Enhancing angular resolution by coherent beamforming of full non-uniformly spaced linear hydrophone array

Abstract

Uniformly-spaced apertures or subapertures of discrete linear receiver arrays are often used in remote sensing to increase signal-to-noise ratio (SNR) by coherent beamforming which reduces noise coming from directions outside the signal beam. To avoid grating lobes in real spatial directions, the uniformly-spaced array inter-element spacing d determines the maximum or cut-off frequency f<c/2d of signals suitable for beamforming with the array, where c is the wave speed. Here we show that a non-uniformly spaced array, for instance formed by combining multiple uniformly-spaced subapertures of a nested linear array, can significantly improve the angular resolution while simultaneously avoiding dominant grating lobes in real angular space, even for signals with frequencies beyond the cut-off. The array gain, beamwidth, and maximum grating lobe height are calculated for the ONR Five Octave Research Array (FORA) for various combinations of its uniformly-spaced subapertures, including the full non-uniformly spaced array. Examples are provided of angular resolution enhancement with resulting non-uniformly spaced array or subarray in both active and passive ocean acoustic waveguide remote sensing over that of the uniformly spaced counterpart, including continental-shelf scale imagery of fish populations, and marine mammal vocalization SNR enhancement which improves detection and bearing-time estimation for passive localization.