Hydrogen Masers as Time and Frequency Standards

Track each other with 100 times more precision than cesium standards. Geographically separated hydrogen masers tracked each other for more than year to within few parts in 10 to the 15th power. Frequency offset of hydrogen-maser output from hyperfine line frequency is function of ambient magnetic field, cavity mistuning, thermal motion of atoms in cavity and collisions of atoms with walls of the cavity. If maser remains at one location some of small offsets in frequency will remain constant throughout life of maser.