Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

The New Generation System of Japan Standard Time at NICT

The New Generation System of Japan Standard Time at NICT Hindawi Publishing Corporation International Journal of Navigation and Observation Volume 2008, Article ID 841672, 7 pages doi:10.1155/2008/841672 Research Article 1 1 1 1 1 Yuko Hanado, Kuniyasu Imamura, Noboru Kotake, Fumimaru Nakagawa, Yoshiyuki Shimizu, 1 2 1 3 Ryo Tabuchi, Yukio Takahashi, Mizuhiko Hosokawa, and Takao Morikawa Space-Time Standard Group, New Generation Network Research Center, NICT, 4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8795, Japan National Institute of Information and Communications Technology, (NICT), 4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8795, Japan Incubation Department, Core Technology R & D Center, ANRITSU Corporation, 5-1-1 Onna, atsugi, Kanagawa 243-8555, Japan Correspondence should be addressed to Yuko Hanado, yuko@nict.go.jp Received 30 July 2007; Accepted 15 January 2008 Recommended by Demetrios Matsakis 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. Copyright © 2008 Yuko Hanado et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. INTRODUCTION achieved by developing 24-channel dual mixer time differ- ence (DMTD) systems. In addition, we aimed to improve the Japan standard time (JST) is generated by the National reliability of JST by new data processing, high redundancy, Institute of Information and Communications Technology and enhanced monitoring. The transition to regular opera- (NICT). JST is defined as UTC(NICT) +9 hours, where tions was made in February 2006 and the system has now UTC(NICT) is a local realization of coordinated universal run over one year without major problems. time (UTC). NICT is continuously generating UTC(NICT) The outline of the JST system is outlined in Section 2, from its atomic clocks at the Koganei headquarters using a with a more detailed description of the new system and its system we termed the “JST system.” comparison with the former system in Section 3. The regular Although improvements are necessary as technology pro- operation performance and a future target are introduced in gresses, any change in the JST system must be made with care Section 4,and asummary is givenin Section 5. since UTC(NICT) is a real-time product that must be con- tinuous so as to satisfy the needs of its users. The JST system 2. JST SYSTEM, BASICS AND FORMER ONE experienced changes three times since its first version became operational in 1976. These were in 1987, 1995, and 1999 [1]. In this section, we present an outline of the JST system and Recently, a new building for time and frequency facilities was also of the former system. The basic data flow is shown constructed, and the development of the fifth JST system was in Figure 1 and the block diagram of the former system is undertaken for the installation. shown in Figure 2(a). The design of this new system was begun in 2002. The UTC(NICT) is a realization of an average atomic time main target of this system was the improvement of a short- calculated from an ensemble of Cs atomic clocks. We call term frequency stability of JST, and this required the in- this timescale “NICT ensemble atomic time” (NET), which troduction of hydrogen masers. Another important target is reported to the International Bureau of Weights and Mea- was the improvement of the measurement precision. It was sures (BIPM) as TA(NICT). In the former system, NET was Data processing program 2 International Journal of Navigation and Observation made from maximum 14 Cs atomic clocks (5071A with high- UTC performance tube, Symmetricom), which are maintained at NICT Koganei headquarters the Koganei headquarters. Adjusting (to follow UTC, as required) To calculate NET, we use the data of regularly measured time differences between clocks. In the former system, they Frequency 5MHz were measured by using the 1 PPS signals of the clocks with a Source adjuster UTC(NICT) time-interval (TI) counter. One clock was chosen as the ref- clock (AOG) JST 1 PPS erence. The signals of the other clocks were selected sequen- Adjusting (to follow NET, once a day) tially by a channel selector, and the measured values were ob- tained by a one-shot measurement without averaging. Average Time differences Timescale Cs clock atomic time The algorithm for NET in the former JST system [2]was between clocks algorithm (NET) based on a standard theory of timescale algorithm [3, 4]. First, the rate of each Cs atomic clock at a moment is esti- Figure 1: Configuration of the JST system. mated 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 System-A AOG Cs weighted average of these errors. In the former system, the 1 PPS UTC(NICT) rate was estimated from the frequency difference between the Cs AOG System-B 5MHz clock and NET during the last 30 days, and the weight was calculated from the Allan deviation at τ = 10 days of this Cs clock. System-A T.S. Ch Since NET is a paper clock, an oscillator is required to 1 PPS of TIC NET NET’ Cs algorithm sel. CS, realize the actual signals of UTC(NICT). The output of an Cs HM, atomic clock (we call it “source clock”) is steered by a fre- Offsets from UTC AOG, quency adjuster used as the oscillator. In the former sys- Cs UTC(NICT) T.S. Ch NET NET’ TIC tem, we used a Cs atomic clock as the source clock, and the sel. algorithm System-B Symmetricom’s auxiliary output generator (AOG) as the fre- quency adjuster. The 5 MHz and 1 PPS signals from AOG were used as the frequency reference and the timing refer- 5MHz ence of UTC(NICT), respectively. The frequency and time 1 PPS differences between the AOG and NET were adjusted once a Data day. (a) Former JST system 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 AOG HM follow UTC are added to NET, to produce a steered timescale 1 PPS NET’. The AOG follows NET’ so that UTC(NICT) should AOG HM UTC(NICT) follow UTC. The AOG allows additional adjustments to be 5MHz HM AOG given as the frequency offset or phase changes. In the op- Offsets from UTC eration of former system, the frequency offset was mainly 1 PPS of NET’ used to meet a 50-nanosecond target of synchronization with CS, UTC. Time links between NICT and other institutes were HM, Ch Cs TIC AOG, performed by GPS common-view method by using the Top- sel. UTC(NICT) Cs con Euro80 multichannel receiver mainly. An extremely important part of the system is built-in re- DMTD T.S. Ref. Cs 5MHz of NET 24ch dundancy to identify and protect against a system trouble. In algorithm CS, DMTD the former system, the measurement device (a channel selec- Cs Ref. HM, tor and a TI counter) and a signal generating unit (a source AOG, DMTD UTC(NICT) clock and an AOG) were duplicated. A pair of these units Ref. was selected as the master system. If malfunction is detected in the master system, the system was quickly changed to the 5MHz backup one. 1 PPS Data 3. MODIFICATIONS FOR THE NEW SYSTEM (b) New JST system A block diagram of the new system is shown in Figure 2(b). Figure 2: Comparison of the former and the new JST system. CS: Cs atomic clock, HM: hydrogen maser, TIC: Time interval counter, Though the basic configuration is similar with that of the Ch Sel.: Channel selector, T.S. algorithm: Timescale algorithm. former system, various upgrades were implemented in the Time comparison Yuko Hanado et al. 3 Table 1: Specification of the hydrogen maser RH401A. Frequency 5, 10, 100 MHz, 1.4 GHz Carrier outputs Level 13 dBm ±2dB Format 1PPS Timing outputs Level TTL −13 1s less than 4 × 10 (auto-tuning off ) −14 10 s less than 4 × 10 (auto-tuning off ) Stability σ −15 100 s less than 5 × 10 (auto-tuning off ) 3 4 −15 10 ∼ 10slessthan2 × 10 −15 Long term drift less than 2 × 10 /day −13 Temperature less than 4 × 10 /degree Sensitivity −9 Magnetic less than 2 × 10 /T −9 Range: 2 × 10 Frequency control −16 Function Resolution: 7 × 10 Auto-tuning No ext. reference required −10 Table 2: Specification of the DMTD5. Input frequency 5 or 10 MHz −11 Beat-down frequency 1 kHz Input channels 24 −12 Period of output 1 s 2ps at 5 MHz −13 Resolution (without averaging) Averaging 1∼ 100 samples −14 −15 change of the source clock from a Cs atomic clock to a hy- drogen maser improves the short-term frequency stability of −16 UTC(NICT) about a hundred times. Though the long-term stability of hydrogen maser is not so good, the long-term sta- −17 01 23 4 5 bility of UTC(NICT) is assured by NET. Average time (log scale, s) 3.2. Measurement system Cs clock H-maser The measurement method in the former system was simple TI counter but had plenty of room for improvement. Firstly, for precise DMTD measurements higher frequency carrier signals are more de- sirable than the 1 PPS signals of the former system. Secondly, Figure 3: Frequency stabilities of atomic clocks and system noise of measurement devices. the simultaneous measurements of all signals are better for the construction of an average atomic time than the sequen- tial measurements of the former system. whole system. We introduce the revised points in the new To solve the problems, 24-channel DMTD system system as compared with the former system. (DMTD5, Japan Communication Equipment Co. Ltd., Yam- ato, Kanagawa, Japan) was developed for the new system [5]. Three DMTD5s are used in the new system, and they 3.1. Clock are the main tools for the measurements. Though a DMTD The new system has 18 Cs atomic clocks (5071A with high- system is a well-known method for a precise measurement performance tube) and 4 hydrogen masers (RH401A, An- of time difference [6–8], multichannel devices are not so ritsu Corp., Atsugi, Kanagawa, Japan). Specifications of the widespread. Our DMTD5 solved the time lag problem in Anritsu hydrogen maser RH401A are shown in Table 1,and the measurements of multiple clocks as well as the precision its frequency stabilities as measured by the new system are problem. shown in Figure 3. The Cs atomic clocks generate NET, and A block diagram of DMTD5 is shown in Figure 4 and the hydrogen masers are used as the source clocks. The its specification is shown in Table 2. It measures the time Allan deviation (log scale) Micro-controller 4 International Journal of Navigation and Observation 4or2 DDS 100 MHz REF multiplier PLL synthesizer 5/10 MHz Comparator REF ±1kHz ΔFLPF Start pulse Start pulse output 1kpps Start REF ±1kHz Time pulse Stop pulse interval DUT counter Ch1 1kpps 5/10 MHz . Output Time DUT interval DUT counter 5/10 MHz Ch24 Figure 4: Block diagram of DMTD5. intervals between the 5 MHz signal of a reference hydrogen 3.3. Processing of measured data maser and that of 24 clocks simultaneously. Input 5 MHz signals are down converted to 1 kHz, so that the phase res- The combination of DMTD5 and TI counter in the mea- olution is magnified 5000 times. The phase resolution of the surements requires special data processing. A newly devel- 1 kHz signal is 10 nanoseconds because the sampling clock oped computer program carries out an anomaly detection inside the DMTD5 is 100 MHz. It means that the relative and data synthesis by using the data of three DMTD5 and phase resolution of 5 MHz signal is 2 picoseconds. An aver- one TI counter. The anomaly detection algorithm attempts age of sequential measurements is output every second. Cur- to identify both bad clocks and bad measurement devices. rently, we use the average of 100 samples. The resolution of This program selects the data of two good devices among the final data is therefore around 0.2 picosecond. Details are four. There was no such function in the former system. The described in [5]. details of the procedure are described as follows: The measurements of 5 MHz signals with a DMTD5 pro- In the new system, the measurement unit consists of four vide more precise data than those of 1 PPS signals with a TI devices (three DMTD5s, and a TI counter). These four de- counter. The measurements of 5 MHz, however, have a risk vices measure the same sources, so that their results should of 200-nanosecond phase ambiguities due to the miscount- be almost same in a normal situation. If an anomaly ap- ing of cycles. The DMTD5 prevents this problem by shift- pears in one device, its measurement differs from the others. ing the signal phase by 2 nanoseconds when the phase differ- The malfunctioning device is identified by comparing the re- ence between the clock and the reference becomes less than sults of all devices. Bad data detection and removal are auto- 1 nanosecond. The limit of the frequency offset to measure matically achieved by an algorithm to be described next. In −9 the signal without cycle slip is 1× 10 .The cyclecount num- this description, the index #i indicates each clock. All clock#i ber is output with each measurement. are Cs atomic clocks, where i = 01,... , 18. The clock#01 This function of DMTD5, however, cannot avoid the risk is a reference clock of the measurement. The indexes A, B, of cycle miscounting if the measurements are temporarily C, and T indicate triplicated DMTD5s and a TI counter, halted. In order to keep a phase continuity of measured data respectively. in such cases, we use the TI counter 1 PPS measurements to- (1) Time difference between clock#i and clock#01 is mea- gether with the DMTD5 5 MHz measurements. The 1 PPS sured by each DMTD5 every second. measurements are made every hour in the same manner as the former system. These data are not so precise, but not am- (2) The time difference between clock#i and clock#01 ev- biguous. We adopt the result of TI counter as the initial phase ery hour is determined as follows. In the case of value of a clock when the operation restarts. In the contin- DMTD5, the time difference of clock#i and clock#01 uous operation, the accumulated phase calculated from the at x o’clock is determined by a linear fit of the data DMTD5 data is used. Details of this process are provided in between x − 1 o’clock and x +1 o’clock. Here, we ex- the next subsection. press this determined time differences as p (t), p (t), Ai Bi The system noise of the DMTD5 and the TI counter are and p (t) for the DMTD5-A, B, and C, respectively. Ci compared in Figure 3. While the TI counter shows much Figure 3 shows that the frequency drifts of the clocks higher noise than that of hydrogen maser RH401A in the are small enough for such linear fit. As for the TI short term, the noise of the DMTD5 is lower than that of counter, the hourly measured value itself is used as the the RH401A in all regions. time difference denoted by p (t). Ti Yuko Hanado et al. 5 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 pro- 4 cess. 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 fre- −10 quency deviation than 5 × 10 against the reference signal is also removed in the process (3). The software allows simple −2 variation of the parameters for these limits. −4 3.4. NET, TA(NICT), and UTC(NICT) −6 −8 The various upgrades described above required many 0 5 10 15 20 25 30 changes in the system. The software, however, was designed Day so that the parameters for calculating the clock rate and weight are easily modified. At present, we use the same values AOG-A (H-maser) as the former system except the rate of a source hydrogen AOG-B (Cs clock) AOG-C (H-maser) maser is estimated from the last 5 days’ frequency difference between the hydrogen maser and NET. Figure 5: Time differences between the AOG outputs 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 (3) Frequency difference between clock#i and clock#01 is not stored, which was very inconvenient for analyzing NET. calculated every hour from the time differences de- In the new system, the clock data using NET as the refer- scribed in (2). In the case of DMTD5-A, f (t) = Ai ence are also archived. The other change is the adoption of a (p (t + τ ) − p (t))/τ.Here, τ is 3600 seconds. The Ai Ai unique NET. The former JST system had two redundant sys- values of f (t), f (t), f (t) are obtained in the same Bi Ci Ti tems (system A and B in Figure 2(a)). Each system made each way. NET from each measurement data and steered each AOG. (4) For each device, the sum of the frequency differences It means that the NET used for making UTC(NICT) was from the other devices is calculated. In the case of changedand atimejumpoccurredinUTC(NICT) when the DMTD5-A, the difference is S =| f − f | + | f − Ai Ai Bi Ai master system was changed. In the former system, this time f | + | f − f |. Similarly, S , S ,and S are calcu- Ci Ai Ti Bi Ci Ti jump was not considered as a serious problem because the lated for other devices. steering errors of AOG were large and masked the time jump. (5) Two devices among four are selected by using the S val- Strictly speaking, this method of operation caused disconti- nuities in UTC(NICT). In the new system, only one NET is ues in (4). If DMTD5-A is out of order, the value of f made from the representative measurement values described is different from f , f , f . As a result, S becomes the B C T A in Section 3.3. biggest value among all S values. We select those who For redundancy, there are three AOGs in the new system. have the smallest and the next smallest S values as two They have different source clocks but are steered so that all reliable devices. the outputs follow the same NET’. The steering to cancel the (6) The representative frequency difference between time offsetbetween AOGoutputand NET’isadjustablein clock#i and clock#01 is calculated from the data of the the new system. Currently, we set the parameter of adjust- two selected devices. In the case that S and S are B C ment so that the time offset will disappear in two days. It selected in (5), the average f (t) = ( f (t)+ f (t))/2 i Bi Ci makes a frequency change of UTC(NICT) in the adjustment is used as the representative frequency difference be- gentle and smooth. In the new system, we aim to synchronize tween clock#i and clock#01. UTC(NICT) with UTC within 10 nanoseconds. (7) By using f (t), representative time difference between TA(NICT), an atomic timescale generated by NICT, was clock#i and clock#01 is obtained as follows: p (t) = reset at the timing of starting the regular operation of new p (t )+ Σ f (t ) · (t − t ). The initial phase p (t ) i 0 k i k k k−1 i 0 JST system. Currently, the NET is used as TA(NICT) and the is obtained from the data of the TI counter when the data are sent to BIPM. regular measurement starts. The time differences between the AOGs and NET’ are In the above procedure, one representative data set is shown in Figure 5. This graph shows the steering errors of made from the four data sets. We can obtain a measurement AOG. To show the difference due to the source clocks, we result if at least one measurement device works properly. If set two hydrogen masers and a Cs clock as the source clocks. only one device remains, we would adopt its data. This pro- In the cases of using hydrogen masers (AOG-A and C), the cess is used for the definition of initial phase. The result of TI steering errors are clearly smaller than that in the case of a Cs counter is adopted if there are no DMTD5 data. Usually, the clock (AOG-B). Time difference (ns) 6 International Journal of Navigation and Observation Table 3: Check points of the new system. Cs clocks H-masers Health check AOGs Data logger for monitoring temperature and humidity Oscilloscopes for monitoring of UTC(NICT) signals DMTD measurements TI counter measurements Status of Regular measurements and Calculations Temperature & humidity TA(NICT) calculation AOG adjusting Figures of 5 MHz & 1 PPS of UTC(NICT) Phase jump of each clock Quality of signals Frequency instability of each clock cycle slip of each clock The atomic clocks are operated in the four special rooms. 06 07 They are shielded against static and AC electromagnetic fields and kept in the constant temperature and humidity at 7e − 15 24+/−0.5degrees andat40+/−10%, respectively. For protec- 53465 53648 20 ns 40 8.3e − 15 3e − 15 2e − 15 tion against external power failure, the atomic clocks and the 54010 54153 main devices are supplied with a large UPS and a generator. 