Angle of arrival estimation of low-frequency sources using a compact hydrophone array towed by an autonomous surface vehicle

Long-range detection and localization in deep water underwater sources with moving platforms typically rely on estimating the angle of arrivals at low frequency (i.e., associated with large wavelengths). Traditional methods typically use a long towed array of hydrophones to provide the required angular directivity at low frequency. However, such arrays are typically costly and complex to operate and require a powerful vehicle capable of towing such arrays. Here, an alternate method is demonstrated using a small autonomous surface vehicle platform capable of low-frequency source localization. The vehicles—Liquid Robotics Wave Gliders—are equipped with a compact four-element tetrahedron-shaped hydrophone array and a CTD suite capable of profiling to 150 m depth. These vehicles were deployed above the New England Seamounts in 2023 and 2024 and recorded low-frequency (200–300 Hz) transmission at ranges upwards of 200 km from bottom-moored sources deployed in the SOFAR channel. Pressure gradient-based signal processing methodology is applied to the compact tetrahedron hydrophone array recordings to emulate the low-frequency angular directionality performance of a vector sensor. The influence of environmental variability and seamount bathymetry on the estimated elevation and azimuthal angles with this compact array is investigated experimentally and numerically using ray-tracing simulation. [Work sponsored by ONR.]