Automated Clock Comparisons and Time Scale Generation in the SIM Region

The second, the base unit of time interval in the International System, is defined in terms of the two hyperfine states of the Cesium atom ground-state energy level. This definition has so far served the metrology community well, and the uncertainty of the best realization of the second has improved by a rate of about one order of magnitude per decade over the past 50?years, reaching a current level of a few parts in 10¹⁶ [1]. This continual reduction in uncertainty has increased the level of performance expected from both time and frequency transfer systems and from the time standards maintained by national metrology institutes (NMIs). During recent years, an automated time comparison network has been developed within the Sistema Interamericano de Metrologia (SIM), a regional metrology organization. The SIM Time Network (SIMTN) allows NMIs to compare their time scales via the Global Positioning System common-view and all-in-view time transfer techniques, and makes results available through the Internet in near real time [2]. The SIMTN has proven to be robust and reliable, and the uncertainty of its comparisons is similar to the uncertainty of the key comparisons published by the Bureau International des Poids et Mesures in its monthly Circular T document. The large number of geographically dispersed clocks measured by the SIMTN made it attractive to develop a SIM time scale (SIMT), which is computed in near real time and immediately made available to the general public via the Internet. This rapid computation allows contributing laboratories to easily monitor their time scales, and to quickly detect short term fluctuations in stability and accuracy. This paper discusses both the SIMTN and the SIMT, focusing primarily on the SIMT algorithm and the results of its performance.     

Problems of the Generation, Quality, and Availability of the International Atomic Time Scale

The computation of the International Atomic Time, TAI, is made in two steps. First, an intermediate time scale, EAL, is established from the data of about 100 clocks (almost all commercially made), in many laboratories. Then TAI is derived from EAL by frequency steering based on the data of a few primary frequency standards. This organization ensures the continuity and the availability of TAI, as well as its accuracy. It was also expected to optimize its stability. However, comparisons with the time scales directly produced by some primary frequency standards in continuous operation (primary clocks) show that EAL may be subject to systematic frequency variations. If it is found that the operation of the commercial clocks is responsible for the variations, should TAI be simply an average of the data of a few primary clocks? The BIH would be reluctant to adopt such a solution because it wishes to maintain as large an international basis as possible for TAI. It tries to identify the source of the systematic variations, in order to use the data of both commercial and laboratory primary clocks, with proper weighting.     

A wavelet-based multiscale ensemble time-scale algorithm

The wide-spread availability of ensembles of high-performance clocks has motivated interest in time-scale algorithms. There are many such algorithms in use today in applications ranging from scientific to commercial. Although these algorithms differ in key aspects and are sometimes tailored for specific applications and mixtures of clocks, they all share the goal of combining measured time differences between clocks to form a reference time scale that is more stable than any of the clocks in the ensemble. A new approach to forming time scales is presented here, the multiscale ensemble timescale (METS) algorithm. This approach is based on a multiresolution analysis afforded by the discrete wavelet transform. The algorithm does not assume a specific parametric model for the clocks involved and hence is well-suited for an ensemble of highly disparate clocks. The approach is based on an appealing optimality criterion which yields a reference time scale that is more stable than the constituent clocks over all averaging intervals (scales). The METS algorithm is presented here in detail and is shown in a simulation study to compare favorably with a time-scale algorithm based on Kalman filtering.     

GPS在轨卫星原子钟性能评估

卫星钟在轨性能评估对于卫星钟差预报与系统完好性监测具有重要作用。利用国际GNSS服务组织发布的事后GPS精密卫星钟差数据,基于频率准确度、漂移率、万秒稳定度及天稳定度对GPS Block IIR、IIR-M、IIF与IIIA 4类卫星的星载钟性能进行评估。结果表明:1) GPS卫星钟的频率准确度与天漂移率分别在10^-13～10^-12量级与10^-15～10^-14量级;2) 星载铷钟的万秒稳定度与天稳定度分别可达10^-14与10^-15量级,比星载铯钟的同类指标高近一个数量级;3) 新型Block IIIA卫星的星载钟的天稳定度比另外3种类型卫星的星载钟的天稳定度更高,达到(3～5)×10^-15的水平;4) 无论对于不同系列卫星还是同一系列卫星,各星载钟之间均存在一定的性能差异,这种差异与卫星钟在轨运行时间长短无显著关系。     

