fuzes due to the lag time in the computer and the minimum range of optical rangefinders. 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. The Auto Barrage Unit The Auto Barrage Unit or ABU, was a specialized gunnery computer and radar ranging system that used Type 283 radar. It was developed to provide computer prediction and radar anti-aircraft fire control to main and secondary armament guns that did not have inherent anti-aircraft capability. The ABU was designed to allow the guns to be pre-loaded with time fused ammunition, and it then tracked incoming enemy aircraft, aimed the guns continuously to track the aircraft, and then fired the guns automatically when the predicted aircraft position reached the preset fuse range of the previously loaded shells. The ABU was also used with guns that were nominally controlled by the HACS to provide a limited blind fire capability. Wartime experience By May 1941, RN cruisers, such as , were engaging the Luftwaffe with stabilized 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. HMS Fiji was sunk in the Battle of Crete after running out of AA ammunition but her HACS IV directed 4-inch AA gun battery fended off Luftwaffe attacks for many hours. Demonstrating the RN's rapid strides in naval AA gunnery, in May 1941, HMS Prince of Wales went to sea with HACS IVGB, with full radar ranging systems, and nine AA associated fire control radars: four Type 285 radar, one on each High Angle Director Tower (HADT) and four Type 282 radar, one on each Mk IV director for the QF 2 pdr (40mm) "pom pom" mounts, and a long range Type 281 radar Warning Air (WA) radar which also had precision ranging panels for aerial and surface targets. This placed HMS Prince of Wales in the forefront of naval HA AA fire control systems at that time. In August and September 1941, HMS Prince of Wales demonstrated excellent long range radar directed AA fire during Operation Halberd. Although the shortcomings of HACS are often blamed for the loss of Force Z, the scope of the Japanese attack far exceeded anything the HACS had been designed to handle in terms of aircraft numbers and performance. The failure of anti-aircraft