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Optical clocks have reached such an impressive accuracy and stability that the future redefinition of the second will be probably based on an optical transition. Consequently, building time scales based on optical clocks has become a key problem. Unfortunately, optical clocks are still laboratory prototypes and are not yet capable of long times of autonomous operation. It is hence critical to understand the impact of this limited optical clock availability on the generated time scale. In this work, after describing a simple and effective optical time scale algorithm, based on the steering of a flywheel oscillator towards the optical clock, we investigate in detail the impact of the limited availability of the optical clock on the performances of the steering algorithm and of the generated time scale through numerical simulations. In particular, we simulate a time scale generated by a hydrogen maser (with a flicker floor of 5.5 × 10−16) steered towards an optical clock, by considering six different scenarios for the availability of the latter, spanning from the ideal one, i.e. continuous operation of the optical clock, to the worst one, i.e. non-uniformly distributed frequency measurements with long unavailability periods. The results prove that the steering algorithm is robust and effective despite its very simple implementation, and it is capable of very good performances in all the considered scenarios, provided that the hydrogen maser behaves nominally. Specifically, they show that a time scale with an accuracy of a few hundreds of picoseconds can be easily realized in the ideal scenario, whereas in a more realistic scenario, with one measurement per week only, the time accuracy is nonetheless of a few nanoseconds, competing with the best time scales currently realized worldwide. The performances degradation due to a non-nominal maser behaviour is also discussed.

Recently, the informatics infrastructure of INRiM Time and Frequency Laboratory has been completely renewed with particular attention to network security and software architecture aspects, with the aims to improve the reliability, robustness and automation of the overall set-up. This upgraded infrastructure has allowed, since January 2020, a fully automated generation and monitoring of the Italian time scale UTC(IT), based on dedicated software developed in-house [1]. We focus in this work on the network and software aspects of our set-up, which enable a robust and reliable automatic time scale generation with continuous monitoring and minimal human intervention.

TWSTFT has been one of the most popular techniques used for comparison of remote atomic clocks. It typically employs the code-phase of a signal modulated by a pseudorandom noise (PN) code sent and received by microwave link via a geostationary telecommunications satellite [1]. The National Institute of Information and Communications Technology (NICT) has developed a Two-Way Carrier-Phase (TWCP) satellite frequency transfer technique [2] and successfully integrated it with PN code-phase for time transfer in a new modem called Software Ranging System (SRS) [3]. Three SRS modems have been deployed to Istituto Nazionale di Ricerca Metrologica (INRiM / IT), LNE-SYRTE (OP) and Physikalisch-Technische Bundesanstalt (PTB) for experimentation in comparison of local realization of Coordinated Universal Time (UTC(k), where k is the laboratory responsible for the generation of the time scale). We will present preliminary code-phase-based and carrier-phase-based results for the links between UTC(IT), UTC(OP) and UTC(PTB). A comparison with other satellite time and frequency transfer techniques currently available at all sides is carried out, i.e. TWSTFT link using SATRE modems and using Software-Defined Radio (SDR) receivers [4]. We find that TWSTFT with SRS modems achieve promising results of about 3e-15 at 1-day averaging time using code-phase measurements. [1] “The operational use of two-way satellite time and frequency transfer employing pseudorandom noise codes”, Recommendation ITU-R TF.1153-4 (08/2015) [2] “Carrier-phase two-way satellite frequency transfer over a very long baseline”, M Fujieda et al 2014 Metrologia 51 253 [3] “Development of a new digital TWSTFT modem”, Miho Fujieda, Ryo Tabuchi and Tadahiro Gotoh, International Journal of Electrical Engineering, Vol.27, No.4 pp.141-145 (2020) [4] “Use of software-defined radio receivers in two-way satellite time and frequency transfers for UTC computation”, Zhiheng Jiang et al 2018 Metrologia 55 685

