Multipass coherent processing on synthetic aperture sonar data

We investigate whether coherent processing can be achieved on synthetic aperture sonar (SAS) data recorded on different passes over the same scene. A major challenge of multipass coherent processing on SAS images is the demand of accurate platform navigation and control. To overcome this we will correlate echoes from multiple passes in order to provide high fidelity navigation between the passes, supporting co-registration of SAS imagery with sub-resolution accuracy. This would provide a door-opener for multipass coherent processing on full resolution SAS images for products such as high resolution bathymetric mapping and coherenct change detection. A dedicated experiment is run using the HISAS 1030 SAS on a HUGIN 1000-MR autonomous underwater vehicle (AUV). On the recorded data we demonstrate multipass coherent processing on ping pairs from different passes and obtain a normalized correlation coefficient exceeding 0.96 between time series of pings recorded one hour apart. We also present, to our knowledge, the first experimental assessment of the effect of cross-track separation on multipass coherent processing of SAS data. Finally we demonstrate that the navigation system can be updated with measurements from multipass coherent processing, and that such updates can reduce the navigation uncertainty between passes down to sub-wavelength scale.