Coherent combination of perpendicular synthetic aperture sonar passes

Many downward-looking synthetic aperture sonar (SAS) systems form a synthetic aperture in the direction of travel (along-track) and a real aperture in the perpendicular direction (cross-track). At lower frequencies and longer ranges, the difference between the synthetic aperture resolution and real aperture resolution may become significant. Images created with such a spatially-varying resolution may provide an inaccurate depiction of important seafloor features or objects of interest. Combining two perpendicular SAS passes into one “effective” pass provides a synthetic aperture in two orthogonal directions, which should enable the region where the two passes overlap to be imaged with a spatially-consistent resolution. The difficulty in this “multi-pass coherent fusion” (MPCF) comes in quantifying and accounting for all of the sources of error that occur when forming a synthetic aperture. This presentation will outline current work towards the coherent combination of perpendicular SAS passes. We will examine factors that impact the coherence between passes and show how this work relates to existing research in repeat-pass SAS, image co-registration, and SAS micronavigation. Imagery and results from both modeled data and field data will be presented. Finally, future work and applications to automatic target recognition will be discussed.