https://iopscience.iop.org/article/10.1088/1681-7575/ab1e68/pdf

The Coordinated Universal Time (UTC)

The relationship and the mutual benefits of timekeeping and Global Navigation Satellite Systems (GNSS) are reviewed, showing how each field has been enriched and will continue to progress, based on the progress of the other field. The role of GNSSs in the calculation of Coordinated Universal Time (UTC), as well as the capacity of GNSSs to provide UTC time dissemination services are described, leading now to a time transfer accuracy of the order of 1–2 ns. In addition, the fundamental role of atomic clocks in the GNSS positioning is illustrated. The paper presents a review of the current use of GNSS in the international timekeeping system, as well as illustrating the role of GNSS in disseminating time, and use the time and frequency metrology as fundamentals in the navigation service.

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Precise time scales and navigation systems: mutual benefits of timekeeping and positioning

Two-way satellite time and frequency transfer (TWSTFT) is a primary technique for the generation of coordinated universal time (UTC). At present, more than 12 timing laboratories around the world use SAtellite Time and Ranging Equipment (SATRE) modems in TWSTFT operations and contribute data for the realization of UTC. The advantages of TWSTFT are its small calibration uncertainty (⩽1.0 ns if the link is calibrated with a TWSTFT mobile station) and its long-term link stability. However, the precision of SATRE TWSTFT in the operational networks is degraded by a daily variation pattern (diurnal) in the TWSTFT results. The diurnal with varying amplitude appears virtually in all SATRE TWSTFT links. The observed peak-topeak variation of the diurnals can reach 2.0 ns in some cases. So far, studies on the sources of the diurnal have not provided conclusive understanding of the diurnal’s dominant origin. Therefore, efforts have been made to reduce the impact of the diurnal variation in TWSTFT for UTC computation. The BIPM has been using the combination of SATRE TWSTFT results and GPS carrier-phase precise point positioning solutions (GPSPPP) for UTC computation since 2010. The combination adjusts the GPSPPP results to long-term averages of TWSTFT and is effectively free from the diurnal variations because the GPSPPP results contain almost no diurnal. Lately, the use of software-defined radio receivers (SDR) in TWSTFT has shown one way of how to reduce the diurnal variations by a factor of two to three in most of the inner-continental SATRE TWSTFT links, and furthermore, how the short-term stability for all UTC SDR TWSTFT links can be improved. In addition, there has been research on the full use of the redundancy in the TWSTFT network to improve the TWSTFT link stability. Recent studies on evaluating indirect links revealed that it is possible to apply a simplified procedure to use the redundancy, in a most effective way, to reduce the diurnal variations in the Europeto-Europe SATRE TWSTFT links by a factor of two to three. Based on these findings, we gained new insights about the diurnals and its dominant origin(s) which are discussed in this paper. The methods of the combination of SATRE TWSTFT and GPSPPP as well as the indirect SATRE TWSTFT links utilize the redundancy in the UTC time transfer network. SDR TWSTFT can largely reduce the diurnal in SATRE TWSTFT, but noticeable residual Metrologia Improving two-way satellite time and frequency transfer with redundant links for UTC generation

https://iopscience.iop.org/article/10.1088/1681-7575/aafced/pdf

Improving two-way satellite time and frequency transfer with redundant links for UTC generation

