Ocean medium and platform effects on synthetic aperture sonar

Abstract

Synthetic aperture methods have been extensively used in radar ground mapping and imaging since the 1950s. They have also been discussed and occasionally used in biomedical and nondestructive testing ultrasound, and in hydrographic sonar since the late 1960s. A recent example of a synthetic aperture sonar (SAS) is that of Gough and Hayes [J. Acoust. Soc. Am. 86, 2328-2333 (1989)]. In order for SAS to form sharp images, both the amplitude and phase content of a collection of sequential echo histories of a target, for example a point target on the ocean bottom, must be known. The collection of echo histories represent data taken over the length of a synthesized aperture, usually from a sonar platform moving at a known speed. If the phase is not accurately measured over the synthesized length, the image resulting from the signal processing will be blurred or even absent. Medium instability, multipath effects, and uncorrected platform motion may all contribute to phase errors that lead to blurred images. This paper will discuss SAS and these related ocean medium and platform influences, and show numerical examples of image degradation due to these effects. [Work supported by the Charles Stark Draper Laboratory, Cambridge, MA.]