Gustavo Bento Mansur , Pierre Sakic , Andreas Brack, Benjamin Männel , Harald Schuh 

Over the past years, the International GNSS Service (IGS) has been putting efforts into extending its service towards 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. The MGEX orbit and clock combination is a product that is still not consolidated inside the IGS and requires studies in order to provide a consistent solution. 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. 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 agreement is generally slightly better with the AC and constellation weighting. A comparison of our combination approach with the official combined IGS final solution using three years of GPS, and GLONASS orbits from the regular IGS processing shows an agreement of better than 5 mm and 12 mm for GPS and GLONASS, respectively. An external validation using Satellite Laser Ranging (SLR) is performed for our combined MGEX orbit solutions with both weighting schemes.