The primary elements of the Midcourse Defense Segment are Ground- Based Midcourse Defense (GMD) and Aegis Ballistic Missile Defense (Aegis BMD).

GMD uses a variety of satellites and radars to obtain information on launch warning, tracking, targeting, and discrimination, This information provides the Ground-Based Interceptor with the ability to locate, identify, and destroy the incoming ballistic missile warhead. The Ground-Based Interceptor is comprised of a booster vehicle and an exoatmospheric kill vehicle. It launches into space based on threat identification and command authority. The booster flies to a projected intercept point and releases the kill vehicle, which uses on-board sensors, with assistance from ground-based assets, to acquire the target. The kill vehicle performs final discrimination and steers itself to collide with the enemy warhead, destroying it by sheer force of impact.

The Aegis Ballistic Missile Defense is the sea-based element of the BMDS. Aegis destroyers, on BMD patrol, detect and track ICBMs and report track data to the missile defense system. This capability shares tracking data to cue other missile defense sensors and provides fire control data to GMD interceptors. Aegis cruisers and long-range surveillance and track destroyers are being equipped with the capability to intercept short- and medium-range, unitary, and separating ballistic missile threats. Engaging missiles in the ascent phase reduces the overall BMD system’s susceptibility to countermeasures.

Terminal Phase — The primary elements in the Terminal Defense Segment are Terminal High Altitude Area Defense (THAAD), PATRIOT Advanced Capability-3 (PAC-3), Arrow (a joint effort between the U.S. and Israel), and Medium Extended Air Defense System (MEADS) (a co-developmental program with Germany and Italy).

The THAAD element will provide the BMDS with rapidly deployable groundbased missile defense components that deepen, extend, and compliment the BMDS to defeat short- to intermediaterange ballistic missiles. THAAD is a land-based element that has the capability to shoot down a ballistic missile, both inside and just outside the atmosphere, using “hit-to-kill” technology (directly hitting the incoming missile to destroy it), providing regional or limited-area terminal defense. THAAD provides an effective defense against ballistic missiles carrying weapons of mass destruction by making it likely that their lethal payloads will be destroyed before reaching the ground.