The idea is to supplement radar and satellites. The helium-filled dirigibles -- about 25 times larger than those seen at sporting events -- could also be used to monitor combat zones overseas.

They would patrol at 65,000 feet, or more than 11 miles up, and be powered by the sun.

``It's another mix in the multi-layered defence sensors,'' Christopher Taylor, a spokesman for the Pentagon's Missile Defence Agency, said Tuesday. The goal is to have a prototype ready to fly in 2006. The contract includes a $50 million option to build one.

The agency will direct the design and construction of the prototype in Akron, Ohio, by Lockheed on behalf of the Department of Defence, the North American Aerospace Defence Command (NORAD) and the Army.

Blimps can peer down into valleys missed by surface radar and, unlike satellites, their surveillance isn't limited by the earth's rotation. They can be aloft for months at a time and, unlike satellites, can land for equipment changes or maintenance.

Depending on tests, the Pentagon will order an undetermined number of the blimps, Taylor said. The Department of Homeland Security, which includes border patrols, also will consider ordering blimps, he said...