About AUVs
Autonomous Underwater Vehicles (AUVs) are robot submarines, which are used to explore the world’s oceans without a pilot, or any tether. Before launch from the mother ship, the AUV’s computers are programmed with instructions on where to go, what to measure, and what depths to go to. With no link to the mother ship, all communications with the AUV are limited to using acoustics (sound) when the AUV is underwater (this typically has a range of a few km) or satellite communications (such as Iridium) can be used when the AUV is floating on the sea surface.
Power
Energy supply for the propulsion system and sensors is a challenge for AUVs. Without the supply of oxygen from the atmosphere, internal combustion engines are not practical. Rather, the AUV must rely on batteries. As the amount of energy available from 1 kg of the best batteries is about ten times less than that available from the same quantity of diesel fuel, currently AUVs are limited in range and speed compared to surface vessels. As the required propulsive power increases very rapidly with operating speed (approximately proportional to the speed cubed), to achieve acceptable range, the solution is usually to go rather slowly. The current Autosub AUVs run at about 1.7 ms-1 (surface ships typically run at 5 to 10 ms-1).
Navigation
Accurate navigation is also a challenge for an AUV. At the sea surface there is no problem: AUVs can be positioned using the satellite based Global Positioning Systems (GPS). However, satellite signals don’t penetrate even millimetres of sea water, and hence other means are needed to navigate the AUVs once they have dived. The Autosub AUVs rely mostly on an approach known as dead reckoning. The AUVs bounce sound off the seabed, and by measuring the Doppler shift of the echoes, they are able to measure their speed relative to the sea bed. For dead reckoning the AUV must also accurately sense its heading. On the Autosub AUV a fibre optic gyro based sensor is used , giving heading accuracy of better than 0.1 degrees. Overall, accuracies of about 1 m error for each 1km travelled are achievable. Navigation accuracy is critical to many survey missions and hence we are researching and developing techniques to further improve upon this performance.
NOC AUVs
Within NOC, the Autonomous Systems group has developed the Autosub range of AUVs. Since the first missions of Autosub back in 1997, the emphasis within the group has been on the autonomous aspects of their scientific missions. Very often the AUVs are operated without contact with any operator for a day or more. There are currently two Autosub AUVs in operation (Autosub3 and Autosub6000, with a third (Autosub Long Range) undergoing engineering trials.
Autosub3
Autosub3 is ready to launch in front of the Pine Island Glacier in the Western Antarctic. In a mission lasting 36 hours it ran 60 km under the ice shelf, into a cave, topped with ice up to 1000 m thick.
AUVs are particularly effective where they carry out missions which cannot be done any other way. Excellent examples of this are the Autosub missions carried out in the Arctic and Antarctic from 1999 until the present , with long missions, beyond 24 hours, operating under sea ice and under the floating ice-tongues of glaciers. Perhaps the most striking example was the campaign of 2009 in the Western Antarctic, where Autosub3 operated beneath the 500 m to 1000 m thick floating ice tongue of the Pine Island Glacier, penetrating into the ice cave by up to 60 km. Using upwards and downwards looking mapping sonar, it was able to map out, for the first time, both the ice above and sea bed depths below the AUV track. These missions were potentially very dangerous for the AUV: unlike open water missions, were the AUV can return to the surface if a fault is detected, for an AUV, with up to 1 km of ice above it, there is no such easy option.
Autosub3 has a total displacement 3.6 tonnes and can travel up to 400 km on a set of batteries. It can dive to a depth of 1600 metres.
Autosub6000
Autosub6000 being prepared for hydrothermal vent hunting in the Caribbean sea. Note the dedicated launch and recovery gantry in the background, and in the nose of the AUV, the scanning sonar system used to help the AUV avoid collision with the very rugged terrain in the area.
Like Autosub3, Autosub6000 has been developed to operated in extreme environments. This time it is to the depths of the oceans, 6000 m deep, where the crushing pressures are 600 times greater than at the surface.
Following its first sea trials in 2007, Autosub6000 has undergone significant development. In the summer of 2008 it undertook its first science missions, surveying deep sea scour features on the deep western European Margin with a multibeam sonar.
In October of 2009, the AUV completed successful trials to an operating depth of 5600 m, and tested a new obstacle avoidance system, based on a scanning sonar, over the rugged terrain of the Casablanca seamount. This was put to good use in when in early 2010, operating off the RRS James Cook, when the AUV was crucial to the effort to locate and pin-point the positions of two hydrothermal vent sites in the mid Cayman rise area of the Caribbean sea.
One of the innovative features of Autosub6000 , is that unlike Autosub3, it does not use strong pressure vessels to protect the batteries from the external pressure. Rather it uses batteries which we have developed which can themselves withstand the pressures at 6000 m depth. These are fitted into cut outs within the buoyancy foam (syntactic foam) which makes up the centre section of the AUV.
Autosub LR
Transparent view of the Autosub Long Range AUV. The forged aluminium spheres (upper hemispheres are not shown), hold the batteries (front sphere) and the control computer (rear sphere). The propulsion motor, which is housed in a dry pressure vessel, is coupled to the propeller via magnetic couplings, eliminating the need for (energy sapping and potentially unreliable) rotating seals.
The Autosub Long Range AUV (or Autosub LR) is a new type of AUV. Although a third the weight of the Autosub3 and the Autosub6000 AUVs , it will be able to travel for more than ten times the distance, and be deployed for over a hundred times greater duration. All this, and with a depth rating of 6000 m. How does it do this?
The key to achieving this performance is efficient propulsion at slow speed (at 0.4 ms-1), and by keeping tight control of the power used by the AUV sensors and control systems. One area in which recent advances in technology has helped make this possible has been in the development of microprocessors for devices such as mobile phones which have ample processing power , but which use very little energy.
With a 6000 km range, an endurance of 6 months and a depth rating of 6000 m, this AUV will be very useful to oceanographers in providing measurements of ocean and seabed properties over ocean scales, and without the need for a research ship. But the scientists won’t need to wait for six months to get access to the data. The AUV will periodically surface and transmit the data back to the scientist via an Iridium satellite data link.
The Autosub Long Range being lowered into the fresh water test tank at NOC Southampton. The AUV runs with only 2 kg buoyancy out a total displacement of 650 kg, hence the buoyancy measurement must be carried out meticulously. Even the small amount of dissolved solids in the Southampton “fresh” water in the test tank affects the buoyancy and must be accounted for.