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M51 Skysweeper was an American 75 mm anti-aircraft gun deployed in the early 1950s by both the U.S. Army and U.S. Air Force. It was the first such gun to combine all of the various systems needed for effective use against high-speed aircraft into a single carriage, namely radar, an analog computer for calculating “lead“, and an autoloader for high-speed fire. Development of what would become the Skysweeper started in 1948. A new 75 mm gun, known as the T83E1 or M35, was developed that had excellent muzzle velocity, along with two ten-round revolver-type magazines and an auto-loader that allowed it to reach 45 rounds/minute—about the same as the much lighter Bofors had managed. This was mounted on a large box-shaped powered traverse, which also mounted the computer and manual gunsights on the right side, and the T-38 radar unit on the left. The T-38 radar had a range of about 30 km[1] and could track aircraft traveling at up to 700 mph (1100 km/h). Deployment started in the early 1950s and a number were rushed to Korea. Field units used the system widely, although its size meant it was largely static and was primarily used for airbase defense. It proved to be an excellent system and was found in various places until the early 1970s. Skysweeper was also integrated with the massive anti-aircraft systems under the command of Army's Anti-Aircraft Command (ARAACOM) in the U.S. Most ARAACOM deployments were around cities and used the 90 mm and 120 mm guns, but the Skysweeper was used near targets that would have to be attacked by low-altitude aircraft. However the development of missile systems like the Nike Ajax meant that the days of the anti-aircraft gun were soon over, and in 1957 the ARAACOM started to dramatically reduce the number of guns in favor of a much smaller number of missile sites. By the end of the year only three Skysweeper battalions remained in the U.S., one at Sault Ste. Marie, Michigan to protect the Soo Locks, two at Savannah River, plus one 90mm and two Skysweeper battalions at Thule Air Base, Greenland. These too were removed by 1959.
