Spray Glider


What is it and why do we use it?

Spray gliders are robotic submarines that navigate underwater without a human crew onboard and without cables connecting them to research vessels at the sea surface. Spray gliders are among a class of ocean instruments known as autonomous underwater vehicles, or AUVs.

These gliders carry a variety of sensors, and are programmed by researchers to go where they are needed to do research. They are used to take vertical profiles of data, giving scientists a clearer understanding of the temperature, salinity, and turbidity of specific areas of the oceans. These measurements are then used to determine and understand ocean circulation and its role and influence on the global climate.

How does the vehicle work?

Each Spray Glider is 2 meters (6.5 feet) in length, and has a wingspan of 1.2 meters (3.9 feet). The gliders have no external moving parts or motors. Instead, they move on a pre-programmed course vertically and horizontally in the water by pumping mineral oil between two bladders, one internal and the other external to the hull. This action changes the volume of the glider, making it denser or lighter than the surrounding water.

Researchers guide the glider by giving it waypoints, or target positions. The glider steers to these waypoints by controlling its buoyancy and orientation, using the lift from its wings to move horizontally. The maximum depth is prescribed independently of the waypoints. Maximum depth is chosen to avoid hitting bottom and to cover the range of depth of interest. In the deep ocean, the basic principal is the deeper the glider goes, the further the horizontal distance per dive.

Except for the external bladder and measurement sensors, located in a plastic tail section, all parts are encased inside a thin, 8-millimeter (1/4-inch) aluminum hull. To descend from the sea surface, approximately one liter (four cups) of mineral oil is drained from the external, pancake-shaped rubber bladder through a short tube and into an internal bladder. The movement of the oil and the subsequent shrinking of the external bladder decreases the glider's volume. Like a balloon drained of air, the glider then sinks.

To ascend, a hydraulic pump moves the oil from the internal bladder back into the external bladder, increasing the glider's volume. Researchers pre-program the computer system within the glider so that it knows the maximum depth to reach before returning to the surface.

At the beginning and the end of each dive, the glider obtains and records its position by rolling on its side to expose a Global Positioning System (GPS) antenna embedded the right wingtip. Researchers obtain data from the glider and send new instructions to it using a satellite phone system and an antenna embedded in the left wingtip.

To steer the glider, information from the compass and altitude sensors is used to move two battery packs, one to control pitch (fore and aft angle) and the second to control roll (rotation around the axis of the glider. This changes the orientation of the wings, similar to the way a pilot guides and operates a hang glider.

What does it come equipped with?

For the present mission (Sept-Oct 2004) from Cape Cod, Mass. to Bermuda, the Spray Gliders is equipped with a CTD (for conductivity, temperature and depth) that measures temperature, salinity and pressure, as well as an optical sensor that measures the turbidity, which is related to the biomass in the water.

Orientation sensors on the glider allow it to calculate a dead-reckoning (DR) displacement for each dive. This provides the velocity of the glider relative to the movement of the water. The difference between the DR displacement and the true displacement give the ocean current.

For the next mission, the glider will carry an Acoustic Doppler Current Profiler (ADCP). These can be combined with the positions taken using GPS to give vertical profiles of velocity. In the near future, researchers expect that the gliders will be equipped with an entire suite of sensors that can provide profiles indicating the presence of dissolved oxygen, carbon dioxide, alkalinity (the capacity of bases to neutralize acids), and nutrients.

What platforms are needed for its use?

Spray Gliders weigh 51 kilograms (112 pounds) each and can be launched from any vessel that can accommodate the weight and is capable of reaching the appropriate deployment sites. For the Gulf Stream crossing, the glider was launched from the R/V Cape Hatteras. It has been launched and recovered using the R/V Tioga and other smaller coastal research vessels.

What are the advantages of using this vehicle?

Spray Gliders can provide a look at entire sections of ocean basins, as well as serve as virtual moorings by keeping station at a single point. Unlike humans, who need to stop for breaks, present gliders can carry out missions as long as six months in duration, traveling at a speed of 23 centimeters per second. That works out to 20 kilometers per day (12.4 miles per day), for a total mission distance of 3,600 kilometers (2,237 miles).

What are the disadvantages of using this vehicle?

Spray Gliders move at modest speeds—up to 25 centimeters per second, or about one-half of a knot - so they are best used for monitoring changes over weeks to months. They are not as well-suited for small-scale surveys near strong fronts or coastal jets where other faster moving AUVs can overcome the strong currents and provide the rapid sampling required.

Spray Gliders are presently limited to an operating depth of less than 1,500 meters (one mile) so they can not make full water depth measurements. They also cannot take water samples for chemical analysis. That is, they are not a replacement for a hydrographic cruise, but they are a lot cheaper to build and operate.

How did it get the name "Spray" glider?

The name "Spray" honors the boat used during the first solo circumnavigation of the planet. In 1895, Captain Joshua Slocum sailed from Boston on the sloop Spray during a three-year, 74,000-kilometer (46,000-mile) voyage.

How was the Spray Glider developed?

The idea for developing a robotic glider like Spray that could travel in the ocean gathering data over long periods of time came from Henry Stommel, an oceanographer known for his contributions to understanding the dynamics of ocean currents, especially the Gulf Stream.

Russ Davis of the Scripps Institution of Oceanography and Breck Owens of the Woods Hole Oceanographic Institution developed the idea by directing the initial design using support from the Office of Naval Research. Jeff Sherman of Scripps also lead operations on the design.

Additional development of sensors was suppported by the National Oceanic and Atmospheric Administration's Climate Observations Program. Breck Owens is directing the ocean observation project in the Gulf Stream using funding from the National Science Foundation.