Gyro rate unit

[2] A gyroscope was attached, via mechanical linkage, to an optical monocular sight to form the gyro rate unit or GRU.

The Royal Navy, after World War I, became increasingly concerned with the threat posed by aerial attack.

In 1930 the RN began equipping ships with the High Angle Control System, a non-tachymetric anti-aircraft fire control system, that would compute the gun laying orders and the time fuze setting of the anti-aircraft guns, to hit the target.

[7] The gyro rate unit box used the measured target motion, range and height, to accurately determine the true direction of movement of the target, including its rate of altitude change, and passed this information to the HACS computer, which then generated the gun laying orders and the correct time fuze setting.

The HACS computer could not directly use rate of altitude change information, so the GRUB would calculate the target altitude, direction and apparent speed for a short interval of time, equal to the loading cycle of the guns, in advance of the actual time and feed this to the HACS computer allowing it to generate correct gunlaying and fuze setting orders.