We present two different approaches to broadcasting information to retrieve the GNSS-to-GNSS time offsets needed by users of multi-GNSS signals. Both approaches rely on the broadcast of a single time offset of each GNSS time versus one common time scale instead of broadcasting the time offsets between each of the constellation pairs. The first common time scale is the average of the GNSS time scales, and the second time scale is the prediction of UTC already broadcast by the different systems. We show that the average GNSS time scale allows the estimation of the GNSS-to-GNSS time offset at the user level with the very low uncertainty of a few nanoseconds when the receivers at both the provider and user levels are fully calibrated. The use of broadcast UTC prediction as a common time scale has a slightly larger uncertainty, which depends on the broadcast UTC prediction quality, which could be improved in the future. This study focuses on the evaluation of two different common time scales, not considering the impact of receiver calibration, at the user and provider levels, which can nevertheless have an important impact on GNSS-to-GNSS time offset estimation.

When observed from the ground, the frequency of the atomic clocks flying on the satellites of a Global Navigation Satellite System is referred to as apparent frequency, because it is observed through the on-board signal generation chain, the propagation path, the relativistic effects, the measurement system, and the clock estimation algorithm. As a consequence, the apparent clock frequency is affected by periodic variations of different origins such as, for example, the periodic component of the J2 relativistic effect, due to the oblateness of the earth, and the clock estimation errors induced by the orbital estimation errors. We present a detailed characterization of the periodic variations affecting the apparent frequency of the Galileo clocks, obtained by applying time–frequency analysis and other signal processing techniques on space clock data provided by the European Space Agency. In particular, we analyze one year of data from three Galileo Passive Hydrogen Masers, flying on two different orbital planes. Time–frequency analysis reveals how the spectral components of the apparent frequency change with time. For example, it confirms that the amplitude of the periodic signal due to the orbital estimation errors depends on the angle between the sun and the orbital plane. Moreover, it allows to find a more precise estimate of the amplitude of the J2 effect, in agreement with the prediction of the general theory of relativity, and it shows that such amplitude suddenly decreases when the corresponding relativistic correction is applied to the data, thus validating the analytical formula used for the correction.

Recently, the official Italian time scale UTC(IT) has been successfully automatically generated in real-time, with a steering algorithm using Rapid UTC (UTCr) as steering reference [1]. This work aims to present the results of an on-line test of other two steering algorithms: one using the INRIM cesium fountain (ITCsF2) [2], and another one using an ensemble of various commercial clocks, as steering references alternative to UTCr [3].

Optical clocks have reached an impressive accuracy, surpassing by orders of magnitude that of microwave clocks, so that the optical transitions on which they are based can be considered as ideal candidates for a possible future redefinition of the second. For these reasons, it is fundamental to develop an effective way to generate time scales based on optical clocks. However, optical clocks are still far from matching the robustness, reliability and long times of autonomous operation typical of microwave clocks. Therefore, the usual way to generate a time scale based on an optical clock, is to use it as a frequency steering reference for a master clock [1], typically an Active Hydrogen Maser (AHM). The master clock is usually referred to as a flywheel oscillator, since it allows the generation of the time scale also when the optical clock is unavailable. It is hence important to understand the impact of the unavailability of the optical clock on the performances of the steering algorithm and of the generated time scale. For example, it is fundamental to understand the minimum availability of the optical clock data needed to guarantee a given level of performances. To this aim, we simulated a time scale steered with an optical clock by considering several possible scenarios for the availability of the latter. By modifying the steering algorithm described in [2], we simulated the measurement data of the frequency offset of an AHM with respect to an optical clock, and we used them to steer the AHM for the time scale generation. Then, we tested the algorithm by considering six different scenarios for the availability of the optical clock, spanning from the ideal one (continuous operation of the optical clock), to the worst one (non-uniformly distributed frequency measurements with long unavailability periods). We also considered a realistic scenario where the optical clock is operated for a few hours once a week, with the possibility of a jitter on the day of the week. Finally, we evaluated the performances by considering the phase offset of the steered time scales with respect to UTC, and we compared and discussed the results obtained within the different scenarios. The results prove that the steering algorithm is robust and effective despite its very simple implementation. As expected, the scenarios giving the best and worst performances are the ideal one and those with long unavailability periods, respectively. More interestingly, the realistic scenario, with one measurement per week only, gives results similar to the ideal scenario ones. This is remarkable, as it means that, even with a much smaller effort dedicated to the optical clock, the final performances of the time scale are still close to the optimal case. This project 18SIB05 ROCIT has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme. References [1] H. Hachisu, F. Nakagawa, Y. Hanado and T. Ido, “Months-long real-time generation of a time scale based on an optical clock,” Scientific Reports, (2018) 8:4243. [2] L. Galleani, G. Signorile, V. Formichella and I. Sesia, “Generating a real-time time scale making full use of the available frequency standards,” Metrologia 2020, in press.