1e − 15 1e − 15 The generator has sufficient fuel to maintain power for three days. The building itself is equipped with a quake-absorbing 53586 structure. 2e − 15 −8e − 15 −20 53920 54089 −3e − 15 −2e − 15 3.6. Time links −40 Time transfer method for the link of UTC(NICT) was also 53371 53491 53611 53731 53851 53971 54091 54211 upgraded [9]. When the new JST system started, February MJD 2006, the Septentrio PolaRX2 was newly adopted as the main Figure 6: Phase stability of UTC-UTC(NICT). 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 3.5. Control systems, monitoring systems between NICT and PTB. These improvements decreased the and facilities time link uncertainty. In the former system, a workstation handled all tasks of de- 4. CURRENT STATUS AND NEXT TARGET vice controls and calculations. These tasks are distributed to several computers in the new system to avoid the task The regular operation of the new system was initiated on concentration. In each task, the computers are duplicated February 7, 2006 (MJD 53773). Figure 6 shows the time dif- or triplicated. All systems clocks are synchronized with ference of UTC-UTC(NICT) reported by the Circular-T. In UTC(NICT) via network time protocol (NTP) in a triply re- the new system, we have so far made 5 additional frequency dundant manner. adjustments to follow UTC. Though the number of adjust- The check points of the system are increased compared ments was the same as that in 2005, the magnitudes of the with the former system. They are listed in Table 3.Wehave adjustmentsweremuchsmaller. found the real-time monitoring of 5 MHz and 1 PPS signals In 2006, UTC(NICT) by the new system was stable and of UTC(NICT) with oscilloscopes to be effective for rapid synchronized with UTC to within almost 10 nanoseconds troubleshooting. The outputs of DMTD5 are also useful to peak to peak. The frequency stability in one year period be- check the precise timing of clock anomalies. The staff are tween February 6, 2006 and February 6, 2007 is shown in notified by email if an anomaly exists, and they can check Figure 7. The stabilities in 2001, 2003, and 2005 are also the system condition on the internet with a newly developed shown for a comparison. The stability in the new system is monitoring program. clearly improved with respect to the former system. Time difference (ns) Yuko Hanado et al. 7 1E − 13 ACKNOWLEDGMENTS The authors deeply appreciate the staff effort made to main- tain the UTC(NICT) generation system, and are grateful to many staff members for their helpful discussions with them. 1E − 14 REFERENCES [1] Y. Hanado, M. Imae, and N. Kurihara, “Generating and mea- surement system for Japan Standard Time,” Journal of the Na- tional Institute of Information and Communications Technology, vol. 50, no. 1-2, pp. 169–177, 2003. [2] Y. Hanado, K. Imamura, and M. Imae, “Upgrading of 1E − 15 UTC(CRL),” in Proceedings of the IEEE International Frequency 1E +05 1E +06 1E +07 Control Symposium and Pda Exhibition Jointly with the 17th Eu- Averaging time (s) ropean Frequency and Time Forum, pp. 296–300, Tampa, Fla, USA, May 2003. 2001 (former system) [3] C. Thomas, P. Wolf, and P. Tavella, “Time scale,” BIPM Mono- 2003 (former system) graphie 94/1, pp. 23–32., 1994. 2005 (former system) 2006 (new system) [4] P. Tavella and C. Thomas, “Comparative study of time scale al- gorithms,” Metrologia, vol. 28, no. 2, pp. 57–63, 1991. Figure 7: Frequency stability of UTC-UTC(NICT). [5] F. Nakagawa, M. Imae, Y. Hanado, and M. Aida, “Development of multi channel dual mixer time difference system to generate UTC (NICT),” IEEE Transactions on Instrumentation and Mea- surement, vol. 54, no. 2, pp. 829–832, 2005. In 2007, UTC(NICT) showed a large drift in February [6] D. W. Allan, “The measurement of frequency and frequency sta- and March (around MJD 54153), and the time difference bility of precision oscillators,” NBS Tech. Note 669, 1975. from UTC reached almost 20 nanoseconds. Then we made a [7] D. W. Allan and H. Daams, “Picosecond time difference mea- phase adjustment on April 24, 2007 (MJD 54214). This phase surement system,” in Proceedings of the 29th Annual Symposium on Frequency Control, pp. 404–411, Atlantic City, NJ, USA, May adjustment is rarely used because it causes a rapid change of frequency in UTC(NICT). Usually, only a frequency adjust- [8] S. Stein, D. Glaze, and J. Levine, “Automated high-accuracy ment is enough for canceling a small phase offset. This time, phase measurement system,” IEEE Transactions on Instrumen- the large offset of 20 nanoseconds was a good opportunity tation and Measurement, vol. 32, no. 1, pp. 227–231, 1982. to test a performance of phase adjustment. The result agreed [9] NICT, “Summary of time and frequency activities at NICT,” with what was expected. Working documents of CCTF 17th meetings, CCTF/06-09, 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 al- gorithm. 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 in- vestigating these methods for their appropriateness improv- ing the long-term frequency stability of UTC(NICT). 5. SUMMARY In the new JST system, better short-term frequency sta- bility 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, how- ever, a frequency drift of UTC(NICT) occurred because of the drifts of some Cs atomic clocks. This provided an oppor- tunity 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. Allan deviation International Journal of Rotating Machinery International Journal of Journal of The Scientific Journal of Distributed Engineering World Journal Sensors Sensor Networks Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 Volume 2014 Journal of Control Science and Engineering Advances in Civil Engineering Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 Submit your manuscripts at http://www.hindawi.com Journal of Journal of Electrical and Computer Robotics Engineering Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 VLSI Design Advances in OptoElectronics International Journal of Modelling & Aerospace International Journal of Simulation Navigation and in Engineering Engineering Observation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2010 Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com http://www.hindawi.com Volume 2014 International Journal of Active and Passive International Journal of Antennas and Advances in Chemical Engineering Propagation Electronic Components Shock and Vibration Acoustics and Vibration Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png International Journal of Navigation and Observation Hindawi Publishing Corporation

Loading next page...
