Self-propelled anti-aircraft weapon

An anti-aircraft vehicle, also known as a self-propelled anti-aircraft gun (SPAAG) or self-propelled air defense system (SPAD), is a mobile vehicle with a dedicated anti-aircraft capability. The Russian equivalent of SPAAG is ZSU, for zenitnaya samokhodnaya ustanovka, ("anti-aircraft self-propelled mount").

Specific weapon systems used include machine guns, , larger guns, or missiles, and some mount both guns and longer-ranged missiles (e.g. the Pantsir-S1). Platforms used include both trucks and heavier combat 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 rate of elevation, for tracking fast-moving aircraft. They are often in dual or quadruple mounts, allowing a high rate of fire. Today, missiles (generally mounted on similar turrets) have largely supplanted anti-aircraft guns.

Anti-aircraft machine guns have long been mounted on trucks, and these were quite common during World War I. A predecessor of the WWII German "88" anti-aircraft gun, the WWI German 77 mm anti-aircraft gun, was truck-mounted and used to great effect against British tanks.

The British QF 3 inch 20 cwt was mounted on trucks for use on the Western Front.

Between the two World Wars the United Kingdom developed the Birch gun, a general purpose artillery piece on an armoured tracked chassis capable of maintaining formation with their current tanks over terrian. The gun could be elevated for anti-aircraft use.

Vickers Armstrong also developed a SPAAG based on the chassis of the Mk.E 6-ton light tank/Dragon Medium Mark IV tractor, mounting a Vickers QF-1 "Pom-Pom" gun of 40 mm. About 26 were sold to Siam and saw action as infantry support guns and AA guns during the Franco-Thai war (1940-1941) along with 30 Vickers Mk.E Type B 6-ton tanks. This was probably the first tracked SPAAG manufactured in series. Later the British also developed a version of the Mk.VI Light Tank armed with four machine guns that was known as Light Tank AA Mk.I. And also a twin 15 mm version based on the Light Tank Mk.V was built.

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