Although the exact architecture of the proposed NMD system is not yet finalized, its general shape is clear, and the components of the system have been chosen. The system will use ground-based interceptors topped with an Exo-atmospheric Kill Vehicle (EKV) that is designed to destroy the incoming warhead by colliding with it at high speed. This collision would take place above the atmosphere, when the warhead is in the mid-course of its trajectory.

The launch of an attacking missile would first be detected by US early warning satellites. The existing satellites, known as DSP (Defense Support Program) satellites, use infrared sensors to detect the hot plume of a missile booster in the early stage of its flight.

Beginning in 2004, the DSP satellites will be replaced by a new system of early warning satellites known as SBIRS-high (Space-Based Infrared System--high-earth orbit), which will also use infrared sensors to detect missile plumes but have improved capabilities.
The data from the early warning satellites would be fed to the NMD Battle Management Center, to be located at Cheyenne Mountain in Colorado.

Once the booster finishes burning, the NMD system would use different sensors to detect the missile and any objects it releases, to track these objects accurately enough to guide the interceptors, and to discriminate the real warhead from decoys or other false targets.
These sensors include five existing early-warning radars, in Massachusetts, California, central Alaska, Greenland, and Britain, which will be upgraded to give them the ability to track targets accurately enough to guide interceptors. In addition, new X-band radars designed specifically for NMD and with much greater discrimination capabilities will be deployed. These ground-based radars will be supplemented by a space-based system of roughly 24 SBIRS-low (Space-Based Infrared System--low-earth orbit) missile-tracking satellites that are designed to provide track data accurate enough to guide interceptors without assistance from other sensors.

At some point in this process, the system must discriminate the actual warhead from the other objects. Otherwise, the NMD system--with a limited number of interceptors--would risk simply running out of interceptors if it attempted to fire at all the objects. Because the NMD interceptors are designed to intercept their targets above the atmosphere, where there is no air resistance and where lightweight objects travel on the same trajectory as a heavy warhead, the system would be particularly vulnerable to countermeasures that use numerous lightweight decoys.

The NMD Battle Management Center would integrate the information from the various sensors and decide which objects the system should try to intercept. The NMD system would then launch interceptors and guide them towards their targets. An In-Flight Interceptor Communications Systems (IFICS), which will consist of several ground stations deployed at forward locations, would relay communications from the battle-management center to interceptors that have flown over the horizon.

As each interceptor nears its assigned target, it would release the EKV, which will use infrared and visible light sensors to detect the target and attempt to discriminate it from decoys or other false targets. Finally, the EKV would home on the target and use thruster rockets to steer itself into the target.

To increase its odds of success, the NMD system would likely fire several interceptors at each target. To conserve interceptors, if time permits, the defense would use a shoot-look-shoot strategy, in which one or more interceptors are fired at the target, and after observing the results of the intercept attempts, additional interceptors are fired if necessary. Current plans reportedly call for firing four interceptors at each target.

The United States plans to build the NMD in three stages, with the capability of the system designed to increase in each stage. The first system configuration--dubbed the "capability-1" or "C-1" system--is designed to defend against an attack of a "few, simple" warheads. This initial system would be augmented to provide a "capability-2" or "C-2" system, designed to defend against a "few, complex" warheads. The stated goal of the NMD program is to deploy a "capability-3" or "C-3" system, designed to defend against "many, complex" warheads. The term "few" refers to five or fewer warheads. The C-3 System is designed to be compatible with further expansions, such as more interceptors, more interceptor sites, and/or space-based lasers (a small R&D program on space-based lasers is ongoing).

The initial site will be either Grand Forks, North Dakota or central Alaska. The site not chosen for initial deployment would likely be used as a second site for the C-3 system. The Clinton administration has indicated it is leaning strongly towards an initial deployment in Alaska.

The exoatmospheric kill vehicle is the star of the Clinton plan, a 130-pound wonder just 54 inches long. It is made by Raytheon at a plant in Tucson. In space, it would guide itself toward the target, its tiny computer analyzing sensor readings and firing thrusters.
Its big challenge is to disregard the decoys amid the nuclear warheads, which the sensor tracks through their heat rays and sees as twinkling points of light, like stars.
Zipping along at about two miles per second, the kill vehicle is to slam into the nuclear warhead in space and demolish it by force of impact.

Antimissile designers praise the kill vehicle as the apex of miniaturization and accuracy. By contrast, they say, the world's first successful hit-to-kill interceptor, in 1984, had to unfurl a 15-foot-wide steel umbrella to raise the odds of collision.

Donald R. Baucom, an antimissile historian at the Pentagon, said the new kill vehicle's deadly agility is rooted in its miniaturized parts and light weight -- pounds versus earlier tons. So firings of its four small thrusters produce fast maneuvers.
Still, even its staunchest backers acknowledge that the kill vehicle is blind to enemy warheads for most of its flight. Raytheon, its maker, says it can pick up the telltale heat emanations of targets only in the last 100 or so seconds before impact.

So the weapon must still rely on radars and satellites to find its quarry. The needed helpers, detailed in April in a Congressional Budget Office report, and in interviews with its author, Geoffrey Forden, include these:

	Early-warning radars. Five existing ones would be improved and a new one built in Asia to help alert the force of interceptor missiles of enemy attack.
	High-resolution radars. These can better resolve targets in space to aid tracking, eliminate decoys and assess whether targeted warheads have been destroyed. Nine would be built.
	Missile-tracking satellites. These detect heat from newly launched missiles and can help estimate flight paths. In time, existing ones would be supplemented by five new ones, all in high orbits.
	Warhead-tracking satellites. From low orbits, 24 of these new spacecraft would aid the hunt for warheads and decoys.
	Command centers. The main one at Cheyenne Mountain, Colo., a bunker hewn out of solid rock, would link all the data, and its officers would fight the defensive war.
	In-flight relays. On the ground, radio transmitters would send navigational signals to missile interceptors heading for battle.

The goal of this network is not only to aid kill vehicles but to push the defensive battle as close as possible to enemy territory so as to give military officers time to fire more than one interceptor at a specific warhead, raising the odds of success.

The tactic is known as shoot-look-shoot. In theory, antimissile officials say, three or more hits might be attempted against a given warhead.

General Kadish, director of the Pentagon's Ballistic Missile Defense Organization, told Congress in February that if interceptors are 80 percent dependable, two tries will provide 96 percent confidence and three will give 99 percent assurance of a successful kill.

The anticipated power of the weapon system is a state secret. But documents that the State Department gave Moscow in January said the full system would be able to destroy up to 50 enemy warheads.

From:
Union of Concerned Scientists Fact Sheet and
A Missile Defense With Limits: The ABC's of the Clinton Plan