The official products delivered to the satellite navigation community by the International GNSS Service (IGS) include highly accurate satellite orbits and clocks for GPS and GLONASS. These products are system-wise combinations based on the individual solutions provided by the Analysis Centers (ACs). The development of new satellite systems such as Galileo, BeiDou, and QZSS in the past years is addressed by the IGS Multi GNSS Experiment (MGEX). Since 2012, several ACs provide multi-GNSS products specific to this section. Until 2018, when the GFZ IGS AC started to investigate methods for a joint combination using all available satellite constellations, there was no combination from MGEX products specifically. The following topics summarize the efforts of the GFZ group:

• Modification of the IGS legacy combination software for multi-GNSS (Sakic et al., 2018);
• Tests and evaluation of the legacy combination strategy (Mansur et al., 2020);
• Development of a new joint combination strategy with least-squares variance component estimation based AC and constellation weighting.

• Mansur, G., Sakic, P., Männel, B., and Schuh, H.: Multi-constellation GNSS orbit combination based on MGEX products, Advances in Geosciences, 50, 57–64, https://doi.org/10.5194/adgeo-50-57-2020, 2020.
• Sakic, P., Mansur, G., Viegas, E., Männel, B., and Schuh, H.: Towards a Multi-Constellation combination: Improving the IGS orbit & clock combination software for MGEX products, in: IGS Workshop, Wuhan, 2018.
• Sakic, P., Mansur, G., & Männel, B. (2020). A prototype for a Multi-GNSS orbit combination. Accepted as presentation of the ENC2020. https://doi.org/10.13140/RG.2.2.31736.60163.

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