bridge instead of weapons. The AVLB's job is to allow armoured or infantry units to cross water, when a river too deep for vehicles to wade through is reached, and no bridge is conveniently located (or sufficiently sturdy, a substantial concern when moving 60-ton tanks). The bridge layer unfolds and launches its cargo, providing a ready-made bridge across the obstacle in only minutes. Once the span has been put in place, the AVLB vehicle detaches from the bridge, and moves aside to allow traffic to pass. Once all of the vehicles have crossed, it crosses the bridge itself and reattaches to the bridge on the other side. It then retracts the span ready to move off again. A similar procedure can be employed to allow crossings of small chasms or similar obstructions. AVLBs can carry bridges of or greater in length. By using a tank chassis, the bridge layer is able to cover the same terrain as main battle tanks, and the provision of armour allows them to operate even in the face of enemy fire. However, this is not a universal attribute: some exceptionally sturdy 6x6 or 8x8 truck chassis have lent themselves to bridge-layer applications. Combat engineer section carriers Combat engineer section carriers are used to transport sappers (combat engineers) and can be fitted with bulldozers' blades and other mine-breaching devices. They are often used as APCs because of their carrying ability and heavy protection. They are usually armed with machine guns and grenade launchers and usually tracked to provide enough tractive force to push blades and rakes. Some examples are the U.S. M113 APC, IDF Puma, Nagmachon, Husky, and U.S. M1132 ESV (a Stryker variant). Air defence vehicles 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. Specific weapon systems used include machine guns, anti-aircraft autocannons, larger anti-air guns, or surface-to-air-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 armored personnel carriers 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. In addition, most anti-aircraft guns