Close-in Weapon Systems (CIWS - usually pronounced "see-whiz") are designed to engage anti-ship cruise missiles and fixed-wing aircraft at short range. Phalanx is the most numerous CIWS in the world and has been exported to many other countries. Like other close-in weapon systems, Phalanx provides ships with a terminal defense against anti-ship missiles that have penetrated other fleet defenses.

Phalanx is very much a self contained system requiring minimal deck space and wiring. Unlike many other CIWS, which have separate, independent systems, Phalanx combines search, detection, threat evaluation, acquisition, track, firing, target destruction, kill assessment and cease fire into a single mounting.

A prototype unit was installed for evaluation purposes on USS King (DLG-10) in 1973. In 1975 another prototype was mounted on the hulked USS Alfred A. Cunningham (DD-752) while several different kinds of missiles were fired against it. All of these missiles, including a Walleye, were destroyed before reaching the ship. A pre-production Phalanx unit underwent operational tests and evaluation onboard USS Bigelow (DD-942) in 1977. These tests showed that the unit exceeded the required maintenance and reliability specifications. The evaluations included tests with high levels of jamming noise during which the unit succeeded in distinguishing small missile-like targets against nearby islands.

Phalanx Block 0 production started in 1978 with orders for 23 USN and 14 Foreign Military Sales (FMS) systems.

Phalanx Block 1 saw service introduction in 1988. Block 1 baseline 0 upgrades included a larger magazine (1,500 rounds), a multiple pulse repetition frequency search radar, an expanded radar search envelope to counter diving targets as well as reliability and maintainability improvements. Block 1 was also able to engage targets diving at steeper angles than Block 0 could handle. Block 1 replaced the 2-D scanning antenna of Block 0 with a four-plate back-to-back antenna that continuously searches from the horizon up to the vertical. The cease-fire protocol was changed to match the burst length to the type of target being engaged, thus conserving ammunition and allowing a greater number of targets to be engaged. In automatic control, the gun will prioritize the first six threats it sees at about 10,000 yards (9,100 m) and engage at 4,000 yards (3,600 m).

Block 1 baseline 1 replaced the hydraulic gun drive with a pneumatic (air-driven) gun drive system that increased the rate of fire to 4,500 rounds per minute. Search radar sensitivity was also improved in this upgrade. Block 1 baseline 2 introduced further reliability upgrades along with a muzzle restraint to decrease dispersion. As installed on multiple non-Aegis and Aegis ships, neither the original Phalanx Block 0 nor the subsequent Block 1 baseline 0, 1, or 2 upgrades were integrated with a ship self-defense system.

The Phalanx Block 1A incorporated a high-order language computer in order to better process engagement algorithms and provided improved performance against maneuvering targets. Block 1A also provided for basic integration with the Ship Self Defense System and enabled RAM missile engagement through the Phalanx detection and track function.

Block 1B improvements include Optimized Gun Barrels (OGB) and a new integrated forward looking infrared system (FLIR). The OGB are electrically controlled, pneumatically driven and fire a new Enhanced Lethality Cartridge (ELC). The Phalanx FLIR provides increased capability to search, track and engage coastal warfare threats as well as provide better anti-ship missile defense. Phalanx Block 1B saw service evaluation in 1999 aboard USS Underwood (FFG-36) and was first operationally installed on USS Taylor (FFG-50) in September 2000.

As of February 2007, about 900 Phalanx systems have been built and deployed in the navies of 22 nations. More than 3 million rounds of ammunition have been manufactured specifically for these weapons.

In recent years, the Vulcan 20 mm gun that is the heart of this weapon has increasingly been seen as not being effective enough against modern missile threats. However, the British Royal Navy did select Phalanx for their new Daring class Type 45 destroyers.

Phalanx is somewhat notorious for having maintenance problems, with the Navy's Material Readiness Database for fiscal years 1997 through 1999 noting that Phalanx Block 1B (all mods) had an availability rate of between 72 and 81% for this time period.

An interesting modification of Phalanx that recently entered service with the US Army is the Counter-Rocket Artillery Mortar system (C-RAM), which is designed to combat the small mortar and rocket threat. This system grew out of a 2004 request from Army Chief of Staff General Peter Schoomaker, who wanted a better means of defending US troops based in Iraq.

The complete C-RAM system networks a ground-based version of Phalanx together with the Army's Lightweight Counter Mortar Radar (LCMR) and Q-36 Target Acquisition Radar (AN/TPQ-36 Firefinder Radar), which detects incoming rounds and determines their point of origin. When C-RAM detects an incoming round, it turns on a set of strobe lights to alert local personnel to take cover, authorizes the modified Phalanx to open fire with explosive bullets to destroy the projectile and dispatches a Hunter UAV equipped with Viper Strike laser-designated munitions to kill whoever fired it. Raytheon is currently (2007) offering a trailer-mounted version of Phalanx as part of C-RAM under the product name of CenturionTM. Unlike the naval version, the C-RAM version of Phalanx does not fire solid tungsten penetrators. Instead, in order to reduce the risk of civilian and friendly casualties, this system uses self-destructing explosive bullets.

The first two C-RAM systems arrived in Iraq in May, 2005 and have been reported as having successfully knocked down incoming mortar rounds.

In September 2006, it was reported that the Israeli military expressed an interest in this weapon as well as in Skyshield, the land version of the Millennium system.

Nomenclature Note: The Phalanx system is designated as the Mark 15. The CIWS mounting itself is designated as the Mark 72.

For additional information, see the Essay on the Naval Technical Board.