# The New Generation System of Japan Standard Time at NICT

Authors: Yuko Hanado; Kuniyasu Imamura; Noboru Kotake; Fumimaru Nakagawa; Yoshiyuki Shimizu; Ryo Tabuchi; Yukio Takahashi; Mizuhiko Hosokawa; Takao Morikawa
Journal: International Journal of Navigation and Observation
 (2008)

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## Abstract

NICT has
completed a set of major upgrades in its systems for the realization of Japan standard time. One of
the most significant changes is the introduction
of hydrogen masers as signal sources for UTC (NICT)
instead of Cs atomic clocks. This greatly improves the short-term stability of
UTC (NICT). Another major change is the introduction
of a newly developed 24-channel dual-mixer-time-difference system
(DMTD) as the main tool for measurements. The reliability
of the system is also improved by enhanced redundancy and monitoring systems. The new JST system has been in regular operation since February 2006.


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## Body

## 1. Introduction

Japan standard time (JST) is generated by the National Institute of
Information and Communications Technology (NICT). JST is defined as UTC(NICT) +9 hours, where UTC(NICT) is a local realization of coordinated universal time (UTC).
NICT is continuously generating UTC(NICT) from its atomic clocks at the Koganei
headquarters using a system we termed the “JST system.”Although improvements are necessary as technology progresses, any change in
the JST system must be made with care since UTC(NICT) is a real-time product
that must be continuous so as to satisfy the needs of its users. The JST system experienced changes three times
since its first version became operational in 1976. These were in 1987, 1995,
and 1999 [1]. Recently, a new building
for time and frequency facilities was constructed, and the development of the
fifth JST system was undertaken for the installation.The design of this new system was begun in 2002. The main target of this
system was the improvement of a short-term frequency stability of JST, and this
required the introduction of hydrogen masers. Another important target was the improvement
of the measurement precision. It was achieved by developing 24-channel dual mixer
time difference (DMTD) systems. In addition, we aimed to improve the reliability
of JST by new data processing, high redundancy, and enhanced monitoring. The
transition to regular operations was made in February 2006 and the system has
now run over one year without major problems.The outline of the JST system is outlined in Section2, with a more detailed
description of the new system and its comparison with the former system in
Section 3. The regular operation performance and a future target are
introduced in Section 4, and a summary is given in Section 5.


## 2. JST System, Basics and Former One

In this section, we present an outline of the JST system and also of the
former system. The basic data flow is shown in Figure1 and the block diagram of
the former system is shown in Figure 2(a).Figure 1
Configuration of the JST system.Comparison of the former and the new JST system.
CS: Cs atomic clock, HM: hydrogen maser, TIC: Time interval
counter, Ch Sel.: Channel selector, T.S. algorithm: Timescale
algorithm.
(a)
Former JST system(b)
New JST systemUTC(NICT) is a realization of an average atomic
time calculated from an ensemble of Cs atomic clocks. We call this timescale “NICT ensemble atomic time” (NET),
which is reported to the International Bureau of Weights and Measures (BIPM) as
TA(NICT). In the former system, NET was made from maximum 14 Cs atomic
clocks (5071A with high-performance tube, Symmetricom), which are maintained at
the Koganei headquarters.To calculate NET, we use the data of regularly
measured time differences between clocks. In the former system, they were
measured by using the 1 PPS signals of the clocks with a time-interval (TI) counter.
One clock was chosen as the reference. The signals of the other clocks were
selected sequentially by a channel selector, and the measured values were
obtained by a one-shot measurement without averaging.The algorithm for NET in the former JST system [2] was based on a standard
theory of timescale algorithm [3, 4]. First, the rate of each Cs atomic clock at
a moment is estimated from the past behavior. The clock reading is predicted by
a linear extrapolation using this rate, and the discrepancy between the
predicted phase and the actual one is treated as the prediction error of each
clock. NET is made from a weighted average of these errors. In the former
system, the rate was estimated from the frequency difference between the clock
and NET during the last 30 days, and the weight was calculated from the Allan
deviation at τ=10 days of this clock.Since NET is a paper clock, an oscillator is required to realize the actual
signals of UTC(NICT). The output of an atomic clock (we call it “source clock”) is steered by a frequency adjuster used as the oscillator. In the former system, we used a Cs atomic
clock as the source clock, and the Symmetricom's auxiliary output generator
(AOG) as the frequency adjuster. The 5 MHz and 1 PPS signals from AOG were used
as the frequency reference and the timing reference of UTC(NICT), respectively.
The frequency and time differences between the AOG and NET were adjusted once a
day.The time difference of UTC-UTC(NICT) is reported
in the “Circular-T,” produced monthly by the BIPM. When the offset from UTC becomes
large, additional adjustments to follow UTC are added to NET, to produce a
steered timescale NET’. The AOG follows NET' so that UTC(NICT) should follow
UTC. The AOG allows additional adjustments to be given as the frequency offset
or phase changes. In the operation of former system, the frequency offset was
mainly used to meet a 50-nanosecond target of synchronization with UTC. Time
links between NICT and other institutes were performed by GPS common-view
method by using the Topcon Euro80 multichannel receiver mainly.An extremely important part of the system is built-in redundancy to
identify and protect against a system trouble. In the former system, the measurement
device (a channel selector and a TI counter) and a signal generating unit (a
source clock and an AOG) were duplicated. A pair of these units was selected as
the master system. If malfunction is detected in the master system, the system was
quickly changed to the backup one.


