Since the 1990s the U.S. Navy has increasingly used AUV (Autonomous Underwater Vehicle) for monitoring the underwater weather and, lately, much else. Newer models like SeaExplorer have more sensors, better controls and more reliable communications than older models. With AUVs like this the navy can inexpensively monitor large expanses of ocean for underwater conditions and all sorts of activity.
Back in 2004 the navy began using SeaGlider. This is a two meter (six foot) long, 52 kg (114 pound) AUV that looks like a torpedo with wings. It can stay at sea for up to six months before needing a battery recharge. As its name implies it glides through the water at up to 20-25 kilometers a day. This AUV, like most of them, is propelled by via a system of shifting weights (oil in an external bladder) and an air tank that is emptied and filled to adjust depth. A pair of wings provides lift as wings do for an aircraft in the air. The SeaGlider moves forward by diving and then coming back up in a forward glide, collecting data all the way.
SeaGliders main mission is to measure of the water, and use a built in satellite phone, every four hours or so, to send the information to anyone in the navy that needs it. SeaGlider also uses the satellite phone to get new orders, and has a built in GPS and other navigation sensors to enable it to find its way to areas it has been ordered to monitor. SeaGlider also collects information on currents, and uses that to glide from place to place.
The composition (temperature, salinity, oxygen content, quantities of biomatter, and so on) of the water in oceans changes slowly. Those characteristics influence the effectiveness of sonars (both active and passive.) If you can monitor the water composition more accurately, your sonars will be more accurate. SeaGlider can be dropped by aircraft or helicopter and spend days, weeks, or months collecting water information (at depths of up to 1,000 meters) before friendly subs show up for action.
At $100,000 each, SeaGlider was a cheap way to keep an eye on large chunks of the ocean. SeaGlider worked because its onboard electronics draw very little power, as does its movement mechanism. SeaGlider wasn’t fast, but it had that most prized UAV/AUV characteristic; persistence. SeaGlider could hang around for a long time, waiting for the enemy to show up. This was a mission submarines were originally designed for. But manned subs were too expensive to put enough of them out there to cover large areas of the ocean. SeaGlider is cheap, efficient, patient and never has to worry about crew morale. What the navy is not discussing is a future version of SeaGlider that wanders around an area looking for hostile submarines as well.
New models like SeaExplorer provide more data and reliability and can be adapted to be launched from a submarine torpedo tube. SeaExplorer is sometimes described as an improved SeaGlider. The two are very similar. SeaGlider could go as deep as 700 meters (2,100 feet) while SeaExplorer could go to 1,000 meters (3,200 feet) and carry more sensors. Given all the data these AUVs now collect it’s essential to get it to manned submarines in the area as quickly as possible. Most modern submarines have satellite communications capability via a small buoy that is sent to the surface from a sub via a cable to control, power and communicate with it. The comm buoy can quickly send and receive data from AUVs and then be pulled back to the sub.