IEEE TRANSACTIONS ON SYSTEMS, MAN AND CYBERNETICS - PART C: APPLICATIONS AND REVIEWS 1 Navigation Technologies for Autonomous Underwater Vehicles Luke Stutters, Honghai Liu Senior Member, IEEE, Carl Tiltman and David J Brown Abstract—With recent advances in battery capacity and the development of hydrogen fuel cells, autonomous underwater vehicles (AUVs) are being used to undertake longer missions that were previously performed by manned or tethered vehicles. As a result, more advanced navigation systems are needed to maintain an accurate position over a larger operational area. The accuracy of the navigation system is critical to the quality of the data collected during survey missions and the recovery of the AUV. Many different methods for navigation in different underwater environments have been proposed in the literature. In this paper, the state of the art in navigation technologies for AUVs is investigated for theoretical and operational systems. Their suitability for use in different environments is compared and current limitations of these methods are identified. In addition, new approaches to address these current problems and areas for future research are suggested. Finally, it is concluded that only geophysically referenced methods will enable AUVs to navigate accurately over large areas and that advances in underwater feature recognition are required before these methods can be implemented in operational AUVs. I. I NTRODUCTION A UTONOMOUS underwater vehicles (AUVs) were ini- tially developed to perform missions which were im- possible for tethered, remotely operated vehicles (ROVs) and are replacing ROVs and towed arrays for many missions [1]. Unlike unmanned underwater vehicles (UUVs) which are usually operated remotely by an acoustic modem link, AUVs present a uniquely challenging navigational problem because they operate autonomously in a highly unstructured environment where satellite-based navigation is not directly available. Unlike autonomous aerial vehicles, AUVs must navigate using other methods when submerged. Their autonomy allows AUVs to be used for missions where a surface vehicle or manned submersible would be at risk, such as mine countermeasure (MCM) or under-ice operations [2]. When performing detailed surveys, AUVs can offer a more stable platform for precision sensors than towed arrays because they are not subject to physical disturbances transmitted along the cable to the surface vessel. This lack of physical attachment also allows AUVs to measure ocean characteristics at specific depths and perform bottom-following missions. For an AUV to successfully complete a typical survey mission, it must follow a path specified by the operator as closely as possible and arrive at a precise location for collection by a surface vessel. If the final position of the AUV This work was supported by funding from DSTL No RD023-02361. L. Stutters, H. Liu and D. J. Brown are with the Institute of Industrial Research, University of Portsmouth, Buckingham Building, PO1 3HE. (e- mail: honghai.liu@port.ac.uk) Carl Tiltman is with Defense Science and Technology Laboratory. is not accurate, the AUV may be unrecoverable. If the AUV does not follow the path accurately during the mission, critical features may not be recorded and the position of any features recorded during the mission will be uncertain. The precision of the navigation system can directly affect the quality of the recorded data if image processing techniques are used to enhance areas which were observed multiple times during the mission and these areas are misaligned because of navigational errors. While established techniques provide AUVs with reason- able navigation for most scientific missions, the operation of AUVs in restrictive environments which require more accurate navigation is more challenging. If an AUV is undertaking a covert mission or operating where the surface is inaccessible, methods which are practical for scientific missions become impossible to use. In these situations, the AUV cannot resur- face to receive a GPS signal or make use of existing sonar beacons so new methods which do not rely on the availability of such signals are needed. The primary challenge in AUV navigation is maintaining the accuracy of an AUV’s position over the course of a long mission. An initially accurate position can quickly become uncertain through variations in the AUV’s motion. This effect can be reduced by using accurate acceleration, heading and velocity sensors but these sensors cannot be made arbitrarily accurate. During long missions, these inaccuracies become significant. Strong currents and other underwater phenomena which affect the motion of the AUV but cannot be precisely modelled lead to greater inaccuracies. If the position of the AUV is not externally referenced, the accuracy of position will inevitably degrade over the course of the mission. The lack of an easily observable, external reference makes AUV navigation very difficult. Any AUV navigation system which provides accurate navigation over long missions must use an external reference. Different methods of providing such a reference are surveyed in this paper. In restrictive environments, some of these methods may not be usable. By considering the problem of providing accurate navigation in these more restrictive environments, techniques may be developed which lead to more effective navigation for all AUV missions. II. NAVIGATIONAL METHODS The different methods which are currently used for AUV navigation can be grouped into three categories [3]: 1) Inertial navigation Inertial navigation uses gyroscopic sensors to detect the