The U.S. Sea-Based X-Band Radar (SBX-1) is a floating, self-propelled, mobile active electronically scanned array early-warning radar station designed to operate in high winds and heavy seas. It was developed as part of the Missile Defense Agency’s Ballistic Missile Defense System.
The radar is mounted on a fifth generation CS-50 twin-hulled semi-submersible drilling rig. Conversion of the vessel was carried out at the AmFELS yard in Brownsville, Texas; the radar mount was built and mounted on the vessel at the Kiewit yard in Ingleside, Texas. It is nominally based at Adak Island in Alaska (though, as of April 2015 has never put into port at Adak). It has spent significant time at Pearl Harbor in test status.

Specifications
Vessel length: 116 meters (381 ft)
Vessel height: 85 meters (279 ft) from keel to top of radome
Vessel draft: approximately 10 meters (33 ft) when in motion or not on station; approximately 30 meters (98 ft) when on station
Vessel stability: remains within 10 degrees of horizontal on station (fully passive stabilization)
Cost: US$900 million
Crew: Approximately 75-85 members, mostly civilian contractors
Radar range: 2,000 kilometers (1,200 mi)
Displacement: 50,000 short tons (45,000,000 kg)

SBX-1 is part of the Ground-Based Midcourse Defense (GMD) system under development by the MDA. The decision to place the system on a mobile sea-based platform was intended to allow the vessel to be moved to areas where it is needed for enhanced missile defense. Fixed radars provide coverage for a very limited area due to the curvature of the Earth. However, the same limitation applies to the SBX. SBX’s primary task is discrimination of enemy warheads from decoys, followed by precision tracking of the identified warheads. Testing has raised doubts about the system’s ability to perform these tasks, to deal with multiple targets, and to report accurately to command authorities.
The vessel has many small radomes for various communications tasks and a large central dome that encloses a phased-array, 1,800 tonne (4,000,000 pound) X band radar antenna. The small radomes are rigid, but the central dome is not – the flexible cover is supported by positive air pressure amounting to a few inches of water. The amount of air pressure is variable depending on weather conditions.

In April 2013, it was reported that SBX-1 was being deployed to monitor North Korea
In November 2015, it was moved to Pearl Harbor for repairs and testing.
In January of 2017 the SBX-1 was deployed into the Pacific during North Korean threats of ICBM and nuclear attacks on other nations.