Space-Time Standards Laboratory

Japan Standard Time Service Group
National Institute of Information and Communications Technology

Japan Standard Time Service Group―Generation, Comparison, andDissemination of Japan Standard Time and Frequency Standards

National Institute of Information and Communications Technology
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The National Institute of Information and Communications Technology (NICT) is responsible for the importanttasks of Generation, Comparison, and Dissemination of Japan Standard Time and Frequency Standards,
which have a direct impact on people’s lives. In this brochure, we first explain how International Atomic Timeand Coordinated Universal Time are calculated. We then look at how the standard time all over the world,including Japan Standard Time, is generated based on them. Finally, we introduce three major functions ofthe Japan Standard Time Service Group: Generation, Comparison, and Dissemination of Japan Standard Time.

(Universal Time 2) (UT2) at 0:00 on January 1, 1958, and thetwo have since drifted apart. TAI is decided by calculating aweighted average time of atomic clocks around the world.

What is the Time?

■Coordinated Universal Time and Leap-Second

Adjustment

Our daily lives are governed by the apparent motion of the Sun.
Since the time scale used in measuring time is atomic time,
there is a need for an atomic time that is close to Universal Time(UT).
This atomic time is called Coordinated Universal Time (UTC).
aaa■ Definition of a Secon
Definition of a second: The duration of the unit “second” wasfirst defined according to ephemeris time, which is tied to therotation and orbital motion of the Earth. Since 1967, however,the second has been defined in terms of the frequency of
atomic radiation under the International System of Units (SI).
The second is defined in the SI as “the duration of 9 192 631
770 periods of the radiation corresponding to the transition
between two hyperfine levels of the ground state of thecesium-133 atom.”■International Atomic Time (TAI)
The time scale created by atomic clocks is referred to as atomictime.
The International Atomic Time (TAI) was synchronized withayaaaaayaaaaaaa

This one second is referred to as a “leap second.”
Leap-second adjustment was introduced after a specialadjustment was carried out in 1972. Leap-second adjustmentsa22a ay2Allaa aa■Japan Standard Time (JST)Japan Standard Time (JST) is defined to set 9 hours forward ofUTC (NICT); the meridian line of Japan is at 135 degrees easternlongitude.

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Generation of JST■ Atomic Clock
JST is obtained using a
18 cesium atomic clocks andfour hydrogen masers in accordance with the definition of thesecond as mentioned above.
Cesium atomic clocks are good for long-term (longer than 5days) stability and hydrogen masers are good for short-term(shorter than 5 days) stability.
The frequency (i.e., the number of oscillations per second) of
each atomic clock is easily affected b y environmental
conditions such as temperature, humidity, and the geomagnetic
To stabilize the frequency, atomic clocks are mounted intemperature-and humidity-controlled “Clock Rooms” withelectromagnetic shielding. Furthermore, each atomic clock isconnected to an uninterruptible power supply unit in casethere is a power outage.

Flow of Time Generation(Left) Hydrogen maser clock (Right) Cesium atomic clock■Generation of JST
Mutual time differences of cesium atomic clocks and hydrogenmasers mounted in four Clock Rooms are regularly measured
every second by a specific measurement system. UTC (NICT) isUTC generated by the National Institute of Information andCommunications Technology (NICT) from averaging andsynthesizing data obtained from atomic clocks once an hour.
This series of procedures for generating JST is fully andautomatically performed by computers. Three redundant units(the main unit and backups) work in parallel, ensuring thecontinual generation of JST in the event of equipment failure.

