ALBERT

Description: Over the past years, the International GNSS Service (IGS) has been putting efforts into extending its service by setting up and running the Multi-GNSS experiment and pilot project (MGEX). Several MGEX analysis centers (ACs) contribute by providing solutions containing not only GPS and GLONASS but also Galileo, BeiDou, and QZSS. As the current IGS combination software can only handle the orbits of one constellation at a time, it requires substantial modifications to obtain a consistent MGEX orbit product. In this contribution, we present a least-squares framework for a Multi-GNSS orbit combination, where the weights used to combine the ACs’ orbits are determined by least-squares variance component estimation. We introduce and compare two weighting strategies, where either AC-specific weights or AC and constellation-specific weights are used. An automated Z-score test is implemented yielding a common set of core satellites that are used to determine the weights. Both strategies are tested using MGEX orbit solutions for a period of two and a half years. They yield similar results with an agreement with the ACs’ orbits at the one centimeter level for GPS and up to a few centimeters for the other constellations. The 3D-RMS is generally slightly better with the AC and constellation weighting. A comparison of our combination approach with the official IGS combination using three years of GPS and GLONASS orbits shows an agreement of better than 5 mm and 12 mm for GPS and GLONASS, respectively, while the agreement of the official IGS combination with the ACs’ GPS solutions is only around 15mm. An external validation using satellite laser ranging shows that the mean residuals of our combined products are around −3mm for Galileo, 6mm for GLONASS, −8mm for BeiDou, and −31mm for QZSS.

Description: Over the past years, the International GNSS Service (IGS) has put efforts into reprocessing campaigns, reanalyzing the entire data collected by the IGS network since 1994. Using state-of-the-art models and software, the goal is to provide a consistent set of orbits, station coordinates, and earth rotation parameters. Unlike the previous campaigns—namely: repro1 and repro2—, the repro3 includes not only GPS and GLONASS but also the Galileo constellation. The main repro3 objective is the contribution to the next realization of the International Terrestrial Reference Frame (ITRF2020). To achieve this goal, several Analysis Centers (AC) submitted their own products to the IGS, which are combined to provide the final solutions for each product type. In this contribution, we focus on the combination of the orbit products. We present a consistent orbit solution based on a newly developed combination strategy, where the weights are determined by a Least-Squares Variance Component Estimation (LSVCE). The orbits are intended to be combined in an iterative processing: firstly, by aligning all the products via a Helmert transformation, secondly by defining which satellites will be used in the LSVCE, and finally by normalizing the inverse of the variances as weights that are used to compute a weighted mean. The combination results show an agreement between the different AC’s input orbits around 10 mm for GPS, 30 mm for GLONASS. The combination also highlights the improvement of the Galileo orbit determination over the past decade, the internal precision decreasing from around 35 mm to 16 mm for the most recent weeks. We used Satellite Laser Ranging (SLR) observations for external validation. The combined orbit has one of the best RMS agreements with respect to the SLR measurements (9.1 mm for GLONASS, and 8.3 mm over the last five years of the processed period).

Description: The combined satellite clocks are one of the products provided by The International GNSS Service (IGS). These products are the result of a combination process that takes as input the individual solutions computed by the Analysis Centers (ACs). So far, only GPS and GLONASS combined products are distributed by the IGS. Since new constellations have been declared operational, such as Galileo in 2016 and BeiDou in 2018, a Multi-Constellation combination has been on focus. Therefore, we propose a method for a Multi-GNSS clock combination based on a least-squares framework, where the weights are determined by least-squares variance component estimation. The necessary alignments implemented in order to harmonize the solutions from the ACs will be demonstrated, such as radial correction between orbits and clocks, alignment of AC clocks to a reference AC through estimation of trends and offsets, and the inter-system bias correction between different systems. In addition, the outlier detection will be shown, which facilitates a reliable definition of a reference AC and consistent AC solutions. The results show that Galileo and GPS clocks have, on average, the best agreement between ACs and the combination, while GLONASS and QZSS have large variations. A validation of the combination is performed using the combined products as input in a Precise Point Positioning (PPP) processing, where the average residuals are below 10 mm, agreeing with the performance of the individual ACs.