Many timing laboratories employ the Two-Way Satellite Time and Frequency Transfer (TWSTFT) technique for the comparison of their high accuracy atomic clocks and time scales and contribute to the Coordinated Universal Time (UTC). Most laboratories use the SATRE (SAtellite Time and Ranging Equipment) modems developed by TimeTech, which is currently the sole European manufacturer of TWSTFT modems. Some laboratories also participate in the 2016 BIPM pilot study on the Software-Defined Radio (SDR) receiver for TWSTFT which was proven to reduce the diurnal variation seen on “traditional” SATRE links. In this work, we developed a prototype for transmitting TWSTFT signal which is compatible with both SATRE modems and SDR receivers and present a preliminary assessment of its compatibility and performance.

The INRIM Time Laboratory is in charge of different important tasks. In this paper the Laboratory and related activities description will be provided. Some improvements have been introduced in the Laboratory during the last year, and the most relevant ones will be hereafter explained. An overview of all the Laboratory institutional, scientific and contractual activities will be also addressed.

The automated generation of a robust real-time time scale with high accuracy, stability, availability and reliability is of crucial importance for several applications. Among them, the local realizations of the Coordinated Universal Time (UTC) by national laboratories, UTC(k), or the generation of the reference time of a Global Navigation Satellite System (GNSS). In this work, we describe the current status and the developments of the generation of the Italian national time scale, UTC(IT), by reporting on the results of a 5-month experiment and on the recent implementation of the new automated procedure for the generation of UTC(IT).

The generation of a real-time time scale with high accuracy, stability, availability and reliability is a very challenging and demanding task. Here, we describe a method applied for the generation of the Italian national time scale UTC(IT), based on the realization of two independent time scales (master and backup) which are then aligned at a high level of accuracy, so that it is possible to switch between the two at any time without discontinuities. This work is mostly focused on the alignment strategy and on the methodology adopted for monitoring and preserving the quality of the final output real-time time scale. Results of a preliminary test in real-time, based on a fully automated process, are also reported and discussed.

Time transfer techniques have been playing a crucial role in time metrology since they allow comparison of oscillators all around the world, contributing to the generation of the Coordinated Universal Time (UTC) at the International Bureau of Weights and Measures (BIPM). The Common View (CV) technique using GPS satellites [1] has been used since the eighties for this purpose to compare local realizations of UTC at timing laboratories (namely UTC(k)); later on GLONASS satellites were also included. Currently the most used technique is the All-in-View (AV) technique [2]. Such technique relies on measurements of all satellites in view of a GNSS receiver connected to the local UTC(k), which are included in a standard file called CGGTTS [3] [4]. Currently with many GNSS being operational, an extension to the standard called CGGTTS V2E [5] has been published to include not only GPS and GLONASS, but also Galileo, Beidou and QZSS; the latter three are still not yet officially used in the frame of UTC and TAI computation [6]. This work presents a MATLAB software developed at INRIM, capable of processing different version of CGGTTS files and to compute the time offset between remote laboratory clocks (baselines) using AV technique. Preliminary results obtained using GPS, GLONASS (GLO), Galileo (GAL) and Beidou (BDS) are reported.

Many timing laboratories are currently employing Two-way Satellite Time and Frequency Transfer technique to compare their respective time scales and contribute to the Coordinated Universal Time calculated monthly by the Bureau International des Poids et Mesures (BIPM). It is not unusual to implement multiple TWSTFT stations or install multiple modems to ensure redundancy. Some SATRE (SAtellite Time and Ranging Equipment) modems, which are widely used in this context, are manufactured with expansion slots for multiple signal receiving (RX) modules, increasing the capability for redundancy. In this case, it would be useful to characterize the different delays introduced between modems and between their RX modules, if applicable, in order to switch devices in the event of a module or system failure while maintaining service continuity and calibration results. This work presents the preliminary measurement results of the delays of various SATRE modems used: #079 and #416 at INRIM and #077 and #321 at LNESYRTE (OP).

