Angular Resolution Enhancement Provided by Nonuniformly-Spaced Linear Hydrophone Arrays in Ocean Acoustic Waveguide Remote Sensing

Abstract

Uniformly-spaced apertures or subapertures of large, densely-sampled, discrete linear receiver arrays are often used in remote sensing to increase the signal-to-noise ratio (SNR) by coherent beamforming that reduces noise coming from directions outside the signal beam. To avoid spatial aliasing or the presence of grating lobes in real spatial directions, the uniformly-spaced array inter-element spacing d sets a limit on the maximum frequency f max < c/2d of signals suitable for beamforming with the array, where c is the medium's wave propagation speed. Here, we show that a nonuniformly-spaced array, for instance, formed by combining multiple uniformly-spaced subapertures of a nested linear array, can significantly enhance the array angular resolution while simultaneously avoiding dominant grating lobes in real angular space, even for signals with frequencies beyond the maximum that the array is designed for. The array gain, beam width, and maximum grating lobe height are quantified for the Office of Naval Research Five Octave Research Array (ONR-FORA) for various combinations of its uniformly-spaced subapertures, leading to nonuniformly-spaced subarrays. Illustrative examples show angular resolution enhancement provided by the nonuniformly-spaced ONR-FORA subarrays over that of its uniformly-spaced individual subaperture counterparts in both active and passive ocean acoustic waveguide remote sensing, drawn from measurements in the Gulf of Maine 2006 Experiment.

The Effect of Attenuation from Fish Shoals on Long-Range, Wide-Area Acoustic Sensing in the Ocean

“…Scattered returns from environmental features are received by a horizontal line array towed by the same research vessel with multiple nested sub-apertures. Three linear apertures of the receiver array, that is, the low-frequency (LF) aperture, the mid-frequency (MF) and the high frequency (HF) aperture, consist of 64 equally spaced hydrophones with respective inter-element spacing of 1.5 m, 0.75 m and 0.375 m. Images are generated by beamforming, matched filtering and charting scattered returns, using non-uniformly-spaced combinations of the LF, MF and HF apertures, as described in Reference [35].…”

Instantaneous 3D Continental-Shelf Scale Imaging of Oceanic Fish by Multi-Spectral Resonance Sensing Reveals Group Behavior during Spawning Migration

“…This leads to a fixed range resolution of roughly 15 m, and an azimuthal resolution varying as λ/(L cos θ) in radians away from array endfire, where θ is the scan angle from array broadside, λ is the acoustic wavelength, and L is the array aperture length. Here, a non-uniformly-spaced aperture [9] combining the LF and MF apertures is used for beamforming to obtain OAWRS images at frequencies centered at 735 and 950 Hz with enhanced angular resolution compared to that of the MF aperture alone. Similarly, OAWRS images are generated at frequencies centered at 1125 Hz with a sub-aperture combining the LF, MF, and HF apertures leading to better angular resolution compared to that of the HF aperture alone by a factor of 2-3 [9].…”

“…Here, a non-uniformly-spaced aperture [9] combining the LF and MF apertures is used for beamforming to obtain OAWRS images at frequencies centered at 735 and 950 Hz with enhanced angular resolution compared to that of the MF aperture alone. Similarly, OAWRS images are generated at frequencies centered at 1125 Hz with a sub-aperture combining the LF, MF, and HF apertures leading to better angular resolution compared to that of the HF aperture alone by a factor of 2-3 [9]. A maximum-likelihood method [10] is used to deconvolve the array beam patterns of the non-uniformly-spaced array apertures used here from the beamformed OAWRS intensities, such that the azimuth-dependent artifacts resulting from the limited angular imaging resolution are further suppressed.…”

Comparing Performances of Five Distinct Automatic Classifiers for Fin Whale Vocalizations in Beamformed Spectrograms of Coherent Hydrophone Array

“…Acoustic pressure-time series measured by sensors across the receiver array were converted to two-dimensional beam-time series by discrete Fourier transform [44]. A total of 64 beams were formed spanning 360 degree horizontal azimuth about the receiver array for data from the ULF subaperture.…”