An Airborne Early Warning (AEW) system is a radar system carried by an aircraft which is designed to detect other aircraft. Used at a high altitude, the radars allow the operators to distinguish between friendly and hostile aircraft hundreds of miles away.
AEW aircraft are used for defensive and offensive air operations. The system is used offensively to direct fighters to their target locations, and defensively to counter attacks.
Many countries have their own AEW systems, although the Boeing E-3 Sentry and Northrop Grumman E-2 Hawkeye are the popular systems worldwide. The Sentry is built by the Boeing Defense and Space Group (now Integrated Defense Systems and is widely considered to be an international benchmark for AEW systems. It is based on the Boeing 707 aircraft. The E-2 Hawkeye, which entered service in 1965, is arguably the most widely used AEW system. The E-3 Sentry is not strictly an AEW system, as it has far more functionality. See Airborne Warning and Control System.
The RAAF are deploying Boeing 737-based aircraft under Project Wedgetail. Unlike the E-2 and E-3, Wedgetail does not have a rotodome . It will probably be marketed towards many existing E-2 customers, who have no choice but to purchase a system intended for an aircraft carrier, due to lack of options.
Related books
AWACS and Hawkeyes: The Complete History of Airborne Early Warning Aircraft (2002). Armistead, L. Motorbooks International. ISBN 0760311404.