In 1936, L. B. C. Cunningham, a scientist at the Royal Aeronautical Establishment (RAE), Farnborough, suggested a "predictor" gun sight using the principle that a gyroscope resists any rotation of its axis. If the device is attached to a normal gun sight, any attempt to follow a crossing target will be resisted with a force proportional to the crossing speed. This idea was not adopted until World War II, when scientists at RAE Farnborough were ordered to produce such a device. The first preproduction Mark I Gyro Gun Sights (GGSs) were installed on Spitfire and Defiant aircraft for trials in 1941. The tests on the resulting GGS Mark I were satisfactory, but there were several operational problems; a redesigned Mark II GGS went into production late in 1943. The pilot selected the type of enemy aircraft on a dial and then adjusted the size of the sighting reticule to match the target's wingspan. This gave an estimate of range and, while the pilot tracked the target, an analog computer in the sight calculated target range and offset the reticule to give the correct deflection. A Mark IIC was employed in bomber turrets.
The GGS was an important technology. Operational experience showed that it approximately doubled the effectiveness of the average squadron; some pilots were five times more likely to score a kill. The GGS Mk II was used by the U.S. Army Air Forces as the K-14 and by the U.S. Navy as the Mk 18.
Clarke, R. Wallace. British Aircraft Armament. 2 vols. Wellingborough, UK: PSL/Haynes, 1993, 1994.; Price, Alfred. Late Marque Spitfire Aces, 1942–45. Botley, UK: Osprey, 1998.