The E-3A, a modified Boeing 707 with a large mushroom-shaped radar dome on its fuselage, provides NATO with crucial battlefield intelligence. The E-3A Sentry is an airborne warning and control system (AWACS), which normally operates at an altitude of 30,000ft. When established at this altitude, a single E-3A can continuously survey the airspace within a radius of more than 400km of the aircraft and, using digital data links, exchange information with ground and sea-based commanders. Geilenkirchen Air Base in Germany is home to NATO's 17 E-3As. Whilst their principal role is air surveillance, they can also help co-ordinate air-to-air refuelling missions, combat search and rescue, and the support and control of friendly aircraft.