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Global navigation satellite system (GNSS) time transfer has been used for four decades in the generation of universal time coordinated (UTC), starting with GPS in the 1980s. Calibration of GNSS receivers is then essential to ensure that time links used in UTC are accurate and remain stable with time. In this paper we study the different calibration schemes based on relative calibration of GNSS receivers that have been operated to fulfill these goals. We specially focus on the current calibration scheme in operation since 2014 and based on a collaboration between the BIPM and the Regional Metrology Organizations. We show that the achieved accuracy and long-term stability of calibration results, thus of UTC time links, are well in line with the stated uncertainties. We study an extended set of absolute calibration results and show that they complete and validate the BIPM scheme, and that relative calibration is still the essential tool to ensure accurate time links for UTC.

Some models for Dark Matter (DM) suggest possible encounters with a period τ of macroscopic dark structures with the Earth, implying transient signatures on clocks onboard GNSS satellites. In this paper we first introduce the GASTON project, an original study dedicated to the search for such ‘DM transients’ using the network of passive H-Maser onboard Galileo satellites. This project is based on a 3-months measurements campaign carried out in the early 2021 where precise satellite clock measurements are supplemented by an intensive laser ranging (SLR) campaign on Galileo satellites. Then, we present preliminary results based on single satellite frequency jumps between consecutive 30s epochs over the 3 months campaign. In particular, we already bring strong constraints on the parameter space for transient DM objects from 105 to 109 km, a region which was never explored previously with GNSS clocks.

The goal of this study is to analyse the stability of the hardware delays of GPS and Galileo signals in multi-GNSS stations dedicated to time transfer. We first demonstrate a significant sensitivity of these delays to external environment (multipath, weather), up to 2 ns in case of snow when the antenna is not protected by a radome. Furthermore, we notice that the long-term stability of the Galileo E5a code and GPS C5 code measurements is poorer than for the other code measurements.

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.

The combined use of multi-GNSS systems in positioning, navigation and timing requires taking into account the variable time offset between the individual system times. This offset can be determined at the user level as an additional unknown in the position-velocity-time (PVT) solution when a sufficient number of satellites is visible. However, a known value of the inter-system bias can also be injected in the PVT algorithm. We investigate in which situation determining the Galileo-to-GPS-Time-Offset (GGTO) or fixing this parameter to a known value provides the best PVT solution and quantify the impact of a biased fixed GGTO value on the PVT solutions. Our results indicate that the recommendation on fixing or determining the GGTO, as well as the impact of a biased fixed GGTO value, heavily depends on the receiver noise level. In high visibility conditions, fixing the GGTO provides a similar or better solution than determining the GGTO if the accuracy of the fixed GGTO is better than 2 ns for a high precision receiver and 7 ns for a smartphone. Furthermore, an extreme bias of 20 ns on the fixed GGTO with respect to the true value induces an increase in the position or timing errors lower than 50% for a smartphone, while up to 150% for a high precision receiver. As a consequence, fixing the GGTO should be preferred in mass-market receivers, while determining the GGTO should be preferred in high precision receivers. The study also shows that there is no significant correlation between the superiority of determining or fixing the GGTO and either the dilution of precision or the number of visible satellites.

Intense solar radio bursts (SRBs) emitted at L-band frequencies are a source of radio frequency interference for Global Navigation Satellite Systems (GNSS) by inducing a noise increase in GNSS measurements, and hence degrading the carrier-to-noise density (C/N0). Such space weather events are critical for GNSS-based applications requiring real-time high-precision positioning.Since 2015, the Royal Observatory of Belgium (ROB) monitors in near real-time the C/N0 observations from the European Permanent Network (EPN). The monitoring allows to detect accurately the general fades of C/N0 due to SRBs over Europe as from 1 dB-Hz. It provides in near real-time a quantification of the GNSS signal reception fade for the L1 C/A and L2 P(Y) signals and notifies civilian single and double frequency users with a 4-level index corresponding to the potential impact on their applications. This service is part of the real-time monitoring service of the PECASUS project of the International Civil Aviation Organization (ICAO) which started end of 2019.Results of this 5-year monitoring will be discussed, including the 3 SRBs of 2015 and 2017, together with the new developments toward a global index using the International GNSS Service (IGS) network. In addition, we will show how the SRB monitoring is sometimes interfered by GPS flex power campaigns on the satellites from blocks IIR-M and IIF, and how it is mitigated . The routine and transient GPS flex power campaigns will be presented in terms of C/N0 variations for the EPN and IGS networks.

