The conventional approach for passive underwater acoustic surveillance is to use uniform linear arrays (ULA) that consist of several nested sub-arrays of pressure measuring omni-directional hydrophones towed behind an underwater or surface platform. Towed arrays have a wide range of applications including underwater warfare, geophysical exploration, marine life research, and underwater acoustic communications. In such arrays, hydrophones are placed at a distance equal to half the wavelength (of the design frequency) apart to achieve an optimal array response in terms of main lobe width and suppressing grating sidelobes. Thus, a typical ULA can have an aperture length of 200 m or more (Barbagelata et al., 2008). In addition, use of omni-directional hydrophones results in port-starboard ambiguity.
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