Evaluating direction of arrival uncertainty in undersea canyons with internal tides

Abstract

The slopes of undersea canyon regions impart complexity to underwater acoustic fields for two reasons: intricate patterns of reflection from the seabed, and sound-speed anomalies from internal tides generated at the slopes. Both may spread the horizontal directional spectrum of sound from “line of sight” to a source. The directional spectrum is tied to the covariance matrix of the field, a fundamental quantity that can be measured or modeled. Here, sound-field horizontal-lag spatial covariance matrices and other derived quantities are generated from time-stepped 3D parabolic equation acoustic simulations made using sound-speed fields from ocean models. The covariance matrices are then inserted into the direction-of-arrival (DOA) estimation problem. Analysis of DOA estimates and error bounds is done for a conventional beamformer and for a Gauss-Markov inverse-based beamformer for a variety of signal-to-noise ratios. The method treats non-line of sight acoustic energy as a form of noise. At low signal-to-noise ratio, the Gauss-Markov estimator can perform better that the other. This analysis of field variability allows performance degradation caused by evolving ocean structures to be directly compared to other detrimental influences such as excess noise and array deformation. Detection, localization, and tracking are affected by the processes examined here.