Underwater gliders
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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 with very low power consumption. While not as speedy, this enables a major increase in range and duration compared to vehicles propelled by electric motors, extending ocean sampling missions from hours to weeks or months, and to thousands of kilometers of range, with a sawtooth path though the water that provides data on temporal and spatial scales unavailable to previous AUVs, and much more costly to sample using traditional shipboard techniques.
The concept of the glider was popularly introduced to the oceanographic community by Henry Stommel in 1989, when he proposed a glider concept called Slocum (after Joshua Slocum, the first solo circumnavigator of the globe by sailboat). His proposal would use the thermal gradient between deep ocean water (2-4 °C) and surface water (near atmospheric temperature) to have globe-circling range, constrained only by battery power onboard for sensors and navigational computers.
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 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. Gliders typically make measurements such as temperature, conductivity (to calculate salinity), currents, chlorophyll fluorescence, optical backscatter, bottom depth, and (occasionally) acoustic backscatter. They navigate with the help of periodic surface GPS fixes, tilt sensors, and magnetic compasses. Vehicle pitch is controllable by movable internal ballast (usually battery packs), and steering is accomplished either with a rudder (as in Slocum) or by moving internal ballast to control roll (as in Spray and Seaglider). Buoyancy is adjusted either flooding or evacuating a compartment with seawater (Slocum) or by moving oil in or out of an external bladder (Seaglider, Spray, and Slocum Thermal). Commands and data are relayed between gliders and shore by satellite.
As of 2006, the US Navy Office of Naval Research is currently 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 [1].
