I was working on something related for work many years ago... commercial gps receivers aren't sufficient. We kept time with GPS disciplined rubidium clocks, but

I was working on something related for work many years ago... commercial gps receivers aren't sufficient. We kept time with GPS disciplined rubidium clocks, but that was only good enough for about 10 seconds of coherent operation before the phase wandered off too far @1GHz. (Not HF exactly, but...) It would be cool to try again with caesium or hydrogen maser ...