Efficient seabed coverage path planning for ASVs and AUVs

Coverage path planning is the problem of moving an effector (e.g. a robot, a sensor) over all points in a given region. In marine robotics, a number of applications require to cover a region on the seafloor while navigating above it at a constant depth. This is the case of Autonomous Surface Vehicles, that always navigate at the water surface level, but also of several Autonomous Underwater Vehicle tasks as well. Most existing coverage algorithms sweep the free space in the target region using lawnmower-like back-and-forth motions, and the inter-lap spacing between these back-and-forth laps is determined by the robot's sensor coverage range. However, while covering the seafloor surface by navigating above it at a constant depth, the sensor's field of view varies depending on the seafloor height. Therefore, to ensure full coverage one would need to use the inter-lap spacing determined by the shallowest point on the target surface, resulting in undesired coverage overlapping among the back-and-forth laps. In this work, we propose a novel method to generate a coverage path that completely covers a surface of interest on the seafloor by navigating in a constant-depth plane above it. The proposed method uses environment information to minimize the coverage overlapping by segmenting the target surface in regions of similar depth features and addressing them as individual coverage path planning problems. A cell decomposition coverage method is applied to each region. The surface gradient is used to determine the best sweep orientation in each cell, and the inter-lap spacing in the lawnmower-like paths used to cover each cell is maximized on a lap-by-lap basis, hence obtaining a shorter, more efficient coverage path. The proposal is validated in simulation experiments conducted with a real-world bathymetric dataset that show a significant increase on path efficiency in comparison with a standard boustrophedon coverage path.