François Lahaye

Experimental assessment of the time transfer capability of precise point positioning (PPP)

In recent years, many national timing laboratories have installed geodetic global positioning system (GPS) receivers together with their traditional GPS/GLONASS common view (CV) receivers and two way satellite time and frequency transfer (TWSTFT) equipment. A method called precise point positioning (PPP) is in use in the geodetic community allowing precise recovery of geodetic GPS receiver position, clock phase and tropospheric delay by taking advantage of the International GNSS Service (IGS) precise products. Natural Resources Canada (NRCan) has developed software implementing the PPP and a previous assessment of the PPP as a promising time transfer method was carried out at Istituto Elettrotecnico Nazionale (IEN) in 2003. This paper reports on a more systematic work performed at IEN and NRCan to further characterize the PPP method for time transfer application, involving data from nine national timing laboratories. Dual-frequency GPS observations (pseudorange and carrier phase) over the last ninety days of year 2004 were processed using the NRCan PPP software to recover receiver clock estimates at five minute intervals, using the IGS final satellite orbit and clock products. The quality of these solutions is evaluated mainly in terms of short-term noise. In addition, the time and frequency transfer capability of the PPP method were assessed with respect to independent techniques, such as TWSTFT, over a number of European and Transatlantic baselines

Further Characterization of the Time Transfer Capabilities of Precise Point Positioning (PPP): The Sliding Batch Procedure

In recent years, many national timing laboratories have installed geodetic Global Positioning System receivers together with their traditional GPS/GLONASS Common View receivers and Two Way Satellite Time and Frequency Transfer equipment. Many of these geodetic receivers operate continuously within the International GNSS Service (IGS), and their data are regularly processed by IGS Analysis Centers. From its global network of over 350 stations and its Analysis Centers, the IGS generates precise combined GPS ephemeredes and station and satellite clock time series referred to the IGS Time Scale. A processing method called Precise Point Positioning (PPP) is in use in the geodetic community allowing precise recovery of GPS antenna position, clock phase, and atmospheric delays by taking advantage of these IGS precise products. Previous assessments, carried out at Istituto Nazionale di Ricerca Metrologica (INRiM; formerly IEN) with a PPP implementation developed at Natural Resources Canada (NRCan), showed PPP clock solutions have better stability over short/medium term than GPS CV and GPS P3 methods and significantly reduce the day-boundary discontinuities when used in multi-day continuous processing, allowing time-limited, campaign-style time-transfer experiments. This paper reports on follow-on work performed at INRiM and NRCan to further characterize and develop the PPP method for time transfer applications, using data from some of the National Metrology Institutes. We develop a processing procedure that takes advantage of the improved stability of the phase-connected multiday PPP solutions while allowing the generation of continuous clock time series, more applicable to continuous operation/ monitoring of timing equipment.

GLONASS ambiguity resolution of mixed receiver types without external calibration

GLONASS processing from mixed receiver types is typically subject to unmodeled inter-frequency phase biases which prevent carrier phase ambiguity parameters from converging to integers. Receiver-dependent values have been proposed to mitigate the contribution of these biases, but are still subject to a number of issues, such as firmware updates. Recent studies have demonstrated that the origin of inter-frequency biases is a misalignment between phase and code observations, and could be calibrated to first order by manufacturers. In this contribution, a calibration-free method for GLONASS ambiguity resolution is presented in which ambiguities naturally converge to integers. A mandatory condition is that two GLONASS satellites with adjacent frequency numbers are observed simultaneously, although this condition can be relaxed once a fixed solution has been obtained. This approach then permits the integration of different receiver types and firmware versions into seamless processing.

Further Characterization of the Time Transfer Capabilities of Precise Point Positioning (PPP)

In recent years, many national timing laboratories (NMIs) have installed geodetic global positioning system (GPS) receivers together with their traditional GPS/GLONASS Common View (CV) receivers and two way satellite time and frequency transfer (TWSTFT) equipment. Many of these geodetic receivers operate continuously within the international GNSS service (IGS), and their data are regularly processed by IGS Analysis Centers. From its global network of over 350 stations and its Analysis Centers, the IGS generates precise combined GPS precise ephemeredes and station and satellite clock time series referred to the IGS Time Scale. A processing method called precise point positioning (PPP) is in use in the geodetic community allowing precise recovery of GPS antenna position, clock phase and tropospheric delays by taking advantage of the IGS precise products. Natural resources Canada (NRCan) has developed software implementing the PPP methodology. A previous assessment of PPP, as a promising time transfer method, was carried out at INRiM (formerly IEN) in 2003 [7], showing better stability over short/medium term than GPS CV and GPS P3 methods. Further analysis was carried out in 2005 [12] where, running continuously for period of up of two weeks, the NRCan PPP software was able to reduce the day-boundary discontinuities, allowing specific time-limited campaigns (PTFs). This paper reports on follow-on work performed at INRiM and NRCan to further characterize the PPP method for time transfer applications, involving some of the National Metrology Institutes considered in 2005. We take advantage of continuous PPP processing to develop a procedure to improve the continuity of solutions and to reduce the solution boundary discontinuities present in the daily PPP results.

