Underwater Gliders
Tech Level: 10

Underwater gliders are a type of autonomous underwater vehicle (AUV) designed for long-endurance oceanic missions that may last many weeks or months. Compare this to more common AUVs, which usually have operational durations measured in hours if not tethered to a surface ship.

The concept of oceanic gliders can be traced to DARPA and to oceanographer Henry Stommel in the late 1980s. In 2003, a working prototype was created by the Webb Research company and the technology has been used for research by a number of institutions since. In 2009, an underwater glider, created by a team from Rutgers University, completed a trans-atlantic crossing with an underwater glider, the first AUV ever to do so, after a 221-day voyage.

Underwater gliders, like their sky-borne cousins, mostly follow the ebb and flow of their medium, flowing with oceanic currents. However, they are not complete dead weights, like buoys. They can use their wings and alterations in their buoyancy to slowly maneuver themselves along at the dizzying speed of about half a knot.

These craft take advantage of thermal stratification, the phenomenon of the water being warmer at the surface than at deeper depths. The glider's engine contains wax tubes that are warmed at the surface. This causes them to expand and push oil from an internal reservoir to an external bladder, forcing a change in buoyancy and helping the glider dive. At lower depths, where the water is colder, the wax compresses and the oil is drawn back into the internal reservoir, causing the glider to rise toward the surface. These slow dives and climbs allow the glider to very gradually move through the water independently of whatever current it may be following.

At the surface portions of its cycle, one of the gliderís wings, each fitted with an antenna, is canted up fully out of the water to transmit data and to receive navigational and GPS signals.

How deep a glider can dive depends on the model, but ranges between 200 and 1500 meters. However, deep-diving gliders are under development which will be able to achieve depths down to 3300 meters.

The craft uses arrays of onboard batteries to control its pitch and orientation, as well as to power its sensors and radios. When the idea of an underwater glider was first being bandied about in the 1980s, it was thought that thermal differentials in the water could help recharge the batteries, allowing the gliders to potentially remain at sea indefinitely. Unfortunately, no one has yet developed this capability, though research is ongoing.

Underwater gliders typically carry sensors such as sonars, hyrdophones, thermal sensors, and others in order to monitor not only the ocean environment but wildlife as well. Because the gliders make almost no operational noise, they are less likely to drive away animals such as whales or dolphins. Scientists use the data collected by gliders for a variety of purposes, which include better understanding of the interactions between the ocean and atmosphere, and of potential climate change impacts on marine ecosystems.

While underwater gliders will no doubt provide a great deal of valuable data on Earthís oceans in the decades to come, it may be in the far future they could be used to explore bodies of water on other worlds such as Europa where their low power requirements and long endurance would prove invaluable.

Cross section of the SPRAY underwater glider.

The dive cycle of an underwater glider. Times and depths vary with different models.







Article added 21 July 2010