Two-way satellite time and frequency transfer (TWSTFT) is a primary technique for the generation of coordinated universal time (UTC). About 20 timing laboratories around the world continuously operate TWSTFT using satellite time and ranging equipment (SATRE19) modems for remote time and frequency comparisons in this context. The precision of the SATRE TWSTFT as observed today is limited by an apparent daily variation pattern (diurnal) in the TWSTFT results. The observed peak-to-peak variation have been found as high as 2 ns in some cases. Investigations into the origins of the diurnals have so far provided no complete understanding about the cause of the diurnals. One major contributor to the diurnals, however, could be related to properties of the receive part in the modem. In 2014 and 2015, it was demonstrated that bypassing the receive part and the use of software-defined radio (SDR) receivers in TWSTFT ground stations (SDR TWSTFT) instead could considerably reduce both the diurnals and the measurement noise.In 2016, the International Bureau of Weights and Measures (BIPM) and the Consultative Committee for Time and Frequency (CCTF) working group (WG) on TWSTFT launched a pilot study on the application of SDR receivers in the TWSTFT network for UTC computation.The first results of the pilot study were reported to the CCTF WG on TWSTFT annual meeting in May 2017, demonstrating that SDR TWSTFT shows superior performance compared to that of SATRE TWSTFT for practically all links between participating stations. In particular, for continental TWSTFT links, in which the strongest diurnals appear, the use of SDR TWSTFT results in a significant suppression of the diurnals by a factor of between two and three. For the very long inter-continental links, e.g. the Europe-to-USA links where the diurnals are less pronounced, SDR TWSTFT achieved a smaller but still significant gain of 30%. These findings are supported by an evaluation of some of the links with an alternate technique based on GPS signals (GPS IPPP) as reported in this paper.Stimulated by these results, the WG on TWSTFT prepared a recommendation for the 21st CCTF meeting, which proposed the introduction of SDR TWSTFT in UTC generation. With CCTF approval of the recommendation, a roadmap was developed for the implementation of SDR TWSTFT in UTC generation. In accordance with the roadmap, most of the stations that participated in the pilot study have updated the SDR TWSTFT settings to facilitate the use of SDR TWSTFT data in UTC generation. In addition, the BIPM conducted a final evaluation to validate the long-term stability of SDR TWSTFT links, made test runs using the BIPM standard software for the calculation of UTC, now including SDR TWSTFT data, and started to calculate SDR TWSTFT time links as backup from October 2017. The use of SDR TWSTFT in UTC generation will begin in 2018.

https://iopscience.iop.org/article/10.1088/1681-7575/aacbe6/pdf

Use of software-defined radio receivers in two-way satellite time and frequency transfers for UTC computation

A time scale is a procedure for accurately and continuously marking the passage of time. It is exemplified by Coordinated Universal Time (UTC) and provides the backbone for critical navigation tools such as the Global Positioning System. Present time scales employ microwave atomic clocks, whose attributes can be combined and averaged in a manner such that the composite is more stable, accurate, and reliable than the output of any individual clock. Over the past decade, clocks operating at optical frequencies have been introduced that are orders of magnitude more stable than any microwave clock. However, in spite of their great potential, these optical clocks cannot be operated continuously, which makes their use in a time scale problematic. We report the development of a hybrid microwave-optical time scale, which only requires the optical clock to run intermittently while relying upon the ensemble of microwave clocks to serve as the flywheel oscillator. The benefit of using a clock ensemble as the flywheel oscillator instead of a single clock can be understood by the Dick-effect limit. This time scale demonstrates for the first time subnanosecond accuracy over a few months, attaining a fractional frequency stability of 1.45 × 10-16 at 30 days and reaching the 10-17 decade at 50 days, with respect to UTC. This time scale significantly improves the accuracy in timekeeping and could change the existing time-scale architectures.

Optical-Clock-Based Time Scale.