 
/lp/hindawi-publishing-corporation/the-new-generation-system-of-japan-standard-time-at-nict-4LmvYe8uxF
Publisher
Hindawi Publishing Corporation
Copyright
Copyright © 2008 Yuko Hanado et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ISSN
1687-5990
DOI
10.1155/2008/841672
Publisher site
See Article on Publisher Site

Abstract

Hindawi Publishing Corporation International Journal of Navigation and Observation Volume 2008, Article ID 841672, 7 pages doi:10.1155/2008/841672 Research Article 1 1 1 1 1 Yuko Hanado, Kuniyasu Imamura, Noboru Kotake, Fumimaru Nakagawa, Yoshiyuki Shimizu, 1 2 1 3 Ryo Tabuchi, Yukio Takahashi, Mizuhiko Hosokawa, and Takao Morikawa Space-Time Standard Group, New Generation Network Research Center, NICT, 4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8795, Japan National Institute of Information and Communications Technology, (NICT), 4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8795, Japan Incubation Department, Core Technology R & D Center, ANRITSU Corporation, 5-1-1 Onna, atsugi, Kanagawa 243-8555, Japan Correspondence should be addressed to Yuko Hanado, yuko@nict.go.jp Received 30 July 2007; Accepted 15 January 2008 Recommended by Demetrios Matsakis 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. Copyright © 2008 Yuko Hanado et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. INTRODUCTION achieved by developing 24-channel dual mixer time differ- ence (DMTD) systems. In addition, we aimed to improve the Japan standard time (JST) is generated by the National reliability of JST by new data processing, high redundancy, Institute of Information and Communications Technology and enhanced monitoring. The transition to regular opera- (NICT). JST is defined as UTC(NICT) +9 hours, where tions was made in February 2006 and the system has now UTC(NICT) is a local realization of coordinated universal run over one year without major problems. time (UTC). NICT is continuously generating UTC(NICT) The outline of the JST system is outlined in Section 2, from its atomic clocks at the Koganei headquarters using a with a more detailed description of the new system and its system we termed the “JST system.” comparison with the former system in Section 3. The regular Although improvements are necessary as technology pro- operation performance and a future target are introduced in gresses, any change in the JST system must be made with care Section 4,and asummary is givenin Section 5. since UTC(NICT) is a real-time product that must be con- tinuous so as to satisfy the needs of its users. The JST system 2. JST SYSTEM, BASICS AND FORMER ONE experienced changes three times since its first version became operational in 1976. These were in 1987, 1995, and 1999 [1]. In this section, we present an outline of the JST system and Recently, a new building for time and frequency facilities was also of the former system. The basic data flow is shown constructed, and the development of the fifth JST system was in Figure 1 and the block diagram of the former system is undertaken for the installation. shown in Figure 2(a). The design of this new system was begun in 2002. The UTC(NICT) is a realization of an average atomic time main target of this system was the improvement of a short- calculated from an ensemble of Cs atomic clocks. We call term frequency stability of JST, and this required the in- this timescale “NICT ensemble atomic time” (NET), which troduction of hydrogen masers. Another important target is reported to the International Bureau of Weights and Mea- was the improvement of the measurement precision. It was sures (BIPM) as TA(NICT). In the former system, NET was Data processing program 2 International Journal of Navigation and Observation made from maximum 14 Cs atomic clocks (5071A with high- UTC performance tube, Symmetricom), which are maintained at NICT Koganei headquarters the Koganei headquarters. Adjusting (to follow UTC, as required) To calculate NET, we use the data of regularly measured time differences between clocks. In the former system, they Frequency 5MHz were measured by using the 1 PPS signals of the clocks with a Source adjuster UTC(NICT) time-interval (TI) counter. One clock was chosen as the ref- clock (AOG) JST 1 PPS erence. The signals of the other clocks were selected sequen- Adjusting (to follow NET, once a day) tially by a channel selector, and the measured values were ob- tained by a one-shot measurement without averaging. Average Time differences Timescale Cs clock atomic time The algorithm for NET in the former JST system [2]was between clocks algorithm (NET) based on a standard theory of timescale algorithm [3, 4]. First, the rate of each Cs atomic clock at a moment is esti- Figure 1: Configuration of the JST system. mated 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 System-A AOG Cs weighted average of these errors. In the former system, the 1 PPS UTC(NICT) rate was estimated from the frequency difference between the Cs AOG System-B 5MHz clock and NET during the last 30 days, and the weight was calculated from the Allan deviation at τ = 10 days of this Cs clock. System-A T.S. Ch Since NET is a paper clock, an oscillator is required to 1 PPS of TIC NET NET’ Cs algorithm sel. CS, realize the actual signals of UTC(NICT). The output of an Cs HM, atomic clock (we call it “source clock”) is steered by a fre- Offsets from UTC AOG, quency adjuster used as the oscillator. In the former sys- Cs UTC(NICT) T.S. Ch NET NET’ TIC tem, we used a Cs atomic clock as the source clock, and the sel. algorithm System-B Symmetricom’s auxiliary output generator (AOG) as the fre- quency adjuster. The 5 MHz and 1 PPS signals from AOG were used as the frequency reference and the timing refer- 5MHz ence of UTC(NICT), respectively. The frequency and time 1 PPS differences between the AOG and NET were adjusted once a Data day. (a) Former JST system 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 AOG HM follow UTC are added to NET, to produce a steered timescale 1 PPS NET’. The AOG follows NET’ so that UTC(NICT) should AOG HM UTC(NICT) follow UTC. The AOG allows additional adjustments to be 5MHz HM AOG given as the frequency offset or phase changes. In the op- Offsets from UTC eration of former system, the frequency offset was mainly 1 PPS of NET’ used to meet a 50-nanosecond target of synchronization with CS, UTC. Time links between NICT and other institutes were HM, Ch Cs TIC AOG, performed by GPS common-view method by using the Top- sel. UTC(NICT) Cs con Euro80 multichannel receiver mainly. An extremely important part of the system is built-in re- DMTD T.S. Ref. Cs 5MHz of NET 24ch dundancy to identify and protect against a system trouble. In algorithm CS, DMTD the former system, the measurement