Somewhere in the deep, dark ocean, a British submarine is always on patrol.
The principle of continuous at-sea deterrence ensures at least one submarine is always on patrol, but only the British prime minister can authorise a missile strike.
The Fire Department dispatches a battalion chief to every rescue incident, and supervisors of the police Emergency Service Unit are always on patrol.
According to www.trooperpay.net, Connecticut has as many troopers per 1,000 residents as a state like New Jersey, where the troopers are always on patrol.
Navy boats are always on patrol around the cruise ships, and their crews can enter the tour vessels at any time to search them.
Mr Alexander's plan, to be spelt out in a speech to the Royal United Services Institute, would end the "continuous at-sea deterrence" offered by the four Trident subs, one of which is always on patrol.
Once he became prime minister, in keeping with the idea that Trident should genuinely be a "minimum" deterrent, Mr Blair reduced the number of warheads carried by each missile and put the one boat that would always be patrolling on notice to fire within days rather than minutes.
At least one of these submarines is always on patrol as a continuous at-sea deterrent, armed with up to 16 Trident missiles and around 48 nuclear warheads, although each submarine can carry up to 160 nuclear warheads with 10 warheads per missile.
They make clear they are divided over whether or not the UK should abandon its continuous at sea deterrence (CASD as it is known) with one Trident submarines always on patrol, and whether the new Trident fleet should consist of three rather than four submarines, and over whether a decision to replace the existing fleet could be postponed.
In the first instance, based on the assumption that the UK is still a functioning state, the chief of the defence staff recommends to the prime minister that nuclear weapons should be launched from a Vanguard-class submarine, one of which is always on patrol.
