Robust adaptive beamforming for hydrodynamic self-noise rejection

White noise gain (WNG) is a metric widely used in the radar community for limiting the degree of adaptivity of an adaptive beamformer (ABF). Constraining adaptation of an ABF algorithm has two key effects: (1) protection against self-nulling associated with steering vector mismatch, and (2) limitation of white noise gain associated with squinting a beampattern to place a null on a mainlobe interferer. Hydrodynamic self-noise, or cable strum, commonly manifests itself as a source of mainlobe interference for passive acoustic towed horizontal line arrays. Cable strum is the result of vibrations excited in the array body by vortex shedding in the presence of flow. Strum is particularly detrimental on beams near endfire and typically exhibits very high dynamic range. As such, it requires an aggressive adaptation strategy for its effective removal. In this work, a white noise gain constraint for the rejection of cable strum is derived based on the scaled orthogonal projection approach of Cox et al. [IEEE Trans. ASSP 35 (1987)]. Tradeoffs involved in balancing strum rejection performance against mismatch-induced signal self-nulling are examined. [Sponsored by the Dept. of the Navy, under Air Force Contract No. F19628-00-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors and not necessarily endorsed by the U.S. Air Force.]