Nov.6-7,2014 DEC Workshop on Participation in Coordinated Universal Time Aimin Zhang National Institute of Metrology (NIM)
Introduction UTC(NIM) at old campus Setup of new UTC(NIM) Algorithm of UTC(NIM) Performance of UTC(NIM) Conclusion
UTC : international standard time More than 400 atomic clocks kept by more than 70 time-keeping labs weighted average EAL Primary frequency standards & secondary frequency standards IERS steer Leap second TAI UTC (k) Circular T UTC UTC(k): local representation of UTC k: acronyms of time-keeping labs
UTC(NIM) : one of the physical realization of UTC, maintained by National Institute of Metrology (NIM) UTC(NIM) together with the Cesium fountain clock NIM#5 are the time and frequency primary standards in China The quantity value of time and frequency should be traceable to UTC(NIM) within China.
Construction of time scale Hardware: 1)clock ensemble- to generate continuous and stable time and frequency signal which can be steered(usually with a micro phase stepper); 2)internal comparison system- to measure the phase differences between the clocks and UTC(k); 3)international comparison system- to take part in TAI computation and realize the time and frequency traceability to UTC Software: 1)algorithm; 2)data storage, process and system monitor.
UTC(NIM) was initially set up at Hepingli Campus in 1980. Until 2009, the clock ensemble consisted of 3 Cesium frequency standards and 2 Hydrogen masers. The maser clock is a Cesium frequency standard and is changed to a Hydrogen maser in 2009. Internal measurement system Time Interval Counter(TIC) Time transfer technique: GPS
In the year of 2009, NIM new campus was put in use. 56-hectare, approximately 40 kilometers northwest of the old Campus, Since then, new UTC(NIM) was constructed at the new campus.
Clock ensemble: 6 H-masers(MHM2010) -- 5 at new campus, 1 at old campus 7 Cs clocks(5071a) 5 at new campus, 2 at old campus
Compare with Cs frequency standard, the H-maser has better short to medium-term stability 5071A (s) Allan deviation y () 1 5.0E-12 10 3.5E-12 100 8.5E-13 1,000 2.7E-13 10,000 8.5E-14 100,000 2.7E-14 5 days 1.0E-14 30 days 1.0E-14 floor 1.0E-14 MHM-2010 (s) Allan deviation y () 1 2.0E-13 10 5.0E-14 100 1.3E-14 1,000 3.2E-15 floor 3.0E-15
Compare with Cs frequency standard, the frequency of H- maser drift for long-term operation, usually in the order of ~10-15 /d, but its frequency drift could be predicted and compensated A H-maser is chose as the master clock of UTC(NIM).
Improvement of ALGOS algorithm at BIPM (1) Frequency drift is taken into account in prediction since Sep. 2011 (2) clock weighting is based on its frequency predictability since Jan. 2014, based on the concept: a good clock is a predictable clock. The improvement of algorithm ALGOS enhances the influences of H-masers
Micro phase stepper: HROG-5(SDI) Frequency resolution: 510-19 Steered by the algorithm and generate UTC(NIM)
Internal measurement system (1) Time Interval Counter(TIC) SR620 (2) Dual Mixer Time Difference (DMTD) TSC MMS
TIC measurement system Clock 1 1PPS Micro phase stepper 1PPS UTC(NIM) Clock 2 Clock 3 Multi-channel switch TIC 1PPS Clock n 1PPS
DMTD measurement system TSC MMS(16-channel) Principle of TSC MMS noise floor of TSC MMS
DMTD measurement system Clock 1 10MHz Micro phase stepper 10MHz UTC(NIM) Clock 2 Clock 3 DMTD measurement system Clock n 10MHz
International comparison system BIPM: the network of time links relies on the observation of GNSS(GPS, GLONASS) satellites and on Two Way Satellite Time and Frequency Transfer(TWSTFT) At NIM GNSS time transfer: at both campus TWSTFT: at new campus and join the Europe Asia link
3 geodetic-type GNSS receivers operate at new campus Code and phase data are reported to BIPM everyday For time transfer, the calibration of time transfer equipments is necessary. BIPM Pilot Project, namely METODE(MEsurement of TOtal DElay) In June 2014, the BIPM travelling Standard visited NIM to calibrate the time links 3 receivers at new campus were involved
Modem: SATRE (TimeTech) Transceiver:KST-2000A,8W Antenna:1.8m Communication satellite for Europe-Asia link: AM2 In June 2014, the TW link was calibrated by BIPM GNSS travelling Standard
TWSTFT link between Asia and Europe was established in 2005, At present, the participating laboratories include NICT, NIM, NPLI, NTSC, PTB, SU and TL In 2013, six Cesium fountain clocks from NIM, NPLI, PTB and SU were compared using TWSTFT and GPS CP techniques, the result shows that the relative frequency differences among all the six PFSs are within 1-sigma(1) of the corresponding combined standard uncertainty in low 10-15 level.
The algorithm plays an important role in the generation of a stable and reliable time scale. Algorithm steering method By steering to make the UTC(k) as close as UTC Frequency steering is better than phase steering There is no best solution, each time-keeping lab can develop its own algorithm based on its facilities
UTC and TAI are the international reference time scales; UTC is a paper time, with a delay up to 45 days. Each month typically at 15 th, the time differences [UTC -UTC(k)] are reported at 5-day intervals in Circular T; UTC(NIM) is steered to UTC in the long term UTCr was realized by BIPM in January 2012, officially published by BIPM from 1 July 2013. UTCr is published every wednesday and disseminated through daily values of [UTCr UTC(k)]. UTCr data are also used as reference in Algorithm of UTC(NIM)
The master clock of UTC(NIM) is a H-maser Frequency prediction y Dt ( t0) y 0 y: predicted frequency y 0 : frequency at time t 0 D: frequency drift
统计偏差 60 previous days data are used for the frequency prediction. 2. 0E- 14 1. 8E- 14 1. 6E- 14 30 天一次拟合 10 天一次拟合 1. 4E- 14 1. 2E- 14 1. 0E- 14 8. 0E- 15 6. 0E- 15 4. 0E- 15 2. 0E- 15 0. 0E+00 0 20 40 60 80 100 120 140 拟合长度 Kalman filtering is used after the linear fitting
时差 ns 20000 18000 16000 14000 12000 10000 8000 6000 Linear fitting to the frequency data 4000 2000 0 0 50 100 150 200 250 300 天数 Free running H-maser Kalman filtering
NIM participate the computation of UTCr, the clock data, GPS data and TWSTFT data are reported to BIPM everyday UTC(NIM) monitor system: The operation of clocks; Temperature and humidity, GPS receivers operation; TWSTFT system operation
GPS monitor TWSTFT monitor
Master clock changed from Cs clock to H- maser in Aug.2009 Data from BIPM Circular T Steered by UTC+UTCr ±10 ns The new UTC(NIM) ran for test in the end of 2010, worked officially in Oct. 2012.
UTC(NIM) has been reconstructed at NIM s new campus. A Hydrogen maser is selected as the master clock, and UTC (NIM) is generated from a phase microstepper steered by UTC and UTCr The time deviation of UTC(NIM) from UTC is within 10 ns, the time stability is less than 1ns/5d. In order to increase the reliability of UTC(NIM), an algorithm based on a Hydrogen maser ensemble and fountain ensemle is under development at NIM.