Autonomous Underwater Vehicles (AUVs) are untethered robotic platforms that operate independently to complete pre-determined surveys. The endurance of AUVs typically range from hours to several days (Huvenne et al. 2018). However, with the rapid development of battery technology long-period deployments ranging from weeks to months are now possible (Furlong et al. 2012; Hobson et al. 2012). Maximum operational depths range from a few hundred metres for the smaller vehicles (Wynn et al. 2014) to over 6000 m for larger units (Huvenne et al. 2009).
Huvenne et al. (2018) classify AUVs as either “cruising” or “hovering” vehicles (Figure 4.1). Cruising AUVs are traditionally torpedo-shaped, driven by a single propeller at speeds up to 2 ms-1, and are optimised to cover large distances along pre-designed survey tracks (Wynn et al. 2014). These cruising AUVs are usually not well suited to photographically surveying high-relief seabed terrain due their lack of vertical agility. Traditionally, cruising AUVs are the main type of AUVs used in the commercial world, with prominent scientific examples including the Autosub series from the National Oceanography Centre (UK), the AsterX and IdefiX from French Research Institute for Exploitation of the Sea (IFREMER; France) and the Dorado series from Monterey Bay Aquarium Research Institute (USA) (Furlong et al. 2012; Rigaud 2007). By contrast, hovering AUVs are equipped with several propellers, which facilitate multi-directional manoeuvrability capabilities, similar to a remotely operated vehicle (ROV). Hovering AUVs are designed for precision operations, slow motion surveys (e.g. seabed photography) and work in distinctly 3-dimensional terrains, such as around high-relief reefs (Williams et al. 2012). Among the best-known scientific examples of hovering AUVs are ABE and Sentry from Woods Hole Oceanographic Institute (USA) (e.g. Tivey et al. 1998; Wagner et al. 2013) and Sirius from Australian Centre for Field Robotics (Australia) (e.g. Bewley et al. 2015; Williams et al. 2016; Williams et al. 2012).
Depending on the size of an AUV they can be equipped with a range of sensors such as conductivity, temperature, depth, acoustic doppler current profilers, chemical sensors, photo cameras, sonars, magnetometers and gravimeters (Connelly et al. 2012; Sumner et al. 2013; Williams et al. 2010). Importantly, on-board battery capacity is the primary limitation to the number of sensors and survey duration for AUVs. Furthermore, AUVs are currently not yet equipped for extensive physical sampling of seabed or fauna, although sampling of the water column can be achieved (Pennington et al. 2016). Overall, AUVs are more suited for survey operations, acquiring sensor data along pre-programmed transects, while ROVs are optimal for high-resolution, highly detailed and interactive work, including high-definition video surveying and physical sampling. An extensive review of the use and capabilities of AUVs for geological research was recently published by Wynn et al. (2014). There is, however, no equivalent review discussing the capabilities of AUVs for ecological research (but see section 3.3 in Wynn et al. 2014; Durden et al. 2016).
This document focuses on hover class AUVs can control their position and heading at very low speeds, which makes them suitable for operations over rough terrain while maintaining an appropriate altitude for imaging small scale targets. When equipped with navigational sensors such as GPS, Ultra Short Baseline Acoustic Positioning System (USBL), acoustic doppler profiler, and forward-looking obstacle avoidance sonar, hover class AUVs enable precise tracking along the pre-programmed routes. These characteristics make them particularly suited to collecting highly detailed sonar and optical images over high-relief seabed terrain, which can be geo-referenced with high precision. These can then be stitched together into photomosaics to focus on large features or specific details on the seafloor.
While most of the well-known AUVs used in scientific research are custom built, technological developments over the last five years have seen a number of ready-built, commercial units becoming available, with examples such as the cruising Iver and hovering Subsea 7 AUVs. The release of these units into the market will likely increase the uptake of AUVs for scientific research.
Figure 4.1: Examples of AUV classes. Left: an example of the cruising class AUV _Nupiri muka _operated by the University of Tasmania (photo credit: Damien Guihen). Right: an example of the hovering class AUV _Sirius _operated by Australian Centre for Field Robotics for Integrated Marine Observing System (Photo credit: Asher Flatt).