Atomic Clocks
Measurement system

Measurement ofAverage

Synthesis
Computer

Set the time forward by +9hours to UTC (NICT)+9hours
Japan Standard Time (JST)TAI (International Atomic Time)
determined by the International Bureau of Weightsand Measures (BIPM)

UTC (Coordinated Universal Time)
determined by the BIPM

UTC (NICT)
determined by the NICTJST (Japan Standard Time)■ Establishment of Distributed Structure
Until recently the generation of JST was carried out only at theNICT headquarters in Koganei City, Tokyo. However, at thetime of emergency, such as a serious natural disaster, inTokyo, the operation of JST in headquarters might bestopped due to infrastructure damage.
To increase reliability, therefore, we are promoting researchand development for decentralization of the JST facilities. As
Hyogo on June 10, 2018.
The equipment for JST sub-station, including cesium atomic 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 theemergency of Tokyo, Kobe Sub-station can function as themain JST station instead of the NICT headquarters.
Development of technology to consolidate multiple timescales will further bring more accuracy of JST by using atomic
clocks of distant stations, including the LF standard time andfrequency transmission stations.

set 9 hours forward to UTC (NICT)
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GPS
satellite

Communication
satellite

Comparisonswith JSTa■Comparison of UTC and JST
The NICT manages UTC (NICT) to appro imate UTC and decrease
the time difference between UTC and UTC (NICT) within 10
nanoseconds (1 nanosecond 1 1,000,000,000 seconds). To
make this adjustment, highly precise international timecomparison methods usingaa aaya lobal ositioning System ( S) andcommunication satellites are implemented.
Standards institutes around the world manage their standard
times, and each institute reports time information for eachatomic clock in terms of time differences to ureau Inter
ational des ods et esures (International ureau of Weightsand easures) I. I
decides the TAI and UTC on thebasis of the data collected from each country.

■International Comparison
The NICT adopts two methods in making precise internationaltime comparisons.

aaaaa(1) Time comparison using:It is possible to obtain the difference between time informationtransmitted bysatellites
aand UTCgenerated by each institute maintaining atomic clocks.
UTC (NICT) and UTC obtained by each institute are comparedusing time information of satellites as an intermediary in thecalculations. The times reported by two standard stations arecompared with precision of about one-hundred millionths of asecond to even ten billionths of a second.
(2) Time comparison using communication satellites:
Two stations simultaneously transmit time information using
communication satellites, and the time difference is calculated.
The precision of this time comparison is about one billionth of asecond to ten billionths of a second.
Nowadays, many highly precise time comparisons are madeusing the two methods.

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Dissemination

time code giving the minute, hour, day of year (counted fromJanuary 1), year (last two digits of the dominical year), and day ofweek. This time code is used for reception devices such as radiocontrolled clocks with automatic time correction functions.
The time code signal is e pected to be used in a wide range ofapplications, such as clocks in home electronic appliances,cameras, and automobiles, and the built-in clocks of measuring
instruments and seismometers. The standard frequency
supplied by the L standard time and frequency transmission
signal is also e pected to serve as a precise frequency standard
for a variety of applications, such as measuring instruments,
communication devices, master standard devices of electronic
manufacturers, and standard oscillators used in ground-based
digital broadcasting.
Although the L standard time and frequency transmission
signal has ordinarily been transmitted for 2 hours a day, it may
temporarily be suspended because of maintenance checkups ofdevices and antennas, or in the event of possible lightning.

of JST■Low- requency Standard Time andrequency Transmission Stationsaaa ayaaaayayaaaaaay ya aaaaa a aaaa aayya aaaaa a aa aaa aa a ya ay aaaa2a aay ayya ayya ya aaaaaaThe status of transmission is announced on the website

https://jjy.nict.go.jp

aaaaa

JJ is the call sign of the radio station and is a registered trademark (T 3557 9) of the NICT.

1,500km40~50dBµV/m
1,000km50~60dBµV/m
1,500km43~53dBµV/m

500km>60dBµV/m

1,000km53~63dBµV/m

500km>63dBµV/m>53dBµV/m

Hagane-yama
LF Standard Time and Frequency
Transmission Station (60 kHz)

Ohtakadoya-yama
LF Standard Time and Frequency
Transmission Station (40 kHz)

Koganei-city, Tokyo

A numerical value below eachdistance (km) shows the value bycalculating theoretically assumed
field strength.

