The M163 VADS (Vulcan Air Defence System) is a self-propelled anti-aircraft gun that uses the M61 Vulcan 20mm rotary cannon mounted on a modified M113 chassis. The system is designed to provide short-range air defence against low-flying aircraft and helicopters. It has a simple range-only radar and an optical sight for target acquisition and tracking, dramatically reducing costs but also capabilities. Vulcan rotary cannons can fire at a high rate of 3,000 RPM or a lower rate of 1,000 RPM, with options for burst fire. The M163 VADS is mobile and can keep up with the armoured units it is assigned to protect. Because of its limited anti-air capabilities, most M163 VADS are used for fire support against ground targets, a job at which they excel. Introduction began in the mid-1960s.