System Degree of dependency
Warhead The UK warhead is a copy of the US W76 warhead.
Arming, fusing and firing system This triggers the explosion. The model used in UK warheads was designed by the US Sandia Laboratory and is almost certainly procured from the USA.
High-explosive (HE) This starts the nuclear explosion. The UK uses a different HE to the USA. Key explosives calculations for the US warhead cannot simply be duplicated so US labs assess the UK HE's long-term performance.
Neutron generator This initiates nuclear fission. The neutron generator used in UK warheads is the MC4380, which is manufactured in the USA and acquired "off the shelf".
Gas reservoir This supplies tritium to boost the fission process. It is most likely that the reservoir used in UK warheads is manufactured in the USA. UK gas reservoirs are filled with tritium in the USA.
Re-entry body shell This is the cone-shaped body which contains the warhead. The UK purchases the Mark 4 re-entry body shell from the USA.
The D5 missile The UK does not own its Trident missiles—they are leased from the USA. UK Trident submarines must regularly visit the US base at King's Bay, Georgia to return their missiles to the US stockpile for maintenance and replace them with others.
Guidance system The Mark 6 guidance system used on the UK's Trident D5 missiles is designed and made in the USA by Charles Stark Draper Laboratories.
Submarines UK Vanguard-class Trident submarines are UK-made, but many aspects of the design are copied from US submarines and many components are bought from the USA.
Navigation The high accuracy of the Trident D5 missile depends on the submarine's position being precisely determined. This is achieved using two systems: GPS, which relies on satellites, and the Electrostatically Supported Giro Navigation System (ESGN), which uses gyroscopes. In both cases UK Trident submarines uses the same US system as the US Navy submarines. The USA has the ability to deny access to GPS at any time, rendering that form of navigation and targeting useless if the UK were to launch without US approval.
Targeting Target packages are designed and formatting tapes produced on shore, then stored on the submarine—using US software at each stage.
Onshore targeting The software installed in the computers at the Nuclear Operations and Targeting Centre in London is based on US models and is probably derived from the US Navy's Submarine Launched Ballistic Missile Integrated Planning System.
Weather and mavity data The US Navy supplies local gravitational information and forecasts of weather over targets, both of which are vital to high missile accuracy, to US and UK submarines.
Fire control system (FCS) Used to assign targets to the warheads on the submarines. UK submarines carry a slightly different model to that on US submarines. However, all the hardware and software used by the system is US-produced. The hardware is produced by General Dynamics Defense Systems. The contracts show that the UK uses similar, if not quite identical, software.
Management British nuclear warheads are designed and made at Aldermaston near Reading. Aldermaston is part managed by the US corporation Lockheed Martin. Repairs to Britian's Trident submarine are carried out at Devonport, which is part managed by another US corporation, Halliburton.
Research and development There is extensive cooperation between Aldermaston and America's nuclear weapon laboratories—Los Alamos in New Mexico and Sandia and Lawrence Livermore in California.
Testing The W76 warhead was tested at the US nuclear test site in Nevada in the early 1990s. The UK has no test site of its own. The missiles are test launched from British submarines under US supervision at Cape Canaveral off the Florida coast. These tests are analysed by the Applied Physics Laboratory (APL) at Johns Hopkins University and by the Charles Stark Draper Laboratories.