Gyro gunsight

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 allowed for automatically, the sight incorporating a gyroscopic mechanism that computes the necessary deflections required to ensure a hit on the target. The sight was developed just before the Second World War for aircraft use during aerial combat.

The sights usually contained more than one reticle to assist in proper aiming point: a fixed one (signifying the direction the guns are pointing), a moving one showing the corrected aiming point, and a reticle to match to a target plane's known wingspan (to adjust the sight for the target's distance).

Gyro gunsights were (for the most part) modifications of the reflector gunsight to aid pilots in hitting targets (other aircraft) that were turning rapidly in front of them. The reflector sight (first used on German fighters in 1918 and widely adopted on all kinds of fighter and bomber aircraft in the 1930s) was an optical device consisting of a 45 degree angle glass beam splitter that sat in front of the pilot and projected an illuminated image of an aiming reticle that appeared to sit out in front of the pilot's field of view at infinity and was perfectly aligned with the plane's guns ("boresighted" with the guns). The optical nature of the reflector sight meant it was possible to feed other information into field of view, such as modifications of the aiming point due to deflection determined by input from a gyroscope.

It is important to note that 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 flightpath was following the flightpath of the target aircraft, as in a dogfight, therefore the input data was close enough.

After tests with two experimental gyro gunsights which had begun in 1939, the first production gyro gunsight was the British Mark I Gyro Sight (left), 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 of that year. With the successful conclusion of these tests the sight was put into production by Ferranti, the first limited-production versions being available by the spring of 1941, with the sights being first used operationally against Luftwaffe raids on Britain in July the same year. The Mark I sight had a number of drawbacks however, including a limited field of view, erratic behavior of the reticle, and requiring the pilot/gunner to put their eye up against an eyepiece during violent maneuvers.

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Wikipedia