A tachymetric
anti-aircraft
Anti-aircraft warfare, counter-air or air defence forces is the battlespace response to aerial warfare, defined by NATO as "all measures designed to nullify or reduce the effectiveness of hostile air action".AAP-6 It includes surface based, ...
fire control system
A fire-control system (FCS) is a number of components working together, usually a gun data computer, a director, and radar, which is designed to assist a ranged weapon system to target, track, and hit a target. It performs the same task as a ...
generates target position, speed, direction, and rate of target range change, by computing these parameters directly from measured data.
The target's range, height and observed bearing data are fed into a computer which uses the measured change in range, height and bearing from successive observations of the target to compute the true range, direction, speed and rate of climb or descent of the target. The computer then calculates the required elevation and bearing of the AA guns to hit the target based upon its predicted movement.
The computers were at first entirely mechanical
analog computers
An analog computer or analogue computer is a type of computer that uses the continuous variation aspect of physical phenomena such as electrical, mechanical, or hydraulic quantities (''analog signals'') to model the problem being solved. In c ...
utilizing gears and levers to physically perform the calculations of protractors and slide rules, using moving graph charts and markers to provide an estimate of speed and position. Variation of target position over time was accomplished with constant-drive motors to run the mechanical simulation.
The term tachymetric should more properly be spelled as "tachometric" which comes from the Greek "takhos" = speed, and "metric" = measure, hence tachometric, to measure speed.
An alternative, non-tachometric, gonometric 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. The observer then corrects the estimate, creating a feed back loop, by comparing the observed target motion against the computer generated motion of his optical sights. When the sights stay on the target, the estimated speed, range, and change of rate data can be considered correct.
An example of tachometric AA fire control would be the USN
Mk 37 system. The early RN High Angle Control System (
HACS
High Angle Control System (HACS) was a British anti-aircraft fire-control system employed by the Royal Navy from 1931 and used widely during World War II. HACS calculated the necessary deflection required to place an explosive shell in the loca ...
) I through IV and the early
Fuze Keeping Clock
The Fuze Keeping Clock (FKC) was a simplified version of the Royal Navy's High Angle Control System analogue fire control computer. It first appeared as the FKC MkII in destroyers of the 1938 ,''Tribal Class Destroyers'', Hodges, p. 27 while late ...
(FKC) were examples of non-tachometric systems.
By 1940 the RN was adding a
Gyro Rate Unit (GRU) 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.
[Weapon Control in the Royal Navy 1935-45, Pout, p104]
Notes
External links
The RN Pocket Gunnery Book BRITISH MECHANICAL GUNNERY COMPUTERS OF WORLD WAR II
{{DEFAULTSORT:Tachymetric Anti-Aircraft Fire Control System
Naval anti-aircraft weapons
Military computers