The robustness and reliability of a reference time scale is becoming more and more crucial in many applications, particularly in global navigation satellite systems (GNSS) and in critical infrastructures with time synchronization needs. This paper proposes two steering algorithms aimed at generating a real-time time scale, the first based on a primary frequency standard (PFS), and the second on an ensemble of clocks. Extensive tests on real data from the Italian National Metrology Institute (INRIM) cesium fountain ITCsF2 and atomic clocks have been carried out. The results show that the PFS algorithm has higher performances, but it requires a cesium fountain, a prototype standard currently available in a few timing laboratories only, whereas the ensemble clock algorithm uses commercially available clocks. A special emphasis is put on the system robustness: data pre-processing and possible combinations of the proposed algorithms have been tested in order to cope with outage periods, while maintaining good performances in terms of stability and accuracy of the resulting time scale. Such algorithms can be used in several applications where a stable time reference is needed, such as for the generation of a local real-time realization of UTC, as well as for any GNSS reference time.

The frequency of atomic clocks flying on board the satellites of global navigation satellite systems, as can be observed on the ground, shows periodic variations of different origins. One of them is related to the J2 relativistic effect due to the Earth oblateness, whose amplitude is now measurable thanks to stable clocks like Galileo’s passive hydrogen masers. Orbital estimation errors are also translated into periodic frequency variations, whose amplitude changes in time and which are superimposed to the J2 signal. In this work we analyze the data of a Galileo satellite clock by using two different techniques, with a twofold objective: to characterize the most evident periodic variations affecting the apparent clock frequency; to estimate the amplitude of the J2 relativistic effect for a comparison with its theoretical value. This also represents a validation of the J2 correction that should be applied to the space clocks’ data, in order to improve the timekeeping/positioning performance of global navigation satellite systems.

Galileo disseminates the GPS to Galileo Time Offset (GGTO) through the navigation message. This broadcast offset allows the user of a combined GSP/Galileo receiver to achieve a unique navigation solution by using measurements from both GPS and Galileo satellites. Nowadays, many positioning systems are available, the topic of interoperability is therefore crucial. The time offsets between the different systems should therefore be made available to the multi-GNSS users. An alternative would be that each constellation broadcasts only the time offset between its time scale and a reference common to all GNSS. In this paper we propose two alternative for, for this reference: either it can be based on the combination of the different GNSS time scales, or it can be directly UTC, as predicted and broadcast by the different systems. This paper presents a first assessment of the performances that can be achieved with each of these two approaches.

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.

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.

International audience; Following a pilot study initiated by the CCTF WG on TWSTFT and coordinated by the BIPM, since the summer of 2016 several laboratories have been testing a new type of TWSTFT receiving equipment based on a Software Defined Radio. This type of receiver has been developed with the aim of mitigate some of the time transfer instabilities observed in the TWSTFT links based on SATRE modem. One of the major feature of the SDR receiver is the low AM-PM conversion, thank to the particular design of the software correlator. At present the SDR system is only used as a receiver in parallel with the regular TWSTFT SATRE system and the investigation is mainly focused on the instability improvement. Nevertheless, considering that in the frame of the TAI network the TWSTFT links are mostly used for their accuracy, it is useful to develop a calibration method for such devices to allow a direct comparison with the traditional links, and to investigate the long term instability.

The Allan variance is the key measure for stability analysis, a fundamental tool to establish the performances of atomic clocks, which are often critical elements in many applications, such as global navigation satellite systems (GNSS). In some applications, only frequency measurements with numerous missing data are available. Because of missing data, the resulting Allan variance can be dramatically different from the expected stability behavior. We have developed an Allan variance estimator which corrects this divergence. We show the effectiveness of our estimator by applying it to frequency measurements with up to 94.4% of missing data.

