Trident, also known as the Trident nuclear programme or Trident nuclear deterrent, covers the development, procurement and operation of submarine-based nuclear weapons in the United Kingdom. Its purpose, as stated by the Ministry of Defence, is to "deter the most extreme threats to our national security and way of life, which cannot be done by other means". Trident is an operational system of four Vanguard-class submarines armed with Trident II D-5 ballistic missiles, able to deliver thermonuclear warheads from multiple independently targetable reentry vehicles. It is operated by the Royal Navy and based at Clyde Naval Base on the west coast of Scotland. At least one submarine is always on patrol to provide "continuous at-sea deterrence". The missiles are manufactured in the United States, while the warheads are produced by the British Atomic Weapons Establishment. Trident began patrols in 1994, and fully replaced its predecessor, the Polaris submarine fleet, in 1996.