Fusion Digital Terrain Models for Synthetic Aperture Sonar Interferometry

High resolution, centimeter-scale synthetic aperture sonar (SAS) imaging of the seafloor relies on a priori knowledge of the bathymetry in order to focus the image correctly. Common approaches to estimate the bathymetry includes using flat or sloped surfaces relative to the sonar altitude measurements or the estimated sidescan bathymetry (SSB), or using the SSB estimates directly. More advanced approaches include iterative processing and various methods of searching for better candidate solutions. In this paper we investigate the use of an additional digital terrain model (DTM) from a multibeam echo sounder (MBES). This external DTM can be used by itself or it can be combined with SSB or SAS bathymetry for a fusion DTM. We compare the results of the various methods by investigating the interferograms and using the number of valid measurements, the interferometric coherence and the phase difference as performance metrics. We also show SAS bathymetry examples of the differences between these approaches.