Recent Development and Utilization of Two-Way Satellite Time and Frequency Transfer

TWSTFT (Two Way Satellite Time and Frequency Transfer) has been developed for a long time, and has become one of the most precise and accurate techniques for comparison of the frequency standards located at remote sites. Since 1999, TWSTFT has been used in TAI (International Atomic Time) generation. More than two-thirds of TAI clocks and almost all the primary frequency standards are transferred using TWSTFT. To increase the time transfer precision and stability, several calibration methods were developed and the possible instability sources were investigated. Due to the high redundancy of the time transfer links and quick developments of independent time transfer techniques (e.g. GPS), much utilization was proposed to enhance the robustness, to reduce the uncertainty, and to reduce the diurnal effect of TWSTFT. For example, one can adopt the concept of network time transfer to improve the short term stability, or combine the data of different time transfer techniques to take their advantages. The numerical results of network time transfer are very promising. For the future development, a newly developed DPN-based TWSTFT method shows competitive performance with the GPS PPP and much less diurnals than the conventional TWSTFT. It is a very promising method for the next-generation TWSTFT. This paper will give an overview of the above topics.     

Impact of new clock technologies on the stability and accuracy ofthe International Atomic Time TAI

The BIPM Time Section is in charge of the generation of the reference time scales TAI (International Atomic Time) and UTC (Coordinated Universal Time). Both time scales are obtained in deferred-time by combining the data front a number of atomic clocks spread worldwide. Since the end of 1992, the quality of the timing data received at the BIPM has evolved rapidly thanks to the extensive replacement of older designs of commercial cesium clocks by the new HP 5071A units and to the use of active auto-tuned hydrogen masers. Consequently, the stability and the predictability of the reference time scales has improved significantly: these are characterized by an Allan standard deviation σ_y(τ) of 2.6×10^-15 for averaging times τ=40 d. The accuracy of TAI is estimated by the departure between the duration of the TAI scale interval and the SI second as produced on the rotating geoid by primary frequency standards. It is now possible to estimate TAI accuracy through the combination of results obtained from six different primary standards: LPTF-FO1, PTB CS1, PTB CS2, PTB CS3, NIST-7, and SU MCsR 102, all corrected for the black-body radiation shift. This led to a mean departure of the TAI scale interval of +2.0×10^-14 s over 1995, known with a relative uncertainty of 0.5×10^-14 (1 σ)     

ACES microwave link requirements

Atomic Clock Ensemble in Space (ACES) is a project of the European Space Agency on-board the future International Space Station (ISS). The payload consists mainly of two atomic frequency standards, one space hydrogen maser (SHM) prepared by the Observatoire de Neuchatel (Switzerland), and one cold atom caesium clock called PHARAO prepared by the CNES (France), with the participation of the BNM-LPTF, the ENS-LKB, and the CNRS-LHA. Because of the anticipated performances of these clocks on-board the ISS, the requirements of the links between the payload and the clocks on the Earth are at the limits of the known potential of the optical or microwave techniques. The microwave link (MWL) requirements are described in this paper. Taking into account the characteristics of the ISS orbit, and fixing an arbitrary limit to the additional noise brought to the clock readings by the MWL, the computation of the required stability leads to two kinds of requirements: the first one at the subpicosecond level over each single continuous pass of the ISS above any Earth station, and the second one at the level of one part in 10(16) and below over a one day or more averaging period. Moreover, the ISS orbit parameters should lead to a knowledge of the ACES clock position at the m level, and of the ACES clock speed at the mm/s level.     

The watt balance: determination of the Planck constant and redefinition of the kilogram

Since 1889, the international prototype of the kilogram has served as the definition of the unit of mass in the International System of Units (SI). It is the last material artefact to define a base unit of the SI, and it influences several other base units. This situation is no longer acceptable in a time of ever-increasing measurement precision. It is therefore planned to redefine the unit of mass by fixing the numerical value of the Planck constant. At the same time three other base units, the ampere, the kelvin and the mole, will be redefined. As a first step, the kilogram redefinition requires a highly accurate determination of the Planck constant in the present SI system, with a relative uncertainty of the order of 1 part in 10(8). The most promising experiment for this purpose, and for the future realization of the kilogram, is the watt balance. It compares mechanical and electrical power and makes use of two macroscopic quantum effects, thus creating a relationship between a macroscopic mass and the Planck constant. In this paper, the operating principle of watt balance experiments is explained and the existing experiments are reviewed. An overview is given of all available experimental determinations of the Planck constant, and it is shown that further investigation is needed before the redefinition of the kilogram can take place. Independent of this requirement, a consensus has been reached on the form that future definitions of the SI base units will take.     

