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Skysweeper, technically Gun, M51, Antiaircraft, was an American 75mm anti-aircraft gun deployed in the early 1950s. 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.

Anti-aircraft guns naturally fall into several categories, each for a different altitude and speed requirement. High-altitude targets require very large guns to get the needed power into the shell to reach those altitudes, but at the same time have the advantage of not needing to move very fast because at that range the change in angle of the target was small – consider the seemingly slow motion of airliner at cruise altitude. At very low altitudes you had only seconds to react, so a hand-swung weapon was the only possibility, no matter how inaccurate. This left an intermediate altitude at which you couldn't use a small gun because the range to the aircraft was too far, and you couldn't use a larger gun because they were moving too fast.

During World War II the US Army filled this niche with the almost-universal Bofors 40mm gun equipped with a US version of the Kerrison Director. But even by the end of the war the performance and accuracy of newer bombers demanded a much higher performance system that was able to attack targets at longer ranges. The Army eventually formulated the definitive requirements; an new gun was needed to defeat aircraft flying at 1,000mph at altitudes up to 20,000 feet. Above that the existing radar-equipped WWII era 90mm and 120mm guns would continue to serve well, with the 120 able to reach as high as 80,000ft.

Development of what would become the Skysweeper started in 1948. A new 75mm gun, known as the T83E1 or M35, was developed that had excellent muzzle velocity, along with two cylindrical 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 radar unit on the left.

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 at airbases. 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 US. Most ARAACOM deployments were around cities and used the 90mm and 120mm 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 favour of a much smaller number of missile sites. By the end of the year only three Skysweeper batallions remained in the US, one at Sault Ste. Marie, Michigan to protect the lockss on the Saint Lawrence Seaway, two at Savannah River, plus one 90mm and two Skysweeper battalions at Thule, Greenland. These too were removed by 1959.