Cesium Atomic Clocks
"...till like a clock worn out with eating time."
John Dryden (1631-1701)
The long term stability of the USNO master clock is determined by an ensemble of cesium beam clocks. The large number of these clocks that are maintained at USNO allow for the robust determination of a very stable time scale with excellent long term performance. The average of these many cesium beam clocks are used to steer the average of the maser clocks.
The cesium beam clocks used at USNO are commercial rack-mounted devices. These clocks feature high-performance cesium beam tubes, with accuracy 1 part in 1012, frequency stability 8 parts in 1014, and a time domain stability of < 2 parts in 1014 with an averaging time of 5 days.
The picture above shows several environmental chambers, each of which holds up to four cesium clocks.