A Joint Stars cruises at 400 to 500 mph (644 - 804 kph) at more than 36,000 feet (10,970 metres). It uses radar to scan for ground objects up to 150 miles (250 km) away and across a 120-degree field of view covering more than 19,000 square miles (50,000 sq km).

The radar equipment is in a 26 ft (7.9 m) long lump under the front part of the fuselage. One radar provides a three-dimensional image map. The other picks out and tracks objects that move. The operators then relay the information to strike aircraft, attack helicopters or ground units and artillery.

The US tested the aircraft to great effect in the Gulf War where it tracked Iraqi forces and Scud missiles. Two operated over Bosnia in 1995. But the mountainous terrain exposed limitations with the radar. The shadow effect of high ground meant that at times, up to half the battlefield picture could not be seen.

Two major problems are that the Joint Stars cannot use the imaging and tracking radars simultaneously and the aircraft needs a very long runway for take off.

On a typical, 10-hour mission there are three flight crew, plus up to 15 air force and three army specialists. Missions can be doubled in length if there is mid-air refuelling and a substitute crew.

Eleven are in use with the 93rd Air Control Wing, which is due to get 17 in total.