Imaging a moving target using forward-look synthetic aperture sonar

Mid-frequency active, surface ship sonar data from a recent shallow-water sea trial have been processed using synthetic aperture sonar (SAS) algorithms. The target of opportunity closed head-on directly toward the surface ship at a relative speed of 13 knots through an acoustic environment that was characterized by a downward refracting sound speed profile in 100-m-depth water over a sand/mud bottom during calm seas. This situation represents an endfire (90-degree squint angle) scenario for which the resolution of any line array (synthetic or real) is considerably degraded relative to the broadside case. The test was not designed for SAS application so the aperture is significantly undersampled. The processing approach utilizes signal adaptive, ping-to-ping correlation autofocusing to correct for the effects of platform and target motion, and for ocean instabilities. The processing is tuned to the specific environment by setting the cross-correlation window size based on the two-way group arrival structure of the channel. The aliasing that results from the undersampled aperture is reduced to acceptable levels through the appropriate use of cross-range matched field processing. Images formed by processing the data from a forward-look synthetic aperture measuring 2200 wavelengths with a mean target distance of 9.5 km are presented.