This paper introduces new tests of fundamental physics by means of the analysis of disturbances on the GNSS signal propagation. We show how the GNSS signals are sensitive to a space variation of the fine structure constant $\alpha$ in a generic framework of effective scalar field theories beyond the Standard Model. This effective variation may originate from the crossing of the RF signals with dark matter clumps and/or solitonic structures. At the macroscopic scale, the subsequent disturbances are equivalent to those which occur during the propagation in an inhomogeneous medium. We thus propose an interpretation of the "measure" of the vacuum permeability as a test of fundamental physics. We show the relevance of our approach by a first quantification of the expected signature in a simple model of a variation of $\alpha$ according to a planar geometry. We use a test-bed model of domain walls for that purpose and focus on the measurable time delay in the GNSS signal carrier., Comment: 15 pages, 6 figures

On top of positioning, Global Navigation Satellite Systems (GNSS) can be used for time determination and synchronization applications. The European GNSS Galileo is therefore good candidate to offer a dedicated Timing Service, with accuracy, robustness and integrity. In this frame the EGALITE (Galileo Timing Service Extension and Consolidation) project was launched in 2018. The EGALITE project was funded by the EC as part of the Horizon 2020 framework programme and aimed at studying the possibility of developing a EGNSS Timing Service based on Galileo with committed end-to-end performances. The project’s consortium is led by GMV with VVA, Physikalisch-Technische Bundesanstalt and Observatoire Royal de Belgique as partners. The aim of this paper is to provide a summary of the main outcomes of the activities developed in the frame of EGALITE project: • A brief description of the project objectives is provided, including the motivation for developing a EGNSS Timing Service and what is understood by Timing Service. • The integrity capability of the future potential EGNSS Timing Service is addressed including aspects previously mentioned: high-level architecture definition, receiver processing, operational concept, standardisation and certification aspects and Cost Benefit Analysis (CBA) results. • The design of a Legal Time Traceability Service is also proposed in the paper. As will be seen in the paper, we propose EGNSS Timing Services based on Galileo with committed end-to-end performances provided EGNSS Timing Receivers are certified.

We relate some models of new physics in the electromagnetic sector to the macroscopic ‘unit system’-free expression of Maxwell’s equations. We illustrate our method in the case of recently introduced models of domain wall dark matter. A new phenomenology for the propagation of GNSS signals is described and preliminary results show measurable effects for time and frequency transfer. Our results pave the way towards tests of fundamental physics using GNSS signals, in addition to the recently developed tests with atomic clocks.

Acquiring reliable estimates of the Antarctic Ice Sheet surface mass balance is essential for trustworthy predictions of its evolution and future contribution to sea level rise. Snow height variations, i.e., the net change of the surface elevation resulting from a combination of surface processes such as snowfall, ablation, and wind redistribution, can provide a unique tool to constrain the uncertainty on mass budget estimations. In this study, GNSS Interferometric Reflectometry (GNSS-IR) is exploited to assess the long-term variations of snow accumulation and ablation processes. Eight antennas belonging to the Polar Earth Observing Network (POLENET) network are considered, together with the ROB1 antenna, deployed in the east part of Antarctica by the Royal Observatory of Belgium. For ROB1, which is located on an ice rise, we highlight an annual variation of snow accumulation in April–May (~30–50 cm) and ablation during spring/summer period. A snow surface elevation velocity of +0.08 ± 0.01 ma−1 is observed in the 2013–2016 period, statistically rejecting the “no trend” null hypothesis. As the POLENET stations are all located on moving glaciers, their associated downhill motion must be corrected for using an elevation model. This induces an increased uncertainty on the snow surface elevation change determined from GNSS-IR. Among the eight stations analyzed, only three of them show a long-term snow height variation larger than the uncertainties. One is located on the Flask Galcier in the Antarctic Peninsula, with a decrease of more than 4 m between 2012 and 2014, with an uncertainty of 2.5 m. The second one is located on the Lower Thwaites Glacier where we observe, between 2010 and 2020, a snow surface drop of 10 m, with a conservative uncertainty of 1 m. The third station, located on the West Antarctic Ice Sheet (WAIS) divide, shows on the opposite an upward motion from 2005 to 2019, of 1.2 m with an uncertainty of 0.4 m. The snow surface change of the other POLENET stations analyzed is smaller than the uncertainty associated with the glacier slope.