device (a channel selec- Cs Ref. HM, tor and a TI counter) and a signal generating unit (a source AOG, DMTD UTC(NICT) clock and an AOG) were duplicated. A pair of these units Ref. was selected as the master system. If malfunction is detected in the master system, the system was quickly changed to the 5MHz backup one. 1 PPS Data 3. MODIFICATIONS FOR THE NEW SYSTEM (b) New JST system A block diagram of the new system is shown in Figure 2(b). Figure 2: Comparison of the former and the new JST system. CS: Cs atomic clock, HM: hydrogen maser, TIC: Time interval counter, Though the basic configuration is similar with that of the Ch Sel.: Channel selector, T.S. algorithm: Timescale algorithm. former system, various upgrades were implemented in the Time comparison Yuko Hanado et al. 3 Table 1: Specification of the hydrogen maser RH401A. Frequency 5, 10, 100 MHz, 1.4 GHz Carrier outputs Level 13 dBm ±2dB Format 1PPS Timing outputs Level TTL −13 1s less than 4 × 10 (auto-tuning off ) −14 10 s less than 4 × 10 (auto-tuning off ) Stability σ −15 100 s less than 5 × 10 (auto-tuning off ) 3 4 −15 10 ∼ 10slessthan2 × 10 −15 Long term drift less than 2 × 10 /day −13 Temperature less than 4 × 10 /degree Sensitivity −9 Magnetic less than 2 × 10 /T −9 Range: 2 × 10 Frequency control −16 Function Resolution: 7 × 10 Auto-tuning No ext. reference required −10 Table 2: Specification of the DMTD5. Input frequency 5 or 10 MHz −11 Beat-down frequency 1 kHz Input channels 24 −12 Period of output 1 s 2ps at 5 MHz −13 Resolution (without averaging) Averaging 1∼ 100 samples −14 −15 change of the source clock from a Cs atomic clock to a hy- drogen maser improves the short-term frequency stability of −16 UTC(NICT) about a hundred times. Though the long-term stability of hydrogen maser is not so good, the long-term sta- −17 01 23 4 5 bility of UTC(NICT) is assured by NET. Average time (log scale, s) 3.2. Measurement system Cs clock H-maser The measurement method in the former system was simple TI counter but had plenty of room for improvement. Firstly, for precise DMTD measurements higher frequency carrier signals are more de- sirable than the 1 PPS signals of the former system. Secondly, Figure 3: Frequency stabilities of atomic clocks and system noise of measurement devices. the simultaneous measurements of all signals are better for the construction of an average atomic time than the sequen- tial measurements of the former system. whole system. We introduce the revised points in the new To solve the problems, 24-channel DMTD system system as compared with the former system. (DMTD5, Japan Communication Equipment Co. Ltd., Yam- ato, Kanagawa, Japan) was developed for the new system [5]. Three DMTD5s are used in the new system, and they 3.1. Clock are the main tools for the measurements. Though a DMTD The new system has 18 Cs atomic clocks (5071A with high- system is a well-known method for a precise measurement performance tube) and 4 hydrogen masers (RH401A, An- of time difference [6–8], multichannel devices are not so ritsu Corp., Atsugi, Kanagawa, Japan). Specifications of the widespread. Our DMTD5 solved the time lag problem in Anritsu hydrogen maser RH401A are shown in Table 1,and the measurements of multiple clocks as well as the precision its frequency stabilities as measured by the new system are problem. shown in Figure 3. The Cs atomic clocks generate NET, and A block diagram of DMTD5 is shown in Figure 4 and the hydrogen masers are used as the source clocks. The its specification is shown in Table 2. It measures the time Allan deviation (log scale) Micro-controller 4 International Journal of Navigation and Observation 4or2 DDS 100 MHz REF multiplier PLL synthesizer 5/10 MHz Comparator REF ±1kHz ΔFLPF Start pulse Start pulse output 1kpps Start REF ±1kHz Time pulse Stop pulse interval DUT counter Ch1 1kpps 5/10 MHz . Output Time DUT interval DUT counter 5/10 MHz Ch24 Figure 4: Block diagram of DMTD5. intervals between the 5 MHz signal of a reference hydrogen 3.3. Processing of measured data maser and that of 24 clocks simultaneously. Input 5 MHz signals are down converted to 1 kHz, so that the phase res- The combination of DMTD5 and TI counter in the mea- olution is magnified 5000 times. The phase resolution of the surements requires special data processing. A newly devel- 1 kHz signal is 10 nanoseconds because the sampling clock oped computer program carries out an anomaly detection inside the DMTD5 is 100 MHz. It means that the relative and data synthesis by using the data of three DMTD5 and phase resolution of 5 MHz signal is 2 picoseconds. An aver- one TI counter. The anomaly detection algorithm attempts age of sequential measurements is output every second. Cur- to identify both bad clocks and bad measurement devices. rently, we use the average of 100 samples. The resolution of This program selects the data of two good devices among the final data is therefore around 0.2 picosecond. Details are four. There was no such function in the former system. The described in [5]. details of the procedure are described as follows: The measurements of 5 MHz signals with a DMTD5 pro- In the new system, the measurement unit consists of four vide more precise data than those of 1 PPS signals with a TI devices (three DMTD5s, and a TI counter). These four de- counter. The measurements of 5 MHz, however, have a risk vices measure the same sources, so that their results should of 200-nanosecond phase ambiguities due to the miscount- be almost same in a normal situation. If an anomaly ap- ing of cycles. The DMTD5 prevents this problem by shift- pears in one device, its measurement differs from the others. ing the signal phase by 2 nanoseconds when the phase differ- The malfunctioning device is identified by comparing the re- ence between the clock and the reference becomes less than sults of all devices. Bad data detection and removal are auto- 1 nanosecond. The limit of the frequency offset to measure matically achieved by an algorithm to be described next. In −9 the signal without cycle slip is 1× 10 .The cyclecount num- this description, the index #i indicates each clock. All clock#i ber is output with each measurement. are Cs atomic clocks, where i = 01,... , 18. The clock#01 This function of DMTD5, however, cannot avoid the risk is a reference clock of the measurement. The indexes A, B, of cycle miscounting if the measurements are temporarily C, and T indicate triplicated DMTD5s and a TI counter, halted. In order to keep a phase continuity of measured data respectively. in such cases, we use the TI counter 1 PPS measurements to- (1) Time difference between clock#i and clock#01 is mea- gether with the DMTD5 5 MHz measurements. The 1 PPS sured by each DMTD5 every second. measurements are