Autonomous Underwater Vehicles

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An Autonomous Underwater Vehicle (AUV) is a robotic device that is driven through the water by a propulsion system, controlled and piloted by an onboard computer, and maneuverable in three dimensions. This level of control, under most environmental conditions, permits the vehicle to follow precise preprogrammed trajectories wherever and whenever required. Sensors on board the AUV sample the ocean as the AUV moves through it, providing the ability to make both spatial and time series measurements. Sensor data collected by an AUV is automatically geospatially and temporally referenced and normally of superior quality. Multiple vehicle surveys increase productivity, can insure adequate temporal and spatial sampling, and provide a means of investigating the coherence of the ocean in time and space.
The fact that an AUV is normally moving does not prevent it from also serving as a Lagrangian, or quasi Eulerian, platform. This mode of operation may be achieved by programming the vehicle to stop thrusting and float passively at a specific depth or density layer in the sea, or to actively loiter near a desired location. AUV’s may also be programmed to swim at a constant pressure or altitude or to vary their depth and/or heading as they move through the water, so that undulating sea saw survey patterns covering both vertical and/or horizontal swaths may be formed. AUV’s are also well suited to perform long linear transects, sea sawing through the water as they go, or traveling at a constant pressure. They also provide a highly productive means of performing seafloor surveys using acoustic or optical imaging systems.
When compared to other Lagrangian platforms, AUV’s become the tools of choice as the need for control and sensor power increases. The AUV’s advantage in this area is achieved at the expense of endurance, which for an AUV is typically on the order of 8- 50 hours. Most vehicles can vary their velocity between 0.5 and 2.5 m/s. The optimum speed and the corresponding greatest range of the vehicle occur when its hotel load (all required power except propulsion) is twice the propulsive load. For most vehicles, this occurs at a velocity near 1.5 m/s.
The degree of autonomy of the robot presents an interesting dichotomy. Total autonomy does not provide the user with any feedback on the vehicle’s progress or health, nor does it provide a means of controlling or redirecting the vehicle during a mission. It does, however, free the user to perform other tasks, thereby greatly reducing operational costs, as long as the vehicle and the operator meet at their duly appointed times at the end of the mission. For some missions, total autonomy may be the only choice; in other cases when the vehicle is performing a routine mission, it may be the preferable mode of operation.