When should we change the definition of the second?

The microwave caesium (Cs) atomic clock has formed an enduring basis for the second in the International System of Units (SI) over the last few decades. The advent of laser cooling has underpinned the development of cold Cs fountain clocks, which now achieve frequency uncertainties of approximately 5×10(-16). Since 2000, optical atomic clock research has quickened considerably, and now challenges Cs fountain clock performance. This has been suitably shown by recent results for the aluminium Al(+) quantum logic clock, where a fractional frequency inaccuracy below 10(-17) has been reported. A number of optical clock systems now achieve or exceed the performance of the Cs fountain primary standards used to realize the SI second, raising the issues of whether, how and when to redefine it. Optical clocks comprise frequency-stabilized lasers probing very weak absorptions either in a single cold ion confined in an electromagnetic trap or in an ensemble of cold atoms trapped within an optical lattice. In both cases, different species are under consideration as possible redefinition candidates. In this paper, I consider options for redefinition, contrast the performance of various trapped ion and optical lattice systems, and point to potential limiting environmental factors, such as magnetic, electric and light fields, collisions and gravity, together with the challenge of making remote comparisons of optical frequencies between standards laboratories worldwide.     

One-way time transfer using geostationary satellite TDF2

The way that geostationary television satellites can easily be used to perform common-view time transfer between remote clocks is presented. Different parameters and sources of error that have to be kept under control in order to achieve time transfer with a precision of 10 ns is reviewed. Two different methods are elaborated to eliminate the influence of satellite residual motion with a final precision that can reach 10 ns. Experimental results obtained on common-view time transfer measurements using the TV satellite TDF2 between four French laboratories show that frequency transfer can be performed with a stability of 1 part in 10¹⁴ for an averaging time of 10 days. The comparison of time transfer results with accurate common-view GPS data available between the four stations and with orbit data provided by the CNES showed measurement biases that are most likely due to errors in the antenna coordinates     

输入估计值的测量不确定度评定

本文所介绍的与1993年由ISO等7个国际组织联合发表的《测量不确定表示指南》（简称《指南》）完全一致，只是指南上的实用于物理测量的绝大多数领域，而本文介绍的仅适用于校准实验的测量。同时本文主要侧重于对输入估计值的A类测量不确定度和B类测量不确定度的评定的介绍，为输出估计值的测量不确定度评定奠定了良好的基础。     

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 (1σ). 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     

Problems in estimating some timing uncertainties of commercialfrequency and time standards

An end user cannot define the different timing errors of current cesium or GPS clocks properly. Conventional satellite-based techniques are able to detect long-term drifts only larger than 2 ns/day, and momentary receiver time deviations of the order of 100 ns can he expected with no practical improvement due to averaging unless extended to one day which precludes a medium-term analysis of local cesium performance. Bias-type errors, reliably found after a three-month test run and seriously hampering synchronization, with less costly user equipment show a typical offset of 200 ns. The generally applied test periods of 20 to 50 days, particularly for drift analysis, seem short since slow 10 ns to 20 ns fluctuations of the timing difference, lasting 20 days, are found. Many laboratories overlook environmental factors during long-term stability tests and thus unreliable results are produced. For a careful analysis of current commercial cesium clocks several colocated high-quality references are needed     

Carrier-phase time transfer

We have conducted several time-transfer experiments using the phase of the GPS carrier rather than the code, as is done in current GPS-based time-transfer systems. Atomic clocks were connected to geodetic GPS receivers; we then used the GPS carrier-phase observations to estimate relative clock behavior at 6-minute intervals. GPS carrier-phase time transfer is more than an order of magnitude more precise than GPS common view time transfer and agrees, within the experimental uncertainty, with two-way satellite time-transfer measurements for a 2400 km baseline. GPS carrier-phase time transfer has a stability of 100 ps, which translates into a frequency uncertainty of about two parts in 10(-15) for an average time of 1 day.     