## 3. Modifications for the New System

A block diagram of the new system is shown in Figure2(b). Though the
basic configuration is similar with that of the former system, various upgrades
were implemented in the whole system. We introduce the revised points in the
new system as compared with the former system.


## 3.1. Clock

The new system has 18 Cs atomic clocks (5071A with high-performance tube)
and 4 hydrogen masers (RH401A, Anritsu Corp., Atsugi, Kanagawa, Japan). Specifications of the Anritsu hydrogen maser
RH401A are shown in Table1, and its frequency stabilities as measured by the
new system are shown in Figure 3. The Cs atomic clocks generate NET, and the
hydrogen masers are used as the source clocks. The change of the source clock from
a Cs atomic clock to a hydrogen maser improves the short-term frequency
stability of UTC(NICT) about a hundred times. Though the long-term stability of
hydrogen maser is not so good, the long-term stability of UTC(NICT) is assured
by NET.Table 1
Specification of the hydrogen maser RH401A.
Carrier outputsFrequency5, 10, 100 MHz, 1.4 GHzLevel13 dBm ±2 dBTiming outputsFormat1PPSLevelTTLStabilityσy1 sless than4×10−13 (auto-tuning off)10 sless than4×10−14 (auto-tuning off)100 sless than5×10−15 (auto-tuning off)103∼104 sless than2×10−15Long term driftless than2×10−15/daySensitivityTemperatureless than4×10−13/degreeMagneticless than2×10−9/TFunctionFrequency controlRange:2×10−9Resolution:7×10−16Auto-tuningNo ext. reference requiredFigure 3
Frequency stabilities of atomic clocks and system noise of measurement devices.


