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Status of IGS Reprocessing Activities at GFZ
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Based on a large network of continuously operated GNSS tracking stations the International GNSS Service (IGS) has a valuable contribution for the realization of the International Terrestrial Reference System (ITRS). In order to contribute to its next realization, the IGS is preparing for a new reprocessing of the GNSS data from 1994 to 2020 including GPS, GLONASS, and – for the first time – Galileo. A first test campaign including single- and multi-system solutions for 2017 and 2018 was performed to derive consistent transmitter phase center corrections for all systems. Preliminary results of the test solutions derived at GFZ show well determined orbits with overlaps of 28 mm for GPS, 67 mm for GLONASS, and 40 mm for Galileo and an overall RMS of satellite laser ranging residuals for Galileo of 58 mm. Using multi-GNSS antenna calibrations (including also E5a and E5b calibrations) horizontal coordinate differences are almost zero between a GPS+GLONASS and a Galileo-only solutions. Due to the mixture of estimated (GPS, GLONASS) and measured (Galileo) transmitter phase center offsets a scale difference of 1.16 ± 0.27 ppb is found between both solutions which agrees nicely to results derived by other analysis centers.
KeywordsGNSS Orbit determination Reprocessing Terrestrial Reference Frame
To provide the best possible GNSS solution for the realization of the International Terrestrial Reference System, the Analysis Centers (ACs) of the International GNSS Service (IGS, Johnston et al. 2017) are preparing for a full reprocessing of GNSS data from 1994 to 2020. Like the previous efforts (repro1 and repro2) the upcoming reprocessing will provide a fully consistent set of orbits, station coordinates and Earth rotation parameters derived with the best and most consistent models available. It is well known that in terms of reference frame parameters the most critical issues for GNSS are, firstly, the transmitter phase center offsets (which are highly correlated with the terrestrial scale, e.g., Zhu et al. 2003) and, secondly, the modeling of the solar radiation pressure on the orbits (main reason for draconitic period in geocenter results, e.g., Meindl et al. 2013). While trying to reduce or to solve both issues several additional topics have to be considered like the 13.63/13.66 day signal in GNSS time series (see for example Ray et al. 2013) or remaining modeling inconsistencies compared to other space geodetic techniques. Compared to the last reprocessing, new satellite systems like Galileo and BeiDou became almost fully operational. As their signals were tracked by an increasing number of IGS stations during the past years, the set of considered systems has to be re-defined from GPS+GLONASS in repro2 to an up-to-date multi-GNSS solution.
During the IGS Analysis Workshop 2019 held in Potsdam, Germany, the IGS ACs agreed to strive for an combined GPS (G), GLONASS (R), and Galileo (E) solution in the upcoming repro3. However, to avoid systematic distortions, so far missing, receiver antenna corrections for the Galileo signals E5a, E5b, and E6 and the GPS L5 frequency as well as consistent transmitter phase center offsets (PCOs) are required (see e.g. Schmid et al. 2016). Whereas the first issue was solved for many antenna types used in the IGS as Geo++ provided robot-based calibrations for these signals it was agreed to solve the second issue by setting up a test campaign. This campaign includes multi- and single-system solutions (if possible GRE, GR, G, R, and E) for 2017 and 2018 which will be used to derive phase center offsets for GPS and GLONASS based on the Galileo offsets which are known thanks to published chamber calibrations (GSA 2017). As these Galileo PCOs are measured – and not estimated from observations itself – they are independent of the terrestrial scale which has to be fixed to the ITRF scale otherwise (Schmid et al. 2007). Therefore, an independent GNSS scale will become available if the final repro3 could be performed with this consistent set of re-estimated and calibrated PCOs. It was also agreed to run a second test campaign using the final repro3 setup including the station selection as benchmark test before starting the processing tasks.
This paper summarizes the current reprocessing status at GFZ during the first test campaign and highlights preliminary outcomes. Section 2 describes investigations regarding station selection and testing some models. Initial results based on the different GFZ solutions in the first test campaign will be presented in Sect. 3. Section 4 provides an outlook to the upcoming tasks.
2 Data and Processing
This section discusses the processed data, the station network, and the selected models for the test campaign but also for the final reprocessing.