The R2CGGTTS software tool developed at the Royal Observatory of Belgium (ROB) to provide clock solutions in the standard Common GNSS Generic Time Transfer Standard (CGGTTS) has been extended to BeiDou Navigation Satellite System (BDS). The BDS includes satellites in three different orbits: 1) Medium Earth Orbit (MEO); 2) Inclined Geosynchronous Satellite Orbit (IGSO); and 3) Geostationary Earth Orbit (GEO). This paper presents first results obtained with this upgraded software, and a comparison between common view (CV) time transfer solutions obtained with either BDS, or GPS or Galileo. These preliminary results indicate that the BeiDou MEO satellites give time transfer results with a higher noise than the GPS results. This additional noise is shown to be due to some elevation-dependent delay in the BDS code measurements. Some biases were furthermore pointed out between the CV results obtained with the different BeiDou MEO satellites when the receivers used in the two stations are of different make. These biases may reach some nanoseconds, and find most probably their origin in the receiver hardware or firmware. It is shown additionally that using the BeiDou IGSO satellites and the GEO satellites, although increasing the number of observations, especially in the Asia-Pacific region, introduces a significant time transfer noise in the CV results.

The paper 'A detailed comparison of two continuous GPS carrier-phase time transfer techniques' (Yao et al 2015 Metrologia 52 666) presents the revised RINEX-shift (RRS) method, a technique using 'classical precise point positioning (PPP)' solutions on sliding batches and aiming at providing continuous time links. The authors claim the superiority of the RRS technique with respect to 'classical PPP' in terms of frequency stability and solving for discontinuities due to data gaps. It is shown here that these conclusions do not rely on physical principles, and are erroneous as they are driven by misinterpreted or corrupted PPP solutions. Using state-of-the-art PPP computation on the same data sets used in Yao et al's paper (2015 Metrologia 52 666), we show that the stability of RRS is at best similar to that of 'classical PPP' (within statistical uncertainties). Furthermore, the RRS method of removing discontinuities in case of data gaps by interpolating the phase data should not be applied systematically as it can cause erroneous clock solutions when the data gaps are associated with a true phase discontinuity.

Absolute calibration procedures have been carried out for many years now, but they have only been focused on GPS signals used by the timing laboratories. In the framework of the “Accurate calibration of multi-system/multifrequency GNSS receiver chain” (AKAL) project, new procedure for calibration of Galileo, GPS and GLONASS signals have been put in place at the ESA Technology center ESTEC in The Netherlands. During the development of the absolute procedures, multiple effects have been considered which have not been previously reported in the literature, especially to identify additional elements that might affect the overall estimated uncertainty. In this paper, we present the results of the validation of this absolute calibration procedure, based on the use of real GNSS signals collected by different stations in different setups. The Validation activities were led by the Royal Observatory of Belgium and consisted in the installation in common clock and travelling of the reference receiver to different Group 1/Group 2 laboratories. The results show some discrepancies between stations in some of the signals, and possible explanations are presented by the authors. The general agreement with relatively calibrated stations from BIPM laboratories has a one nanosecond bias, which is deemed as very good, considering that the absolute calibration of the BIPM reference receiver was done almost 15 years ago.

A GNSS timing receiver can generate a time pulse aligned to GNSS System Time (GNSST) or to the predicted UTC transmitted by GNSS. Using GNSST or its associated UTC realization is enough for many synchronization and time stamping applications, but in some cases (for example in the financial sector) it is necessary to trace the timing solution to the legal time scale of the user’s country, which is normally a national UTC(k) laboratory as for example UTC(NIST) in the USA, UTC(PTB) in Germany, or UTC(ROA) in Spain. Traceability to a UTC(k) time scale can be achieved a-posteriori by means of Common-View (CV) time transfer between the receiver and the UTC(k) laboratory. This paper provides practical recommendations for CV traceability computations, focused on affordable, single-frequency timing receivers.

