Surface reflection caustics and their impact on communications in the littoral zone

Abstract

Gravity waves with wavelengths longer than a Fresnel zone create cusps and caustics in the surface-reflected acoustic field. The properties of these coherent focal regions have been measured in the surf zone and the 20-m-deep littoral waters off Martha’s Vineyard. The time-dependent scattering function shows features consistent with a theoretical analysis of the focal region micro-path structure. The rapid amplitude, phase, and Doppler shifts associated with individual micro-paths present challenges to the reliable operation of phase-coherent underwater communications systems that must implicitly or explicitly track the fluctuations in the impulse response. Data from the experiments will be presented and the implications for underwater communications systems will be discussed. [This work has been undertaken in close collaboration with Dr. James Preisig at the Woods Hole Oceanographic Institution and Dr. Chris Tindle at the Auckland University Physics Department, New Zealand, and supported by the Office of Naval Research.]