two parallel linear arrays. In all studied cases, the post-filter is shown to improve the resolution and background-noise suppression capabilities of the baseline beamformers. It is further shown that, in the dual-line configuration, the post-filter may improve the beamforming performance near the end-fire direction as well as further resolve the port-starboard ambiguity, which is a known drawback of towed linear arrays. Lastly, it is worth noting the relatively low computational complexity of the proposed technique compared to the more demanding adaptive beamformers.