Description: The International GNSS Service (IGS) provides combined satellite and station clock products, which are generated from the individual clock solutions produced by the analysis centers (ACs). Combinations for GPS and GLONASS are currently available, but there is still a lack of combined products for the new constellations such as Galileo, BeiDou, and QZSS. This study presents a combination framework based on least squares variance component estimation using the ACs’ aligned clock solutions. We present the various alignments required to harmonize the solutions from the ACs, namely the radial correction derived from the differences of the associated orbits, the alignment of the AC clocks to compensate for different reference clocks within each AC solution, and the inter-system bias (ISB) alignment to correct for different AC ISB definitions when multiple constellations are used. The combination scheme is tested with IGS MGEX and repro3 products. The RMS computed between the combined product and the aligned ACs’ solutions differ for each constellation, where the lowest values are obtained for Galileo and GPS with on average below 45 psec (13 mm) and reaching more than 150 psec (45 mm) for QZSS. The same behavior is repeated when the process is performed with the repro3 products. A clock and orbit combination validation is done using precise point positioning (PPP) that shows ionosphere-free phase residuals below 10 mm for all constellations, comparable with the AC solutions that are in the same level.

Description: This study presents the mathematical framework and experiments to achieve an orbit and clock combination using Multi-GNSS products. Over the past years, the International GNSS Service (IGS) has been putting efforts into extending its service by setting up and running the Multi-GNSS Experiment and Pilot Project (MGEX). Several MGEX Analysis Centers (ACs) contribute by providing solutions containing not only GPS and GLONASS but also Galileo, BeiDou, and QZSS. As the current IGS combination software can only handle one constellation at a time, obtaining a consistent MGEX orbit and clock product requires substantial modifications. Therefore is presented a least-squares framework for a Multi-GNSS orbit and clock combination, where the weights used to combine the ACs’ products are determined by least-squares variance component estimation. Two different weighting strategies are proposed and discussed: AC specific weights or AC and constellation specific weights. An automated Z-score test is implemented, yielding a common set of core satellites that are used to determine the weights. Both strategies are tested using MGEX orbit and clock solutions for a period of two and a half years, where the two data processing yielded similar results. The agreement with the ACs’ orbits is around one centimeter level for GPS and up to a few centimeters for the other constellations. The 3D-RMS is generally slightly better with the AC and constellation weighting. As a validation of the strategies, a comparison of our combination approach with the official IGS combination using three years of GPS and GLONASS orbits shows an agreement of better than 5 mm and 12 mm for GPS and GLONASS, respectively. An external validation using Satellite Laser Ranging (SLR) shows that the mean residuals of our combined products are around −3 mm for Galileo, 6 mm for GLONASS, −8 mm for BeiDou, and −31 mm for QZSS. For the clock products, the agreement with the IGS final product is around 32 ps and the Precise Point Positioning (PPP) analysis showed that the combined product agrees with the residuals of the ACs with values of less than 10 mm. The MGEX clock combination showed that most of the ACs agreement is below 80 ps for GPS and Galileo, and for the other constellations, the values are more sparse. In addition, it is noted that SHA and JAX presented big discrepancies compared with the other ACs. The validation is performed again in comparison IGS official combination where the agreement with the presented solutions is around 32 ps. A PPP processing is also performed using the combined solutions, showing the suitability of the products. The ACs products from the IGS third reprocessing campaign are also used as input in the combination and discussed. Finally, an extension of the orbit combination is done for the combination of Low Earth Orbit (LEO) satellites. The results show that the proposed combination can deliver reliable products following the standards of the IGS.

