Long time members will know I like to build frequency references & time receivers.
I just come across this while surfing: https://circuit-diagramz.com/kalunndborg-10-mhz-frequency-reference/
There is a transmitter in Denmark operating on 243Kc, and is frequency controlled very accurately.
Using kiwi sdr receiver sites further away to this from me I get a good signal on this 243Kc.
Looks like another project in the midst.
The interesting and useful thing about this signal is that there is no mod at all, which makes locking an oscillator to it fairly simple.