## 3.2. Measurement System

The measurement method in the former system was simple but had plenty of room
for improvement. Firstly, for precise measurements higher frequency carrier signals
are more desirable than the 1 PPS signals of the former system. Secondly, the simultaneous
measurements of all signals are better for the construction of an average atomic
time than the sequential measurements of the former system.To solve the problems, 24-channel DMTD system (DMTD5, Japan Communication
Equipment Co. Ltd., Yamato, Kanagawa,
Japan) was developed for the new system [5]. Three
DMTD5s are used in the new system, and they are the main tools for the measurements.
Though a DMTD system is a well-known method for a precise measurement of time
difference [6–8], multichannel devices are not so widespread. Our DMTD5
solved the time lag problem in the measurements of multiple clocks as well as the precision  problem.A block diagram of DMTD5 is shown in Figure4 and its specification is
shown in Table 2. It measures the time intervals between the 5 MHz signal of a
reference hydrogen maser and that of 24 clocks simultaneously. Input 5 MHz signals
are down converted to 1 kHz, so that the phase resolution is magnified 5000
times. The phase resolution of the 1 kHz signal is 10 nanoseconds because the
sampling clock inside the DMTD5 is 100 MHz. It means that the relative phase
resolution of 5 MHz signal is 2 picoseconds. An average of sequential
measurements is output every second. Currently, we use the average of 100
samples. The resolution of the final data is therefore around 0.2 picosecond.
Details are described in [5].Table 2
Specification of the DMTD5.
Input frequency5 or 10 MHzBeat-down frequency1 kHzInput channels24Period of output1 sResolution2 ps at 5 MHz(without averaging)Averaging1∼100 samplesFigure 4
Block diagram of DMTD5.The measurements of 5 MHz signals with a DMTD5 provide
more precise data than those of 1 PPS signals with a TI counter. The measurements
of 5 MHz, however, have a risk of 200-nanosecond phase ambiguities due to the miscounting
of cycles. The DMTD5 prevents this problem by shifting the signal phase by 2 nanoseconds
when the phase difference between the clock and the reference becomes less than
1 nanosecond. The limit of the frequency offset to measure the signal without
cycle slip is1×10−9. The cycle count number is output with each
measurement.This function of DMTD5, however, cannot avoid the
risk of cycle miscounting if the measurements are temporarily halted. In order
to keep a phase continuity of measured data in such cases, we use the TI counter
1 PPS measurements together with the DMTD5 5 MHz measurements. The 1 PPS measurements
are made every hour in the same manner as the former system. These data are not
so precise, but not ambiguous. We adopt the result of TI counter as the initial
phase value of a clock when the operation restarts. In the continuous
operation, the accumulated phase calculated from the DMTD5 data is used. Details
of this process are provided in the next subsection.The system noise of the DMTD5 and the TI counter are compared in Figure3. While
the TI counter shows much higher noise than that of hydrogen maser RH401A in
the short term, the noise of the DMTD5 is lower than that of the RH401A in all
regions.


## 3.3. Processing of Measured Data

The combination of DMTD5 and TI counter in the measurements requires
special data processing. A newly developed computer program carries out an anomaly
detection and data synthesis by using the data of three DMTD5 and one TI
counter. The anomaly detection algorithm attempts to identify both bad clocks and
bad measurement devices. This program selects the data of two good devices
among four. There was no such function in the former system. The details of the
procedure are described as follows:In the new system, the measurement unit consists of four devices (three
DMTD5s, and a TI counter). These four devices measure the same sources, so that
their results should be almost same in a normal situation. If an anomaly
appears in one device, its measurement differs from the others. The malfunctioning
device is identified by comparing the results of all devices. Bad data
detection and removal are automatically achieved by an algorithm to be
described next. In this description, the
index#i indicates each  clock. All clock#i  are Cs atomic clocks,
where
i=01,…,18. The clock#01
is a reference clock of the measurement. The indexes A, B, C, and T indicate triplicated
DMTD5s and a TI counter,  respectively.(1)
Time difference between clock#i  and clock#01 is measured by each DMTD5 every second.(2)
The time difference between clock#i  and clock#01 every hour is determined as follows. In the case of DMTD5, the
time difference  of clock#i  and clock#01 at x o'clock is
determined by a linear
fit of
the data between x−1 o'clock and x+1 o'clock. Here, we express
this determined time  differences as pAi(t), pBi(t),
and pCi(t) for
the DMTD5-A, B, and C, respectively. Figure 3 shows that the frequency drifts of the clocks are small enough for such
linear fit. As for the TI counter, the hourly measured value itself is used as
the time difference denoted by pTi(t).(3)
Frequency difference between clock#i  and clock#01 is calculated every hour from the time differences described in
(2). In the case of DMTD5-A, fAi(t)=(pAi(t+τ)−pAi(t))/τ. Here, τ is 3600 seconds. The values of fBi(t),
fCi(t), fTi(t) are
obtained in the same way.(4)
For each device, the sum of the frequency differences
from the other devices is calculated. In the
case of DMTD5-A, the difference isSAi=|fAi−fBi|+|fAi−fCi|+|fAi−fTi|. Similarly, SBi, SCi, and STi are calculated for other devices.(5)
Two devices among four are selected by
using theS values in (4). If DMTD5-A is out of order, the value of fA is different from fB, fC, fT.
As a result, SA becomes the biggest value among all S values. We select those who have the smallest and the next smallest S values as two reliable devices.(6)
The representative frequency
difference between clock#i  and clock#01 is calculated from the data of the
two selected devices. In the case that SB and SC are selected in (5), the average fi(t)=(fBi(t)+fCi(t))/2 is used as the representative
frequency difference between clock#i  and clock#01.(7)
By usingfi(t), representative
time difference between clock#i  and clock#01 is obtained as follows: pi(t)=pi(t0)+Σkfi(tk)⋅(tk−tk−1). The initial phase pi(t0) is obtained from the data of the TI counter when the regular measurement
starts.In the above procedure, one representative data set is made from the four
data sets. We can obtain a measurement result if at
least one measurement device works
properly. If only one device remains, we would adopt its data. This process
is used for the definition of initial phase. The result of TI counter is
adopted if there are no DMTD5 data. Usually, the data of TI counter are not selected because their
errors are larger than those of DMTD5.The anomalies of the clock are detected in the above process. If a phase
datum in the process (1) is larger than a limit, the datum is removed. Currently,
the limit is set to the 10 times of the standard deviation. Any clock with a larger
frequency deviation than5×10−10 against the reference signal is also
removed in the process (3). The software allows simple variation of the parameters
for these limits.


