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An airborne early warning and control (AEW&C) system is an airborne radar picket system designed to detect aircraft, ships and vehicles at long ranges and perform control and command of the battle space in an air engagement by directing fighter and attack aircraft strikes. AEW&C units are also used to carry out surveillance, including over ground targets and frequently perform C2BM (command and control, battle management) functions similar to an Air Traffic Controller given military command over other forces. When used at altitude, the radar on the aircraft allows the operators to detect and track targets and distinguish between friendly and hostile aircraft much farther away than a similar ground based radar. Like a ground based radar, it can be detected by opposing forces, but because of its mobility, it is much less vulnerable to counter-attack.<br />
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AEW&C aircraft are used for both defensive and offensive air operations, and are to the NATO and US forces trained or integrated Air Forces what the Command Information Center is to a US Navy warship, plus a highly mobile and powerful radar platform. The system is used offensively to direct fighters to their target locations, and defensively, directing counterattacks on enemy forces, both air and ground. So useful is the advantage of command and control from a high altitude, the United States Navy operates Northrop Grumman E-2 Hawkeye AEW&C aircraft off its Supercarriers to augment and protect its carrier Command Information Centers (CICs). The designation "airborne early warning" (AEW) was used for earlier similar aircraft, such as the Fairey Gannet AEW.3 and Lockheed EC-121 Warning Star, and continues to be used by the RAF for its Sentry AEW1, while AEW&C (airborne early warning and control) emphasizes the command and control capabilities that may not be present on smaller or simpler radar picket aircraft. AWACS (Airborne Warning And Control System) is name of the specific system installed in the E-3 and Japanese E-767 AEW&C airframes.<br />
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http://en.wikipedia.org/wiki/Airborne_early_warning_and_control