The International Atomic Time Research Articles

Time: Astronomical time - atomic time - redefining second based on constant

The evolution history of modern time from the astronomical time to the atomic second is briefly reviewed herein. The progress and status of realizing the current definition of the second using cesium cold atomic fountain primary frequency standards, the process of generating the international atomic time (TAI) and coordinated universal time (UTC), and trend of releasing the tolerance on inserting leap-seconds into the UTC are sketched. The present situation on steering the TAI regularly from January 2016 by cesium fountain frequency standards from five countries is reported. Research progress of local and global optical frequency standards is summarized, and steering TAI by six optical frequency standards starting from 2016 is reported. Requirements and responses on steering TAI regularly by optical frequency standards are introduced according to the roadmap, which is under discussion internationally, or the future redefinition of the second. Furthermore, two types of optical frequency standards, namely the basic-research and the metrological type are suggested, and the performance and technical qualifications for the metrological type of optical frequency standards are recommended. For the optical frequency standards of the metrological type, with uncertainties better than 3×10−17, not only static relative gravity but also the alternative tide frequency shifts should be considered in their frequency evaluations. Finally, a scheme employing frequency comparison, across a long longitude-distance, between two highly stable optical frequency standards to demonstrate the relativistic tidal frequency shift directly and experimentally is proposed.

Research on time scale algorithm based on hydrogen masers

In order to ensure the long-term stability of the time scale, the traditional timekeeping clock ensemble is mainly composed of a cesium clock and a few hydrogen masers. Since 2014, the Bureau International des Poids et Mesures (BIPM) has changed the weighting method of atomic clocks, using the predictability of atomic clocks to weight the global atomic clocks to obtain the International Atomic Time (TAI) and Coordinated Universal Time (UTC). Because of the good predictability of hydrogen masers, less than one third of the number of atomic clocks in TAI/UTC calculation is hydrogen masers which has gained 75% weight. Effective utilization of hydrogen masers has become a research hotspot in timekeeping laboratories. In order to give full play to the advantages of hydrogen masers in timekeeping, the time scale of hydrogen masers is tested in this paper. Firstly, the frequency drift parameters of hydrogen masers are dynamically estimated by using the minimum error theory. Then the estimated parameters are applied to a real-time exponential filtering time scale algorithm. The results show that the stability of the time scale produced by this method can reach 6e-15/day, and the absolute maximum deviation from TA(NTSC) within a month can be kept within 5 ns, which is better than the time scales produced by a traditional fixed frequency drift parameters method and the classical weighted averaging algorithm ALGOS.

Characteristics of the Long-Term Instability of the New-Generation Hydrogen Frequency and Time Standards

The latest changes in the algorithms for the formation of the International Atomic Time (TAI) scale are reported in terms of estimating the weights of the clocks involved in the formation of TAI. The characteristics of the long-term instability of new-generation hydrogen frequency and time standards from the State Primary Standard of time and frequency units and national time scale, GET 1-2018, were studied. The study was based on the results of processing the frequency difference with respect to TAI. It is confirmed that presently, new-generation hydrogen standards of frequency and time show significantly less long-term instability compared with similar and other quantum frequency standards.

Research on the Methods of Performance Monitoring and Evaluation for Active Hydrogen Maserstwo

Active hydrogen maser is the main frequency standard for establishing and maintaining the time scale. It has the characteristics of high short-term stability and low phase noise. At present, it plays an increasingly important role in the international atomic time (TAI) and various local time scales. Firstly, in combination with the internal state parameters of an active hydrogen maser, the correlation between the internal state parameters and the comparison data of a hydrogen maser is analyzed, and a method for the performance monitoring of a hydrogen maser is proposed. Secondly, according to the characteristics of hydrogen maser performance, a method for evaluating the performance of a hydrogen maser is given. The hydrogen maser performance includes mainly two aspects, namely the frequency stability and predictability. The performances of two types of active hydrogen masers (CH1-75 and MHM-2010) are evaluated by this method. The correlation analysis of the atomic clock state parameters and comparison data shows that the state parameter monitoring can effectively predict the variation of the hydrogen maser performance. The evaluation results of atomic clock frequency stability and predictability show that the atomic clock with a higher medium-and-long term frequency stability has a better predictability. There are two methods for the predictability evaluation, one is based on the data published by BIPM (Bureau International des Poids et Measures), and another one is based on the quadratic model. Both methods are consistent with the weights published by BIPM. Therefore, the two methods can be used as a quantitative method to evaluate the predictability of a hydrogen maser.

