Home > What we do > Mission of Japan Standard Time Group > Generation of Japan Standard Time (JST)

What we do

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).

At present, the generation of JST is carried out only at the NICT headquarters in Koganei City, Tokyo. However, in the case of an emergency, such as natural disasters, the operation of JST system in headquarters may be stopped due to the problems caused. Therefore, in order to increase reliability, we are promoting research and development of a distributed structure of JST. The first distributed station has been constructed in Advanced ICT Research Institute in Kobe City, Hyogo Prefecture. We are preparing for generating and supplying JST even from this station; atomic clocks and the measurement system have been installed to always compare the time between headquarters and the distributed station by a satellite. In addition to enhancement of reliability, more precise JST can be expected as the number of atomic clocks available is increased by the distributed structure.