After years of successful tests, the American GMD (Ground-Based Midcourse Defense) anti-missile system is running into some serious problems. Two tests last year (in January and December) failed because of problems with the warhead. As a result, work on eleven new warheads has been halted, until quality control or design issues can be addressed. Twelve warheads, of the same design as those still under construction, have already been delivered. These may have to be modified.
GMD tests tend to be launched from California, while the target missile is launched from Kwajalein Island in the Marshall Islands, far to the west. The program was formerly known as GBI (Ground Based Interceptor).
The GMD system consists of a powerful radar system, and 12.7 ton ballistic missiles that deliver a 64 kg (140 pound) "kill vehicle" that will intercept a ballistic missile before it begins its descent into the atmosphere. The GMD kill vehicle can maneuver to destroy the incoming missile, while avoiding decoys. The U.S. has built fifteen GMD silos in Alaska and another 15 in California. The program has cost about $31 billion so far.
The GMD can receive target information from a variety of sources, but the main sensor is a large X-band radar and space based sensors (that can detect ballistic missiles during their initial launch.) Each GMD missile costs over $100 million (up to several hundred million dollars, depending on how many are built and how you allocated development costs.) The GMD can intercept ballistic missiles launched from as far away as 5,000 kilometers.
One of the long range radars that provide targeting information for GMD (the SBX, or Sea Based X Band radar) is based on a floating platform, that can be towed to wherever it is needed.