IGS Combination of Precise GPS Satellite Ephemerides and Clocks

Since January 1, 1994, the Geodetic Survey Division (GSD) of Natural Resources Canada (formerly Energy Mines and Resources, EMR) has been combining and comparing the GPS satellite ephemerides and clock corrections produced by the seven Analysis Centres contributing to the International GPS Service for Geodynamics (IGS). The IGS ephemeris/clock combination is produced weekly and provides information on the internal and external consistency between results of the IGS Analysis Centres. 7-day orbital fits characterize the internal consistency whereas comparisons between properly re-aligned daily satellite orbit solutions provide the external validation; the statistics are summarized in weekly reports which, along with the combined orbits and the corresponding Earth Orientation Parameters (EOP), are available electronically from the IGS Global Data Centres. Improved weighting, editing and handling of multiple reference clock resets produced significantly improved clock combinations. Steady improvements of results by all Analysis Centres and other improvements of combination techniques have resulted in accuracies approaching 10 cm for IGS combined orbits and 1 ns for combined satellite clocks under Anti-Spoofing (AS).

Near real-time comparison and monitoring of time scales with precise point positioning using nrcan ultra-rapid products

This paper experimentally evaluates the assessment of precise point positioning (PPP) using the Natural Resources Canada (NRCan) Ultra-Rapid GPS products to serve as a short latency time-transfer tool to assist timing laboratories in operational maintenance of frequency standards and time scale dissemination. An automated data exchange and processing system has been set up to serve the international community for efficient, nearly real-time clock comparison and monitoring purposes.

Model comparison for GLONASS RTK with low-cost receivers

GLONASS ambiguity resolution in differential real-time kinematic (RTK) processing is affected by inter-frequency phase biases (IFPBs). Previous studies empirically determined that IFPBs are linearly dependent on the frequency channel number and calibration values have been derived to mitigate these biases for geodetic receivers. The corresponding IFPB-constrained model is currently the de facto approach in RTK, but the growing market of GNSS receivers, and especially low-cost receivers, makes calibration and proper handling of metadata a complex endeavor. Since IFPBs originate from timing offsets occurring between the carrier phase and the code measurements, we confirm other studies that show that IFPBs are not exactly linearly dependent on the frequency channel number, but rather linearly dependent on the channel wavelength, which calls for a modification in the GLONASS functional model. As an alternative to calibration, we revisit a calibration-free method for GLONASS ambiguity resolution and provide new insights into its applicability. A practical experiment illustrates that the calibration-free approach can offer better ambiguity fixing performance when the uncertainty on the IFPB parameter is large, unless partial ambiguity resolution is performed.

Precise Point Positioning

Since its introduction in 1997, precise point positioning (PPP ) offers an attractive alternative to differential global navigation satellite system (GNSS ) positioning. The PPP approach uses undifferenced, dual-frequency, pseudorange and carrier-phase observations along with precise satellite orbit and clock products, for standalone static or kinematic geodetic point positioning with centimeter precision. This chapter introduces the PPP concept and specifies the required models needed to correct for systematic effects causing centimeter-level variations in the satellite-to-user range. For completeness, models and methods for processing single-frequency GNSS data are presented and specific aspects of GLONASS (Global’naya Navigatsionnaya Sputnikova Sistema) and new GNSSs are also described. Furthermore, recent developments in fixing undifferenced carrier-phase ambiguities, which can considerably shorten or nearly eliminate the initial delay for PPP convergence, are highlighted. Existing web applications and real-time corrections services enabling post-mission and real-time PPP are presented. Finally, typical PPP precision and accuracy estimates are discussed, including the solution of station tropospheric zenith path delays and receiver clocks, with millimeter and nanosecond precision respectively.

Near real-time comparison of UTC(k)'s through a Precise Point Positioning approach

The time-transfer technique based on Precise Point Positioning (PPP) allows the comparison of atomic clocks with precisions at the level of hundred picoseconds. This paper presents the results obtained in near real-time using satellite orbit and clock information from IGS real-time streams and from the NRCan Ultra Rapid products (EMU). These results can be considered quite interesting for the National Metrology Institutes, providing a new tool for monitoring their UTC(k)s. Two different PPP software tools have been used in this study, namely “Atomium” developed and operated at ORB and the NRCan-PPP soon to be operated at INRIM and LNE-SYRTE-OP in near real-time mode. This paper presents a quantification of the degree of equivalence of the results obtained from the analysis conducted using each software as well as the operational web page available at the Royal Observatory of Belgium, providing PPP solutions for the comparison of UTC(k)s in near real-time.

PPP using NRCan Ultra Rapid products (EMU): Near real-time comparison and monitoring of time scales generated in time and frequency laboratories

The Precise Point Positioning (PPP) time-transfer technique requires the availability of precise estimates of GNSS satellite orbits and clock offsets. Several such products are available from the International GNSS Service (IGS), each having their own characteristics: robustness, update rate, latency and satellite clock offset time interval. The most frequently updated IGS products are the Ultra Rapid products, which are generated four times a day with a latency of three hours. Natural Resources Canada (NRCan) contributes its own Ultra Rapid GPS product to the IGS for combination. However, the underlying processes running at NRCan generate products much more frequently − 24 times a day — with a latency of 90 minutes, offering an opportunity for more timely time-transfer results when used in PPP. INRIM and NRCan hereby assess the potential of using the PPP with the NRCan Ultra Rapid GPS products to serve as a short latency time-transfer tool. A specific experiment has been set up, where the NRCan Ultra Rapid GPS products, as well as all currently available IGS products, are used in PPP time transfer between selected IGS stations collocated in timing laboratories. Results and relative merits are compared in light of their respective delivery and frequency stability characteristic, in view of designing an automated near real-time monitoring system to assist timing laboratories in operational maintenance of frequency standards and time scales dissemination to external users.