A few years ago I stood close enough to brush my shoulder against NIST-F1 cesium fountain atomic "clock", the primary frequency standard at the time. (No pun intended).This was a qty2 full cesniun atomic clocks similar to those used by WWV (two large consoles). orNIST https://www.time.gov/. They received th 10Mhz signal and did a phase comparison with the 10Mhz signal out of Colorado. you could the ionosphere move causing a phase shift to be recorded on the chart recorder. When we adjusted the crystal oven in the higher dollar equipment we would display the10Mhz standard and measure drift in nanosecond s over a fixed time. this tested both check accuracy and stability.I is odd I never mentioned WWV (the call sign for the
NIST but it is assumed I did, I wouldn't be surprised if the atomic clock output is not used for the10Mhz base freq. It would make sense.AWe used the 10mhz standard factory wide Most godd equipment has an external time base input.
Note that the cesium clock is really not a clock; it is a very precise frequency reference. The actually clocks at NIST are hydrogen MASER's that are periodically calibrated to the the cesium standard.
There are surplus rubidium time standards available:
Finally, you can get access to a very accurate time reference using a GPS receiver. Some modules have a 1Hz output. maybe some of these do but I am not sure: