The latest test of the U.S. GMD (Ground-Based Midcourse Defense) ballistic missile interceptor was successful. This test included verifying new capabilities that enable a GBI (Ground Based Interceptor) missile to be launched using an Aegis radar tracking a ballistic missile in conjunction with tracking also provided by the longer range X-Band radar. This test also used a new Kill Vehicle (the third stage of the GMD missile that actually homes in and hits the incoming nuclear warhead.) This is all to test the ability of the three stage interceptor missile to receive and use new data after launch. So far the GMD has succeeded in 80 percent of 81 tests. However GMD missed its target during a 2013 test as did the two previous tests. Before the recent test the last successful test was in 2008. The primary purpose of the GMD is to defend against nuclear missiles from hostile foreign nations. There are some 30 GBI anti-ballistic missiles deployed so far, all on the west coast of North America.
Target missiles are usually launched from Kwajalein Atoll in the Pacific Ocean west towards the United States. To intercept the missile, the Exoatmospheric Kill Vehicle is launched using a three-stage solid fuel GBI rocket. The Exoatmospheric Kill Vehicle seeks out the target using multicolor sensors and guidance data from ground radar and must also deal with potential decoys. It intercepts the target by colliding with it head on, destroying it through kinetic (impact) energy, as opposed to fragments from a high explosive warhead. This leaves very little margin of error, though it reduces the weight of the interceptor.
So far the program costs over $35 billion and has its origins in the "Star Wars" defense program. It is supplemented by the AEGIS anti-ballistic missile system, used by US naval warships. The U.S. plans to build 14 more GMD batteries for $72 million each.