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Journal ArticleDOI

Combining GPS and GLONASS in all-in-view for time transfer

31 May 2013-Metrologia (IOP Publishing)-Vol. 50, Iss: 3, pp 277-287
TL;DR: 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.
Abstract: 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.
Citations
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Journal ArticleDOI
TL;DR: The associated extended standard, named CGGTTS for Common GNSS Generic Time Transfer Standard, and the corresponding Version 2E of the format are details.
Abstract: The standard for GNSS time transfer was first defined in 1984, associated to the use of GPS signals, which were at that time degraded by the Selective Availability. It was updated at a few instances to follow the evolution of GPS, of the receivers, and the inclusion of GLONASS. With the emergence of additional navigation systems like Galileo, BeiDou, QZSS, the standard has to be further adapted. This paper prepared by the CCTF Working Group on GNSS Time Transfer details the associated extended standard, named CGGTTS for Common GNSS Generic Time Transfer Standard, and the corresponding Version 2E of the format.Full paperThe full paper is available online at: http://metrologia.bipm.org/guides-stds-conventions/2015/G1.pdf

83 citations


Cites methods from "Combining GPS and GLONASS in all-in..."

  • ...The latest version provides CGGTTS files in Version 02, for both GPS and GLONASS satellites [8], starting from RINEX files version 2....

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Journal ArticleDOI
TL;DR: A receiver clock offset model is presented that considers the correlation of the receiver clock offsets between adjacent epochs using an a priori value and concludes that all RT-PPP solutions with different real-time products are capable of time transfer.
Abstract: Thanks to the international GNSS service (IGS), which has provided an open-access real-time service (RTS) since 2013, real-time precise point positioning (RT-PPP) has become a major topic in the time community. Currently, a few scholars have studied RT-PPP time transfer, and the correlation of the receiver clock offsets between adjacent epochs have not been considered. We present a receiver clock offset model that considers the correlation of the receiver clock offsets between adjacent epochs using an a priori value. The clock offset is estimated using a between-epoch constraint model rather than a white noise model. This approach is based on two steps. First, the a priori noise variance is based on the Allan variance of the receiver clock offset derived from GPS PPP solutions with IGS final products. Second, by applying the between-epoch constraint model, the RT-PPP time transfer is achieved. Our numerical analyses clarify how the approach performs for RT-PPP time and frequency transfer. Based on five commonly used RTS products and six IGS stations, two conclusions are obtained straightforwardly. First, all RT-PPP solutions with different real-time products are capable of time transfer. The standard deviation (STD) values of the clock difference between the PPP solutions with respect to the IGS final clock products are less than 0.3 ns. Second, the STD values are reduced significantly by applying our approach. The reduction percent of STD values ranges from 4.0 to 35.5%. Moreover, the largest improvement ratio of frequency stability is 12 as compared to the solution of the white noise model. Note that the receiver clock offset from IGS final clock products is regarded as a reference.

53 citations

Journal ArticleDOI
TL;DR: A new CP approach based on a combination of GPS, BeiDou (BDS), and Galileo satellite systems is proposed, and the mathematical model for the obtained unique time transfer solution is discussed.
Abstract: The carrier-phase (CP) technique based on the Global Navigation Satellite System (GNSS) has proved to be a useful spatial tool for remote and precise time transfer. In order to improve the robustness and stability of the time transfer solution for a time link, a new CP approach based on a combination of GPS, BeiDou (BDS), and Galileo satellite systems is proposed in this study. The mathematical model for the obtained unique time transfer solution is discussed. Three GNSS stations that can track GPS, BeiDou, and Galileo satellites were used, and two time links are established to assess the performance of the approach. Multi-GNSS time transfer outperforms single GNSS by increasing the number of available satellites and improving the time dilution of precision. For the long time link, with a geodetic distance of 7537.5 km, the RMS value of the combined multi-system solution improves by 18.8%, 59.4%, and 35.0% compared to GPS-only, BDS-only, and Galileo-only, respectively. The average frequency stability improves by 12.9%, 62.3%, and 36.0%, respectively. For the short time link, with a geodetic distance of 4.7 m, the improvement after combining the three GNSSs is 6.7% for GPS-only, 52.6% for BDS-only, and 38.2% for Galileo-only.

53 citations


Cites methods from "Combining GPS and GLONASS in all-in..."

  • ...Therefore, the root mean square (RMS) value for the smooth result is always employed [13]....