Two-Way Satellite Time and Frequency Transfer (TWSTFT or TW for short) is one of the primary techniques for the realization of Coordinated Universal Time (UTC), which is computed by the International Bureau of Weights and Measures (BIPM). TWSTFT is carried out continuously by about 20 timing laboratories around the world. One limiting factor of the TWSTFT performance is the daily pattern (diurnals) in the TWSTFT data. Their peak-to-peak variations were observed as up to 2 ns in some extreme cases. They must be attributed to the equipment on ground or the satellite transponder, but no clear understanding has been achieved for some years. In 2014 and 2015, it was demonstrated inlinks between Asian stations that the use of Software-Defined Radio (SDR) receivers for TWSTFT could considerably reduce the diurnals and also the TWSTFT measurement noise.
In 2016, BIPM and the Consultative Committee for Time and Frequency (CCTF) working group (WG) on TWSTFT launched a pilot study on the application of SDR receivers in the Asia to Asia, Asia to Europe, Europe to Europe and Europe to USA TWSTFT networks. 
The very first results of the pilot study have been reported to the PTTI 2017. The results show, 1) for continental (short) links: SDR TWSTFT demonstrates a significant gain in reducing the diurnals by a factor of two to three; 2) for inter-continental (very-long) links: SDR TWSTFT displays a small gain of measurement noise at short averaging times; 3) SDR receivers show superior or at least similar performance compared with SATRE† measurements for all links. 
The CCTF WG on TWSTFT prepared a recommendation “On Improving the uncertainty of Two-Way Satellite Time and Frequency Transfer (TWSTFT) in UTC Generation” for the 21st CCTF meeting. One of the recommended items is to introduce the use of SDR TWSTFT in UTC generation. The recommendation was approved by CCTF during the meeting in June 2017.
In the current setups, a SDR receiver and the collocated SATRE modem receive the signals transmitted by remote SATRE modems, and the two devices independently determine the arrival time of the received signal. Thus, a few setup changes are necessary to implement SDR receivers into operational TWSTFT ground stations. On the other hand, the data computation, e.g. for calibrated time transfer, needs some caution in the data processing and provision, which are not trivial. Thus, an Ad hoc Group has been established to work out a procedure for the use of the SDR TWSTFT in UTC computation. 

This paper reports on the progress of the work for using SDR TWSTFT in UTC computation. Section 1 introduces SDR TWSTFT and the pilot study. Section 2 presents the considerations for using SDR TWSTFT in UTC generation. Section 3 and 4 show the methods for analysing SDR TWSTFT and the analysis results. Sections 5 and 6 discusses the further improvement of SDR TWSTFT and summarizes the work towards using SDR TWSTFT in UTC generation.

/pdf/implementation-of-sdr-twstft-in-utc-computation-5ala3y3gfh.pdf

Implementation of SDR TWSTFT in UTC Computation

A remote comparison campaign of Caesium fountain clocks from four National Metrology Institutes in Europe and Asia was carried out in May 2013. Six fountains at Physikalisch-Technische Bundesanstalt (PTB) Germany, Scientific Research Institute of Physical-Technical and Radio Technical Measurements (VNIIFTRI-SU) Russia, National Physical Laboratory of India (NPLI) India and National Institute of Metrology (NIM) China were compared by Two-Way Satellite Time and Frequency Transfer (TWSTFT) and GPS Carrier Phase (GPS CP) techniques. The frequency differences and the comparison uncertainties between the compared fountain pairs were evaluated. The comparison uncertainty (uc) results from combining the uncertainties of the two compared fountain clocks and the uncertainty introduced by the comparison link. All the frequency differences between the fountains agreed within the 1-sigma uncertainty in the low 10−15 level. We present in this paper the measurements and the evaluation of the campaign in details.

Comparison of Caesium fountain clocks in Europe and Asia

A new algorithm has been developed for the determination and elimination of jumps in the Melbourne–Wubbena combination, formed from the code and phase measurements in global navigational satellite systems. The algorithm more reliably, in comparison with analogous known algorithms, detects jumps in the data that are associated with time losses in the radio-receiving devices for phase capture of RF signals transmitted from satellites. Determination of the positions and values of jumps in a Melbourne–Wubbena combination is performed against the noise component and outliers. The algorithm was tested for actual measurements in global navigational satellite systems at two carrier frequencies.

Andrey Naumov

Papers

Use of software-defined radio receivers in two-way satellite time and frequency transfers for UTC computation

Implementation of SDR TWSTFT in UTC Computation

Comparison of Caesium fountain clocks in Europe and Asia

An Algorithm for Cycle-Slip Detection in a Melbourne–Wübbena Combination Formed of Code and Carrier Phase GNSS Measurements