Study of the GPS inter-frequency calibration of timing receivers

When calibrating Global Positioning System (GPS) stations dedicated to timing, the hardware delays of P1 and P2, the P(Y)-codes on frequencies L1 and L2, are determined separately. In the international atomic time (TAI) network the GPS stations of the time laboratories are calibrated relatively against reference stations. This paper aims at determining the consistency between the P1 and P2 hardware delays (called dP1 and dP2) of these reference stations, and to look at the stability of the inter-signal hardware delays dP1–dP2 of all the stations in the network. The method consists of determining the dP1–dP2 directly from the GPS pseudorange measurements corrected for the frequency-dependent antenna phase center and the frequency-dependent ionosphere corrections, and then to compare these computed dP1–dP2 to the calibrated values.Our results show that the differences between the computed and calibrated dP1–dP2 are well inside the expected combined uncertainty of the two quantities. Furthermore, the consistency between the calibrated time transfer solution obtained from either single-frequency P1 or dual-frequency P3 for reference laboratories is shown to be about 1.0 ns, well inside the 2.1 ns uB uncertainty of a time transfer link based on GPS P3 or Precise Point Positioning. This demonstrates the good consistency between the P1 and P2 hardware delays of the reference stations used for calibration in the TAI network.The long-term stability of the inter-signal hardware delays is also analysed from the computed dP1–dP2. It is shown that only variations larger than 2 ns can be detected for a particular station, while variations of 200 ps can be detected when differentiating the results between two stations. Finally, we also show that in the differential calibration process as used in the TAI network, using the same antenna phase center or using different positions for L1 and L2 signals gives maximum differences of 200 ps on the hardware delays of the separate codes P1 and P2; however, the final impact on the P3 combination is less than 10 ps.

First international comparison of fountain primary frequency standards via a long distance optical fiber link

We report on the first comparison of distant caesium fountain primary frequency standards (PFSs) via an optical fiber link. The 1415 km long optical link connects two PFSs at LNE-SYRTE (Laboratoire National de métrologie et d’Essais—SYstème de Références Temps-Espace) in Paris (France) with two at PTB (Physikalisch-Technische Bundesanstalt) in Braunschweig (Germany). For a long time, these PFSs have been major contributors to accuracy of the International Atomic Time (TAI), with stated accuracies of around . They have also been the references for a number of absolute measurements of clock transition frequencies in various optical frequency standards in view of a future redefinition of the second. The phase coherent optical frequency transfer via a stabilized telecom fiber link enables far better resolution than any other means of frequency transfer based on satellite links. The agreement for each pair of distant fountains compared is well within the combined uncertainty of a few 10−16 for all the comparisons, which fully supports the stated PFSs’ uncertainties. The comparison also includes a rubidium fountain frequency standard participating in the steering of TAI and enables a new absolute determination of the 87Rb ground state hyperfine transition frequency with an uncertainty of .This paper is dedicated to the memory of André Clairon, who passed away on 24 December 2015, for his pioneering and long-lasting efforts in atomic fountains. He also pioneered optical links from as early as 1997.

Frequency ratio measurement of ^171Yb and ^87Sr optical lattice clocks

The frequency ratio of the (1)S(0)(F = 1/2)-(3)P(0)(F = 1/2) clock transition in (171)Yb and the (1)S(0)(F = 9/2)-(3)P(0)(F = 9/2) clock transition in (87)Sr is measured by an optical-optical direct frequency link between two optical lattice clocks. We determined the ratio (ν(Yb)/ν(Sr)) to be 1.207 507 039 343 341 2(17) fractional standard uncertainty of 1.4 × 10(-15) [corrected]. The measurement uncertainty of the frequency ratio is smaller than that obtained from absolute frequency measurements using the International Atomic Time (TAI) link. The measured ratio agrees well with that derived from the absolute frequency measurement results obtained at NIST and JILA, Boulder, CO using their Cs-fountain clock. Our measurement enables the first international comparison of the frequency ratios of optical clocks. The measured frequency ratio will be reported to the International Committee for Weights and Measures for a discussion related to the redefinition of the second.