2.1 Data Selection
2.2 Processing of the First Test Campaign
Summary of estimation and processing strategy (repro3 test campaign); time span 2017.0–2019.0; the used ANTEX was provided by A. Villiger and the IGS ANTEX working group (IGS-ACS-1233 mail)
Modeling and a-priori information
Ionosphere-free linear combination formed by undifferenced GPS observations
GPT2 meteo values mapped with VMF1 (Böhm et al. 2006)
First order effect considered with ionosphere-free linear combination, second order effect corrected using the International Magnetic Reference Field (11th realization, Finlay et al. 2010)
GNSS phase center
Dedicated multi-GNSS ANTEX applied (igsR3_2057.atx)
Zero-mean condition for satellites and selected stations
EGM2008 up to degree and order 12 (Pavlis et al. 2012)
Solid Earth tides
According to IERS 2010 Conventions (Petit and Luzum 2010)
Conventional tide free
Ocean tide model
FES2004 (Lyard et al. 2006)
FES2004 (Lyard et al. 2006)
Tidal: S1 and S2 corrections (Ray and Ponte 2003)
High-frequent EOP model
Desai-Sibois model (Desai and Sibois 2016)
Mean pole tide
Linear mean pole as adopted by the IERS in 2018
No-net-rotation w.r.t. IGS14 (Rebischung and Schmid 2016)
Zenith wet delays for 0.5 h intervals; two gradient pairs per station and day
GPS orbit modeling
Six initial conditions + nine ECOM2 parameters, pulses at 12 h
Rotation pole coordinates, pole-rates and LOD for 24 h intervals, UT1 tightly constrained to a priori Bulletin A
Pre-eliminated every epoch, ISB per station for Galileo, per station and satellite for GLONASS
Ambiguity fixing for GPS and Galileo
Antenna Phase Center
Estimated for GPS, GLONASS, and Galileo but tightly constrained to values given in ANTEX
2.3 Updating Models for Repro 3
According to the discussions between the IGS ACs and the IERS, several models have to be updated for the final reprocessing compared to Table 1. For computing the rotational deformation (pole tide) the linear mean pole will be used as adopted by the IERS in 2018. Regarding the gravity field, a static gravity field up to degree and order 12 was used whereas a time-variable gravity field should be used in the reprocessing. The ocean tides and ocean loading model will be updated to a more recent FES2014b model (Carrere et al. 2016). In order to consider high-frequency variations in Earth orientation parameters (EOP) it was agreed to use the model provided by Desai and Sibois (2016) instead of the model provided in the IERS Conventions.
3 Initial Results
Orbit overlaps: average and standard deviation over all satellites and weeks; time span 2017.0–2019.0; unit: mm
Full sol. (GRE)
27.8 ± 2.9
67.6 ± 26.3
40.5 ± 2.9
28.5 ± 2.8
65.4 ± 27.4
28.4 ± 2.8
40.5 ± 3.6
4 Summary and Outlook
The presented repro3 test campaign was performed at GFZ as contribution to the re-determination of transmitter phase center offsets for GPS and GLONASS in preparation for the final reprocessing. The current setup for this reprocessing is a three-system solution (GPS, GLONASS, Galileo) while some models like the time-variable gravity field are still in discussion between the IGS ACs and the IERS. The preliminary results show acceptable overlap errors for the individual satellites with on average 28 mm, 67 mm and 40 mm for GPS, GLONASS, and Galileo. An SLR orbit validation revealed also good orbit quality without significant biases for the Galileo satellites. Comparing the derived station coordinates a good agreement between the GR- and the Galileo-only solution can be found in the horizontal components with a scale difference of around 1.1 ppb. According to the reprocessing schedule, a second test campaign will be performed after final decisions on the models. In addition, a few open questions have to be addressed like the reason for large orbit overlap errors for some GLONASS satellites or the coordinate difference for some stations equipped with JAVRINGANT_G3T and JAVRINGANT_G5T antennas.
Available also at http://acc.igs.org/repro3/repro3, accessed January 2020.
Stations provided by other networks, like SONEL (3), OAFA (1), GREF (1), EPN (4), NGS (6), UNAVCO (2), are not considered for this initial assessment but will be processed in the final reprocessing.
The four satellites launched in July 2018 are not included as they were not operational before January 2019.
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