HACS

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 ellipse method had the advantage of requiring very little in the way of mechanical computation and essentially modelled target position in real-time with a consequent rapid solution time.

[16] In March 1936, six Queen Bee targets were destroyed by the RN Mediterranean Fleet during intensive AA practice at a time of extreme tension between the UK and Italy.

[29] Starting in 1938 all new RN destroyers, from the Tribal class onwards, were fitted with a FKC and continuous prediction fuse setting trays for each main armament gun.

[30] WWII experience from all navies showed that dive bombers could not be engaged successfully by any remote computer-predictive AA system using mechanical fuzes[31][32] due to the lag time in the computer and the minimum range of optical rangefinders.

[33] In common with other contemporary navies, pre-war designed RN destroyers suffered from a lack of short-range, rapid-fire AA with which to engage dive bombers.

[35][36] By May 1941, RN cruisers, such as HMS Fiji, were engaging the Luftwaffe with stabilised HACS IV systems with GRU/GRUB and Type 279 radar with the Precision Ranging Panel, which gave +/- 25 yd accuracy out to 14,000 yds.

In August and September 1941, HMS Prince of Wales demonstrated excellent long range radar directed AA fire during Operation Halberd.

The HACS was originally designed with Atlantic conditions in mind and Prince of Wales's AA FC radars had become unserviceable in the extreme heat and humidity in Malayan waters and her 2-pdr ammunition had deteriorated badly as well.

This metric wavelength system employed six yagi antennas that could take ranges of targets, and take accurate readings of bearing using a technique known as "lobe switching" but only crude estimates of altitude.

It could not, therefore, "lock on" to aerial targets and was unable to provide true blindfire capabilities, which no other navy was able to do until the USN developed advanced radars in 1944 using technology transfers from the UK.

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.

Anti-aircraft cruiser HMS Scylla . Her four twin 4.5-inch gun mountings are controlled by the two High Angle Director Towers, one sited behind the bridge and the other abaft the after funnel.
HACS deflection screen and table with the deflection screen operator in the foreground. The solid line through the centre of the ellipse shows the wire aligned with the aircraft's course (moving right to left) of approximately 295 degrees; the Deflection Screen operator has his right hand on the Lateral Deflection Control which is aligning the vertical line with the ellipse, and the aircraft track wire, and he is also using his left hand to align the Vertical Deflection Control and a horizontal wire, (which cannot be seen) so that it also intersects the aircraft track wire at the edge of the ellipse.
HADT on HMS King George V . The control officer is shown looking through his binoculars, while the rangetaker's face is hidden.
High Angle Control System fire control computer (table) Mk IV, aboard HMS Duke of York . The Deflection operator is seated in front of the deflection screen. The range plot operator stands directly opposite.