MK 15 Phalanx Close - In Weapons System ( CIWS ) The MK 15 Phalanx Close - In Weapons System ( CIWS - pronounced " sea - whiz " ) is a fast - reaction , rapid - fire 20 - millimeter gun system that provides US Navy ships with a terminal defense against anti - ship missiles that have penetrated other fleet defenses . Designed to engage anti - ship cruise missiles and fixed - wing aircraft at short range , Phalanx automatically engages functions usually performed by separate , independent systems such as search , detection , threat evaluation , acquisition , track , firing , target destruction , kill assessment and cease fire . Phalanx underwent operational tests and evaluation onboard USS Bigelow in 1977 , and exceeded maintenance and reliability specifications . Phalanx production started in 1978 with orders for 23 USN and 14 Foreign Military Sales ( FMS ) systems . Phalanx is a point - defense , total - weapon system consisting of two 20mm gun mounts that provide a terminal defense against incoming air targets . CIWS , without assistance from other shipboard systems , will automatically engage incoming anti - ship missiles and high - speed , low - level aircraft that have penetrated the ship primary defense envelope . As a unitized system , CIWS automatically performs search , detecting , tracking , threat evaluation , firing , and kill assessments of targets while providing for manual override . Each gun mount houses a fire control assembly and a gun subsystem . The fire control assembly is composed of a search radar for surveillance and detection of hostile targets and a track radar for aiming the gun while tracking a target . The unique closed - loop fire control system that tracks both the incoming target and the stream of outgoing projectiles gives CIWS the capability to correct its aim to hit fast - moving targets , including ASMs . The intent is to destroy the warhead on incoming missile . As a secondary measure , should it fail to hit the warhead , CIWS ' s rate of fire is intended to blow holes in the missile body , causing it to break up in air . The gun subsystem employs a gatling gun consisting of a rotating cluster of six barrels . The gatling gun fires a 20mm subcaliber sabot projectile using a heavy - metal ( either tungsten or depleted uranium ) 15mm penetrator surrounded by a plastic sabot and a light - weight metal pusher . The gatling gun fires 20mm ammunition at either 3 , 000 or 4 , 500 rounds - per - minute with a burst length of continuous , 60 , or 100 rounds . As a defensive weapon , the Close In Weapons System ( CIWS ) has special significance for Navy ships and their crews . Battle tested by the British during the Falkand War in the early 1980s , CIWS proved remarkably effective . Navy ship crews routinely test and operate CIWS to ensure the system is working correctly . While most testing involves tracking and firing at a simulated target , the real excitement starts when the fire control teams can fire at a real target . CIWS has been a mainstay self - defense system aboard nearly every class of ship since the late 70 ’ s . It was originally designed to defeat low altitude antiship cruise missiles ( ASCMs ) . As antiship cruise missiles became more complex in maneuvers and ability to be detected , and warfare areas moved from open ocean to littoral environments , CIWS has evolved to meet the threat . Block 0 incorporated on - mount search and track radars , the M61A1 gatling gun capable of firing at a rate of 3 , 000 rounds per minute , and a 980 - round magazine . Block 1 incorporated a new search antenna to detect high altitude missiles , improved search sensitivity , increased the ammunition available for firing by 50 percent , a pneumatic gun drive which increased the firing rate to 4500 rounds per minute , and started using tungsten ammunition in place of depleted uranium . Block I improvements provide increased elevation coverage , larger magazine space for increased round capacity , a variable and higher gun fire rate , and improved radar and processing capabilities . 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 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 the baseline 1 upgrade . Block I baseline 2 introduced further reliability upgrades and a muzzle restraint to decrease dispersion . 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 are integrated with a ship self - defense system . A January 1992 Chief of Naval Operations decision requires replacement of Phalanx with the new ESSM system in new construction DDG ships . Though it initially appeared that DDG - 79 would be the first new construction DDG to receive Evolved Sea Sparrow Missile in lieu of Phalanx , it now appears that , due to a slippage in the ESSM development program , DDG - 85 will be the first . The Navy plans to install the Phalanx Block 1 baseline 2 configuration as temporary installations on DDG - 79 through 84 until ESSM is produced . Block 1A incorporated a new High Order Language Computer ( HOLC ) to provide more processing power over the obsolete general purpose digital computer , improved fire control algorithms to counter maneuvering targets , search multiple weapons coordination to better manage engagements , and an end - to - end testing function to better determine system functionality . Block 1A provides for basic integration with the Ship Self Defense System and enables RAM missile engagement through the Phalanx detection and track function . As of mid - March 2000 , Block 1A installations had been completed on 20 DDG ( Aegis ) destroyers , 2 LHD , 2 FFG - 7 , and 9 LSD 41 / 49 class ships . In addition , LHD - 7 ( currently under construction ) will commission with Block 1A . Block 1B Phalanx Surface Mode ( PSUM ) upgrade allows engagement of small , high - speed , maneuvering surface craft and low , slow - moving aircraft , and hovering helicopters . This upgrade incorporates a thermal imager , an automatic acquisition video tracker , and a stabilization system for the imager , providing both day and night detection of threats . The thermal imager improves the system ' s ability to engage anti - ship cruise missiles by providing more accurate angle tracking information to the fire control computer . Additionally , the FLIR assists the radar in engaging some ASCM ’ s bringing a greater chance of ship survivability . The thermal imager Automatic Acquisition Video Tracker ( AAVT ) and stabilization system provide surface mode and electro - optic ( EO ) angle track . Operational evaluation of Block 1B , conducted aboard USS Underwood ( FFG - 36 ) and the Self - Defense Test Ship , was completed in August 1999 . According to Phalanx Program Office plans , Block 1B will be installed in 11 other FFG - 7 CORT ships between June 2000 and July 2002 . Baseline 2C improvements provide an integrated multi - weapon operations capability . During integrated operations , the command system controls CIWS sensors , target reports , mode employment , and doctrine . The sensors are utilized to provide 360 degree search and track coverage , while providing track data to , and receiving designations from , the Command system . This CIWS installation includes a conversion kit for each weapon group to facilitate ease and safety of maintenance ; the " maintenance enclosure " kit installs the below - deck equipment for a gun mount in a prefabricated enclosure with the mount located above it . According to the Navy ' s Material Readiness Database for fiscal years 1997 through 1999 , the SLQ - 32 electronic warfare system , NATO Sea Sparrow Surface Missile System ( NSSMS ) , Phalanx Close - in Weapon System , and the SPS - 48E radar system were among the ship self - defense systems with the lowest availability rates . The Navy ' s measure of effectiveness for Equipment Operational Capability ( availability ) is classified in the following manner : Operable = Greater than 0 . 8 ; Minor problems = 0 . 7 - 0 . 8 ; Limited capability = 0 . 5 - 0 . 6 ; Major problems = 0 . 3 - 0 . 4 ; Inoperative = 0 - 0 . 2 .