As far as I know, the UK's nuclear submarine fleet used (or still uses?) the presence of a Radio 4 broadcast signal as evidence for the continued existence of the UK. Hopefully they have other means at this point!
> During the time following a perceived attack, a series of progressive checks are made by submarine crews leading up to the captain opening the letter. These include trying to listen for radio transmissions from various levels of Royal Navy and Ministry of Defense command using multiple methods, and most famous of all, listening for new radio broadcasts by BBC Radio channel four, and specifically new episodes of BBC Today. […]
* https://www.twz.com/7300/letters-of-last-resort-are-post-apo...
Also:
I'm not privy to the accuracy of that but it makes sense given the low frequency. In my earlier post to this story, that comes under the umbrella of what I said about that there were strategic reasons for not shutting down the LW Service and governments having nanny goats for advisors.
As well as tbese low frequencies being better to communicate with submarines—although frequencies ten times lower (~18kHz) are much better—they travel long distances and can be very reliable over their given service areas (the ionosphere being more predictable at these low frequencies).
Other reasons are that LW would provide more reliable communications during emergencies such as a mass communication outage with say the internet and other comms services being out. This could happen during nuclear war with EMP destroying much of the electronic infrastructure, similarly a repeat of the 1859 Carrington Event would deviate most existing communications infrastructure. It's dead easy to radiation-harden a LF transmitter if it used only valves (it's halfway there now as its RF output stage already uses valves).
It's a bit like losing communication with one host, and pinging another to see if it's you or them - the 'other' isn't loadbearing.