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American M42 Duster self-propelled anti-aircraft gun

A self-propelled anti-aircraft weapon (SPAA, also self-propelled air defense, SPAD, or self-propelled anti-aircraft gun, SPAAG) is an anti-aircraft gun or surface-to-air missile launcher mounted on a mobile vehicle chassis. The Russian equivalent of SPAAG is ZSU, for zenitnaya samokhodnaya ustanovka, ("anti-aircraft self-propelled mount").

Specific weapon systems include machine guns, autocannon, larger guns, or missiles, and some mount both guns and longer-ranged missiles. Platforms used include both trucks and heavier armored fighting vehicles such as APCs and tanks, which add protection from aircraft, artillery, and small arms fire for front line deployment.

Anti-aircraft guns are usually mounted in a quickly-traversing turret with a high range of elevation, for tracking fast-moving aircraft. They are often in dual or quadruple mounts, allowing a high rate of fire. Today, missiles have largely replaced or supplanted anti-aircraft guns, but are generally mounted on similar turrets.

Anti-aircraft machine guns have long been mounted on trucks, and these were quite common during World War I. Larger guns followed on larger trucks, but these mountings generally required off-truck setup in order to unlimber the stabilizing legs these guns needed. One exception to this rule was the Italian Cannone da 90/53 which was highly effective when mounted on trucks, a fit known as the "autocannoni da 90/53". The 90/53 was a feared weapon, notably in the anti-tank role, but only a few hundred had been produced by the time of the armistice in 1943. Even medium caliber weapons like the Bofors 40 mm gun were too large for truck mounting, not so much due to their weight but due to their need to be trained rapidly, which caused similar problems as the larger weapons. Interest in mobile AA turned to heavier vehicles with the mass and stability needed to easily train weapons of all sizes.

The concept of an armored SPAAG was pioneered mainly by Germany during World War II, with their "flakpanzer" series. German World War II SPAAGs include the Möbelwagen, Wirbelwind, Ostwind and Kugelblitz. Other forces followed with designs of their own, notably the American M16 created by mounting quadruple M2 machine guns on a M3 Half-track. The British Army also introduced their own SPAAG late in the war, the Crusader III AA Mark I, which mounted the excellent Bofors 40 mm gun. On occasion SPAAGs have been used as direct fire weapons against infantry, for example by American forces during late World War II and the Vietnam War when the opposing force had little or no attack aircraft of their own.

Modern weapons include the Russian ZSU-23-4 Shilka and Tunguska-M1, Chinese Type 88 SPAAG, Swedish CV9040 AAV, Polish PZA Loara, American M6 Bradley Linebacker and M1097 Humvee Avenger, Canadian ADATS, German Gepard, Japanese Type 87 SPAAG and similar versions with the British Marksman turret (which was also adapted for a number of other users), Italian SIDAM 25 and Otomatic, and versions of the French AMX-13. Older post-war examples include the ZSU-57-2, the U.S. M42 Duster, M163 VADS and failed M247 Sergeant York.

• Man-portable air-defence system