The computer then calculates the required elevation and bearing of the AA guns to hit the target based upon its predicted movement.
Variation of target position over time was accomplished with constant-drive motors to run the mechanical simulation.
An alternative, non-tachometric, gonometric [3][4] method of AA prediction is for specially trained observers to estimate the course and speed of the target manually and feed these estimates, along with the measured bearing and range data, into the AA fire control computer which then generates change of bearing rate and change of range data, and passes them back to the observer, typically by a "follow the pointer", indicator of predicted target elevation and bearing or by remote power control of the observer's optical instruments.
When the sights stay on the target, the estimated speed, range, and change of rate data can be considered correct.
[7] By 1940 the RN was adding a Gyro Rate Unit (GRU)[8] which fed bearing and elevation data to a Gyro Rate Unit Box computer (GRUB), which also received ranging data to calculate target speed and direction directly, and this tachometric data was then fed directly to the HACS fire control computer, converting the HACS into a tachometric system.