Direct comparison of a Ca^+ single-ion clock against a Sr lattice clock to verify the absolute frequency measurement

Optical frequency comparison of the (40)Ca(+) clock transition ν(Ca)((2)S(1/2-)(2D(5/2), 729 nm) against the (87)Sr optical lattice clock transition ν(Sr) ((1)S(0)-(3)P(0), 698 nm) has resulted in a frequency ratio ν(Ca) / ν(Sr) = 0.957 631 202 358 049 9(2 3). The rapid nature of optical comparison allowed the statistical uncertainty of frequency ratio ν(Ca) / ν(Sr) to reach 1 × 10(-15) in 1000s and yielded a value consistent with that calculated from separate absolute frequency measurements of ν(Ca) using the International Atomic Time (TAI) link. The total uncertainty of the frequency ratio using optical comparison (free from microwave link uncertainties) is smaller than that obtained using absolute frequency measurement, demonstrating the advantage of optical frequency evaluation. We note that the absolute frequency of (40)Ca(+) we measure deviates from other published values by more than three times our measurement uncertainty.

Time scales in the context of general relativity

Towards 1967, the accuracy of caesium frequency standards reached such a level that the relativistic effect could not be ignored anymore. Corrections began to be applied for the gravitational frequency shift and for distant time comparisons. However, these corrections were not applied to an explicit theoretical framework. Only in 1991 did the International Astronomical Union provide metrics (then improved in 2000) for a definition of space-time coordinates in reference systems centred at the barycentre of the Solar System and at the centre of mass of the Earth. In these systems, the temporal coordinates (coordinate times) can be realized on the basis of one of them, the International Atomic Time (TAI), which is itself a realized time scale. The definition and the role of TAI in this context will be recalled. There remain controversies regarding the name to be given to the unit of coordinate times and to other quantities appearing in the theory. However, the idea that astrometry and celestial mechanics should adopt the usual metrological rules is progressing, together with the use of the International System of Units, among astronomers.

A Research on the Algorithm of Local Atomic Time

The goal of all time laboratories’ pursue is to produce and keep a stable, precise and reliable time scale. The long-term stability of the time scale is mainly taken into account in the traditional ALGOS algorithm, while the local atomic time scale should give consideration to both the long-term and shortterm stabilities. From the analysis and research on the atomic clock noise model and under the condition that the long-term stability of the local atomic time scale does not drop, a complete algorithm is proposed which is suitable for the calculation of the local time scale TA (NTSC) carried out at the time laboratory with the time-keeping clocks of a unitary type and being close in performance at the National Time Service Center, called the NTSC for short, of Chinese Academy of Sciences. The data of all the clocks in the NTSC which participated in the calculation of the International Atomic Time (TAI) all the year round in 2008 are applied to the test and verification of the new algorithm, with the result showing that both the short- and long-term stability indexes of the obtained TA (NTSC) are improved. The research result is suitable for the calculation of the atomic time scale of the time laboratory whose time keeping system is similar to that of the NTSC.

