Northrop Grumman has demonstrated a missile defense system that is attached to a UAV and watches for missiles as it circles approximately 10 miles above an airport. The system uses sensors to detect missile launches and lasers to blind the infrared seekers that home in on a hot jet exhaust.

This method was proven in tests on Sept. 8 and 9 at Holloman Air Force Base in New Mexico. From an altitude of 50,000 feet, an infrared sensor was able to detect a missile launch, relay the information to an onboard missile tracker and then hit the missile with a blast from a laser.

Cost has been a major obstacle in the effort to protect commercial airliners from the threat of shoulder-fired missiles, but UAV-mounted missile defense systems might make protecting airports more affordable.

For it would be cheaper to build a fleet of missile-jamming UAVs than to install missile defenses on each of the nation's 6800 commercial airliners.

The Department of Homeland Security (DHS) has spent at least $230 million since 2004 on contracts with Northrop, BAE Systems, United Airlines, FedEx and other companies to develop and test missile defense systems. Laser jammers that would be installed on individual aircraft emerged as the preferred technology and the department set a goal of reducing the cost of missile-jamming systems to $1 million or less per airliner. Northrop has met that goal, but equipping the U.S. airline fleet with missile jammers would cost approximately $6 billion. Airlines, reeling under increased fuel costs and massive financial losses, are in no position to purchase this defense systems. The deficit-plagued U.S. government is also unwilling to do so.

Looking for ways to cover the most aircraft with the least money, DHS hit on the idea of putting missile defense systems on UAVs.

Loitering at 65,000 feet, a single UAV could monitor an area 50 miles across. In the Washington area, for example, a single UAV could provide missile defense for Dulles, Reagan National and Baltimore-Washington airports.

By contrast, a ground-based system might not be able to protect an entire airport where planes are landing at one end and taking off at the other.