Experimental results of 300 kHz shallow water synthetic aperture sonar

Abstract

The design of a shallow water synthetic aperture sonar (SAS) requires an understanding of key system and environmental issues. The main factors which limit SAS performance are micronavigation accuracy, where micronavigation is defined as the problem of estimating the acoustic path lengths to allow the focusing of the aperture, multipath effects and view angle differential effects which degrade shadow classification performance. Micronavigation accuracy is successfully addressed by the gyrostabilized displaced phase

centre antenna technique, which combines data-driven motion estimates with external attitude sensors. Multipath effects in shallow water are effectively countered by narrow vertical beams. View angle differential effects are mitigated by increasing the frequency and by designing the system with a minimum grazing angle of about 6 deg. The combination of these factors led to the choice of a 300 kHz centre frequency and of a multipath mitigation scheme which uses multiple vertical beams. Experimental results obtained with a sonar incorporating these features have produced SAS images with 1.6 cm x 5 cm resolution in range x cross-range and high shadow contrast, up to 170 m range in 20 m water depth.