Description: Diese Studie stellt den mathematischen Rahmen und die Experimente vor, um eine Kombination von Umlaufbahn und Uhr mit Hilfe von Multi-GNSS-Produkten zu erreichen. In den letzten Jahren hat der International GNSS Service (IGS) Anstrengungen unternommen, um seinen Dienst durch die Einrichtung und Durchführung des Multi-GNSS-Experiment- and Pilot Project (MGEX) zu erweitern. Mehrere MGEX Analysis Centers (ACs) tragen dazu bei, indem sie Lösungen anbieten, die nicht nur GPS und GLONASS, sondern auch Galileo, BeiDou und QZSS enthalten. Da die derzeitige IGS-Kombinationssoftware jeweils nur eine Konstellation verarbeiten kann, sind für den Erhalt eines konsistenten MGEX-Bahn und Uhrenprodukts erhebliche Änderungen erforderlich. Daher wird ein Least-Squares-Rahmen für eine Multi-GNSS-Bahn- und Uhrenkombination vorgestellt, bei dem die zur Kombination der AC-Produkte verwendeten Gewichte durch eine Least-Squares-Varianzkomponentenschätzung bestimmt werden. Es werden zwei verschiedene Gewichtungsstrategien vorgeschlagen und diskutiert: AC-spezifische Gewichte oder AC- und konstellationsspezifische Gewichte. Es wird ein automatischer Z-Score-Test durchgeführt, der einen gemeinsamen Satz von Kernsatelliten ergibt, die zur Bestimmung der Gewichte verwendet werden. Beide Strategien werden anhand von MGEX-Bahn- und Uhrenlösungen über einen Zeitraum von zweieinhalb Jahren getestet, wobei die beiden Datenverarbeitungen ähnliche Ergebnisse erbrachten. Die Übereinstimmung mit den Bahnen der ACs liegt bei GPS bei etwa einem Zentimeter und bei den anderen Konstellationen bei bis zu einigen Zentimetern. Der 3D-RMS ist im Allgemeinen etwas besser mit der AC- und Konstellationsgewichtung. Zur Validierung der Strategien zeigt ein Vergleich unseres Kombinationsansatzes mit der offiziellen IGS-Kombination unter Verwendung von GPS- und GLONASS-Umlaufbahnen aus drei Jahren eine Übereinstimmung von besser als 5 mm und 12 mm für GPS bzw. GLONASS. Eine externe Validierung mittels Satellite Laser Ranging (SLR) zeigt, dass die mittleren Residuen unserer kombinierten Produkte bei etwa −3 mm für Galileo, 6 mm für GLONASS, −8 mm für BeiDou und −31 mm für QZSS liegen. Bei den Uhrenprodukten liegt die Übereinstimmung mit dem IGS-Endprodukt bei etwa 32 ps, und die Analyse der präzisen Precise Point Positioning (PPP) ergab, dass das kombinierte Produkt mit den Residuen der ACs mit Werten von weniger als 10 mm übereinstimmt. Die MGEX-Uhrenkombination zeigte, dass die meisten ACs unter 80 ps für GPS und Galileo übereinstimmen, und für die anderen Konstellationen sind die Werte spärlicher. Darüber hinaus ist festzustellen, dass SHA und JAX im Vergleich zu den anderen ACs große Diskrepanzen aufweisen. Die Validierung erfolgt wiederum im Vergleich mit der offiziellen IGS-Kombination, bei der die Übereinstimmung mit den vorgestellten Lösungen bei etwa 32 ps liegt. Eine PPP-Verarbeitung wird ebenfalls unter Verwendung der kombinierten Lösungen durchgeführt, was die Eignung der Produkte zeigt. Die ACs-Produkte aus der dritten IGS-Reprocessing-Kampagne werden ebenfalls als Input für die Kombination verwendet und diskutiert. Schließlich wird eine Erweiterung der Orbit-Kombination für die Kombination von Low Earth Orbit (LEO)-Satelliten durchgeführt. Die Ergebnisse zeigen, dass die vorgeschlagene Kombination zuverlässige Produkte gemäß den Standards des IGS liefern kann.