Ballistic missile trajectories are commonly divided into three phases of flight: boost, midcourse, and terminal. Each element will play an important role in a robust system intended to defend against hostile missiles in any phase of flight.

• Boost Phase:The boost phase defenses can defeat ballistic missiles of all ranges including Intercontinental Ballistic Missiles (ICBMs), but it is the most difficult phase in which to engage a missile. The intercept "window" is only from one to five minutes. Although the missile is easiest to detect and track in the boost phase because its exhaust is bright and hot, missile defense interceptors and sensors must be in close proximity to the missile launch. Early detection in the boost phase allows for a rapid response and intercept early in its flight, possibly before any countermeasures can be deployed.
• Midcourse PhaseThe midcourse phase begins when the enemy missile's booster burns out and it begins coasting in space towards its target. This phase can last as long as 20 minutes, allowing several opportunities to destroy the incoming ballistic missile outside the earth's atmosphere. Any debris remaining after the intercept will burn up as it enters the atmosphere. The Ground-based Midcourse Defense element is now deployed in Alaska and California to defend the U.S. homeland against a limited attack from rogue nations. This system can only defend against intermediate and long-range ballistic missiles. The Aegis sea-based missile defense element utilizes existing Aegis cruisers and destroyers armed with interceptor missiles designed to defend against short- to medium-range ballistic missiles, and has been successfully tested against an intermediate range missile. A network of advanced sensors, radars and command, control, battle management, and communication components provide target detection, tracking and discrimination of countermeasures to assist the interceptor missile in placing itself in the path of the hostile missile, destroying with hit-to-kill technology. These sensors and radars include transportable X-band radars, as well as advanced radars aboard Aegis cruisers and destroyers capable of operating in the world's oceans. We have also built the largest X-band radar in the world, the Sea-Based X-band, which is mounted on a floating platform allowing it to traverse the world's oceans. This radar provides precise tracking of target missiles of all ranges and discriminates between actual missiles and countermeasures that could be deployed with a hostile missile.
• Terminal PhaseThe terminal phase is very short and begins once the missile reenters the atmosphere. It is the last opportunity to make an intercept before the warhead reaches its target. Intercepting a warhead during this phase is difficult and the least desirable of the phases because there is little margin for error and the intercept will occur close to the intended target. Terminal phase interceptor elements include the Terminal High Altitude Area Defense (THAAD) now being delivered to the U.S. Army, the Aegis BMD near-term Sea-Based Terminal Defense capability using the SM-2 Block IV missile, and the U.S. Army's PATRIOT Advanced Capability-3 (PAC-3) now deployed worldwide. These mobile systems defend against short- to medium-range missiles.

Following are the ten basic steps involved in intercepting a ballistic missile target.

1. DETECT: The adversary launches a missile. The adversary’s missile will be detected by overhead Sensors as soon as it launches.


2. CUE:Those sensors cue the rest of the Missile Defense System by sending the initial information to our forward based Radars to help them find the adversary’s missile in the sky.


3. TRACK:The radars put the threat into track so the system will have regular updates on the adversary’s missile’s position. As those forward radars begin tracking the Missile, they also analyze the information and cue additional high resolution radars down range.


4. CLASIFY:Radars with sufficient resolution and information to classify and discriminate the lethal object in the cluster will do so as shown.


5. FIRE CONTROL: That additional information along with updated track information is sent to Fire Control Node where an intercept solution will be calculated and the warfighters will determine the response.


6. INTERCEPTOR LAUNCH:Fire Control Node calculates the precise launch time and parameters needed to best ensure a successful mission and if the warfighter determines the incoming missile is a credible threat, a command is sent to the missile field to launch the interceptor. The interceptor launched could be any one of our defensive missiles, each designed to work in a different phase of flight, which is the timeline of the adversary’s missile.


7. UPDATE INTERCEPTOR:As the interceptor is racing through space, the rest of the system remains up and supporting. Commands can be sent to the interceptor to alter its path as needed, based on additional information from other sensors.


8. KILL VEHICLE TRACKING & MANEUVER: The interceptor, such as the Ground-Based Interceptor, has the ability to internally track the incoming missile and adjust its own course to ensure intercept.


9. INTERCEPT: The interceptor strikes the threat. This kinetic event is referred to as "hit-to-kill."


10. VERIFY HIT: After intercept we verify the incoming adversary threat was eliminated, that is we verify the hit, using off-board sensors in multiple locations. If all goes well the threat will be eliminate. If not, Fire Control Node will already have built a plan for multiple opportunities to intercept the incoming threat.