The precise details of how a British Prime Minister would authorise a nuclear strike remain secret, although the principles of the Trident missile control system is believed to be based on the plan set up for Polaris in 1968, which has now been declassified. A closed-circuit television system was set up between 10 Downing Street and the SSBN Control Officer at the Northwood Headquarters of the Royal Navy. Both the Prime Minister and the SSBN Control Officer would be able to see each other on their monitors when the command was given. If the link failed – for instance during a nuclear attack or when the PM was away from Downing Street – the Prime Minister would send an authentication code which could be verified at Northwood. The PM would then broadcast a firing order to the SSBN submarines via the Very Low Frequency radio station at Rugby. The UK has not deployed control equipment requiring codes to be sent before weapons can be used, such as the US Permissive Action Link, which if installed would preclude the possibility that military officers could launch British nuclear weapons without authorisation.