An overview of unmanned underwater vehicle noise in the low to mid frequency bands.

Abstract

Unmanned (autonomous) underwater vehicles offer a unique, cost-effective platform for performing ocean acoustic measurements and surveys because multiple systems can be deployed from a single research vessel. Various data surveys can be performed including on-the-bottom geo-acoustic surveys over large areas, sub-sea-surface turbulence and microbubble structure surveys, and bi-static fish population surveys. To take advantage of the autonomous survey capabilities of underwater vehicles, sufficient signal-to-noise ratio and acoustic aperture (resolution) are required for acoustic measurements. The most commonly used vehicle sonar systems provide images utilizing high frequency hull mounted arrays and sources. In the lower-frequency band (100 Hz–10 kHz), however, vehicle noise levels and aperture remain the two most significant challenges, especially for passive systems. Previous experimental and analytical work has shown that a towed array with synthetic aperture processing can be used to obtain the necessary aperture. The major challenge of vehicle radiated noise is the focus of this paper, and both measured and archival results on vehicle noise are presented including an overview of levels, spectral character, and noise mechanisms for several vehicles. In particular, fundamental vehicle propulsion system noise is discussed along with implications on measurement performance and possible mitigation strategies.