Abstract
A conventional coherent plane wave beamformer exploits the phase evolution of the wave field across the array aperture, using the phase at one element as reference, to obtain information on source location. Alternatively, in a multipath environment, the phase of one mode or multipath can be used as reference for the phase evolution of the other multipaths in the wave field. This ‘‘intermode interferometry’’ is the basis of an incoherent beamformer presented here. It involves summing together individual element autospectra, after performing a frequency-domain dilatation/contraction operation based upon a hypothesized source location. Because it uses autospectra rather than element time series, this processing provides no gain of signal over background noise. Therefore, its usefulness in its basic form is restricted to localization of strong signals. However, it is not limited to line arrays with equal element spacing, and the array element positions need only be known to a fraction of an interference wavelength rather than an acoustic wavelength. Examples of the incoherent beamforming results from simulation and actual ocean acoustic data collected during the shallow-water (200 m) SWellEx-3 experiment will be presented. Emphasis is placed on 2D passive synthetic aperture applications. [Work supported by ONR, Code 321US.]
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