A $150 million test of the American GBI (Ground Based Interceptor) failed when the floating radar system failed (in unspecified ways). The test simulated shooting down a long range Iranian ballistic missile. The GBI was launched from California, while the target missile was launched from Kwajalein in the Marshall Islands, far to the west.
The GBI system consists of a powerful radar system, and 12.7 ton ballistic missiles that deliver a 140 pound "kill vehicle" that will intercept a ballistic missile before it begins its descent into the atmosphere. The GBI kill vehicle can maneuver to destroy the incoming missile, while avoiding decoys. The U.S. is installing GBIs in Alaska and in California.
The GBI can receive target information from a variety of source, but the main source is a large X-band radar and space based sensors (that can detect ballistic missiles during their initial launch.) The U.S. plans to install 5-10 GBIs a year over the next few years, until 30 are in service. Each GBI costs over $100 million (up to several hundred million dollars, depending on how many are built and how you allocated development costs.) The GBI can intercept ballistic missiles launched from as far away as 5,000 kilometers.
One of the long range radars that provide targeting information for GBI is based on a floating platform, that can be towed to wherever it is needed. This is called the SBX (Sea Based X Band radar). It was a something in the SBX that failed in the recent test.