Since 2009 the U.S. Navy has been developing and testing a series of robotic mini-submarines, or AUVs (Autonomous Undersea Vehicle) that are silent, very small, and able to operate on their own for up to a year. The first models were two meters (six feet) long and weighed 59 kg (130 pounds) and built to operate completely on its own collecting valuable information about underwater “weather”. What this AUV does is automatically move slowly (30-70 kilometers a day) underwater, collecting data on salinity and temperature and transmitting back via a satellite link every hour or so as the AUV briefly reaches the surface. This data improves the effectiveness of sonars used by friendly forces, making it easier to detect and track enemy submarines. That’s because the speed of sound travelling through water varies according the temperature and salinity of the water. Having more precise data on salinity and temperature in a large body of water makes your underwater sensors (sonar, which detects sound to determine what is out there) more accurate. The current navy AUVs can dive as far down as 200 meters (620 feet) but new models will be able to go down to 1,000 meters or more.
These AUVs use a unique form of propulsion. They have wings, and a small pump, that fills and empties a chamber. This changes its buoyancy, causing it to glide down, then back up. This maneuver moves the AUV forward. Equipped with GPS and a navigation and communications computer, the AUV is programmed (or instructed via the sat link) to monitor a particular area. The small pump uses less electricity than a propeller (to move it at the same speed). Thus these UAVs can remain at sea for up to a year on one battery charge. Before the battery runs out the navy has to direct the UAV and a ship to a rendezvous where the AUV will remain on the surface and the ship will haul it aboard, replace the battery and perform any other needed maintenance. Small AUV maintenance detachments (of two or three sailors) can be flown to a ship that is close enough to make the rendezvous. In some cases you can direct the AUV to move close to land, which makes it even easier to find a boat to go out and get the AUV. These AUVs can be launched from ships or shore. In 2009 an AUV of this type crossed the Atlantic on its own, as part of a civilian research project.
The navy currently has 75 of these AUVs and plans to have at least 150 by 2015. This is part of a plan to have UAVs replace many of the ocean survey ships currently used for this kind of work. The survey ships take temperature and salinity reading from instruments deployed from the ship as well as a global network of several thousand research buoys. Unlike the survey ships the AUVs could be deployed in areas where hostile subs are believed to be operating, and be kept at it as long as needed. If successful in regular use, larger versions are planned, equipped with more sensors and longer duration.