made every hour in the same manner as the former system. These data are not so precise, but not am- (2) The time difference between clock#i and clock#01 ev- biguous. We adopt the result of TI counter as the initial phase ery hour is determined as follows. In the case of value of a clock when the operation restarts. In the contin- DMTD5, the time difference of clock#i and clock#01 uous operation, the accumulated phase calculated from the at x o’clock is determined by a linear fit of the data DMTD5 data is used. Details of this process are provided in between x − 1 o’clock and x +1 o’clock. Here, we ex- the next subsection. press this determined time differences as p (t), p (t), Ai Bi The system noise of the DMTD5 and the TI counter are and p (t) for the DMTD5-A, B, and C, respectively. Ci compared in Figure 3. While the TI counter shows much Figure 3 shows that the frequency drifts of the clocks higher noise than that of hydrogen maser RH401A in the are small enough for such linear fit. As for the TI short term, the noise of the DMTD5 is lower than that of counter, the hourly measured value itself is used as the the RH401A in all regions. time difference denoted by p (t). Ti Yuko Hanado et al. 5 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 pro- 4 cess. 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 fre- −10 quency deviation than 5 × 10 against the reference signal is also removed in the process (3). The software allows simple −2 variation of the parameters for these limits. −4 3.4. NET, TA(NICT), and UTC(NICT) −6 −8 The various upgrades described above required many 0 5 10 15 20 25 30 changes in the system. The software, however, was designed Day so that the parameters for calculating the clock rate and weight are easily modified. At present, we use the same values AOG-A (H-maser) as the former system except the rate of a source hydrogen AOG-B (Cs clock) AOG-C (H-maser) maser is estimated from the last 5 days’ frequency difference between the hydrogen maser and NET. Figure 5: Time differences between the AOG outputs 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 (3) Frequency difference between clock#i and clock#01 is not stored, which was very inconvenient for analyzing NET. calculated every hour from the time differences de- In the new system, the clock data using NET as the refer- scribed in (2). In the case of DMTD5-A, f (t) = Ai ence are also archived. The other change is the adoption of a (p (t + τ ) − p (t))/τ.Here, τ is 3600 seconds. The Ai Ai unique NET. The former JST system had two redundant sys- values of f (t), f (t), f (t) are obtained in the same Bi Ci Ti tems (system A and B in Figure 2(a)). Each system made each way. NET from each measurement data and steered each AOG. (4) For each device, the sum of the frequency differences It means that the NET used for making UTC(NICT) was from the other devices is calculated. In the case of changedand atimejumpoccurredinUTC(NICT) when the DMTD5-A, the difference is S =| f − f | + | f − Ai Ai Bi Ai master system was changed. In the former system, this time f | + | f − f |. Similarly, S , S ,and S are calcu- Ci Ai Ti Bi Ci Ti jump was not considered as a serious problem because the lated for other devices. steering errors of AOG were large and masked the time jump. (5) Two devices among four are selected by using the S val- Strictly speaking, this method of operation caused disconti- nuities in UTC(NICT). In the new system, only one NET is ues in (4). If DMTD5-A is out of order, the value of f made from the representative measurement values described is different from f , f , f . As a result, S becomes the B C T A in Section 3.3. biggest value among all S values. We select those who For redundancy, there are three AOGs in the new system. have the smallest and the next smallest S values as two They have different source clocks but are steered so that all reliable devices. the outputs follow the same NET’. The steering to cancel the (6) The representative frequency difference between time offsetbetween AOGoutputand NET’isadjustablein clock#i and clock#01 is calculated from the data of the the new system. Currently, we set the parameter of adjust- two selected devices. In the case that S and S are B C ment so that the time offset will disappear in two days. It selected in (5), the average f (t) = ( f (t)+ f (t))/2 i Bi Ci makes a frequency change of UTC(NICT) in the adjustment is used as the representative frequency difference be- gentle and smooth. In the new system, we aim to synchronize tween clock#i and clock#01. UTC(NICT) with UTC within 10 nanoseconds. (7) By using f (t), representative time difference between TA(NICT), an atomic timescale generated by NICT, was clock#i and clock#01 is obtained as follows: p (t) = reset at the timing of starting the regular operation of new p (t )+ Σ f (t ) · (t − t ). The initial phase p (t ) i 0 k i k k k−1 i 0 JST system. Currently, the NET is used as TA(NICT) and the is obtained from the data of the TI counter when the data are sent to BIPM. regular measurement starts. The time differences between the AOGs and NET’ are In the above procedure, one representative data set is shown in Figure 5. This graph shows the steering errors of made from the four data sets. We can obtain a measurement AOG. To show the difference due to the source clocks, we result if at least one measurement device works properly. If set two hydrogen masers and a Cs clock as the source clocks. only one device remains, we would adopt its data. This pro- In the cases of using hydrogen masers (AOG-A and C), the cess is used for the definition of initial phase. The result of TI steering errors are clearly smaller than that in the case of a Cs counter is adopted if there are no DMTD5 data. Usually, the clock (AOG-B). Time difference (ns) 6 International Journal of Navigation and Observation Table 3: Check points of the new system. Cs clocks H-masers Health check AOGs Data logger for monitoring temperature and humidity Oscilloscopes for monitoring of UTC(NICT) signals DMTD measurements TI counter measurements Status of Regular measurements and Calculations Temperature & humidity TA(NICT) calculation AOG adjusting Figures of 5 MHz & 1 PPS of UTC(NICT) Phase jump of each clock Quality of signals Frequency instability of each clock cycle slip of each clock The atomic clocks are operated in the four special rooms. 06 07 They are shielded against static and AC electromagnetic fields and kept in the constant temperature and humidity at 7e − 15 24+/−0.5degrees andat40+/−10%, respectively. For protec- 53465 53648 20 ns 40 8.3e − 15 3e − 15 2e − 15 tion against external power failure, the atomic clocks and the 54010 54153 main devices are supplied with a large UPS and a generator. 