Low-altitude terrain following and collision avoidance in a flight-class autonomous underwater vehicle

An autonomous underwater vehicle (AUV), Autosub6000, has been shown to operate safely at altitudes as low as 3 m above rugged and complex sea floor environments. This capability is essential for future AUV missions in such environments, e.g. high-resolution surveys using colour photography or multi-beam sonar bathymetry. This was achieved through the development of an obstacle avoidance system for the AUV, incorporating relatively low-cost off-the-shelf components and simple algorithms. This paper details the specification, design, and testing at sea of Autosub6000's obstacle avoidance system. It describes how the specification of the system was influenced by the need to retrofit it into the existing control architecture, together with the pragmatic need to minimize overall complexity. The sensor used in the obstacle avoidance system is a mechanically scanned forward-looking sonar, and the control algorithm is based upon the detection of the range and elevation of the horizon relative to the AUV. The avoidance behaviour is by default to fly over obstacles but, if this is not possible, a turn-around and retry collision avoidance algorithm is invoked. Results are presented of the system's performance during recent deep-water trials of the AUV over the Casablanca Seamount region of the Atlantic Ocean