Noncoherent autofocus of single-receiver broad-band synthetic aperture sonar imagery

A significant problem in Synthetic Aperture Sonar (SAS) imaging is compensating for unknown errors in the sonar path trajectory. Unknown deviations from the ideal sonar trajectory have the effect of blurring and smearing the sea-floor image. In typical operating conditions, the blurring can completely obscure details in the imaged scene. Techniques for estimating path deviations from the recorded data have been developed. Once estimated, the effects of deviations are compensated for and the blurring reduced. The process of estimating path-deviations and removing image blurring is called autofocus. In this paper, we present a broad-band scheme for a bulk-motion estimation in the absence of inertial navigation system (INS) data. Once the effect of bulk-motion error is removed, alternate autofocus methods may be used to yield diffraction-limited imagery. In addition, the bulk autofocus allows a check on the validity of the motion measured by any inertial systems installed on the sonar tow-fish. The noncoherent autofocus operates by exploiting the envelope correlation between adjacent sonar echos to provide path deviation information. This puts the method into the same class of algorithm as the well-known shear-average autofocus, which exploits ping-to-ping phase correlation for its operation. The algorithm differs from shear-average by using only the base-band envelope of the echo data and not phase information. The algorithm has been tested on both real and simulated data and has some promise as a first step in any autofocusing system. In particular, the system should provide good starting information for any image quality based autofocus methods. Accuracy of the method is limited by biasing caused by strong target scatterers. Noncoherent autofocus also performs better than coherent shear-average autofocus for the very large path perturbations investigated.