Gyro gun sight, GGS Mk I. Fighter sight with Ross prismatic sight. In production by mid 1941.

Prismatic sight linked to gyroscope which allowed graticule to automatically indicate lead angle. Limited head movement and for daylight use only. For turret use, fitted with Ross Prismatic sight on anti-vibration mounting. This was the first gyro gun sight to go into service.

The limitations of the previous sights, necessitating a number of accurate estimations emphasized the need for a predicting gunsight and the first one, the Mark I Gyro Gunsight was designed and developed at the R.A.E., and was in production by mid-1941. The mechanism of the sight was such that if the line of sight was continuously on the target, and assuming the range had been correctly set, then the guns would be pointing in the right direction to hit the target with an unknown crossing speed. Only one estimation (i.e. range) had to be made. The principle of the sight was shown by the graticule of the prismatic sight being linked mechanically to a gyroscopic prediction mechanism. The sight did not come into general use because of the optical limitations, but however, the prediction principal was used in all later versions of the G.G.S.