Using a Towed Array to Localise and Quantify Underwater Sound Radiated by the Tow-vessel

This paper presents the results of a study aimed at determining the feasibility of using a towed array of hydrophones to localise and quantify sound sources on the tow-vessel. The method requires the tow-vessel to execute a manoeuvre in order to bring the array into a suitable geometry to allow it to image the tow-vessel. Previous work has focussed on a scenario where the tow-vessel executes a U-turn manoeuvre, resulting in rapid relative motion between the tow-vessel and hydrophones. In this paper a simulation is used to compare the performance of different beamforming algorithms in a scenario where the tow-vessel executes a constant radius turn. This scenario has the advantage of allowing longer integration times than the U-turn manoeuvre. Introduction Test Scenario It is advantageous for Navy vessels, particularly submarines, to have a means of measuring their own acoustic signature and localising the primary sources of radiated acoustic noise that contribute to that signature. This is conventionally done using fixed acoustic ranges, which require the vessel to divert to wherever the range is located, or air-dropped sonobuoys, which require a cooperating aircraft. This paper builds on previous work reported in [1-3] aimed at determining the feasibility of using the vessel’s own towed hydrophone array to make acoustic signature and source localisation measurements without the need for fixed ranges or sonobuoys. Vessel and towed array The tow-vessel was assumed to be 100 m long and operating at a depth of 100 m below the water surface. The simulated manoeuvre consisted of a 225 m radius turn to starboard at a speed of 1 m/s. The towed array used in this simulation had an overall length of 750 m and consisted of four sections: a tow-cable, a forward vibration isolation module, an acoustic section, and an aft vibration isolation module. The diameter and density of each section were typical of a towed array and the water density was 1025 kg.m. The acoustic section was 150 m long and contained 64 equally spaced hydrophones spread over its entire length, giving a hydrophone spacing of 2.381 m. The hydrophones were assumed to be omni-directional at all frequencies. The feasibility of performing this type of measurement was demonstrated in the work described in [1] which included a field trial in which a surface vessel towing an array of hydrophones carried out a series of Uturn manoeuvres. A disadvantage of the U-turn manoeuvre is that there is rapid relative motion between the hydrophones and the vessel when the geometry is most favourable for beamforming (when the acoustic section is broadside to the vessel). This makes tracking the hydrophone locations extremely critical and also limits the integration time available for beamforming. -50 0 50 100 150 200 250 300 350 400 -200 -150 -100 -50 0 50 100 150 Athwartships position relative to towpoint (m, +ve to stbd) Fo re a nd a ft po si tio n re la tiv e to to w -p oi nt (m , + ve fo rw ar d) The work described in this paper uses simulations to investigate an alternative scenario in which the towvessel executes a constant radius turn. This results in the acoustic section of the array taking up a stable position relative to the vessel, reducing the hydrophone tracking requirements and allowing for longer integration times. The performance of several array processing algorithms are investigated for three acoustic source configurations: a single point source, a pair of coherent sources, and a rectangular piston. Figure 1 Steady-state horizontal plane array shape relative to tow-vessel. The thick black line is the centre-line of the tow-vessel and the solid red line is the acoustic section of the array. Proceedings of ACOUSTICS 2004 3-5 November 2004, Gold Coast, Australia The steady-state array shape was calculated using the program described in [2] and is plotted in figures 1 and 2. 0 100 200 300 400 500 600 700 800 -20