## 1 Introduction  
The Multi Global Navigation Satellite System (Multi-GNSS) Experiment (MGEX) [1] project was initiated by the International GNSS Services (IGS) in 2011 to prepare the IGS for multi-GNSS processing in view of the emergence of BeiDou Navigation Satellite System (BDS), Galileo and Quasi-Zenith Satellite System (QZSS). With several years' development, MGEX has now built up a global multi-GNSS network of more than 300 stations to collect all available GNSS signals. Based on the observations of the MGEX network, precise orbit and clock products are now routinely generated by seven MGEX analysis centers (ACs). Among all the MGEX ACs, the Center for Orbit Determination in Europe (CODE) [2], GeoForschungsZentrum Potsdam (GFZ) [3], Technische Universitat Munchen (TUM) [4] and Wuhan University (WHU) [5] provide MGEX products for all the emerging constellations including BDS, Galileo and QZSS. The Centre National d'Etudes Spatiales (CNES) and Collecte Localisation Satellites (CLS) only provide Galileo products [6], and the Japan Aerospace Exploration Agency (JAXA) [7] only provides QZSS products. In addition, the Shanghai Observatory (SHAO) recently joined the group of MGEX ACs by providing rapid products covering Galileo and BDS [8]. In view of the variety of MGEX orbit and clock products and the absence of combined MGEX products [9], it could be confusing for multi-GNSS users to choose the most appropriate products and to understand their positioning performances. In this context, assessments of the MGEX products--particularly the new BDS, Galileo, and QZSS--are of great importance and necessity in order to properly utilize the current MGEX products and adequately prepare for the upcoming generation of combined MGEX products. [PERSON] et al. previously performed an assessment of Galileo products from CNES/CLS, CODE, GFZ and TUM for a 20-week period in 2013. They reported a general orbit consistency of 5-30 cm among these four ACs [10]. [PERSON] et al. further assessed the quality of MGEX orbit and clock products during 2013-2015 for the emerging Galileo, BDS and QZSS constellations. From this study, satellite laser ranging (SLR) validations demonstrated an orbit accuracy of about 10 cm for the Galileo and BDS-2 inclined geosynchronous orbit (IGSO) and medium earth orbit (MEO) satellites, and about 20 cm for QZSS IGSO J01 [11]. Thereafter, in 2017, [PERSON] et al. assessed the orbit and clock products of the new constellations for the first half of 2016. Signal-in-space range error values of 3-7 cm were reported for Galileo and BDS-2 IGSO/MEO satellites [9]. Since January 2018, BDS has launched 28 satellites including 26 BDS-3 satellites and two BDS-2 satellites. At the end of 2018, BDS-3 began a global service. In parallel, following the launch of three QZSS satellites in June 2017, QZSS completed a four-satellite constellation and announced an official start to QZSS services in November 2018. Galileo also launched four new satellites into orbit in 2018, and all of them have since started signal transmission. Most of these new satellites have been included in the MGEX products, but their orbit and clock performances have not been fully investigated in current literature. In addition, several critical issues have been continuously studied for BDS, Galileo and QZSS precise orbit determination (POD) and precise clock estimation (PCE), including antenna parameters [12], yaw-attitude models [13,14], solar radiation pressure (SRP) models [15,16,17], inter system bias (ISB) [14] and ambiguity fixing [18,19]. Moreover, the European GNSS Service Centre (EGSC) and Cabinet Office (CAO), Government of Japan, published the metadata of the Galileo and QZSS satellites, including the phase center offsets and variations (PCOs and PCVs), attitude laws and physical characteristics [20,21,22,23,24]. Satellite metadata are helpful for advanced POD and PCE processing [25,26]. With these refined models and new information, several updates have been implemented by MGEX ACs.