Gyro rate unit refers to a fire-control computer developed by the Royal Navy of the United Kingdom in 1937, and which was used extensively on British warships in World War II. In the 1930s the Royal Navy began to investigate the possibility of combining gyroscopes with optical sights to directly and accurately measure target aircraft speed and direction[1] and began development of the GRU in 1937.[2] A gyroscope was attached, via mechanical linkage, to an optical monocular sight to form the gyro rate unit or GRU.
Gyroscopes, when spinning, keep their spin axes pointed in a given direction if they are undisturbed. The basic premise of the GRU was that as the cross-hairs of the optical sight were kept centred on the moving target aircraft, the mechanical linkage would pull the gyroscope in the direction of the aircraft movement. The force required to move the gyroscope is proportional to the observed target movement across the line of sight. This force was measured by the deflection of a spring-loaded device and the deflection measurement was combined with rangefinder, and/or, radar measured target range and altitude in a specialized computer, the gyro rate unit box (GRUB).