The United Kingdom's Polaris programme provided the first submarine-based British nuclear weapons system. "Polaris" itself was an operational system of four Resolution-class submarines, each armed with 16 UGM-27 Polaris A-3 ballistic missiles, with each missile able to deliver three ET.317 thermonuclear warheads around a single target. This configuration was later upgraded to carry two hardened warheads along with a range of decoys. Polaris patrols continued until May 1996, by which time the phased handover to the replacement Trident system had been completed.