Bottom crawling synthetic aperture sonar for very shallow water mine countermeasures

Synthetic aperture sonar (SAS) is a promising technology for rapidly detecting and classifying underwater mines. Previous SAS systems have been mounted on swimming vehicles that typically undergo smoothly varying motion. EDO Electro-Ceramics built an SAS for us and Foster-Miller, Inc. mounted it on one of their underwater crawling vehicles. This SAS-Crawler operates at 160 kHz, has a theoretical resolution of 2.54 cm (1 in) in range and 1.27 cm (0.5 in) in cross-range (azimuth) out to 50 m, and is intended to operate in the very shallow water and surf zones. The motion of a bottom crawling SAS is expected to be jerky as it follows the rough terrain. This presents new problems in motion estimation and correction. Furthermore, the acoustic environment, e.g., coherence properties, of the very shallow water and surf zones is less stable than the deeper waters. The slow speed of the vehicle in conjunction with a relatively high ping rate, however, yields many overlapping, or redundant, phase centers, which should help us overcome the motion estimation and coherence problems