Hydrogen Maser

Hydrogen maser at the Westford Radio telescope. 

Hydrogen masers are the gold standard of stable clocks. Their frequency is derived from the spin-state transition of the electron of a hydrogen atom, which is corresponds to an emission at 1,420,405,751.786 Hz. This clock is very stable on the time scale less than or equal to a day. The Allan standard deviation deviation (standard deviation of the frequency error) on the scale of hours can be 10^-15.

The clock shown above is located at the MIT Haystack Observatory and used as the frequency standard for the 20 GHz Westford Radio telescope that is used for very long baseline geodesic measurements, ie., measurements of the drifts of the continents and the Earth's orbital parameters using radio stars as references.