Air Observation Post

Air Observation Post (AOP) is the term used by the Royal Air Force and other services of the United Kingdom and Commonwealth air forces for an aeroplane or helicopter used in the role of artillery spotter. In this role, either the pilot of the aircraft or a passenger acts both just as an observer watching for targets on the ground, and as a Forward Observation Officer directing the fire, by radio, of artillery on the ground (or calling in tactical ground-attack aircraft).