DEMETRA, the 'DEMonstrator of Egnss services based on Time Reference Architecture', is an European Project funded by the European Union in the Horizon 2020 program. The main aim of the project was to develop and test a prototype of Time Service Provider based the European Global Navigation Satellite System (GNSS) providing innovative or improved time services based on different technologies and embedded in a modular architecture. The main innovative features are time certification, non-repudiation, integrity, reliability, resilience. The demonstrator has been developed and tested during the years 2015-2016. Nine different time services were developed and tested also in real environment at users premises. The paper reports the results of the experimental campaigns and the main lesson learned also by the trials with potential users.

The H2020 research project, DEMETRA (DEMonstrator of EGNSS services based on Time Reference Architecture) is funded by the European Union and aims to develop and test several time services based on the European Global Navigation Satellite System (GNSS) and introducing important new features such as certified time stamping, improved accuracy, or integrity, not yet provided by GNSS systems. The demonstrator has been successfully assembled at INRIM premises by the end of February 2016, and it will be tested with two validation test campaigns. The first closed loop test finished in May 2016, the first experimental results to validate the time services performances are here reported. The second test campaign will be conducted in a realistic case at user premises, to test the real advantages and feasibility of the proposed time services.

DEMETRA aims to be a prototype of an European time dissemination service, based on the timing signal of the European Galileo system, adding particular features like - among the others - certification, calibration, or integrity, that could be of interest to a wide range of users belonging to different sectors as traffic control, energy distribution, finance, telecommunication, and scientific institutions. Nine time dissemination services are proposed. The paper will report about the development status and the first experimentation results of the system, showing potentialities and limits of the proposed time dissemination services, aiming to foster the exploitation of the European GNSS for timing applications.

In the rubidium atomic frequency standard (RAFS), an rf-discharge lamp produces the device's atomic signal. As a consequence of the light-shift effect, variations in the lamplight's intensity result in variations in the RAFS' output frequency. While the basic physics of the light-shift is reasonably well understood, its operational implications for GNSS performance are only beginning to be fully appreciated. Here, we summarize results examining decade-long histories of on-orbit GPS RAFS lamplight variations. In addition to a deterministic variation that is well modeled by the sum of an exponential and linear trend, the stochastic variations appear to be of three types: lamplight jumps, repeating lamplight ramps, and non-stationary lamplight noise. Employing an estimated GPS RAFS light-shift coefficient, these variations can be used to infer lamplight-induced RAFS frequency variations.

In the rubidium atomic frequency standard (RAFS), an rf-discharge lamp produces the device's atomic signal. As a consequence of the light-shift effect, variations in the lamplight's intensity result in variations in the RAFS' output frequency. While the basic physics of the light-shift is reasonably well understood, its operational implications for global navigation satellite system (GNSS) performance is only beginning to be fully appreciated. Here, we describe first results examining decade-long histories of on-orbit GPS RAFS lamplight variations and GPS RAFS frequency variations. Our preliminary analyses have focused on one space vehicle's RAFS, and our conclusions are tempered by that present limitation. Nevertheless, our analyses suggest that a RAFS' long-term frequency stability (i.e., τ 106 sec) is likely lower-bounded by the lamp's intensity fluctuations. Moreover, considering the light-shift coefficient for this one particular RAFS over 12 years, we find that the data do not support Camparo's hypothesis regarding RAFS frequency aging and a time-varying light-shift coefficient.

Due to its small size, low weight, and low power consumption, the Rb atomic frequency standard (RAFS) is routinely the first choice for atomic timekeeping in space. Consequently, though the device has very good frequency stability (rivaling passive hydrogen masers), there is interest in uncovering the fundamental processes limiting its long-term performance, with the goal of improving the device for future space systems and missions. The ac Stark shift (i.e., light shift) is one of the more likely processes limiting the RAFS' long-term timekeeping ability, yet its manifestation in the RAFS remains poorly understood. In part, this comes from the fact that light-shift induced frequency fluctuations must be quantified in terms of the RAFS' light-shift coefficient and the output variations in the RAFS' rf-discharge lamp, which is a nonlinear inductively-couple plasma (ICP). Here, we analyze the light-shift effect for a family of 10 on-orbit Block-IIR GPS RAFS, examining decade-long records of their on-orbit f...

