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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 an hour.
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
determined by NICT
JST (Japan Standard Time)
set 9 hours forward to UTC(NICT).

Until recently the generation of JST was carried out only at the NICT headquarters in Koganei City, Tokyo. However, at the time of emergency, such as a serious natural disaster, in Tokyo, the operation of JST in headquarters might be stopped due to infrastructure damage.
To increase reliability, therefore, we are promoting research and development for decentralization of the JST facilities. As the first step, JST sub-station was established in Kobe City, Hyogo on June 10, 2018. The equipment for JST sub-station, including cesiu m ato mic clocks and high precision satellite time and frequency transfer systems, were installed there. The two accurate time scales generated in Tokyo and Kobe are always compared. At the emergency of Tokyo, Kobe Sub-station can function as the main JST station instead of the NICT headquarters. Development of technology to consolidate multiple time scales will further bring more accuracy of JST by using atomic clocks of distant stations, including the LF standard time and frequency and development for decentralization of the JST facilities. As transmission stations.