## 4 Conclusions  
In this study, the qualities of precise orbit and clock products for Galileo, BDS and QZSS were assessed. Galileo orbit/clock comparisons showed a consistency of 3-5 cm for orbit and 0.1-0.3 ns for clock among most ACs. The orbit differences of GRG with respect to ESM significantly improved after DOY280 2018 when GRG started to calculate Galileo orbits with an undifferenced ambiguity fixing strategy. BDS-2 orbit comparisons showed agreements of 12-18 cm for IGSOs and 8-9 cm for MEOs among ESM, GFZ, SHA and WUM. COD and TUM showed large orbit differences with other ACs in the along-track and cross-track directions. The orbit differences of BDS-3 satellites manufactured by SECM were obviously larger than those of BDS-3 satellites manufactured by CAST. The RMSs of the BDS clock differences were 0.2-0.5 ns for IGSOs and MEOs, and about 1 ns for GEOs for most ACs. As for QZSS, the orbit differences of J01, J02 and J03 were within 11-16 cm among COD, ESM and JAX. However, the orbit differences of GEO satellite J07 were able to reach several meters. The clock differences of QZSS satellites varied from 0.3 to 1.1 ns. SLR validations showed an accuracy of 3-4 cm for Galileo. For BDS-2, the STD values of MEO satellites were mostly 3-4 cm, and the STD values of IGSO satellites were mostly 5-6 cm in COD, GFZ, WUM and SHA products. The orbit accuracy of the BDS-2 GEO satellite is significantly which improved in WUM solution by applying an improved SRP model for BDS GEO satellites. For BDS-3, the STD values of SLR residuals were mostly between 4 and 6 cm. For QZSS IGSO satellites, an accuracy of 5-10 cm was achieved in COD, ESM and JAX products, while the STD values of the QZSS GEO satellite J07 were around 20 cm. GPS-only, BDS-only, Galileo-only and GPS+QZSS PPP were performed to further evaluate the qualities of multi-GNSS products provided by different ACs. For static PPP solutions, the positioning accuracies in Galileo-only mode were 3-5 mm in horizontal directions, and 11 mm in the up direction, with different products used. As for BDS-only PPP, the positioning errors in the up direction depended  
Figure 13: Positioning errors of kinematic PPP at the KAT1 station using different products (unit: dm). on the number of available BDS satellites included in the orbit and clock products. Since WUM products included all available BDS-2 and BDS-3 satellites, static PPP in BDS-only mode achieved the best positioning accuracy of 9 mm in the up direction using WUM products. As to kinematic PPP results, positioning errors in Galileo-only mode were 17-19 mm in the north, 13-16 mm in the east and 74-81 mm in the up direction, respectively. With WUM products used, positioning accuracies of 14, 14 and 49 mm were achieved in the east, north and up directions in BDS-only mode, respectively. Methodology, X.L., Y.Z., K.Z. and Y.Y. ; Project administration, X.L. ; Investigation, Y.Z., K.Z. and Y.Y. ; Validation, G.L. and Y.X. ; Writing--original draft, Y.Z. ; Writing--review & editing, X.L., K.Z., Y.Y. and G.L. All authors have read and agreed to the published version of the manuscript. This study is financially supported by the National Natural Science Foundation of China (Grant No. 41774030, Grant 41974027), the Hubei Province Natural Science Foundation of China (Grant No. 2018 CFA081) and the National Youth Thousand Talents Program. We are grateful to IGS-MGEX for providing multi-GNSS data and products. Thanks also go to ILRS for providing SLR observation data. The authors declare no conflict of interest. The following abbreviations are used in the text: