An autonomous underwater vehicle towed array for ocean acoustic measurements and inversions

A novel experimental method involving an autonomous underwater vehicle (AUV) with a towed hydrophone array has been developed to measure the single path interaction from the surface, bottom and volume of the shallow water waveguide. The system is designed to operate from the low (/spl sim/100 Hz) to the mid frequencies (/spl les/10 kHz) with a directional source. The effects of surface, volume, and bottom scattering on the coherency of direct and direct-reflected signals are difficult to measure and a mobile directional receiver provides an adaptive capability. For example, the quantification of the role micro-bubble layers near the surface and in surface ship wakes plays on the coherency and scattering of sound could be measured with this device. In addition, an AUV with a hydrophone array can be easily and rapidly deployed and because of its mobility can provide area wide characterization. However, AUV radiated noise measurements indicated vehicle noise would limit the ability of hull-mounted or interior hydrophone arrays and thus suggested the use of a low noise towed array. The AUV discussed here is the Remus vehicle, a tested as well as a readily-available tow platform for a small low drag array. To demonstrate the ability of Remus to act as a low noise tow vehicle, radiated noise measurements were made on the vehicle at Dodge Pond acoustic test facility. The vehicle was rotated on a shaft at a depth of 8 meters and calibrated noise levels measured at 13 meters distance. At the maximum rpm of the AUV, the 1/3/sup rd/ octave noise level, when converted to source level by the calibrated transmission factor, was 130 dB re 1/spl mu/Pa at 1 m. This would represent the radiated noise source level for a vehicle moving at 3 knots. A small-diameter (2.8 cm O.D.) fluid-filled hydrophone array has been developed with 6 channels spaced evenly at 0.75 meters, each channel having a receiving sensitivity of -174 dB re 1/spl mu/Pa/V from 100 Hz to 10 kHz. The recording system consisted of three commercial off-the-shelf (COTS) battery powered mini-disc recorders with a 20 kHz band pass, sampling frequency of 44 kHz and a 16 bit analog to digital converter providing storage capability of 90 minutes of raw data. The prototype system was deployed in a "proof-of-concept" test at the Dodge Pond test facility. The vehicle was programmed to navigate along paths that provided straight courses as well as several turns to determine the operational characteristics of the system. Results on capabilities of the vehicle with the towed system are presented including vehicle and array noise, beamforming, and element localization.