## 4 Discussion
The improvement (with a statistic of 18-90%) on the convergence time is significant after adding the GPS observations to the GLONASS-only, Galileo-only and BDS-only PPP processing in both static and kinematic modes in both real-time and post-processed scenarios. The post-processed GRECJ-PPP shortens the static convergence time by 42-59%/46-52%/54-57% over the post-processed GPS-only PPP in the three directions, respectively. As for the effect of employed post-processed precise products on the static convergence time, it is ignorable for GRECJ-PPP, and the difference is smaller than 1 min when adopting the precise products from different ACs. The convergence time improvement in the static mode is 28%/22%/31% for real-time GREC-PPP over real-time GPS-only PPP in the three directions, respectively. Similar to the post-processed PPP results, the static positioning accuracy of real-time GPS-only PPP is comparable to the multi-system combined cases, but the other three real-time single-system PPP cases obtain worse static positioning accuracy by several millimeters. The positioning performance of real-time static PPP is only slightly worse than that of post-processed static PPP for GPS-only, Galileo-only and dual-system combined PPP, but it is not the case for GLONASS-only and BDS-only PPP. The post-processed static GRECJ-PPP shortens the convergence time by 21-32%/40-50%/37-44%, and improves the positioning accuracy by 50-67%/63-75%/15-23% over the real-time static GREC-PPP in the three directions, respectively. Different from the static PPP solutions, both the positioning accuracy and convergence time of post-processed and real-time kinematic PPP solutions can benefit from the multi-system combination. By introducing GPS observations, we can achieve a convergence time improvement of 18-90% and 39-86%, as well as an accuracy improvement of 48-75% and 49-64% for dual-system combined PPP over single-system PPP in the kinematic mode in the post-processed and real-time scenarios, respectively. Compared with the post-processed kinematic GPS-only PPP, the post-processed kinematic GRECJ-PPP reduces the convergence time by 59-72%/73-74%/60-69%, and improves the positioning accuracy by 56-71%/53-64%/40-43% in the east/north/up directions, respectively, while the corresponding improvement is 42%/49%/43% and 48%/38%/31% for real-time kinematic GREC-PPP over real-time kinematic GPS-only PPP in the three directions, respectively. When using the post-processed precise products from different ACs, several combination cases exhibit distinct kinematic positioning performance, and the difference can be as large as 1 cm and 10 min in terms of position accuracy and convergence time, respectively. However, the corresponding difference, which is smaller than 0.1 cm and 2 min, is not obvious for the post-processed kinematic GREC-PPP. Compared with the static post-processed PPP results, both the positioning errors and convergence time of the corresponding kinematic ones are increased by a factor of 3.5 times on average. The real-time kinematic PPP even has slightly better positioning performance compared with some cases of post-processed kinematic PPP results. The post-processed kinematic GRECJ-PPP shortens the convergence time by 25-41%, 42-52% and 38-40%, and improves the positioning accuracy by 35-41%, 50% and 28-31% in comparison with the real-time kinematic GREC-PPP in the three directions, respectively. The real-time kinematic PPP increases the position errors and convergence time by a factor of 2.7 times on average over the real-time static PPP. Conceptualization, L.P.; methodology, L.P.; software, X.L. ; validation, X.L. ; formal analysis, X.L. ; investigation, X.L. ; resources, L.P.; data curation, X.L. ; writing--original draft preparation, X.L. ; writing--review and editing, L.P.; visualization, X.L. ; supervision, L.P.; project administration, L.P.; funding acquisition, L.P. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China (Grant No.