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HACS, an acronym of High Angle Control System, was a British anti-aircraft fire-control system employed by the Royal Navy from 1931 onwards and used widely during World War II. HACS calculated the necessary "aim off" required to place an explosive shell in the location of a target flying at a known height, bearing and speed. Unfortunately it was not a predictive, or "tachymetric" system, which was to prove a fatal flaw. It was not until later in the war that a Gyro Rate Unit (GRU) was added to the HACS system, an analogue computer capable of calculating rates of change in speed and direction. This still assumed straight and level flying on the part of the target, but the skill of the air defence control officer could be used to correct for this. Other flaws in the system included the lack of suitable fuse-setting machines and good dual-purpose weapons suitable for ships of destroyer size; for much of the war 40 ° was the maximum elevation of the 4.7 inch guns equipping such ships, totally inadequate for dealing with dive bombers.

HACS utilised various director towers that were generally equipped with Radar Type 285 as it became available. This metric wavelength system employed six yagi antennae that could take ranges of targets, and take crude readings of bearings and altitude using a technique known as "lobe switching". It could not however "lock on" to targets, and therefore was unable to provide true blindfire capabilities. This situation was not remedied until the introduction of the HACS Mark VI director in 1944 that was fitted with centimetric Radar Type 275. Another improvement was the addition of Remote Power Control (RPC), in which the anti-aircraft guns automatically trained with the director tower, with the necessary changes in bearing and elevation to allow for convergent fire. Previously the gun crews had to follow mechanical pointers that indicated where the director tower wanted the guns to train.

• The British High Angle Control System (HACS) http://navweaps.com/index_tech/tech-066.htm

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