The Galileo In-Orbit Validation Element (GIOVE) is an experiment led by the European Space Agency (ESA) aimed at supporting the on-going implementation of Galileo, the European global navigation satellite system (GNSS). Among the objectives of the GIOVE Mission are the validation and characterization of the on-board clock technologies. The current baseline technologies for on-board clocks are the rubidium atomic frequency standard (RAFS) and the passive hydrogen maser (PHM). Both technologies have been validated and qualified on ground and are now being further validated in a representative in-orbit environment aboard 2 spacecrafts, GIOVE-A and GIOVE-B. This paper presents the results obtained in the frame of the GIOVE experimentation. The behavior and performances of the clock technologies on board both spacecrafts has been investigated and analyzed in terms of operation, frequency stability, and clock prediction error after more than 3 years of operation for GIOVE-A and almost one year for GIOVE-B. In addition, relativistic frequency shifts of GIOVE spacecrafts have been investigated.

The ability of the Allan variance (AVAR) to identify and estimate the typical clock noise is widely accepted, and its use is recommended by international standards. Recently, a time-varying version called Dynamic Allan variance (DAVAR) was suggested and exploited.Currently, the AVAR is commonly used in applications to space and satellite systems, in particular in monitoring the clocks of the Global Positioning System, and also in the framework of the European project Galileo. In these applications stability estimation, either AVAR or DAVAR (or other similar variances), presents some peculiar aspects which are not commonly encountered when the clock data are measured in a laboratory. In particular, data from space clocks may typically present outliers and missing values. Hence, special attention has to be paid when dealing with such experimental measurements.In this work we propose an estimation algorithm and its implementation in a robust software code (in MATLAB® language) able to estimate the AVAR in the case of missing data, unequally spaced data, outliers, and with long periods of missing observation, so that the Allan variance estimates turn out unbiased and with the maximum use of all the available data.

The Allan variance (AVAR) is widely used to measure the stability of experimental time series. Specifically, AVAR is commonly used in space applications such as monitoring the clocks of the global navigation satellite systems (GNSSs). In these applications, the experimental data present some peculiar aspects which are not generally encountered when the measurements are carried out in a laboratory. Space clocks’ data can in fact present outliers, jumps, and missing values, which corrupt the clock characterization. Therefore, an efficient preprocessing is fundamental to ensure a proper data analysis and improve the stability estimation performed with the AVAR or other similar variances. In this work, we propose a preprocessing algorithm and its implementation in a robust software code (in MATLAB language) able to deal with time series of experimental data affected by nonstationarities and missing data; our method is properly detecting and removing anomalous behaviors, hence making the subsequent stability analysis more reliable.

DEMETRA (DEMonstrator of EGNSS services based on Time Reference Architecture) is a project co-funded by the EU through the research and innovation programme 'Horizon 2020'. The DEMETRA project started in January 2015 and lasts 24 months. Based on the current practice of national metrological laboratories, identifying the additional necessities of the timing users, DEMETRA will define and develop a prototype of a European time disseminator, based on EGNSS. Nine different time services are proposed for demonstration by consortium partners. The DEMETRA partnership, including Scientific Institutions, GNSS Industries, and a Service Provider cover the different faces of the project, including an analysis of commercial potential in terms of market and business development.

A preliminary test to base the generation of the Italian standard time UTC(IT) also on a primary Cesium fountain has been carried out and the results are reported also in comparison to a steering strategy based only on UTC or rapid UTC data. In addition, the results of the first months of a hardware implementation of the fountain steering algorithm currently in operation in INRIM is also reported. This work represents the first step towards the realization of UTC(IT) with a complete set of primary and commercial frequency standards.

International audience; In the Galileo FOC phase (Full Operational Capability), GMV is the prime contractor for the Time and Geodetic Validation Facility (TGVF), a contract of the European Space Agency (ESA). Within the TGVF, the Time Validation Facility (TVF) is the subsystem in charge of steering Galileo System Time (GST) to UTC, among other duties. The new TVF is operated at GMV headquarters in Madrid, Spain. TVF operations rely on the contribution of five European timing laboratories, located at INRiM, OP, PTB, ROA, and SP. This paper provides a general description of the TVF element and its related activities for the FOC phase, and presents the main results and findings of the TVF operation until now.

