Drone Sub-Hunter to Patrol Seas - Even without a captain at the helm, the Pentagon hopes this drone will chase down enemy subs.

Anti-submarine warfare has long been accomplished by steely-eyed captains who search the oceans before dropping countermeasures like depth charges or shipboard torpedoes to knock out enemy subs. The job requires skill and experience, plus the latest in sonar and radar technology.

But now the Pentagon wants to build a drone sub-hunter that can chase enemy craft for up to two months at a time without any human operator at the helm.

Instead of being launched at sea, as smaller ocean-going drones are at present, the "Continuous Trail Autonomous Vessel" will leave its berth, patrol along the U.S. coastline and then chase enemy subs until they leave. The only time a human will be involved is navigating the robot ship in and out of crowded harbors.

The drone will not be armed, nor will it hide from its opponent.

"The challenge is to create a planning system that is able to track the submarines and at the same time to avoid surface traffic in a way that confirms to the rules of the road," said John Dolan, principal systems scientist at the National Robotics Engineering Center at Carnegie Mellon University, which is working with the Virginia-based contractor SAIC on the $58 million, three-year contract.

Dolan said CMU roboticists will be trying to build something new, a vessel that "doesn't give up" no matter what kind of weather conditions it faces at sea or how its prey is behaving.

"This thing has to be out on its own for a long period of time without human intervention," Dolan said. "Even if the unforeseen happens."

The exact specifications of the DARPA project are not known, such as length or power supply. CMU scientists are working on the autonomy and control systems, while SAIC is building the platform. The vessel has to be able to navigate the ocean while pursuing a submarine, and sending back updates to naval commanders back home or nearby.

The reasoning behind building such a ship is simple, according to one naval expert: money.

"For any nation, building a warship is among the most expensive capital things you can do," said Cmdr. Bill Sommer, program officer for undersea warfare at the Naval Postgraduate School in Monterey, Calif.

Sommer said the size of the naval fleet is shrinking over time, while each ship has to do more at at sea.

"That's why we need autonomy," Sommer said. "We've got to have more ears and cover the ground reliably."

For engineers building drones, whether on land, in the air or at sea, one of the biggest problems to overcome is the so-called "sense-and-avoid" issue, or building a system that can detect other vessels or airplanes and move away.

Right now, for example, federal aviation authorities won't let drone aircraft fly over U.S. airspace, with a few exceptions.

International maritime laws say that each ship, whether it's supertanker, fishing boat or pleasure craft, must be able to "maintain an adequate watch," according to Sommer, and be able to avoid a collision. How that watch will be maintained with a robot ship is yet to be determined.

Once it is up and running, the robot ship will be able to sail for up to 80 days and travel 6,200 kilometers (3,852 miles) without refueling, according to DARPA documents.