Trajectory generation for vessels using a reachability graph

In this work an approach to generate collisions-free trajectories for vessels is presented. The approach consist of a global path planner and a local evasive trajectory planner. The global path planner uses chart information from ENCs to estimate waypoints for a global path avoiding obstacles defined in a chart. The local evasive trajectory planner uses information of a target tracking system to proof for a subsection of the global path if a collisions with other vessels will probably occur. In case of possible collisions the local evasive trajectory planner estimates an evasive trajectory to avoid the dynamic obstacles and lead back to the global path after the avoidance manoeuvre. For the global path planning algorithm a grid based reachability graph is applied to consider the turning circle of the own vessel. The local planner uses a grid based search algorithm that takes into account the turning circle of the vessel also. The result of both search algorithms is a sequence of waypoints defining a collision free path. A Bézier curve interpolation is applied to these waypoints to achieve a path with continuous curvatures suitable for path following algorithms.