Their builders call them ‘gliders’, but of course these vehicles move through water instead of air. Two new robotic gliders, or autonomous underwater vehicles, powered by changes in their own buoyancy produced by heat engines as the vehicles cross the thermocline are now undergoing testing. Developed with support from the United States’ Office of Naval Research, the gliders will participate in Navy and Marine Corps exercises in September 2003.
The System
Webb Research of East Falmouth, MA, a company with long experience in the design and building of oceanographic instruments, has built a system it calls the ‘Slocum Glider’. Conceived as a replacement for fixed buoys and capable of accomplishing many of the survey tasks otherwise done by a hydrographic vessel, the Slocum Glider has a transoceanic range. It uses a proprietary Webb Research heat engine that employs a phase change material to augment a pump that inflates or deflates a gas-filled bladder (the working gas is typically nitrogen) which in turn alters the vehicle’s buoyancy. Wings on the glider impart forward motion, and thus the vehicle moves in a saw-tooth pattern. The Slocum Glider cycles thousands of times between the surface and a programmed depth, getting the energy it needs to change its buoyancy from the heat flow of the surrounding water. This long-range deep ocean glider is designed to cruise for five years in a vertical zigzag from the surface to depths of about 5,000 feet and back. The vehicle is capable of carrying a variety of sensors to measure salinity and temperature, plot currents and eddies, count microscopic plants, or record biological sounds. An earlier battery powered model is used to study coastal waters up to 656 feet in depth, for up to thirty days at a time. Webb Research has named the glider in honour of Captain Joshua Slocum, who sailed alone around the world in a 37 foot sloop between 1895 and 1898.The specifications of the Slocum Glider are:
- Length: 2m
- Diameter: 30cm
- Wingspan: 90cm
- Weight: 56.7 kilos, dry
- Propulsion: Battery or thermocline-driven heat engine
- Range: 1,500km
- Endurance: 30 days
- Maximum Dive Depth: ~200m (battery-powered version), -1,500m (heat-engine powered version)
- Communication: line-of-sight and satellite
- Sensor packages: Conductivity and temperature
- Navigation: GPS
The second robot, the University of Washington Applied Physics Laboratory’s Seaglider, is propelled by buoyancy control and wing lift to alternately dive and climb along slanting glide paths. It dead reckons underwater between Global Positioning System (GPS) navigation fixes obtained at the surface, and so glides through a sequence of programmed way points. It transmits data and receives commands via satellite data telemetry when it exposes an antenna above the sea surface for a few minutes between dive cycles. Seaglider has enough range to cross an entire ocean basin in missions that last months, all the while diving and rising between the surface and waters as deep as 3,500 feet. Like the Slocum Glider, it can be launched and recovered manually from a small boat with a crew of two, and so doesn’t rely on costly ships for its deployment. Seaglider collects high resolution profiles of physical, chemical, and bio-optical properties of the ocean. Its specifications are as follows:
- Length: 1.8m
- Diameter: 30cm maximum
- Wingspan: 1m
- Weight: 52kg, dry
- Maximum dive depth: ~1,000m
- Maximum range: 6,000km
- Duration: 900 dives
- Sensor packages: conductivity/ temperature/dissolved oxygen; fluorometer and optical backscatter, surface current assessment
- Navigation: GPS and dead reckoning
The Office of Naval Research is interested in systems like Seaglider and the Slocum Glider because they offer the Navy and Marine Corps potential tools for collecting data about regions of the ocean necessary for mine countermeasures and other tasks important to expeditionary warfare. These systems represent advanced science and technology being developed to detect, identify, and classify mines in shallow and very shallow waters. They exploit recent advances in sensors (especially lasers, sonar and television-like imaging), robotics (the sensors are installed on a variety of autonomous underwater vehicles-submarine robots), networking and signal processing. Developed by teams of government, industry and academic partners, the mine countermeasures technologies offer the prospect of ultimately reducing or eliminating the need for sailors and marines to enter the dangerous shallow waters just off-shore in order to clear mines in preparation for expeditionary operations.
Autonomous vehicles deploying a variety of mission-tailored sensor packages represent a new direction in mine countermeasures. The Office of Naval Research’s Organic Mine Countermeasures Future Naval Capability programme has been developing such systems for some time now, in close partnership with the operating forces. The ultimate goal of the programme is to let the robots do the dangerous work. Autonomous vehicles like Seaglider and the Slocum Glider offer the prospect of affordable, long-duration underwater surveillance in areas that might otherwise be denied it.
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