From Raytheon:

At sea, the Phalanx® Close-In Weapon System—a rapid-fire, computer-controlled, radar-guided gun system—is designed to defeat anti-ship missiles and other close-in air and surface threats. The Land-based Phalanx Weapon System is part of the U.S. Army’s Counter Rocket, Artillery and Mortar systems used to detect and destroy incoming rounds in the air before they hit their ground targets. It also helps provide early warning of attacks.