The present Master Clock is based on a system of dozens of independently operating cesium atomic clocks and 5 to 10 hydrogen maser clocks. (See)
These clocks are distributed over 20 environmentally controlled clock vaults, to insure their stability. By automatic intercomparison of all clocks every 100 seconds, the time scale can be computed which is not only reliable but also extremely stable.
Its rate does not change by more than about 100 picoseconds (0.000 000 000 1 seconds) per day from day to day.รพ On the basis of this computed time a clock reference system can be steered to produce clock signals which serve as the Master Clock.
The clock reference system is driven by a hydrogen maser atomic clock. Hydrogen masers are extremely stable clocks over short time periods (less than one week). They provide the stability, reliability and accessibility needed to maintain the accuracy of the Master Clock System.