Simulation of subdiurnal variation in the earth’s rotation

On the basis of the celestialmechanical approach, the mathematical model of subdiurnal tidal variation in the rotation of the deformable Earth is developed taking into account the gravity-tidal lunar and solar disturbances, which is adequate for astrometric mea� surements of the International Earth Rotation Service (IERS) (1). The subdiurnal and nearly diurnal varia� tions of the Earth's axial rotation are investigated using the third Euler-Liouville equation and the results of these measurements within a day (IGS). The model parameters were identified by the least squares method, and the features of the Earth's axial rotation within a day were revealed. The combinational fre� quencies confirmed by the IERS observations were found. For solving very actual problems of celestial mechanics and astrometry, it is necessary to create a precision model of the motion of Earth'srotation with respect to the center of mass. The analysis of equations of motion (2, 3) and observation data (1, 4) indicates the necessity of taking into account the dis� turbing moments of forces of various physical natures and the considerable deformability of the Earth's fig� ure. The gravity-tidal moments of forces from the Sun and the Moon exert the major effect. However, a sig� nificant correction of the model results in the compli� cation of processes of the filtration of unknown parameters including the estimation of the initial data. In a number of important problems of astrometry, navigation, and geophysics, the precision forecast of the Earth's rotation is of substantial significance for relatively short time intervals. For applications, it is the extreme precision forecast for the intervals from 1-2 to 10-30 days of length that can be of interest. Analysis of the theoretical model indicates the fact that an accuracy of about 10 -4 -10 -5 s can be achieved. It is necessary to accomplish the construction of an ade� quate set of basic functions, the choice of optimum length of the interpolation interval, and the adjust� ment of the filtration algorithm by the method of "weighed" least squares. The error in the IERS data (1) amounts to several μs. The achieved very high accuracy of measurements is sufficient for construct� ing an adequate model of the Earth's rotation (1-3). 1. The universal time (UT1) related to the Earth's rotation is a reasonably important value, which requires continuous measurements. Because the aver� age solar time, and consequently, the UT1 is not suffi� ciently accurate time scale, the atomic time scale (TAI) having a relative stability of 10 -14 can be used as such a scale on reasonably short intervals (several years). The creation of the unified atomic time scale accepted as the international standard, precisely, the international atomic time (TAI), made it possible to accept it as the practical timescale standard. It is adapted for supporting the relation to the scale UT1 determined by the Earth's rotation and is known as the coordinated universal time (UTC).

Two-way satellite time and frequency transfer networks in Pacific Rim region

A two-way satellite time and frequency transfer (TWSTFT) network in the Pacific Rim region is under construction to contribute to the calculation of the international atomic time (TAI). Four major time and frequency institutes in this region have been conducting long-term TWSTFT experiments. In addition to these institutes, several others in the region are planning to join the network. A new type of time transfer modem for TWSTFT is also described.

Recent results of Physikalisch-Technische Bundesanstalt's primary clock CS1

This paper describes the current performance of the Physikalisch-Technische Bundesanstalt's (PTB) primary clock CS1. After major reconstruction during 1995 and 1996, routine clock operation was started in May 1997. An evaluation of the CS1 Type B uncertainty yielded 7.10(-15) (1sigma). The performance of the CS1 is illustrated by results of a comparison lasting 2 yr with other clocks at PTB and with the international Atomic Time (TAI) and free atomic time scales. With reference to TAI, e.g., a frequency instability of 3.9.10(-15) at an averaging time of 30 d was observed.

Time and Frequency Transfer Using GPS Codes and Carrier Phases: Onsite Experiments

Recent studies have shown the capabilities of Global Positioning System (GPS) carrier phases for frequency transfer based on the observations from geodetic GPS receivers driven by stable atomic clocks. This kind of receiver configuration is the kind primarily used within the framework of the International GPS Service (IGS). The International GPS Service/Bureau International des Poids et Mesures (IGS/BIPM) pilot project aims at taking advantage of these GPS receivers to enlarge the network of Time Laboratories contributing to the realization of the International Atomic Time (TAI). In this article, we outline the theory necessary to describe the abilities and limitations of time and frequency transfer using the GPS code and carrier phase observations. We report on several onsite tests and evaluate the present setup of our 12-channel IGS receiver (BRUS), which uses a hydrogen maser as an external frequency reference, to contribute to the IGS/BIPM pilot project. In the initial experimental setup, the receivers had a common external frequency reference; in the second setup, separate external frequency references were used. Independent external clock monitoring provided the necessary information to validate the results. Using two receivers with a common frequency reference and connected to the same antenna, a zero baseline, we were able to use the carrier phase data to derive a frequency stability of 6 × 10−16 for averaging times of one day. The main limitation in the technique originates from small ambient temperature variations of a few degrees Celsius. While these temperature variations have no effect on the functioning of the GPS receiver within the IGS network, they reduce the capacities of the frequency transfer results based on the carrier phase data. We demonstrate that the synchronization offset at the initial measurement epoch can be estimated from a combined use of the code and carrier phase observations. In our test, the discontinuity between two consecutive days was about 140 ps. © 1999 John Wiley & Sons, Inc.

