At a cost of $2 billion each, the B-2 Stealth bomber is the crown jewel of Air Force bombers. But concerns are growing among military analysts that the B-2 Stealth, which is supposed to be invisible to enemy radar, could be identified by a new kind of radar — and may not be invisible after all.

"Ultimately if there is something here, and if it can be made to work and if it can be made exportable, down the road you would have to worry," says Dan Goure of the Lexington Institute, an Arlington, Va., think tank that favors a strong national defense.

The potential threat is known as passive radar, a system being developed by China, Russia, and companies in the U.S. and Europe.

Unlike conventional radar, which bounces its own signal off of an aircraft, a passive system relies on television transmitters or cell-phone towers and the low-frequency radio waves they constantly send out.

High-speed computers analyze a variety of signals and pick out those radio waves reflecting off of moving objects. One U.S. company has already used passive radar to track a commercial jetliner.

That could spell trouble for Stealth aircraft, and non-Stealth aircraft as well, such as the F15s patrolling Iraq. While pilots can detect and take countermeasures against conventional radar, they can’t against passive radar.

"The problem is, in a sense, that noise will be like the constant hum of a television set or an air-conditioning unit," Goure said. "It's there, but there is nothing you can do about it, so you have to ignore it essentially."

The Pentagon isn’t ignoring the threat, even though analysts believe the system won’t be perfected for at least 10 years.

"Our desire to have Stealth technology remain effective in any part of the world is something that we are constantly going to care about very, very much," Pentagon spokesman Rear Adm. Craig Quigley said.

And to that end, Pentagon analysts are already working on countermeasures for the threat. And now the race for passive radar is on.