Coherence estimation for repeat-pass interferometry

Repeat-pass interferometry is an extension of Synthetic Aperture Sonar (SAS) that exploits the temporal characteristics of seafloor reverberation by coherently combining echoes from multiple passes of the sonar platform. In order to achieve pass-to-pass coherence, the spatial separation between passes must be less than a value known as the critical baseline, which depends on the sonar parameters and the range-dependent imaging geometry. The temporal separation between passes must also be less than the coherence time of the seabed, which is highly environmentally dependent and which defines the maximum time interval between passes. It is also necessary to coregister the complex seabed images to within a fraction of a pixel (usually on the order of several millimeters for the centimeter resolution SAS used in mine countermeasures). This imposes strict requirements on the navigation accuracy of the sonar platform that cannot be met using inertial or underwater positioning sensors alone. This paper presents a data-driven method for estimating the repeat-pass coherence map using local coregistration of the complex SAS images from each pass. The method can be applied to any SAS without requiring access to raw acoustic data or post-processing of data from a high grade inertial navigation system. Experimental results are presented for a repeat-pass survey using AquaPix, a wideband 300 kHz interferometric SAS, installed on an ISE Arctic Explorer AUV during recent sea trials with Defence Research and Development Canada.