We formulate a new theoretical model which paves the way for methods of detection of particular types of dark matter using GNSS networks. Our model consists in a (topo-logically) massive version of Maxwell theory in a medium which admits topological defect solutions. Hence it describes hypothetical dark matter consisting in Earth-sized structures like monopoles, vortices or domain walls which could induce disturbances in the GNSS networks. In the future our model will then be constrained by GNSS measurements.

By the Galileo Initial Services Declaration on 15 December 2016, Galileo has officially started delivering services to users around the world. In addition to the positioning service, Galileo is able to disseminate an accurate time scale referring to UTC or providing to users interoperability with GPS and improving resilience. Galileo OS users are thus able to obtain a UTC SIS realization by applying the UTC conversion parameters to Galileo System Time (GST), broadcast in Galileo Open Service (OS) navigation message additional parameters, according to the GST-UTC conversion algorithm as described in the GALILEO ICD [1]. In order to monitor Galileo OS Time performance, the French Space Agency (CNES) which is involved in the performance monitoring from the early steps of the program has developed specific independent means using calibrated GPS/Galileo stations. In this context, three Key Performance Indicators (KPIs) are monitored: the offset between the Galileo System Time and UTC, the OS dual-frequency UTC dissemination accuracy and the Galileo to GPS Time Offset (GPGA) dissemination accuracy. This paper presents the developed monitoring means; explains the used approach for the KPIs computation and shows the results of one year of monitoring at CNES.

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.

This paper is presenting results from experiments aiming at quantifying the multipath effects on GNSS antennas in some moderate and extreme implementation cases. In Observatoire de Paris (OP), the multipath due to a metallic plate nearby the GNSS antenna might generate a relative calibration offset reaching about 3 ns on P-code delays. But proper Common-View (CV) software is able to get rid of such offsets, reaching P3 CV deviation mean values below 100 ps. At the Royal Observatory of Belgium (ORB), with three different receiver types, different antenna positions are showing that P-code offsets might reach about 350 ps in P1 and 750 ps in P2, hence a significant bias in relative calibration results. The results are showing that a more extensive work is required before trying to fix an upper limit for the multipath uncertainties in the guidelines uncertainty budget for GNSS relative calibration.

A software tool was developed at the Royal Observatory of Belgium for building the CGGTTS products from the RINEX observation and navigation files provided by the GNSS receivers. This software has been completely re-built in order to increase the robustness, the computation speed, and to include GPS, GLONASS, BeiDou and Galileo. The first part of the paper describes this new software tool. Then some time transfer experiments are realized for a short and a long baseline, comparing the results obtained from single-constellation with the multi-constellation solutions. We show that even with an incomplete constellation, Galileo already provides time transfer results with nearly the same quality as GPS for the short baseline, and 1.5 times better for the long baseline time link. Combining GPS and Galileo satellites furthermore improves the time transfer link with respect to the GPS-only or Galileo-only solutions, reducing the noise by a factor of about 1.4. Adding moreover BeiDou satellites in the combination gives a solution similar to the solution obtained from GPS+Galileo.

This paper presents a study of the consistency of the values used as reference for the hardware delays of GPS P1 and P2 for calibration in the TAI network. The inter-signal biases dP1-dP2 are computed from the pseudorange measurements, after applying an ionospheric delay correction, and then compared to the calibrated values. It is shown that the differences between the computed dP1-dP2 and the calibrated values are well inside the expected combined uncertainty. A similar conclusion is reached comparing the computed dP1-dP2 with absolutely calibrated stations. We also show that using a same antenna phase center for the calibration 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, but the final impact on the P3 combination is less than 10 ps. The computed dP1-dP2 are then also used to look at the stability of the inter-signal hardware delays of some GPS stations dedicated to time transfer. It is shown that only variations larger than 2 ns can be detected in the computed dP1-dP2 for a particular station, while variations of 200 ps can be detected when differentiating the results between two stations.

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.

Time and navigation are intimately linked and rely on each other. Global navigation satellite system (GNSS ) positioning is based on the measurement of time intervals needed by the signal to travel from satellites to the receiving station on the Earth or nearby. The precision of GNSS positioning is reached thanks to atomic frequency standards onboard the satellites and the possibility to determine their synchronization differences at the subnanosecond level. Time is thereby the core of GNSS. Inversely GNSS is widely used for accurate time and frequency dissemination, as well as for the comparison of distant clocks as needed for time and frequency metrology. All these aspects of using GNSS for time/frequency applications will be presented in this chapter.

