High Angle Control System

High Angle Control System (HACS) 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 deflection required to place an explosive shell in the location of a target flying at a known height, bearing and speed.

The HACS was first proposed in the 1920s and began to appear on RN ships in January 1930, when HACS I went to sea in HMS Valiant. HACS I did not have any stabilization or power assist for director training. HACS III which appeared in 1935, had provision for stabilization, was hydraulically driven, featured much improved data transmission and it introduced the HACS III Table. The HACS III table (computer) had numerous improvements including raising maximum target speed to 350 knots, continuous automatic fuze prediction, improved geometry in the deflection Screen, and provisions for gyro inputs to provide stabilization of data received from the director. The HACS was a control system and was made possible by an effective data transmission network between an external gun director, a below decks fire control computer, and the ship's medium calibre AA guns.

The bearing and altitude of the target was measured directly on the UD4 Height Finder/Range Finder, a coincidence rangefinder located in the High Angle Director Tower (HADT). The direction of travel was measured by aligning a binocular graticule with the target aircraft fuselage. The early versions of HACS, Mk. I through IV, did not measure target speed directly, but estimated this value based on the target type. All of these values were sent via selsyn to the HACS in the High Angle Calculating Position (HACP) located below decks. The HACS used these values to calculate the range rate (often called rate across in RN parlance), which is the apparent target motion across the line of sight. This was also printed on a paper plot so that a range rate officer could access its accuracy.

This calculated range rate was fed back to the UD4 where it powered a motor to move prisms within the UD4. If all the measurements were correct, this movement would track the target, making it appear motionless in the sights. If the target had apparent movement, the UD4 operator would adjust the range and height, and in so doing would update the generated range rate, thereby creating a feedback loop which could establish an estimate of the target's true speed and direction. The HACS also displayed the predicted bearing and elevation of the target on indicators in the Director tower, or on later variants, the HACS could move the entire Director through Remote Power Control so that it could continue to track the target if the target became obscured.

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