Beacons seen as undersea robot navigation aid.

• Author: Tadjdeh, Yasmin

Unmanned underwater vehicles have become increasingly popular within the military, but finding ways to stealthily navigate undersea has been a perennial issue.

Draper Labs--operating under the Defense Advanced Research Projects Agency's positioning system for deep ocean navigation program contract that was awarded in March--is working in earnest to solve that problem.

The program is "intended to revolutionize navigation underwater like GPS did in terms of revolutionizing navigation above the water," said Neil Adams, director for defense systems at Draper Labs.

To remain stealthy, UUVs use inertial sensors to navigate underwater. However, during long missions these sensors can accumulate errors, rendering navigation systems inaccurate, Adams said.

"Inertial sensors... come in a lot of different varieties but the one thing they all have in common is that they have systematic biases in them," he said. "Their measurement sources drift over time and you need some external absolute source to discipline an inertial instrument."

Even the best inertial systems can't operate for extended periods of time without having updates to correct the drift, he said.

GPS is one way to bring a system back into line, he said. However, such signals are opaque underwater. "The only way you can use it for underwater operations really is to surface, get a fix and then submerge again," he said.

A potential solution could come in the form of a constellation of underwater antennas that can transmit...