Reflections of the GNSS signal around the antenna induce an error in the measurement of the satellite---receiver distance and therefore should be avoided as much as possible. One solution often used to mitigate these reflections is to apply radio frequency (RF) absorbing material to the antenna, its support or its site. Such material could however alter the antenna phase delay and, in turn, alter the position as calculated from the GNSS observations. We explain under which conditions the RF material will or will not alter the antenna phase delay, and hence in which conditions a re-calibration of the antenna is necessary after the installation of absorbing material. Furthermore, rules of thumb are given to install the material in such a way that re-calibration can be avoided. Some basic theory and measurements of the influence of RF material are reviewed. An application to a real life absorber setup similar to one of the International GNSS Service reference stations is then discussed, and the position offset due to the absorbing material is demonstrated. The topics discussed can serve station managers to limit effects of absorbing material and take precautions to avoid a position bias.

Using global navigation satellite system (GNSS) signals for accurate timing and time transfer requires the knowledge of all electric delays of the signals inside the receiving system. GNSS stations dedicated to timing or time transfer are classically calibrated only for Global Positioning System (GPS) signals. This paper proposes a procedure to determine the hardware delays of a GNSS receiving station for Galileo signals, once the delays of the GPS signals are known. This approach makes use of the broadcast satellite inter-signal biases, and is based on the ionospheric delay measured from dual-frequency combinations of GPS and Galileo signals. The uncertainty on the so-determined hardware delays is estimated to 3.7 ns for each isolated code in the L5 frequency band, and 4.2 ns for the ionosphere-free combination of E1 with a code of the L5 frequency band. For the calibration of a time transfer link between two stations, another approach can be used, based on the difference between the common-view time transfer results obtained with calibrated GPS data and with uncalibrated Galileo data. It is shown that the results obtained with this approach or with the ionospheric method are equivalent.

The time transfer technique based on Precise Point Positioning (PPP) has proved to be a very effective technique allowing the comparison of atomic clocks with a precision of a hundred picoseconds, and with latency of 2 days. Using satellite orbit and clock information from the IGS real-time products, it is now possible to compute very precise time transfer solutions based on PPP in near real-time, i.e., with a latency down to some minutes. We present PPP-based time transfer results obtained in near real time with the new version of the GNSS data processing software Atomium, and the satellite products mentioned above. We additionally analyze the results that can be obtained with low latency using the NRCan Ultra-Rapid products (EMU) available with a delay of 90---150 min. From the statistics of the results, it is concluded that the real-time IGS products allow detection of a clock jump larger than 1.5 ns after some minutes; the detection threshold falls down to 0.8 ns when using the EMU products about 2 h later. It is also shown that in near real time, a frequency change larger than 2e-14 can be detected when looking at the last 24 h data or larger than 2e-13 when looking at the last 2 h. As demonstrated, the quality of the monitoring depends on the distance between the two stations, so that distances shorter than about 3,000 km should be preferably used for a near real-time comparison of atomic clocks based on PPP.

This paper studies the use of multiple GNSS constellations for time transfer. The performances of each constellation are compared to one another. We show that, with its current constellation, Galileo already provides time transfer results with the same or better performances than GPS, for which some time links are affected by a diurnal variation. The paper also investigates different ways of combining the constellations into one global time transfer solution. We show that in a global AV solution, only one constellation should be calibrated, due to the inherent inter-system biases in the multi-GNSS clock products. Our results show that the best combination is to use the Galileo solution as a reference, and to combine the observations applying one bias (with respect to Galileo) per satellite of the other constellations. This allows reducing the impact of diurnal variations present in some GPS clock solutions. Combining Galileo and GPS satellites in this way improves the time transfer link with respect to the GPS-only or Galileo-only solutions. Adding moreover BeiDou-2 and GLONASS satellites gives, depending on the link, a solution similar or worse than the solution obtained from GPS and Galileo.

