Unmanned Vehicles Gliding Undersea

September 10, 2009 · Filed Under 2009, Journey

CDR William Sommer, U.S. Navy

In the early hours of August 8, a U.S. Navy detachment aboard U.S. Coast Guard Cutter Healy launched a SeaGlider into the Chuckchi Sea. The SeaGlider is one model of a class of Unmanned Undersea Vehicles used to sense and report ocean characteristics, including optical properties as well as conductivity (salinity), temperature and depth, also known as CTD. Gathered data are used to evaluate the performance of its oceanographic forecasts.

As the name suggests, this undersea vehicle “glides” through the water column as it moves from location to location. A small pump and motors adjust the buoyancy (weight) and balance of the vehicle causing it to rise and fall through the water. As it rises or falls, the wings convert some of the vertical motion to a forward thrust just as an aero-glider trades altitude for air speed. When the glider completes a full dive and surfacing cycle, it will point an antenna skyward and “phone home” to its pilot at the Glider Operations Center (GOC) at the Naval Oceanographic Office at Stennis Space Center in Mississippi. During this communications period, it sends the recently sensed data, receives a new set of instructions from the pilot, and then begins the next cycle.

The glider is not fast-it is designed to conserve power through gliding. The lack of an active propulsion system conserves enough power to allow the glider to remain at sea for upwards of six months. It is this persistence which makes the glider so useful. Once deployed from a ship, it provides high-quality observations for months, all the while unattended. A single operator at the GOC can manage several gliders simultaneously. In the case of the glider launched from Healy, it will remain at sea for six weeks before it is recovered.

CDR Sommer prepares glider for launch. Click image for details.

The glider also offers a superior quality of data since the vehicle’s small shape does not disturb or mix the water column as a ship’s hull and propellers do. Complex and often very small scale ocean features remain largely undisturbed by the gliders transit. The glider is portable and can be assembled, launched, and recovered easily by a two-man team with just a few hours of training.

Gliders are also a very cost effective tool for observing the ocean. It can cost upwards of $40,000 a day to keep a ship at sea. To complete a CTD cast to 1000 meters depth, it might take two hours or almost $3,400 not including the cost of the instrument itself or the cost to move the ship to the site being studied. While conducting a CTD cast, a ship is also stopped in the water and cannot conduct most other research operations. Costing in the low $100,000s, the glider effectively pays for itself through the recovered ship time in a matter of days. Use of a glider frees the ship to conduct research more appropriate to the ship’s design, such as hydrographic and bathymetric survey operations.

As mentioned, all of the data from the glider and others like it world wide are sent to the Naval Oceanographic Office. There, the Navy operates a supercomputing facility producing daily oceanographic forecasts for the nation’s maritime forces (Navy and Coast Guard) and other key government partners and agencies. These forecasts serve both scientific research and U.S. Navy operational concerns. The observations gathered by the glider will both improve the quality of the ocean forecasts and aid in determining the quality of those modeled forecasts.

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