Waveguide Invariant Focusing for Broadband Adaptive Beamforming in a Shallow Water Channel

Focusing methods are effective broadband techniques that can reduce the observation time required for adaptive beamforming. However, often focusing methods designed for planewave signals do not prevent broadband covariance matrix rank inflation for shallow-water multipath wavefronts. The waveguide invariant is a robust channel parameter that describes the dispersive characteristics of an oceanic waveguide. Using the relationship between the waveguide invariant and horizontal wavenumber differences, this paper introduces a new broadband waveguide invariant focusing (WIF) method that can mitigate rank inflation for multipath signals. The new WIF method involves summing narrowband snapshots from array apertures which are judiciously shifted as a function of frequency. The shifted apertures may be obtained naturally as a result of towed horizontal array motion or via sub-aperture processing of a fixed long array. Simulation results for WIF against end-fire interference in a shallow-water waveguide are given to illustrate the performance improvement against conventional approaches.