Wideband Multipath Rejection in Synthetic Aperture Sonar Imaging

Synthetic aperture sonar (SAS) is an emerging technology capable of providing high resolution seafloor imaging, which has the appealing property of range and frequency independent spatial resolution. However, for SAS systems with low frequency signals in shallow water environments, there are strong sea surface and bottom reflected wideband multipath components that interfere with the desired echo signal. Based upon a small vertically-displaced hydrophone array, several spatial processing algorithms have already been proposed for multipath reduction. Unfortunately, all of these algorithms are only applicable to narrowband signals, while wideband signals are always utilized in low-to-medium-frequency SAS systems for achieving high range resolution. Thus there is a great demand to solve the wideband multipath rejection problem in SAS systems. This paper discusses wideband multipath mitigation using a small vertical array and presents a steered robust Capon beamforming (SRCB) approach, together with conventional delay-and-sum beamforming (DSB), for shallow water wideband multipath rejection in SAS imaging. The designed performance improvements, mainly measured by direct-to-multipath-ratio (DMR) enhancements, are verified by numerical simulations. In addition, the limitations of these algorithms are also discussed.