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 research ship, the AUV’s computers are programmed with instructions of 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) while the AUV is underwater (this typically has a range of a few kilometres), or satellite communications (such as Iridium) when the AUV is floating on the sea surface.
The NOC has been developing the Autosub range of AUVs for many years with the first missions taking place in 1997. You can read about the history of the development of Autosub.
Power and propulsion
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 1kg 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), the solution is usually for the Autosub to travel slowly to enable sufficient range. The current Autosub AUVs run at approximately 1.7ms−1 (surface ships typically run at 5 to 10ms−1).
Accurate navigation is also a challenge for an AUV. At the sea surface, AUVs can be positioned using the satellite-based Global Positioning Systems (GPS), however, satellite signals are not able to penetrate even the top few millimetres of the ocean 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 seabed. 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 one metre error for each kilometre travelled are achievable. Navigation accuracy is critical to many survey missions, and hence, the NOC are researching and developing techniques to further improve upon this performance.
The NOC Autosub fleet
The NOC operates three AUVs: Autosub3, Autosub6000 and Autosub Long Range. Each has its own niche strengths.
AUVs are particularly effective when they are carrying out missions that could not be achieved via any other means. Excellent examples of include the Autosub missions carried in the Arctic and Antarctic from 1999 until the present, with long missions (greater than 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 500m to 1000m thick floating ice tongue of the Pine Island Glacier, penetrating into the ice cave by up to 60km. Using upwards and downwards looking mapping sonar, Autosub3 was able to map out both the ice above and sea bed depths below the AUV track. These missions were potentially very dangerous for the AUV, as unlike in open water missions where the AUV can return to the surface if a fault is detected, an AUV with up to a kilometre of ice above it has no such option.
As with Autosub3, Autosub6000 has been developed to operate in extreme environments, this time it is to the depths of the oceans, 6000m 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 multi-beam sonar.
In October of 2009, the AUV completed successful trials to an operating depth of 5600m, and tested a new obstacle avoidance system, based on scanning sonar, over the rugged terrain of the Casablanca Seamount. This was put to good use in early 2010, operating off the RRS James Cook, where 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 that we have developed, which can themselves withstand the pressures at 6000m depth. These are fitted into cut-outs within the buoyancy foam (syntactic foam) which makes up the centre section of the AUV.
Autosub Long Range
The Autosub Long Range AUV (or Autosub LR) is a new type of AUV with a depth rating of 6000m. Although a third the weight of the Autosub3 and the Autosub6000 AUVs, it is able to travel greater than ten times the distance and can be deployed for over a hundred times the duration.
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 6000km range, an endurance of six months and a depth rating of 6000m, this AUV is very useful to oceanographers in providing measurements of ocean and seabed properties over ocean scales, and without the need for a research ship. The AUV periodically surfaces and transmits the data back to the scientists via an Iridium satellite data link.