Synthetic Scan Formation for Underwater Mapping With Low-Cost Mechanical Scanning Sonars (MSS)

Mechanical Scanning Sonars (MSS) are popular devices for Unmanned Underwater Vehicles (UUV), i.e., Remotely Operated Vehicles (ROV) and Autonomous Underwater Vehicles (AUV), as they function under low visibility conditions and over extended ranges. They are comparatively low-cost and easy to integrate. But they require motion-compensation due to the low updates rates caused by the mechanical scanning and the slow speed of sound. We present here a new form of scan formation for MSS where the data from single beams is embedded into a pose-graph. The rendering of scans is not as usual based on only core navigation sensors, but it can improve in the spirit of a synthetic aperture. To this end, online Simultaneous Localization and Mapping (SLAM) is used to form scans from the single beams. These can be optimized and improved scans in turn lead to improved registration results in subsequent steps. This Synthetic Scan Formation (SSF) leads to better mapping results than state-of-the-art SLAM with MSS. The method is validated with several real-world experiments. First, different trajectories with precise ground-truth in a pool with a gantry set-up are used. Second, results from field trials in a WW-II submarine bunker are presented. It is shown that there are clear quantitative and qualitative improvements, and that SSF can be used in real-time for mapping during a mission.