An underwater glider is a type of autonomous underwater vehicle (AUV) that uses small changes in its buoyancy in conjunction with wings to convert vertical motion to horizontal, and thereby propel itself forward with very low power consumption. While not as fast as conventional AUVs, gliders using buoyancy-based propulsion represent a significant increase in range and duration compared to vehicles propelled by electric motor-driven propellers, extending ocean sampling missions from hours to weeks or months, and to thousands of kilometers of range. Gliders follow a sawtooth path though the water, providing data on temporal and spatial scales unavailable to previous AUVs, and much more costly to sample using traditional shipboard techniques.
By 2005, not only had a working thermal-powered glider (Slocum Thermal) been demonstrated by Webb Research, but they and other institutions had introduced battery-powered gliders with impressive duration and efficiency, far exceeding that of traditional survey-class AUVs. The University of Washington Seaglider and Scripps Institution of Oceanography Spray vehicles have performed feats such as crossing the Gulf Stream from the mainland USA to Bermuda, and, together with the Webb Slocum, conducting sustained, multi-vehicle collaborative monitoring of oceanographic variables in Monterey Bay.
Originally conceived as testbed for the thermal-power gliders before developing into a platform of their own, the Webb Slocum electric gliders have been widely deployed since 2003. As one of its earliest users, Rutgers University has flown its fleet of gliders over 45,000 km (as of June 2008) in three of the five major oceans. In January 2007, Slocum glider RU06 became the first autonomous underwater vehicle to fly across the Antarctic Circle into Antarctica, on its way completing a 22 day, 553 km mission along the western shelf of the Antarctic Peninsula.
Gliders vary in the pressure they are able to withstand. The coastal Slocum model is rated for 200 meters depth. Spray can operate to 1500 meters, Seaglider to 1000 meters, and Slocum Thermal to 1200. In December 2006, a Deep Glider variant of the Seaglider achieved a repeated 3300-meter operating depth.
As of 2006, the US Navy Office of Naval Research is developing the world's largest glider, the Liberdade XRay, which uses a blended wing body hullform to achieve hydrodynamic efficiency. It is intended to quietly track diesel electric submarines in littoral waters, remaining on station for up to 6 months, with major field testing beginning in August 2006 .