## 3.4. NET, TA(NICT), and UTC(NICT)

The various upgrades described above required many changes in the system. The
software, however, was designed so that the parameters for calculating the
clock rate and weight are easily modified. At present, we use the same values as the
former system except the rate of a source hydrogen maser is estimated from the
last 5 days' frequency difference between the hydrogen maser and NET.There are two changes in the way of making UTC(NICT) from NET. One is in
the clock data archive. In the former system, NET' was used as the reference of
archived clock data. The time differences between clocks and NET were not
stored, which was very inconvenient for analyzing NET. In the new system, the clock
data using NET as the reference are also archived. The other change is the
adoption of a unique NET. The former JST system had two redundant systems (system
A and B in Figure2(a)). Each system made each NET from each measurement data
and steered each AOG. It means that the NET used for making UTC(NICT) was changed
and a time jump occurred in UTC(NICT) when the master system was changed. In
the former system, this time jump was not considered as a serious problem because
the steering errors of AOG were large and masked the time jump. Strictly
speaking, this method of operation caused discontinuities in UTC(NICT). In the
new system, only one NET is made from the representative measurement values
described in Section 3.3.For redundancy, there are three AOGs in the new system. They have different
source clocks but are steered so that all the outputs follow the same NET'. The
steering to cancel the time offset between AOG output and NET' is adjustable in
the new system. Currently, we set the parameter of adjustment so that the time
offset will disappear in two days. It makes a frequency change of UTC(NICT) in
the adjustment gentle and smooth. In the new system, we aim to synchronize
UTC(NICT) with UTC within 10 nanoseconds.TA(NICT), an atomic timescale generated by NICT,
was reset at the timing of starting the regular operation of new JST system.
Currently, the NET is used as TA(NICT) and the data are sent to BIPM.The time differences between the AOGs and NET' are shown in Figure5. This
graph shows the steering errors of AOG. To show the difference due to the
source clocks, we set two hydrogen masers and a Cs clock as the source clocks.
In the cases of using hydrogen masers (AOG-A and C), the steering errors are
clearly smaller than that in the case of a Cs clock (AOG-B).Figure 5
Time differences between the AOG outputs and NET'.