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Journal ArticleDOI
TL;DR: A receiver clock offset model, considering the correlation of the receiver clock offsets between adjacent epochs using an a priori value, is employed to improve multi-GNSS PPP time and frequency (scheme2), which outperforms single GNSS by increasing the number of available satellites and improving the time dilution of precision
Abstract: Thanks to the international GNSS service (IGS), which has provided multi-GNSS precise products, multi-GNSS precise point positioning (PPP) time and frequency transfer has of great interest in the timing community. Currently, multi-GNSS PPP time transfer is not investigated with different precise products. In addition, the correlation of the receiver clock offsets between adjacent epochs has not been studied in multi-GNSS PPP. In this work, multi-GNSS PPP time and frequency with different precise products is first compared in detail. A receiver clock offset model, considering the correlation of the receiver clock offsets between adjacent epochs using an a priori value, is then employed to improve multi-GNSS PPP time and frequency (scheme2). Our numerical analysis clarify how the approach performs for multi-GNSS PPP time and frequency transfer. Based on two commonly used multi-GNSS products and six GNSS stations, three conclusions are obtained straightforwardly. First, the GPS-only, Galileo-only, and multi-GNSS PPP solutions show similar performances using GBM and COD products, while BDS-only PPP using GBM products is better than that using COD products. Second, multi-GNSS time transfer outperforms single GNSS by increasing the number of available satellites and improving the time dilution of precision. For single-system and multi-GNSS PPP with GBM products, the maximum improvement in root mean square (RMS) values for multi-GNSS solutions are up to 7.4%, 94.0%, and 57.3% compared to GPS-only, BDS-only, and Galileo-only solutions, respectively. For stability, the maximum improvement of multi-GNSS is 20.3%, 84%, and 45.4% compared to GPS-only, BDS-only and Galileo-only solutions. Third, our approach contains less noise compared to the solutions with the white noise model, both for the single-system model and the multi-GNSS model. The RMS values of our approach are improved by 37.8–91.9%, 10.5–65.8%, 2.7–43.1%, and 26.6–86.0% for GPS-only, BDS-only, Galileo-only, and multi-GNSS solutions. For frequency stability, the improvement of scheme2 ranges from 0.2 to 51.6%, from 3 to 80.0%, from 0.2 to 70.8%, and from 0.1 to 51.5% for GPS-only, BDS-only, Galileo-only, and multi-GNSS PPP solutions compared to the solutions with the white noise model in the Eurasia links.

33 citations

Journal ArticleDOI
TL;DR: In this paper, a comprehensive investigation of China's BDS-3 real-time PPP with Centre National d'Etudes Spatiales (CNES) realtime products (SSRA00CNE0) from the perspective of time transfer, positioning, and tropospheric delay retrieval was conducted.

30 citations

References
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Proceedings ArticleDOI
28 May 1980
TL;DR: This method is the subtracting the received times of arrivals as measured by clocks A and B at the two sites while simplest and least accurate.
Abstract: Summary First, Clock A and a GPS receiver are used to Even though the GPS is primarily a navigation deduce from a GPS sate1 1 ite' s ephemeris, from system, if two clocks at known coordinates A and B clock .A's location, and from received GPS time are in common-view of a single GPS satellite, receivers at these two clock sites may coinciden- decoded from the same satellite, the time differtally receive transmitted GPS clock times. By ence (Clock A - GPS time). This method is the subtracting the received times of arrivals as measured by clocks A and B at the two sites while simplest and least accurate (estimated to be compensating for the propagation delays, one has better than about 100 ns with respect to GPS an accurate measure of the time difference between time),2 but has global coverage, is in the receiveclock A and clock B. When all of the error contributions are only mode, requires no other data, yields receiver

350 citations


"Combining GPS and GLONASS in all-in..." refers methods in this paper

  • ...Measurements from Global Navigation Satellite Systems (GNSS) have been used since the 1980s [1] to perform precise and accurate time and frequency transfer (TFT)....

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  • ...As at the beginning, GNSS data were collected from onechannel receivers, the initial approach for time transfer was based on common view (CV hereafter) [1], i....

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Journal ArticleDOI
TL;DR: Modifications to the CCTF procedure are proposed to adapt it for the links between geodetic receivers, in order to take advantage of the P codes available on L1 and L2, and the results indicate that they do not significantly improve the results at the present level of precision.
Abstract: 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.

130 citations

Journal ArticleDOI
TL;DR: By comparing the GPS CV and AV with the independent and more accurate TW and PPP time transfer techniques, the gain that can be obtained on a given time link is quantified.
Abstract: Since the 1980s, GPS time links have been essential to the TAI computation and, until 2006, the Common View (CV) technique has been used for this purpose. Recent advances in obtaining precise satellite orbits and clock parameters now permit us to obtain better results using another technique, which we name All in View (AV). By comparing the GPS CV and AV with the independent and more accurate TW and PPP time transfer techniques, we quantify the gain that can be obtained on a given time link. The AV technique also allows us to choose a more efficient network of GPS links between the tens of laboratories participating in TAI, which further improves the uncertainty in the access to UTC. The BIPM TAI software has been updated and the AV technique has been effectively used since the computation for the month of September 2006.

113 citations

Journal ArticleDOI
TL;DR: The principles of the GNSS Common-View and All in View time transfers are recalled and the technical issues for the use of GLONASS in UTC are presented, i.e. short- and long-term stabilities, frequency biases, calibration and its practical implementation.
Abstract: At present, the applicable spatial techniques used in UTC (Coordinated Universal Time) computation are GPS, TWSTFT (Two-Way Satellite Time and Frequency Transfers) and GLONASS. To enable accuracy and robustness for the generation of UTC, a multi-technique strategy for UTC time transfer is indispensable. Over the last two decades efforts have been made to use GLONASS for accurate time transfer. The first GLONASS time link that presents in UTC was introduced in November 2009, BIPM Circular T 263.For present and future accurate time transfers, GLONASS is comparable to GPS with the same types of observations. In this paper, we first recall principles of the GNSS Common-View and All in View time transfers; we present the technical issues for the use of GLONASS in UTC, i.e. short- and long-term stabilities, frequency biases, calibration and its practical implementation. Finally, we outline the prospects for the use of GLONASS in accurate time transfer.

43 citations