Underwater acoustic measurements of Doppler‐shifted Bragg scattering in the vertical plane from interaction with the ocean surface.

High resolution beamformer output in the vertical plane for received fields from narrowband acoustic tones transmitted at 280, 370, 535, and 695 Hz shows evidence of Doppler‐shifted Bragg scattering by the ocean surface waves. The sidebands in the beamformer output are offset in frequency by amounts corresponding to ocean surface wave frequencies, while deviations in vertical arrival angle agree with those predicted by the Bragg condition through first‐order perturbation theory using measured directional surface wave spectra and measured acoustic mode wave numbers. The acoustic measurements were made by a horizontal hydrophone array on the ocean bottom in a shallow water waveguide under a variety of ocean surface conditions. [Work supported by ONR, Code 321(US).]