1e − 15 1e − 15 The generator has sufficient fuel to maintain power for three days. The building itself is equipped with a quake-absorbing 53586 structure. 2e − 15 −8e − 15 −20 53920 54089 −3e − 15 −2e − 15 3.6. Time links −40 Time transfer method for the link of UTC(NICT) was also 53371 53491 53611 53731 53851 53971 54091 54211 upgraded [9]. When the new JST system started, February MJD 2006, the Septentrio PolaRX2 was newly adopted as the main Figure 6: Phase stability of UTC-UTC(NICT). 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 3.5. Control systems, monitoring systems between NICT and PTB. These improvements decreased the and facilities time link uncertainty. In the former system, a workstation handled all tasks of de- 4. CURRENT STATUS AND NEXT TARGET vice controls and calculations. These tasks are distributed to several computers in the new system to avoid the task The regular operation of the new system was initiated on concentration. In each task, the computers are duplicated February 7, 2006 (MJD 53773). Figure 6 shows the time dif- or triplicated. All systems clocks are synchronized with ference of UTC-UTC(NICT) reported by the Circular-T. In UTC(NICT) via network time protocol (NTP) in a triply re- the new system, we have so far made 5 additional frequency dundant manner. adjustments to follow UTC. Though the number of adjust- The check points of the system are increased compared ments was the same as that in 2005, the magnitudes of the with the former system. They are listed in Table 3.Wehave adjustmentsweremuchsmaller. found the real-time monitoring of 5 MHz and 1 PPS signals In 2006, UTC(NICT) by the new system was stable and of UTC(NICT) with oscilloscopes to be effective for rapid synchronized with UTC to within almost 10 nanoseconds troubleshooting. The outputs of DMTD5 are also useful to peak to peak. The frequency stability in one year period be- check the precise timing of clock anomalies. The staff are tween February 6, 2006 and February 6, 2007 is shown in notified by email if an anomaly exists, and they can check Figure 7. The stabilities in 2001, 2003, and 2005 are also the system condition on the internet with a newly developed shown for a comparison. The stability in the new system is monitoring program. clearly improved with respect to the former system. Time difference (ns) Yuko Hanado et al. 7 1E − 13 ACKNOWLEDGMENTS The authors deeply appreciate the staff effort made to main- tain the UTC(NICT) generation system, and are grateful to many staff members for their helpful discussions with them. 1E − 14 REFERENCES [1] Y. Hanado, M. Imae, and N. Kurihara, “Generating and mea- surement system for Japan Standard Time,” Journal of the Na- tional Institute of Information and Communications Technology, vol. 50, no. 1-2, pp. 169–177, 2003. [2] Y. Hanado, K. Imamura, and M. Imae, “Upgrading of 1E − 15 UTC(CRL),” in Proceedings of the IEEE International Frequency 1E +05 1E +06 1E +07 Control Symposium and Pda Exhibition Jointly with the 17th Eu- Averaging time (s) ropean Frequency and Time Forum, pp. 296–300, Tampa, Fla, USA, May 2003. 2001 (former system) [3] C. Thomas, P. Wolf, and P. Tavella, “Time scale,” BIPM Mono- 2003 (former system) graphie 94/1, pp. 23–32., 1994. 2005 (former system) 2006 (new system) [4] P. Tavella and C. Thomas, “Comparative study of time scale al- gorithms,” Metrologia, vol. 28, no. 2, pp. 57–63, 1991. Figure 7: Frequency stability of UTC-UTC(NICT). [5] F. Nakagawa, M. Imae, Y. Hanado, and M. Aida, “Development of multi channel dual mixer time difference system to generate UTC (NICT),” IEEE Transactions on Instrumentation and Mea- surement, vol. 54, no. 2, pp. 829–832, 2005. In 2007, UTC(NICT) showed a large drift in February [6] D. W. Allan, “The measurement of frequency and frequency sta- and March (around MJD 54153), and the time difference bility of precision oscillators,” NBS Tech. Note 669, 1975. from UTC reached almost 20 nanoseconds. Then we made a [7] D. W. Allan and H. Daams, “Picosecond time difference mea- phase adjustment on April 24, 2007 (MJD 54214). This phase surement system,” in Proceedings of the 29th Annual Symposium on Frequency Control, pp. 404–411, Atlantic City, NJ, USA, May adjustment is rarely used because it causes a rapid change of frequency in UTC(NICT). Usually, only a frequency adjust- [8] S. Stein, D. Glaze, and J. Levine, “Automated high-accuracy ment is enough for canceling a small phase offset. This time, phase measurement system,” IEEE Transactions on Instrumen- the large offset of 20 nanoseconds was a good opportunity tation and Measurement, vol. 32, no. 1, pp. 227–231, 1982. to test a performance of phase adjustment. The result agreed [9] NICT, “Summary of time and frequency activities at NICT,” with what was expected. Working documents of CCTF 17th meetings, CCTF/06-09, 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 al- gorithm. 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 in- vestigating these methods for their appropriateness improv- ing the long-term frequency stability of UTC(NICT). 5. SUMMARY In the new JST system, better short-term frequency sta- bility 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, how- ever, a frequency drift of UTC(NICT) occurred because of the drifts of some Cs atomic clocks. This provided an oppor- tunity 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. Allan deviation International Journal of Rotating Machinery International Journal of Journal of The Scientific Journal of Distributed Engineering World Journal Sensors Sensor Networks Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 Volume 2014 Journal of Control Science and Engineering Advances in Civil Engineering Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 Submit your manuscripts at http://www.hindawi.com Journal of Journal of Electrical and Computer Robotics Engineering Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 VLSI Design Advances in OptoElectronics International Journal of Modelling & Aerospace International Journal of Simulation Navigation and in Engineering Engineering Observation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2010 Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com http://www.hindawi.com Volume 2014 International Journal of Active and Passive International Journal of Antennas and Advances in Chemical Engineering Propagation Electronic Components Shock and Vibration Acoustics and Vibration Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation Hindawi Publishing Corporation http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014 http://www.hindawi.com Volume 2014

Journal

International Journal of Navigation and ObservationHindawi Publishing Corporation

Published: Apr 9, 2008

References