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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 forward with very low power
consumption. While not as speedy, this enables a major 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.
The concept of the glider was 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). He proposed
harnessing energy from the thermal gradient between deep ocean water (2-4 °C)
and surface water (near atmospheric temperature) to achieve globe-circling
range, constrained only by battery power onboard for communications, 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 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 only as testbed for the thermal-power gliders, the Webb
Slocum electric-powered shallow water gliders have been widely deployed since
2003. As one of its earliest users, Rutgers University Coastal Ocean
Observations Lab has flown over 33,000 km (as of September 3, 2007) in five of
the seven continents with its fleet of gliders. 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
Antarctic shelf.
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, pressure sensors, 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 by using a piston to flood/evacuate a compartment
with seawater (Slocum) or by moving oil in/out of an external bladder (Seaglider,
Spray, and Slocum Thermal). Commands and data are relayed between gliders and
shore by satellite.
Gliders vary in the pressure they are able to withstand. The coastal Slocum
model is rated for 200 meters depth. Spray and Seaglider can dive to 1000
meters, and Slocum Thermal to 2000. 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
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