Underwater acoustic azimuth angle estimation with a millimetre diameter flexible hydrophone towed array

Abstract This paper investigates a millimetre diameter hydrophone towed array cable for acoustic direction finding. The mechanism of the underwater acoustic waves received by this linear array without any mandrel structure is deduced and it is demonstrated that the amplitude of its response varies with the direction of the sound wave. An anechoic pool experiment is carried out to test the performance of such a hydrophone array. The hydrophone unit consists of a sensing fiber between adjacent UWFBGs, coiled into 9 cm diameter fiber rings, with arbitrary 6 fiber rings, called a 6-element hydrophone array, suspended horizontally in the sound-absorbing wedge to receive acoustic signals. An average sensitivity of − 141.2 dB re rad/µPa at frequencies from 2.5 kHz to 6.3 kHz was achieved. Direction detection at an angle of 75° is realised, validating the detection of the azimuth of underwater acoustic waves. The ultra-thin towing cable system, with free structure, high sensitivity and underwater target detection capability that it has demonstrated has great potential for future Unmanned Underwater Vehicle (UUV) applications.