A gyro gunsight (G.G.S.) is a modification of the non-magnifying reflector sight in which target lead (the amount of aim-off in front of a moving target) and bullet drop are calculated automatically. The first examples were developed in Britain just before the Second World War for use during aerial combat, and more advanced models were common on Allied aircraft by the end of the war. The amount of lead required to hit a target is a function of the rate of turn of the attacking aircraft and the range to the target. The former is measured using a gyroscope in the sight, while the latter is estimated by the pilot by moving a dial or pointer so that a reticle in the sight matches the wingspan of the target. Post-war models added a small radar to automate the range measurement; these are known as radar gunsights. Gyro sights usually contained more than one reticle to assist in proper aiming: a fixed one, often just a dot, signifying the direction the guns are pointing, a moving one showing the corrected aiming point, and a ring to match to a target plane's known wingspan. A particularly advanced model, the K-14 found in the North American P-51 Mustang, had separate projectors and displays for air and ground attacks. History In 1936 Royal Aircraft Establishment scientist Leslie Bennet Craigie Cunningham suggested using a gyroscope's resistance to rotation to modify the aiming point in a gun sight to compensate for deflection caused by a turning aircraft. This arrangement meant the information presented to the pilot was of his own aircraft, that is the deflection/lead calculated was based on his own bank-level, rate of turn, airspeed etc. The assumption was that the flight path was following the flight path of the target aircraft, as in a dogfight, therefore the input data was accurate enough to provide useful output data to the pilot. British developments Mark I After tests with two experimental gyro gunsights which had begun in 1939, the first production gyro gunsight was the British Mark I Gyro Sight, developed at Farnborough in 1941. To save time in development the sight was based on the already existing type G prismatic sight, basically a telescopic gun sight folded into a shorter length by a series of prisms. Prototypes were tested in a Supermarine Spitfire and the turret of a Boulton Paul Defiant in the early part