The National Institute of Metrological Research (INRIM) in Italy has been deeply involved in the last 15 years in the development of the timing system of the European Satellite Navigation System Galileo, collaborating with the European metro-logical time laboratories, space industries, and the European Space Agency. In 2010–2013, INRIM designed, developed and operated the Time Validation Facility (TVF), a crucial timing element used for the In Orbit Validation (IOV) Galileo phase carried out with the first 4 satellites in 2013.

Timekeeping is crucial in Global Navigation Satellite Systems (GNSS), being the positioning accuracy directly related to a time measurement. As a consequence, the typical expertise of time metrology laboratories is necessary in many different aspects of a navigation system. This paper presents the experience of INRIM in the development of the Galileo navigation system from the earlier studies till the very recent In Orbit Validation phase showing how the time metrology practice has been useful in the understanding of time aspects of navigation and showing as well how the navigation service perspective has stimulated new ideas and better understanding of time measures.

In Global Navigation Satellite Systems (GNSS) the role of atomic clocks is essential for the determination of the user position and due to their excellent stability they are fundamental to ensure the adequate performances required in the positioning service. Since a degradation of the clock stability would immediately impact the navigation performances, the on board clock behaviour has to be continuously monitored. In this work, we illustrate a software tool developed at INRIM, optimized for GNSS clock characterization and monitoring. Although some commercial software are available for clock characterization, special attention has to be paid when dealing with space clocks. In fact, the space clock behaviour analysis may be complicated by different aspects: data from satellite clocks often present gaps and outliers as well as periodic fluctuations. On the contrary, data from timing laboratories are equally spaced and usually not affected by many outliers. Hence, often, the typical methods for clock characterization currently used in timing laboratories are not sufficient for space applications: new approaches, more suitable for GNSS applications, are required. To overcome these limitations, we developed the Clock Analysis Tool: a software, developed in Matlab, for clock characterization to be used either in metrological laboratories or for space applications. To fulfill GNSS needs, the software has been enhanced and extended, including new routines and functionalities of particular interest when characterizing the space clock behaviour, such as the dynamic stability analysis, as well as the addition of the uncertainty evaluation in the clock parameter estimation, or the inclusion of the system noise estimate in the frequency stability analysis. Moreover, the algorithms commonly used for clock characterization, such as the Allan Deviation computation and the frequency drift estimation, have been adapted to deal with space clock features, namely data gaps and outliers, treating missing data with the best possible procedure to allow the estimate of the stability over long observation intervals. In addition, the software has been provided with a graphical user interface, allowing an easy handling of satellite clock data and permitting a quick estimate and graphic representation of the clock key parameters.

The Galileo In-Orbit Validation Element (GIOVE) is an experiment led by the European Space Agency (ESA) aimed at supporting the on-going implementation of Galileo, the European Global Navigation Satellite System (GNSS). Among others, the objectives of the GIOVE Mission are the validation and characterization of the on-board clock technologies. The today baseline technologies for on-board clocks are the Rubidium Atomic Frequency Standard (RAFS) and the Passive Hydrogen Maser (PHM). Both technologies have been validated and qualified on ground and are now being further validated in a representative in-orbit environment on-board two spacecrafts, GIOVE-A and GIOVE-B. This paper presents the results obtained on both spacecrafts, after more than three years (GIOVE-A) and almost one year (GIOVE-B) of operation.

Within the GIOVE Mission (GIOVE-M), two experimental satellites called GIOVE-A and GIOVE-B have been launched by the European Space Agency. This paper analyses the different issues involved in GPS/GIOVE interoperability for positioning and timing, including GGTO (the GPS to Galileo time offset) and timing biases, and presents practical experience and results related to EGGTO, the GIOVE-M experimental version of GGTO broadcast within the GIOVE navigation messages.

