Passive synthetic aperture processing with an autonomous underwater vehicle towed hydrophone array

Synthetic aperture processing techniques are presented and applied to results from an at-sea trial of an autonomous underwater vehicle (AUV) towed hydrophone array system in Nantucket Sound. The experiment consisted of a source deployed from a moored ship and the AUV towing the array outward from the source on a straight radial track. During the experiment, a ferry passed through the shipping channel from the main land of Cape Cod to Nantucket Harbor and provided a "target of opportunity" emitting both broadband and narrow band sound. This "target of opportunity" proved to be a good basis for testing passive broadband synthetic aperture bearing estimation routines. Estimates of the limits of coherent processing with a synthetic aperture are presented which indicate sufficient coherence in the shallow water multi-path environment to form a reasonable synthetic aperture. Both conventional and Kalman filter tracking techniques are theoretically presented and applied to the radiated noise of the vehicle showing that a passive synthetic aperture at low (~1 kHz) frequencies formed using an AUV towed array can successfully be used to track a noise source. Further, the benefit of using the joint estimation scheme is shown by a decrease in variance of the bearing estimate for the target when compared to the conventional scheme