Carine Bruyninx

Enhancement of the EUREF Permanent Network Services and Products

This paper describes the EUREF Permanent Network (EPN) and the efforts made to monitor and improve the quality of the EPN products and services. It is shown that the EPN is becoming a multi-GNSS tracking network and that the EPN Central Bureau and the Analysis Centers are preparating to include the new satellite signals in their routine operations.

GPS Time and Frequency Transfer: PPP and Phase-Only Analysis

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.

PPP and Phase-only GPS Time and Frequency transfer

The Royal Observatory of Belgium (ROB) developed the software Atomium to perform GPS-based time and frequency transfer. Originally dedicated to perform Precise Point Positioning (PPP) based on a combined analysis of dual-frequency carrier phase and code measurements, Atomium has recently been adapted to allow a phase-only analysis, providing a continuous solution 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 continuous solution improves the stability of the time link for averaging times smaller than 7 days, but that the phase-only solution is drifting with respect to the combined code-carrier phase solution; this drift is station-dependent.

Regional densification of the IGS in europe using the EUREF permanent GPS network (EPN)

Abstract In 1995 EUREF took the initiative to coordinate the activities related to existing permanent GPS stations in Europe and created the EUREF Permanent Network (EPN) for the maintenance of the European Terrestrial Reference System (ETRS89). Presently, the data from most of the EPN stations is available within a 24-hour delay and additionally one third of the stations provides hourly data. The performance evaluation of the EPN data flow indicates that the reliability of the hourly data flow and the development of fallback procedures in case of unreachable data centres are requiring further improvements. Half of the EPN stations belong to the IGS network and as a consequence, improvements of latency, availability and reliability within EUREF will benefit the IGS too. Each of the twelve analysis centres process part of the EPN and obtain weekly estimated coordinates and their covariance info. The combination of these individual solutions constitutes the EPN weekly combination. The quality of this solution is in average 1.6, 1.7 and 4.9 mm for the North, East and up components, respectively. A multi-year combination of the weekly EPN solutions was submitted to IERS for inclusion into the ITRF2000. As a first accuracy assessment this solution was compared to the ITRF2000 contribution of CODE and showed to be consistent with the internal precision of the weekly solutions. The establishment of an EPN Coordination Group, in June 2000, allows the continuation of an efficient management of the expanding GPS tracking network and the growing array of related multi-disciplinary projects.

Noise and Periodic Terms in the EPN Time Series

The EUREF Permanent Network (EPN) has been installed in 1996 with some 30 stations and now includes more than 190 permanent GNSS sites (see Fig. 1). The network is operated according to the standards of the International GNSS Service (IGS) and it is considered as a regional densification of the ITRF (International Terrestrial Reference Frame). The EPN is primarily a geodetic reference network, but its results are also widely used for geophysical studies. In order to better serve the user needs, the EUREF Time Series Analysis special project monitors the weekly combined SINEX solutions, cleans the individual station coordinate series, and maintains and publishes the database of the detected coordinate offsets and outliers. Using this info, cleaned cumulative solutions are then computed with the CATREF software (Altamimi et al 2004). The estimated coordinates and velocities, together with the outlier and offset database are regularly updated and published on the EPN CB website (www.epncb.oma.be ).

Time and Frequency Transfer Using GPS Codes and Carrier Phases: Onsite Experiments

Recent studies have shown the capabilities of Global Positioning System (GPS) carrier phases for frequency transfer based on the observations from geodetic GPS receivers driven by stable atomic clocks. This kind of receiver configuration is the kind primarily used within the framework of the International GPS Service (IGS). The International GPS Service/Bureau International des Poids et Mesures (IGS/BIPM) pilot project aims at taking advantage of these GPS receivers to enlarge the network of Time Laboratories contributing to the realization of the International Atomic Time (TAI).In this article, we outline the theory necessary to describe the abilities and limitations of time and frequency transfer using the GPS code and carrier phase observations. We report on several onsite tests and evaluate the present setup of our 12-channel IGS receiver (BRUS), which uses a hydrogen maser as an external frequency reference, to contribute to the IGS/BIPM pilot project.In the initial experimental setup, the receivers had a common external frequency reference; in the second setup, separate external frequency references were used. Independent external clock monitoring provided the necessary information to validate the results. Using two receivers with a common frequency reference and connected to the same antenna, a zero baseline, we were able to use the carrier phase data to derive a frequency stability of 6 × 10−16 for averaging times of one day. The main limitation in the technique originates from small ambient temperature variations of a few degrees Celsius. While these temperature variations have no effect on the functioning of the GPS receiver within the IGS network, they reduce the capacities of the frequency transfer results based on the carrier phase data. We demonstrate that the synchronization offset at the initial measurement epoch can be estimated from a combined use of the code and carrier phase observations. In our test, the discontinuity between two consecutive days was about 140 ps.

On the influence of RF absorbing material on the GNSS position

Abstract 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.

The European Reference System coming of age

More than ten years ago, the advantages of the GPS technology were recognized and a first GPS campaign covering the western part of Europe was organized in order to establish a uniform European Reference Frame (EUREF). Through successive GPS campaigns, the network has been extended towards eastern parts of Europe and various countries have undertaken densification campaigns. The international co-operation within Europe has resulted in the establishment of a high accuracy, threedimensional geodetic network with links to global and national reference systems.

Time Transfer Experiments Using GLONASS P-code Measurements from RINEX Files

We have used GLONASS P-code measurements from different geodetic GPS/GLONASS receivers involved in the IGEX campaign to perform frequency/time transfer between remote clocks. GLONASS time transfer is commonly based on the clock differences between GLONASS system time and the local clock computed by a time transfer receiver. We choose to analyze the raw P-code data available in the RINEX files. This also allows working with the data from geodetic receivers involved in the IGEX campaign. As a first point, we show that the handling of the external frequency in some of the IGEX receivers is not suited for time transfer applications. We also point out that the GLONASS broadcast ephemerides give rise to a considerable number of outliers in the time transfer, compared to the precise IGEX ephemerides. Due to receiver clock resets at day boundaries, which is a characteristic of the R100 receivers from 3S-Navigation, continuous data sets exceeding one day are not available. Invthis context, it is therefore impossible to perform RINEX-based precise frequency transfer with GLONASS P-codes on a time scale longer than one day. Because the frequencies used by GLONASS satellites are different, the time transfer results must be corrected for the different receiver hardware delays. After this correction, the final precision of our time transfer results corresponds to a root-mean-square (rms) of 1.8 nanoseconds (ns) (maximum difference of 11.8 ns) compared to a rms of about 4.4 ns (maximum difference of 31.9 ns) for time transfer based on GPS C/A code observations.

Time Transfer for TAI Using a Geodetic Receiver: An Example with the Ashtech ZXII-T

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.