Assessment of Shallow Water Performance Using Interferometric Sonar Coherence

Interferometry is used for bathymetric mapping with side scan and synthetic aperture sonars. The principle is based on estimating the time difference between arrivals for two vertically displaced receivers. An important additional feature with interferometric sonar is that an estimate of the signal-to-noise ratio can be derived from the coherence between overlaid data series from the two receivers. Interferometric sonar coherence therefore allows for an effective real-time estimate of quality for side scan sonar and synthetic aperture sonar measurements. We have conducted a series of trials in shallow waters along with Kongsberg Maritime using a HISAS 1030 interferometric sonar mounted on a HUGIN autonomous underwater vehicle. We have also developed a tool for shallow water performance assessment based on a simple ray model. In this paper we first demonstrate how the interferometric sonar coherence relates to the image quality of synthetic aperture images. We then associate the coherence from real data with results from the model in order to better understand the different signal contributions. For the shallow water environment of investigation, we identify multipath returns as the limiting factor for image quality. We conclude that the interferometric sonar coherence yields an effective estimate of quality for interferometric side scan sonar and synthetic aperture sonar measurements. It allows for a simple real-time estimate of the imaging range and in combination with simulations provides the means required for autonomous on-the-fly change of the sonar geometries and settings for optimal performance.