Self Defense Test Ship (also known as SDTS) was originally the guided-missile destroyer Decatur (DDG-31), which was stricken from the Naval Vessel Register in March 1988. The ship, now in the status of "equipment", was ordered converted for weapons trials purposes a month later. This work took several years, and SDTS achieved initial operational capability in October 1994.

SDTS subsequently operated out of Port Hueneme, California, with a mission of providing the Navy "with an at sea, remotely controlled, self defense weapons engineering, test and evaluation platform for the conduct of advanced weapons evaluation without risk to personnel". Powered by outboard drive diesel engines, SDTS carries a variety of advanced guided-missiles, guns and sensors for engaging incoming anti-shipping missiles, but has also been used for other test and evaluation purposes. She can operate either manned or unmanned, depending on the risks involved in engaging the "real-world" weapons that are sent her way.

The Self Defesne Test Ship (ex-Decatur) is scheduled for retirement at the end of September 2003, when the decommissioned Spruance class destroyer Paul F. Foster (DD-964) will assume her role. It is then intended to expend the original SDTS as a target.