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■Internet Time Servicea aaation Service by Networks” to thetime-dissemination enterprises aaaaa
Through this service, stable time information is provided bydirectly connecting users servers with an NT server linked up
with JST. The JST also offers an “Internet Time Service” togeneral users so that they too are able to use an NT server(ntp.nict.jp) that links up with JST.

In today s computerized society, most computers transfer
many data via networks. During these processes, the file
update time is usually defined by the built-in clocks ofindividual computers. These built-in clocks of personalcomputers, however, are not highly precise, and so, without
regular periodic time synchronization, inconsistencies tend tooccur in file information.
h NT (Network Timerotocol) is widely used to synchronize the times of
networked computers.

aArchitecture of Time Information Service through Internet

National Institute ofInformation andCommunications
Technology

NTP service

Internetand others

Users

Leased LineNTP stratum 2 server

UTC(NICT)Time information service
through Networks

NTP stratum 2 server

NTP stratum 1Server■ TelephoneInternetInternet Time ServiceService

Conventionally, high-frequency ( ) standard transmissions
were widely used to synchronize clocks for business applications, such as the master clocks of broadcasting stations. OnAugust 1, 1995, a new system came into operation, supplying
a high-precision time signal electronically via analog
telephone lines (telephone JJ ). A personal computer, modem,
and commercial communications software are all that isrequired for users to acquire time information with precise
adjustment for delays in the telephone line.
Dedicated receivers for commercial useyausing JST with precision of less than one millisecond,equivalent to the precision of conventional commercial
receivers, by measuring and correcting the delay
in communication
lines
using
thebi-directional
telephone connection. edicated receivers are widely used incommercial applications, especially at broadcasting stations.
aMa2 2 aaaa2 2y5

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■aa: Japan Calibration Service System using National Standard

jcssaaMay 2aaayaaa y2aaa
Ma: International Committee for Weights and easures

CIa: utual ecognition Arrangement

Aaaaa■ aaaaaaaaaayyaaay a aaaay ayaaay aa2ayaaaaaa aa aa
M M
Commissioned calibrationaaMa aa aaa ayaa a aa
M aaayy a aa a aaa2aaaaaaaaayayaa aa ayaayaaaa2 2a a aaay a aa a a aa

https://c .nict.go.jp a ayaaaaaay a aa aayy aa2aaaaaaaa a

2 2aayya2aaayyya

M aa

Carried-inRemoteayrequency Standard Calibration System

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AnaInformation
TechnologySociety■Electronic Time Authentication
The concept of incorporating precise time into a aa
y is referred to as new words “timebusiness,” which mainly consists of “time dissemination”
and “time authentication” business. The IT community
has been encouraged to adopt precise time; e.g., as atimestamp.
a aaayy aa
which came into effect in April 2005, requires time stamps tobe put on electronic documents to certify that they have notbeen altered, and it is inevitable for precise time information
to be used for the time stamps.
yaa aa aaa ya aaya a aaaayaaa y2aayaa yaayaaayyaaaayaaaaayy 2 2aaay a a aaaa aaaaayaaaayayaaaaaaa aa yaa aa aaa aaaaaaa aaaaaaaaaythe JSTG has promoted the use of traceability chainsof UTC (NICT) to the aand the structureof the time dissemination and the audit carried out by thea. Technical requirements for thea
have been established in JapaneseIndustrial Standards (JIS X 5094) and also established in ISO/IEC 18014-4 by the International Organization forStandardization.

thori ed time erti i ation
ervi e provider

TClock of provider

TJJYoon e

Telephone JJY
Telephone JJYInternet and others

Providing Method

d t the timedi eren e to ithine ond o TTTime Stamp server
National Institute of Information and Communications Technologyaaaa aaSpace-Time Standards Laboratory, Japan Standard TimeGroupyThis brochure is printed on recycled paper.

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The 1st edition:
2004
Revised: May 2 22

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