This WWII Ministry of Information film shows the Ack Ack gun crew using their mobile gun. The narrator speaks about the diverse backgrounds of the recruits and discusses how the gun (most likely a 40mm Bofors cannon) can fire a shell up to 30,000 feet. A height-finder and a predictor are used to come up with a firing solution to hit incoming aircraft. The commentator also notes that enemy planes often fly a zig-zag or other evasive course to avoid being hit, but this can also have a positive outcome in that it prevents bombers from making accurate attacks. A command post is seen at the 3:30 mark, with the enemy and friendly planes being tracked at all times. As nighttime approaches, radar is also used to spot enemy aircraft.
The Kerrison Predictor was one of the first fully automated anti-aircraft fire-control systems. The predictor could aim a gun at an aircraft based on simple inputs like the observed speed and the angle to the target. Such devices had been used on ships for gunnery control for some time, and versions such as the Vickers Predictor were available for larger anti-aircraft guns intended to be used against high-altitude bombers, but the Kerrison's electromechanical analog computer was the first to be fast enough to be used in the demanding high-speed low-altitude role, which involved very short engagement times and high angular rates.
Anti-aircraft warfare or counter-air defence is defined by NATO as "all measures designed to nullify or reduce the effectiveness of hostile air action." They include ground-and air-based weapon systems, sensor systems, command and control arrangements and passive measures (e.g. barrage balloons). It may be used to protect naval, ground, and air forces in any location. However, for most countries, the main effort has tended to be 'homeland defence'. NATO refers to airborne air defence as counter-air and naval air defence as anti-aircraft warfare. Missile defence is an extension of air defence as are initiatives to adapt air defence to the task of intercepting any projectile in flight.
In some countries, such as Britain and Germany during the Second World War, the Soviet Union and NATO's Allied Command Europe, ground based air defence and air defence aircraft have been under integrated command and control. However, while overall air defence may be for homeland defence including military facilities, forces in the field, wherever they are, invariably deploy their own air defence capability if there is an air threat. A surface-based air defence capability can also be deployed offensively to deny the use of airspace to an opponent.
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