A hybrid SLAM representation for dynamic marine environments

We present a hybrid SLAM system for marine environments that combines cubic splines to represent the trajectories of dynamic objects, point features to represent stationary objects and an occupancy grid to represent land masses. This hybrid representation enables SLAM to be applied in environments with moving objects, where solutions using point features alone are computationally prohibitive or where dense objects e.g. landmasses can not be represented correctly using point features. Estimation is achieved using a sliding window framework with reversible data-association and reversible model-selection. Our main contributions are: (i) a hybrid representation of the environment; (ii) occupancy grid fusion is continually refined for the duration of the sliding window; (iii) the trajectories of dynamic objects are represented using cubic splines and (iv) radar scans are re-rendered at a sub-scan resolution to compensate for the egomotion during the scan acquisition period. We show that the continual refinement of the occupancy grid greatly improves the quality of the resultant map, leading to a better estimate of the egomotion and therefore better estimates of the trajectories of dynamic objects. We also demonstrate that the use of cubic splines to represent trajectories has two major advantages: (i) the state space is compressed i.e. many vehicle poses can be represented using a single spline section and (ii) the trajectory becomes continuous and so fusing information from asynchronous sensors running at multiple frequencies becomes trivial. The efficacy of our system is demonstrated using real marine radar data, showing that it can successfully estimate the positions/velocities of objects and landmasses observed during a typical voyage on a small boat.