Anti-aircraft warfare, counter-air, anti-air, AA, flak, layered air defence or air defence forces is the counter to aerial warfare. It is defined by NATO as "all measures designed to nullify or reduce the effectiveness of hostile air action". It includes surface based, subsurface , and air-based weapon systems, associated sensor systems, command and control arrangements, and passive measures . 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. Missile defence is an extension of air defence, as are initiatives to adapt air defence to the task of intercepting any projectile in flight.