Simple Time and Frequency Dissemination Method Using Optical Fiber Network

This paper describes a simple and cost-effective method of frequency dissemination. In current digital communication networks, node clocks are hierarchically synchronized to the atomic master clock through fiber links. This synchronized network is used as an intermediate link for remote calibration services like the global positioning system common-view method. A prototype reference signal generator has been developed for recovering the communication clock signal and synthesizing a 10-MHz signal from it. The generator output frequency at the client site can be traced to coordinated universal time (UTC) National Metrology Institute of Japan (NMIJ) with some uncertainty, depending on the stability of the node clocks and the distance from the master clock. The stability performance of the generated reference signal has been tested at Okinawa-the farthest prefecture from Tokyo, where the master clock is located (baseline distance of 1500 km). The primary rate (1.544 MHz) for telecommunication services was chosen for the 10-MHz signal generation in the experiment. A sinusoidal phase fluctuation within a one-day period is dominantly observed. This fluctuation is mainly caused by fiber expansion and contraction due to normal daily temperature changes. It degrades the stability (Allan deviation) to the level of 5 X 10^-13 (t = 40 000 s, which is almost half a day). However, the major part of the phase fluctuation can be canceled by averaging a full day's data. In this case, the Allan deviation becomes 1 X 10^-13, which is obtained at Okinawa over ten consecutive days of measurement. The worst average frequency offset relative to UTC (NMIJ) (one-day averaging) is -6.3 X 10^-13. The results indicate that this method promises to be suitable for most applications, providing an uncertainty of less than 1 X 10^-12 at an averaging time of one day.     

INRIM Tool for Satellite Clock Characterization: Frequency Drift Estimation and Removal

In Global Navigation Satellite Systems (GNSS), atomic clocks are fundamental for their excellent stability. Being the distance measured from the time, any error on the measure of time leads to an error in the positioning: accurate and stable atomic clocks need to be employed on board satellites. Hence, the on board clock behaviour has to be continuously monitored and any malfunctioning has to be immediately detected. In this work, we illustrate a software tool developed at the National Institute of Metrological Research (INRIM) for GNSS clock characterization and monitoring. In particular we focus on the functionality of frequency drift estimation and removal, including the uncertainty evaluation. Actually, the frequency drift evaluation and the monitoring of its evolution over time is extremely important in GNSS applications to ensure the adequacy of the timing system to the integrity requirements of the positioning service. The software has been optimized for space clock data, which are different from the ones from timing laboratories, since often present missing data and outliers. The tool allows to easily handle satellite clock data, and get a quick estimate and graphic representation of clock key parameters, such as the clock frequency drift.     

An Automated Time-Keeping System

The fully automated time-keeping system of the Van Swinden Laboratory (VSL) is described. The microprocessor-based system compares and measures clocks and time signals at programmed fixed times. Thus a flexible system is created, which makes it possible to carry out a large number of measurements over short periods and to process the measurement data automatically. For this purpose a series of special devices have been constructed. The uncertainty in the whole system is of the order 1 ns.     

European two-way satellite time transfer experiments using theINTELSAT (VA-F13) satellite at 307°E

A two-way satellite time and frequency transfer (TWSTFT) experiment performed between six European laboratories is reported. Transfers were made on a regular basis over a period of six months. TWSTFT was demonstrated to be a useful method to compare a relatively large number of atomic clocks internationally, as an alternative to the method of common view of global positioning system (GPS) satellites. The GPS common view time transfer method and TWSTFT method were compared over the same link for a period of 150 days. Three TWSTFT's have been performed simultaneously. Time transfers were performed over periods of up to 8 h, enabling the measurement noise of the TWSTFT system and the stability of the atomic clocks to be examined. Time transfers were made in consecutive successions between clocks at three distant Earth stations. Closing errors of up to approximately 4 ns were observed. The magnitude of these closing errors justified further examination. The uncertainties in the determination of the closing errors have been examined along with their origin, characteristics, and dependence on operating parameters     

Mathematical Model of a Territorially Distributed Collective Time and Frequency Standard

A method for combining remote atomic clocks into one collective network is described. A synthesized model gives the uncertainty of the collective standard as a function of the frequency instability versus the number of clocks in the network and the measuring time. The results of an investigation of that model are reported.