To better understand how receiver antenna calibration models contribute to GPS positioning error budget, we compare station positions estimated with different calibration models: igs05.atx, igs08.atx and individual antenna calibrations. First, the impact of switching from the igs05.atx antenna calibration model to the igs08.atx antenna calibration model is investigated using the EUREF Permanent Network historical data set from 1996 until April 2011. It is confirmed that these position offsets can be effectively represented by the igs05.atx to igs08.atx latitude-dependent model. Then, we demonstrate that the position offsets resulting from the use of individual calibrations instead of type mean igs08.atx calibrations can reach up to 1 cm in the up component, while in the horizontal, the offsets generally stay below 4 mm. Finally, using six antennas individually calibrated by a robot as well as in an anechoic chamber, we observe a position agreement of 2 mm in the horizontal component and a bias of 5 mm in the up component. Larger position offsets, dependent on the antenna/radome type, are, however, found when these individual calibrations are compared to type mean calibrations of two tested antennas.

GPS code measurements have been used for three decades for remote clock comparison, also called Time Transfer. Initially based on a technique using common-view (CV) single-frequency measurements, GPS time transfer now mostly uses dual-frequency measurements from geodetic receivers processed in all-in-view (AV). With the completion of the GLONASS constellation, it has been possible to readily use it in the CV single-frequency mode, providing results similar to GPS for short-distance time links. However GLONASS results are not readily equivalent to GPS in the dual-frequency AV mode, necessary for any moderate- to long-distance link, and this paper shows how to achieve this. We first present the GLONASS upgrade of the R2CGGTTS software, a tool to provide dual-frequency measurements in a format dedicated to time transfer named CGGTTS (Common GPS GLONASS Time Transfer Standard). The GLONASS navigation files are used to determine satellite clocks and positions, and dual-frequency pseudorange measurements are linearly combined to compute the CGGTTS results in a similar way as for GPS. In a second part, we present the combination of GPS and GLONASS into one unique time transfer solution based on AV. The results are first corrected using precise satellite orbit and clock products delivered by the IGS analysis centre ESOC, and characterized by the same reference for the GPS and GLONASS satellite clocks. Then, the need to introduce satellite-dependent hardware delays in GLONASS results is emphasized, and a procedure is proposed for their determination. The time transfer solutions obtained for GPS-only and GPS+GLONASS are then compared. The combination of GPS and GLONASS results in AV provides a time transfer solution having the same quality as GPS only. Furthermore, comparisons show that even when increasing the number of observations in CV thanks to the combination of the two constellations, the AV remains superior to the CV solution in terms of noise and short term stability, especially for long baselines.

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.

Using dual-frequency data from 36 GPS stations from the EUREF Permanent Network (EPN), the influence of the October 30, 2003 Halloween geomagnetic storm on kinematic GPS positioning is investigated. The Halloween storm induced ionospheric disturbances above the northern part of Europe and Scandinavia. It is shown that kinematic position repeatabilities for this period are mainly affected for stations in northern Europe with outliers reaching 12 cm in the horizontal, and 26 cm in the vertical. These magnitudes are shown to be possibly due to the second-order ionospheric delays on GPS signals, not accounted for in the kinematic GPS positioning analysis performed. In parallel, we generate hourly TEC (Total Electron Content) maps on a 1° × 1° grid using the dense EPN network. These TEC maps do not use any interpolation but provide a high resolution in the time and space and therefore allow to better evidence small structures in the ionosphere than the classical 2-hourly 2.5° × 5° grid Global Ionospheric TEC Maps (GIM). Using the hourly 1° × 1° TEC maps, we reconstruct and refine exactly the zones of intense ionosphere activity during the storm, and we show the correlation between the ionospheric activity and assess the quality of GPS-based kinematic positioning performed in the European region.

There were four 1.5-hour sessions of Division I business meetings during the XXVIIth IAU General Assembly. The first three were devoted to the reports of Commissions, Working Groups and services associated with the Division, discussion about plans for the next triennium and future structure of the Division. Scientific presentations on the future space astrometric mission Gaia were made at the fourth session.

