The Phalanx CIWS (Close-in Weapon System, pronounced see-whizz) is an anti-missile system designed and manufactured by Raytheon Systems Company. It is used by the US Navy and on the vessels of twenty "allied nations".
It was developed as the final line of defense (terminal defense) against anti-ship missiles (ASM), including maneuvering sea-skimmers. The first system was offered to the US Navy for evaluation in 1977. It was accepted and production started in 1978, the first ship fully fitted out was the USS Coral Sea in 1980.
The basis of the system is a 20mm M61 Vulcan firing at over 3,000 rounds per minute. The gun is mounted in a self-contained turret along with a automated fire control system. The system automatically searches, detects, tracks and engages targets using its computer-controlled radar system. The entire unit weighs 5625 kg or 6120 kg.
CIWS has been developed through a number of different configurations. The basic style is the Block 0. The Block 1 (1988) offers various improvements in radar, ammunition, rate of fire and computing. Block 1A introduced a new computer system. The Block 1B PSuM (Phalanx Surface Mode, 1999) adds a forward looking infrared (FLIR) sensor to the Block 1A to allow the weapon to be used against surface targets, this addition was developed to attack modern smaller vessel threats in littoral waters and to improve the weapon's performance against slower low-flying aircraft. The Block 1B also allows for human intervention to identify and target threats.