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Mission of Japan Standard Time Group



Generation of Japan Standard Time (JST)

Cesium atomic clock and hydrogen-maser clock
     (The left picture)Hydrogen-maser clock
     (The right picture)Cesium atomic clock

Japan Standard Time (JST) is generated by using a maximum of 18 sets of cesium atomic clocks and 4 hydrogen masers in accordance with the definition of one second mentioned above.
Cesium atomic clocks are good for a long-term (longer than a day) stability and hydrogen masers are good for a short-term (shorter than a day) stability.
The frequency (the number of oscillations per second) of each atomic clock is easily influenced by the environmental conditions such as temperature, geomagnetic field and others.
To prevent the frequency from changing, the atomic clocks are mounted in temperature- and humidity-controlled “Clock Rooms” with electromagnetic shielding. Furthermore, each atomic clock connects with an uninterruptible power supply unit in case of a power outage.

The mutual time differences of the cesium atomic clocks and hydrogen masers mounted in 4 “Clock Rooms” are measured regularly with a specific measurement system. UTC (NICT) is generated by averaging and synthesizing (frequency control) the times of atomic clocks once a day.
This series of procedures for generating JST is fully and automatically performed by several computers. Redundant units (main and backup) work in parallel to generate the time, ensuring continual measurement of JST even in the event of an incident such as equipment trouble.


TAI (International Atomic Time)
determined by International Bureau of Weights and Measures (BIPM)
UTC (Coordinated Universal Time
determined by BIPM
UTC(NICT)
determined by NICT
JST (Japan Standard Time)
set 9 hours forward to UTC(NICT).