Bistatic spatial coherence for micronavigation of a downward-looking synthetic aperture sonar

Combining multiple passes of a downward-looking synthetic aperture sonar (SAS) may open new possibilities for sonar imaging, if the passes can be precisely aligned. Previous work suggests coherence-based micronavigation may help achieve this alignment. However, existing coherence-based techniques require assumptions that break down in many downward-looking imaging geometries. This breakdown leads to a loss of coherence that has not been widely studied, and consequently has hindered the application of micronavigation to downward-looking SAS systems. An accurate model of spatial coherence is needed to better understand the performance of coherence-based micronavigation techniques. This work describes the development of a broadband, bistatic model for the spatial coherence of seafloor scattering. This model removes assumptions that may not be appropriate for common downward-looking SAS geometries. The development of the model focuses on interface scattering, but the framework for modeling returns from a volume of sediment is presented as well. When compared with existing methods, it is shown that the proposed coherence model produces the expected results. Finally, the work’s relevancy for efforts in acoustic positioning and repeat-pass imaging is discussed along with future work to expand the model.