Sensing the underwater acoustic environment with a single hydrophone onboard an undersea glider

Abstract

Undersea gliders are autonomous vehicles that propel themselves through the ocean by changing their buoyancy. Acoustic platform noise (or self-noise) generation occurs only briefly (and predictably) when the buoyancy change and trim adjustment mechanisms are activated on these quiet platforms. Also, hydrodynamic flow noise is minimal due to the slow (≤0.5 m/s) travel speed of the glider. This paper demonstrates the utility of a single hydrophone onboard a glider that samples the underwater acoustic environment in space and time for the purpose of detecting human activity during a glider mission. In May 2009, acoustic sensor data from a single hydrophone onboard a Slocum glider were recorded during the glider's deployment in a shallow water environment. Time-frequency signal analysis of the underwater acoustic data results in the detection of a helicopter, a surface vessel and the presence of tone burst transmissions from an underwater sound beacon. In addition, echolocation sound pulses (or clicks) emitted by the biological sonars of dolphins permeate the data. Cepstrum analysis of the acoustic data containing the surface vessel's radiated noise (as the signal) provides an estimate of the time delay between the direct path and multipath arrivals of the vessel's broadband acoustic energy at the hydrophone. Given the multipath time delay estimate, along with the known (measured) water depth, hydrophone depth, and sound speed in water, a single hydrophone multipath passive ranging method is able to estimate the instantaneous range of the surface vessel from the glider. The variation with time of the range estimates provides the distance and occurrence time of the closest point of approach of the vessel to the glider. The onboard underwater sound sensor, coupled with the passive ranging method, provides the glider with a self-protection capability in that the separation distance between an encroaching maritime vessel and the glider would indicate the likelihood of the vessel encountering the underwater vehicle, which could adversely affect the operation of the glider.