"Loneliness of the Long Distance Glider", Input/Output, ME Departments, March 2005

input/output

by Gayle Ehrenman, Associate Editor

Loneliness of the Long Distance Glider

The term "glider" may bring to mind something simple made of balsa wood. Maybe you tossed one out of a second-story window to see what kind of distance you could get.

On the other hand, when the glider in question is expected to travel underwater from New England to Bermuda, you know that it will be anything but simple. And last fall, a small oceangoing glider called Spray did just that, and became the first autonomous vehicle to cross the Gulf Stream underwater.

Launched September 11 last year about 100 miles south of Nantucket Island, the 6-foot-long orange submersible with a 4-foot wingspan looks like a model airplane with no propeller. It made its way slowly toward Bermuda at about one-half knot, roughly half a mile an hour or a little more than 12 miles per day, measuring various properties of the ocean as it glided up to the surface and then back down to a depth of 1,000 meters three times a day. Scientists recovered the vehicle in early November north of Bermuda.

The crew of the R/V Cape Hatteras uses the ship's crane to launch the glider. Brian Guest steadies the craft with a pole. The white frame at his feet is the cradle in which the glider is stowed.

"It has been exciting, to say the least," said Breck Owens, one of the developers of the robot for Woods Hole Oceanographic Institution. "This trip proved we can use gliders to monitor circulation patterns and major current." Designers of the vessel included Russ Davis and Jeff Sherman of the Scripps Institution of Oceanography at the University of California, San Diego, and there were contributions as well by the Office of Naval Research.

The vessel was developed to provide a small, autonomous platform for long-term ocean measurements. It has no external moving parts. A hydraulic pump powered by lithium batteries transfers mineral oil between two bladders. One bladder is inside the aluminum hull. The other is outside the hull enclosed in the tail section. When the external bladder is pumped full of oil, it displaces seawater from the tail. The craft's mass remains the same, but its effective volume has increased, so it rises. When the oil is pumped the other way, the craft submerges. Spray glides forward through the water as it sinks and rises.

Two battery packs move inside the hull. One can be moved fore and aft to control pitch. The other, for roll, turns axially to steer the Spray. An attitude sensor and compass in the nose keep the craft oriented.

Spray was the first autonomous vehicle to cross the Gulf Stream underwater.

The glider has a range of approximately 3,500 miles, which means that it could potentially cross the Atlantic Ocean. Its mission is to provide a relatively low-cost way for scientists to observe large-scale changes under the ocean surface that might otherwise go unobserved, according to Owens.

There are GPS and Iridium phone antennas in the wingtips, and when it surfaces, the sub rolls one of its wings out of the water to get a reading from the global positioning system and then communicates, using the Iridium modem, back to shore. When the glider surfaces every seven hours to report its position and information about ocean conditions, waypoints and other parameters defining its mission can be updated at the same time.

On the mission last fall, Spray was equipped with instrumentation to measure temperature, salinity, and pressure of the ocean, and with an optical sensor for measuring the turbidity in the water, which is an indicator of biomass. Its next mission is slated for early this year.