Bistatic spatial coherence for micronavigation of a volumetric synthetic aperture sonar

Combining multiple passes of a volumetric synthetic aperture sonar (SAS) may open new possibilities for sonar imaging, if the passes can be precisely aligned. Previous work suggests that coherence-based micronavigation is a potential approach to achieve this alignment. However, existing coherence-based techniques require assumptions that break down in many volumetric 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 volumetric SAS systems. This work will explore the impact of sensing range and bistatic separation on spatial coherence in downward-looking, volumetric sonar applications where the far-field assumption no longer holds. As signals transmitted from downward-looking sonars may penetrate into the sediment floor, this analysis will consider both interface and volumetric scattering. First, using both simulated and experimental sonar data, we will quantify the loss in coherence that occurs in these near-field geometries. Then, methods to regain lost coherence will be examined in an effort to enable spatial coherence-based navigation for a volumetric SAS. Finally, the work’s relevancy for ongoing efforts in acoustic navigation and repeat-pass imaging will be discussed.