## 3.5. Control Systems, Monitoring Systems  and Facilities

In the former system, a workstation handled all tasks of device controls
and calculations. These tasks are distributed to several computers in the new
system to avoid the task concentration. In each task, the computers are duplicated
or triplicated. All systems clocks are synchronized with UTC(NICT) via network
time protocol (NTP) in a triply redundant manner.The check points of the system are increased compared with the former
system. They are listed in Table3. We have found the real-time monitoring of
5 MHz and 1 PPS signals of UTC(NICT) with oscilloscopes to be effective for
rapid troubleshooting. The outputs of DMTD5 are also useful to check the precise
timing of clock anomalies. The staff are notified by email if an anomaly
exists, and they can check the system condition on the internet with a newly
developed monitoring program.Table 3
Check points of the new system.
Health checkCs clocksH-masersAOGsData logger for monitoring temperature and humidityOscilloscopes for monitoring of UTC(NICT) signalsStatus of Regular measurements
and CalculationsDMTD measurementsTI counter
measurementsTemperature & humidityTA(NICT) calculationAOG adjustingQuality of signalsFigures of 5 MHz & 1 PPS
of UTC(NICT)Phase jump of each clockFrequency instability of each clockcycle slip of each clockThe atomic clocks are operated in the four special
rooms. They are shielded against static and AC electromagnetic fields and kept
in the constant temperature and humidity at 24+/ −0.5 degrees and at 40+/ −10%,
respectively. For protection against external power failure, the atomic clocks
and the main devices are supplied with a large UPS and a generator. The generator
has sufficient fuel to maintain power for three days. The building itself is
equipped with a quake-absorbing structure.


## 3.6. Time Links

Time transfer method for the link of UTC(NICT) was also upgraded [9]. When
the new JST system started, February 2006, the Septentrio PolaRX2 was newly adopted as the main GPS receiver instead of Euro80. By this replacement, both P3 and
multichannel CCTF data can be obtained from the same receiver. Since March
2007, two-way satellite time and frequency transfer method has been used for
the time link between NICT and PTB. These improvements decreased the time link
uncertainty.


## 4. Current Status and Next Target

The regular operation of the new system was initiated on February 7, 2006
(MJD 53773). Figure6 shows the time difference of UTC-UTC(NICT) reported by the Circular-T. In the new
system, we have so far made 5 additional frequency adjustments to follow UTC.
Though the number of adjustments was the same as that in 2005, the magnitudes
of the adjustments were much smaller.Figure 6
Phase stability of UTC-UTC(NICT).In 2006, UTC(NICT) by the new system
was stable and synchronized with UTC to within almost 10 nanoseconds peak to
peak. The frequency stability in one year period between February 6, 2006 and
February 6, 2007 is shown in Figure7. The stabilities in 2001, 2003, and 2005
are also shown for a comparison. The stability in the new system is clearly
improved with respect to the former system.Figure 7
Frequency stability of UTC-UTC(NICT).In 2007, UTC(NICT) showed a large drift in February and March (around MJD
54153), and the time difference from UTC reached almost 20 nanoseconds. Then we
made a phase adjustment on April 24, 2007 (MJD 54214). This phase adjustment is
rarely used because it causes a rapid change of frequency in UTC(NICT). Usually,
only a frequency adjustment is enough for canceling a small phase offset. This
time, the large offset of 20 nanoseconds was a good opportunity to test a
performance of phase adjustment. The result agreed with what was expected.The drift in 2007 was caused by unexpected large drifts of some Cs clocks. We
are now trying to solve this problem. Some anomaly checks should be added to
the timescale algorithm. Several methods were tested with simulations, and some
of them show promise of reducing the frequency drift of NET to almost half of
the present value. We are further investigating these
methods for their appropriateness improving the long-term frequency stability
of UTC(NICT).


## 5. Summary

In the new JST system, better short-term frequency
stability of UTC(NICT) and more precise measurement were achieved by the hydrogen
maser and the newly developed multichannel DMTD system. The reliability was
improved through upgraded monitoring, increased redundancy and improved data
processing.Since the start of a regular operation of the system in February 2006, the
frequency stability of UTC(NICT) in 2006 was better than that in the former
system. In 2007, however, a frequency drift of UTC(NICT) occurred because of
the drifts of some Cs atomic clocks. This provided an opportunity to test a phase adjustment and reconsider the timescale algorithm. Together with the reliable regular operation of the system, an investigation on improvements to the algorithm to make UTC(NICT) more stable is in progress.



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*Source: 841672-2008-04-09.xml*