The dynamic Allan variance (DAVAR) method has been recently proposed as an extension of the classical Allan variance, with the aim of defining the instantaneous stability of an atomic clock. It is in fact known that atomic clocks may undergo sudden failures, are influenced by environmental factors, and they eventually age and stop working. Hence the stability of an atomic clock may vary with time, and in some demanding applications, such as navigation systems, a quick identification of clock failures is of the utmost importance. Working on the first preliminary data coming from the Galileo System Test Bed (GSTB) V2, some criticalities were encountered in the DAVAR evaluation and its graphical representation, when dealing with long periods of missing data. A new estimation method has been developed, allowing to evaluate the DAYAR in case of missing data, not equally spaced data and with large periods of missing observation. The new estimation method is presented here. The experimental results obtained by applying the improved DAVAR on the Galileo experimental satellite are also shown. Also experimental data obtained by using the two-way satellite time and frequency rransfer (TWSTFT) method are analyzed and discussed.

The experimental precise timing station generating the reference time scale for the first experimental phase of the Galileo system is described, together with the first experimental results.

In this paper the noise characterization of the GPS receivers (both timing and geodetic) hosted at the Time and Frequency Laboratory of the Istituto Elettrotecnico Nazionale (IEN) "Galileo Ferraris" was taken into account. In particular, a specific algorithm as well as two different approaches (a classical GPS "common view" technique and a geodetic- based Precise Point Positioning, PPP) employed to process the data coming from such receivers were described. Moreover, the paper focused on the preliminary timing performance of the PPP geodetic method by comparing them with the International GPS Service (IGS) estimates on the link between two European metrological institutes, such as IEN and PTB (Physikalisch-Technische Bundesanstalt), Braunschweig, Germany.

International audience; During 2014 (June 5th to August 17th), a campaign intended for the calibration of the time transfer links at the nanosecond uncertainty level established via Two-Way Satellite Time and Frequency Transfer (TWSTFT) between the Galileo Precise Timing Facilities PTF1 and PTF2, and five UTC(k) laboratories associated to the Galileo Full Operational Capability Time and Geodetic Validation Facility (hereinafter referred to as UTC(k)_labs) was performed across Europe. This calibration exercise was performed coinciding with the deployment and operations (FOC) phase of the Galileo Program, according to the Time Validation Facility (TVF) Statement of Work, issued by the TVF industrial prime GMV, Tres Cantos, Spain. In their statements of work, all UTC(k) agreed to participate in the necessary calibration campaigns. The calibration involved a travelling TWSTFT station subcontracted to TimeTech, GmbH, Stuttgart, Germany. This paper presents part of the results, specifically those calibration results for links between these involved UTC(k) laboratories: INRIM, OP, PTB, ROA, SP. As a result of the visit of five UTC(k) laboratories (besides the two Galileo Control Centers, and the fixed station at TimeTech), after about 10000 km of distance travelled, and 600 hours of effective measurement, the calibration report includes the results of 10 time links using the site method. This paper also includes the calibration results for links between the laboratories previously mentioned and USNO (in total 4 links) using the triangle closure analysis method. For theses links new CALR values were provided, based on the triangles UTC(k)_labs ? PTB and PTB ? USNO (calibration by means of the USNO portable X-band TWSTFT station, in June 2014), respectively. The final report of this calibration campaign has been used as an example (Annex I) of the TWSTFT Calibration Guidelines for the UTC Time Links (v3.0) (available on BIPM website, open area of CCTF WG on TWSTFT). The TGVF contract is being carried out under a programme of and funded by the European Union, and managed by ESA. The views expressed herein can in no way be taken to reflect the official opinion of the European Union and/or ESA.

Recently an European Consortium of 15 partners from 7 different countries, started to work on a research project, DEMETRA (DEMonstrator of EGNSS services based on Time Reference Architecture), funded by the European Union in the frame of the Horizon 2020 program, aiming to develop and experiment time services based on the European Global Navigation Satellite System (GNSS). DEMETRA aims to be a prototype of an European time dissemination service, based on the timing signal of the European Galileo system, adding particular features like certification, calibration, or integrity, that could be of interest to some specific users like traffic control, energy distribution, finance, telecommunication, and scientific institutions.