An Open-Source Solution for Fast and Accurate Underwater Mapping with a Low-Cost Mechanical Scanning Sonar

An open-source software framework is presented that allows real-time underwater mapping with popular marine robotics components, namely a BlueRobotics BlueROV2 with its standard Ping360 Mechanical Scanning Sonar (MSS) and a A50 Doppler Velocity Log (DVL), which are low-cost devices for their respective types - if not even the most affordable ones on the market. The software runs with low computational power on a Raspberry Pi4. The framework builds upon Synthetic Scan Formation (SSF) where single MSS beams or scan-lines are embedded into a pose-graph. The rendering of scans is not only based on navigation, but based on the graph itself. Scans formed from scan-lines can be optimized by online Simultaneous Localization and Mapping (SLAM) and result in improved scans, based on the current state of the graph. In subsequent steps this leads to improved registration results. To this end, a combination of two different types of loop-closures is presented. Namely a consecutive loop closure, and a proximity based loop closure, which reduces the overall drift. The framework is validated in three different test-environments, namely a pool, a test-tank with a gantry for ground truth motion, and the flooded basement of a WW-II submarine bunker. Among others, it is shown that there is an increased accuracy compared to conventional SLAM and that the software is usable in real-time during a mission with the low-cost hardware.