This paper presents a time transfer method based on a least-square analysis of the GPS code and carrier-phase measurements in common view (CV), constrained by TWSTFT (TW hereafter) data. Combining GPS and TWSTFT considerably increases the robustness of the remote clock comparisons thanks to the complete independence between these two techniques. An additional goal of the combination (named CV + TW) is to provide a time transfer solution, which benefits from the high short-term stability and high resolution of the GPS data and from the high accuracy of the TW data. The final solution is then calibrated by the TW data rather than by the GPS pseudoranges. As a consequence, the large day-boundary discontinuities that can exist in GPS time transfer solutions disappear in the combined solution, and are replaced by smaller discontinuities the magnitude of which depends on the noise level of the TW link. For a very noisy link, we found jumps up to 1 ns, while for the very quiet link NIST–OP, they are all lower than 200 ps.

In high-precision geodetic time and frequency transfer, which requires precise modeling of code and carrier phase GPS data, the ionosphere-free combinations P 3 and L 3 of the codes and carrier phases, made on the two GPS frequencies, are used to remove the first-order ionospheric effect. We quantify the impact of the residual second- and third-order ionospheric effects on geodetic time and frequency transfer solutions for continental and intercontinental baselines. All time transfer computations are done using the ATOMIUM software, developed at the Royal Observatory of Belgium. In order to avoid contamination by some imperfect modeling of the second- and third-order ionospheric effects in the satellite clock products, only single-difference, common-view processing is used, based on code and carrier phase measurements. The results are shown for weak and strong solar activity, as well as for particular epochs of ionospheric storms. Second-order ionospheric delays can lead to corrections up to about 10 ps in the common-view clock solution of intercontinental baselines with very different longitudes. However, realistic values of the geomagnetic field in the ionosphere are required to assess the amplitude of second-order ionospheric effects in time and frequency transfer during an ionospheric storm.

To compute precise point positioning (PPP) and precise time transfer using GPS code and phase measurements, a new software named Atomium was developed by the Royal Observatory of Belgium. Atomium was also adapted to perform a phase-only analysis with the goal to obtain a continuous clock solution which is independent of the GPS codes. In this paper, the analysis strategy used in Atomium is described and the clock solutions obtained through the phase-only approach are compared to the results from the PPP mode. It is shown that the phase-only solution improves the stability of the time link for averaging times smaller than 7 days and that the phase-only solution is very sensitive to the station coordinates used. The method is, however, shown to perform better than the IGS clock solution in case of changes in the GPS receiver hardware delays which affects the code measurements.

Aleksander Brzezinski, P-C19 (Poland), Joseph A. Burns, P-C7 (USA), Pascale Defraigne, P-C31 (Belgium), Dafydd Wyn Evans, VP-C8 (UK) Toshio Fukushima, P-C4, PP (Japan), George H. Kaplan, VP-C4 (USA), Sergei A. Klioner, P-C52 (Germany), Zoran Knezevic, VP-C7 (Serbia) Irina I. Kumkova, P-C8 (Russia), Chopo Ma, VP-C19 (USA), Richard N. Manchester, VP-C31 (Australia), and Gérard Petit, VP-C52 (France)

When neglecting calibration issues, the accuracy of GPS-based time and frequency transfer using a combined analysis of code and carrier phase measurements highly depends on the noise of the GPS codes. In particular, the pseudorange noise is responsible for day-boundary discontinuities which can reach more than 1 ns in the time transfer results obtained from geodetic analysis. These discontinuities are caused by the fact that the data are analyzed in daily data batches where the absolute clock offset is determined by the mean code value during the daily data batch. This pseudorange noise is not a white noise, in particular due to multipath and variations of instrumental delays. In this paper, the pseudorange noise behavior is characterized in order to improve the understanding of the origin of the large day-boundary discontinuities in the geodetic time transfer results. In a first step, the effect of short-term noise and multipath is estimated, and shown to be responsible for only a maximum of 150 ps (picoseconds) of the day-boundary jumps, with only one exception at NRC1 where the correction provides a jump reduction of 300 ps. In a second step, a combination of time transfer results obtained with pseudoranges only and geodetic time transfer results is used to characterize the long-term evolution of pseudorange errors. It demonstrates that the day-boundary jumps, especially those of large amplitude, can be explained by an instrumental effect imposing a common behavior on all the satellite pseudoranges. Using known influences as temperature variations at ALGO or cable damages at HOB2, it is shown that the approach developed in this study can be used to look for the origin of the day-boundary discontinuities in other stations.

