REMEDYING THE EFFECTS OF ARRAY SHAPE DISTORTION ON THE SPATIAL-FILTERING OF ACOUSTIC DATA FROM A LINE ARRAY OF HYDROPHONES

Traditionally, the spatial filtering (or beamforming) of towed array d ata proceeds on the assumption that the horizontal array is always str aight. However, if the geometric configuration of the hydrophones adop ts a nonlinear shape so that the hydrophone positions are no longer co llinear, the acoustic performance of the sonar system degrades. Using real towed array data acquired during a sea experiment, this paper sho ws the effects of both small perturbations and large deformations to t he array's shape on both conventional and adaptive beamformers for two frequencies: The lower frequency is approximately equal to the spatia l Nyquist frequency (or design frequency) of the array, while the high er frequency is about three times greater. Large shape deformations le ad to a decrease in the conventional beamformer's output power for a b eam steered in the direction of the signal source, together with an in crease in the sidelobe levels (or secondary maxima), while small pertu rbations in the array shape have little effect. Signal suppression is observed to be far greater for the adaptive beamformer because it is m uch more sensitive (than the more robust conventional beamformer) to s ystem errors, which arise in the present case from imperfect knowledge of the hydrophone positions. The imposition of a weight norm constrai nt on the adaptive beamformer reduces the signal suppression only for small shape perturbations, while array shape estimation techniques nee d to be invoked to reduce signal suppression for large shape deformati ons. The adverse effects of a nonlinear array shape on both convention al and adaptive beamforming are shown to be substantially reduced by a pplying techniques that estimate the coordinates of the hydrophones pr ior to beamforming. The two array shape estimation techniques consider ed here require an acoustic source to be present in the far field and use only the acoustic data from the hydrophones themselves to estimate their positions. These techniques do not require data from non-acoust ic sensors such as heading and depth sensors distributed along the len gth of the array to estimate its shape.