Autofocus of multi-band, shallow-water synthetic aperture sonar imagery using shear-averaging

A significant problem with Synthetic Aperture Sonar (SAS) imaging is the compensation of compensating for unknown errors in the sonar path trajectory. Unknown path errors in SAS have the effect of blurring and smearing the sea-floor image. Inertial navigation systems as used in Synthetic Aperture Radar (SAR), are not accurate enough for use in SAS. To deal with this problem, techniques for estimating and compensating the path errors from the gathered data (autofocus algorithms) have been developed. In this paper we present enhancements to an existing sonar autofocus algorithm presented in 1995 by Johnston et al. These enhancements help prevent the autofocus being biased by strong targets. Improvement in autofocus is significant and more apparent whenever an extended prominent target is far stronger than the surrounding seafloor clutter signal. We have tested both algorithms using and simulated data and the results are presented in this paper. In addition, we demonstrate the advantage of using single pass multiband imagery to improve the autofocus result.