Accuracy evaluation of the optically pumped Cs frequency standard at NRLM

The optically pumped cesium (Cs) frequency standard at the National Research Laboratory of Metrology, Ibaraki, Japan (NRLM), NRLM-4, was modified in various ways in order to improve frequency stability and accuracy, We determined the frequency stability by comparing the phase with that of a hydrogen maser: the Allan deviation was /spl sigma//sub y/(/spl tau/)=5/spl times/10/sup -13///spl radic//spl tau/. We estimated the frequency corrections and uncertainties due to various sources, and the total uncertainty was found to be 2.9/spl times/10/sup -14/. The frequency difference between NRLM-4 and the international atomic time (TAI) was also measured.

Comparison of remote time scales in the CCTF format: Initial results with a specialized GPS receiving system based on OEM technology

A report is given on the development and initial results of tests of a specialized GPS system for receiving time signals. The system is designed mainly for highly accurate comparison of remote metrological-class time scales and frequency standards. The fact that the method of operation of the system and the format of the output data conform to CCTF requirements ensures high-precision access to the time scales of any time laboratories which participate in the formation of the International Atomic Time (TAI) scale and the Universal Time (Coordinated) (UTC) scale.

光ポンピング方式セシウム周波数標準器による国際原子時の校正

The optically pumped cesium (Cs) frequency standard at NRLM (NRLM-4) was developed. We determined the frequency stability by comparing the phase with that of a hydrogen maser: the Allan deviation was σy(τ)=5×10-13/√τ. We estimated the frequency corrections and uncertainties due to various sources, and the total uncertainty was found to be 2.9×10-14. The frequency difference between NRLM-4 and the international atomic time (TAI) was also measured.

Progress in Realization of the Time Unit and Scales

AbstractThe establishment of the TAI is done in two steps: the construction of the EAT from clocks in laboratories, then its steering by primary frequency standards. Great progress in the precision and stability of EAL was made by the generalized use of GPS time transfer and by the introduction of hydrogen masers and new very performant clocks. Only two primary frequency standards are sufficiently accurate to steer the frequency of TAI, but new standards are now being assessed. The present stability of TAI is 5.10−15 and the accuracy 2.10−14. Improvements should occur in the years to come.The establishment of the International Atomic Time (TAI) and the resulting Coordinated Universal Time (UTC) by the time section of BIPM is done in two steps (Guinot and Thomas, 1988). At first, a free atomic scale, EAL (Echelle atomique libre) is built in two month blocks combining data from about 200 atomic clocks kept by 60 laboratories and regularly reported to the BIPM by 45 centres which maintain a local coordinated universal time, UTC(k). Then, the duration of the scale interval of EAL is compared with data from primary caesium standards producing the SI second which, in turn, is converted on the rotating geoid as the unit scale of TAL A linear function of time with the necessary slope is added to EAL to ensure the accuracy of the TAI scale interval.

Recent Progress in Cs Beam Frequency Standards at the NRLM

Some causes and cures of frequency uncertainty in the cesium beam frequency standard, NRLM-II, are described. Accuracy reevaluation indicates about 2.2 × 10-13, and the standard frequency is about 8 × 10-14 below that of the International Atomic Time (TAI) scale. The frequency stability is estimated as better than 8 × 10-12 τ-1/2 for short-term and 4 × 10-14 for 1-2 day averages.

Recent progress in primary Cs beam frequency standards at the PTB

The deflection of cesium (Cs) atomic beams using quadrupole and hexapole magnets, or combinations of both, has been investigated. The beam efficiency and the atomic velocity distribution has been calculated for magnets with a cylindrical, conic or parabolic form, respectively. A combination of quadrupole and hexapole magnets allows the selection of atoms in the beam with a relative velocity range of a few percent. A new rectangular frequency modulation method has been developed using modulation widths of multiples of the line width enabling the detection of very small asymmetries of the Cs resonance. By improving the beam optics and lowering the average atomic velocity in the beam to less than 100 m/s, the relative uncertainty of the primary Cs standard CSI of the PTB could be reduced from 1.5 × 10−13 to 1 × 10−13. Measurements of the frequency of the International Atomic Time (TAI) since 1973 showed that on an average the relative frequency deviation from the nominal value is +(10.3 ± 1) × 10−13. TAI frequency measurements with CSI carried out between 1969 and 1973 had revealed a negative TAI-frequency drift.