Precise point positioning (PPP) is a zero-difference single-station technique that has proved to be very effective for time and frequency transfer, enabling the comparison of atomic clocks with a precision of a hundred picoseconds and a one-day stability below the $1e-15$ level. It was, however, noted that for some receivers, a frequency difference is observed between the clock solution based on the code measurements and the clock solution based on the carrier-phase measurements. These observations reveal some inconsistency either between the code and carrier phases measured by the receiver or between the data analysis strategy of codes and carrier phases. One explanation for this discrepancy is the time offset that can exist for some receivers between the code and the carrier-phase latching. This paper explains how a code-phase bias in the receiver hardware can induce a frequency difference between the code and the carrier-phase clock solutions. The impact on PPP is then quantified. Finally, the possibility to determine this code-phase bias in the PPP modeling is investigated, and the first results are shown to be inappropriate due to the high level of code noise.

Precise Point Positioning (PPP) is a zero-difference single-station technique that has proved to be very effective for time and frequency transfer, enabling the comparison of atomic clocks with a precision of a hundred picoseconds and a one day stability below the 1e-15 level. It was however noted that for some receivers, a frequency difference is observed between the clock solution based on the code measurements and the clock solution based on the carrier phase measurements. These observations reveal some inconsistency between the code and carrier phases measured by the receiver. One explanation of this discrepancy is the time offset that can exist for some receivers between the code and carrier phase latching. This paper explains how a code-phase bias in the receiver hardware can induce a frequency difference between the code and the carrier phase clock solutions. The impact on PPP is then quantified. Finally, the possibility to determine this code-phase bias in the PPP modeling is investigated, and the first results are presented.

The classical time transfer method used to realize International Atomic Time (TAI) is based on the common view technique, with GPS observations collected by C/A code receivers. The resulting clock offsets between the laboratory clock and GPS time are obtained from a fixed procedure defined by the Consultative Committee for Time and Frequency (CCTF). A similar procedure can be applied to the Receiver INdependent EXchange (RINEX) observation files produced by geodetic receivers driven by a stable external frequency. If the link between the receiver clock and the external clock is stable and precisely determined, the geodetic receivers can then be used for time transfer to TAI. In that case, we propose some modifications to the CCTF procedure to adapt it for the links between geodetic receivers, in order to take advantage of the P codes available on L1 and L2. This new procedure forms the ionosphere-free combination of the P1 and P2 codes as given by the 30 s RINEX observation files, the standard of the International GPS Service. The procedure is tested using the Ashtech Z-XII3T geodetic receivers and the results are compared with those obtained with the classical CCTF procedure based on the C/A code by computing the fractional frequency stability (Allan deviation) of the time links. Over short baselines, the two techniques are equivalent, while the new technique provides a factor 2 improvement for a transatlantic time link. For time links between a time receiver and a geodetic receiver, the differential satellite delays (P1-C/A or P2-C/A) must additionally be introduced. We show here that these biases do not, however, alter the long-term (>3 days) stability of the time transfer results. The corrections associated with tidal station displacement are also investigated, and the results indicate that they do not significantly improve the results at the present level of precision.

The International Atomic Time scale (TAI) is computed by the Bureau International des Poids et Mesures (BIPM) from a set of atomic clocks distributed in about 40 time laboratories around the world. The time transfer between these remote clocks is mostly performed by the so-called GPS common view method: The clocks are connected to a GPS time receiver whose internal software computes the offsets between the remote clocks and GPS time. These data are collected in a standard formal called CCTF. In the present study we develop both the procedure and the software tool that allows us to generate the CCTF files needed for time transfer to TAI, using RINEX files produced by geodetic receivers driven by an external frequency. The CCTF files are then generated from the RINEX observation files. The software is freely available at ftp://omaftp.oma.be/dist/astro/time/RINEX_CCTF. Applied to IGS (International GPS Service) receivers, this procedure will provide a direct link between TAI and the IGS clock combination. We demonstrate here the procedure using the RINEX files from the Ashtech Metronome (ZXII-T) GPS receiver, to which we apply the conventional analysis to compute the CCTF data. We compared these results with the CCTF files produced by a time receiver R100-30T from 3S-Navigation. We also used this comparison with the results of a calibrated time receiver to determine the hardware delay of the geodetic receiver. © 2001 John Wiley & Sons, Inc.