The USA has two main time labs: the USNO, which provides time and navigation for the DoD, including the GPS; and NIST which provides time for civilian purposes, including WWV. NIST tends to do more research into new kinds of atomic clock (eg optical clocks, chip-scale clocks) whereas the USNO does more work on earth orientation.

The USNO atomic clock ensemble includes caesium beam clocks, hydrogen masers, and rubidium fountains. NIST uses mostly hydrogen masers, and fewer caesium beam clocks, though their primary frequency standards are caesium fountains.

I get the confusion for the US Navy though, as the clock is at the US Naval Observatory.

If you ever need the time, just call (719) 567-6742

"US Naval Observatory, Master Clock, at the tone, Mountain daylight time, nine hours, sixteen minutes, fifteen seconds...beep!"

And not just one, millions of them.