Unmanned aerial vehicles (UAVs) have proved to be an invaluable assets to the Army troops who need to monitor miles of hostile terrain in Afghanistan and Iraq. But now the Navy hopes to take UAVs to a new — and very deep — level.
Its Office of Naval Research is helping to fund the development of new types of unique autonomous underwater vehicles (AUVs) that could help make monitoring the vast high seas more manageable.
These new experimental AUVs are strikingly similar to the Predator and Global Hawk drones used in the on-going ground conflict in Afghanistan. With slim streamlined shapes and wings, these pilot-less watercraft can be programmed to "glide" through certain routes to gather various bits of information using instruments stored inside their hulls.
But what separates these robotic gliders from other drones and underwater vehicles is that neither of them uses motors or fuel to move through the ocean depths. Instead, they rely upon changes in buoyancy.
In other words, they "swim" when they "sink" from — and "float" back to — the ocean's surface.
Sink to Swim
The Seaglider from the University of Washington in Seattle weighs about 110 pounds and is nearly 6 feet long. Inside the hull are computerized controls, sensor equipment, a GPS receiver, satellite communications system, a bank of lithium ion batteries and an empty reservoir tank.
Before launching the Seaglider, researchers can program the computer with the coordinates of which parts of the ocean it should go and measure. Once the AUV gets its bearing from the GPS satellites, a small electric pump transfers about 100 cubic centimeters of oil from an external bladder into the reservoir, making the Seaglider heavier and dense enough to sink.
As the vessel dives, a small motor pushed the bank of batteries slightly forward, shifting it into a nose down attitude. The water that rushes over the "wings" of the glider pushes the craft forward as it falls through the water.
Once it reaches a pre-determined depth, the process is reversed and the Seaglider begins to rise. As it goes up, the wings continue to give the Seaglider forward momentum, pushing it further along through the water.
To change from a straight-line course, the batteries are rolled from side to side inside the hull. The shift in weight causes the glider to "bank" and turn like an airplane.
The glider travels in this vertical up-and-down "sawtooth" pattern and navigates to its programmed destination using compass readings and "dead reckoning" — figuring out where it is based on how fast it's traveled since its last fix from the GPS satellites.
Once at its destination, the AUV collects the requested data — water temperature and salinity, for examples — using the on-board instruments. The glider then rises to the surface and transmits its finding back to the lab using the Iridium satellite communications system. Researchers can then send back new destinations and instructions to the glider.
Slow, But Long-Lasting
Charles Eriksen, an oceanography professor and one of the developers of the Seaglider, says that such a propulsion system isn't fast. At best, the glider can make about half a knot — slightly more than half a mile an hour.
But since it will use only one-half watt of electrical energy to produce that speed, Eriksen says the Seaglider has a range of "thousands of kilometers" and remain in the ocean gather data for much longer.