## 4 Results and Validations
### ZTD Validation with the Final Tropospheric Products  
Based on the IGS-RT multi-GNSS orbit and clock products, the operational RT ZTD can be retrieved. In this section, we investigate and compare the impact of different systems, different multi-GNSS combination and different positioning modes (i.e., fixing coordinate and kinematic modes) on the performance of the multi-GNSS RT ZTD estimates [8]. By processing observation data from the selected 30 tracking stations equipped with the four-system receivers as described in Section 2, the RT ZTD estimates are obtained every five seconds during the time period from March 28 to April 11 2017. The final troposphere products provided by the U.S. Naval Observatory (USNO) are taken as the reference data sets here. The USNO products with the sampling rate of five minutes are generated by using PPP with the IGS final orbit/clock products [33]. Figure 6 shows the RT ZTD results at station ONS1 (Onsala, Sweden, 57.39\({}^{\circ}\) N, 11.92\({}^{\circ}\) E) for different IGS-RT services with processing the GPS-only, the combined GPS/GLONASS, and the combined GPS/GLONASS/Galileo/BDS observations in fixing coordinate and kinematic processing modes over the first two hours of DOY 090, 2017. From Figure 6, we can clearly see that, for either fix coordinate or kinematic positioning mode, there is no big difference between the RT ZTDs derived from different IGS-RT services after 40 min, no matter which of the multi-GNSS combination are employed. Figure 5: The averaged STD values of the differences between the IGS-RT clocks and the reference for GPS, GLONASS, Galileo, and BDS. In the GPS-only solution with fixinTg the coordinates, the shortest convergence time of less than five minutes for RT ZTD estimates can be achieved by GFZC2 and GFZD2 compared to that of about eight minutes for other IGS-RT services [8]. In contrast, the results in the kinematic mode exhibit longer convergence period and worse stability than those in the fixing coordinate mode. For example, the initial convergence time of RT ZTDs for GFZC2 and GFZD2 in the kinematic mode is about ten minutes. With introducing more GNSS observations, the performance of ZTD estimates can be improved in the combined GPS/GLONASS and GPS/GLONASS/Galileo/BDS solutions compared to the GPS-only one, especially for the convergence time in kinematic positioning mode. Similar to the GPS-only case, the GFZC2 and GFZD2 provide the best results of RT ZTDs in the combined solutions. By using different IGS-RT service and different combination of multi-GNSS observations in the fixing coordinate mode, we provide the averaged initial convergence time of RT ZTDs for all  
Figure 6: RT ZTD estimates at station ONS1 derived from the GPS-only (“GPS”, **a**), the combined GPS/GLONASS (“G/R”, **b**), and the combined GPS/GLONASS/Galileo/BDS (“G/R/E/C”, **c**) solutions in fixing coordinate (left panels) and kinematic processing (right panels) modes by employing different IGS-RT service over the first 2 h of DOY 90, 2017.  
stations as summarized in Figure 7. The initialization process is considered to be completed when the differences between the estimated RT ZTDs and the post-processed USNO ZTDs become and remain smaller than a given value, which is set to be 20 mm here following the mean of the threshold and goal value of ZTD for weather nowcasting [34]. With the increased number of observations, the initialization time for the dual-system combination (GPS/GLONASS) and the four-system combination (GPS/GLONASS/Galileo/BDS) become shorter than that for the single-system solution (GPS-only). In addition, the results for GFZC2 and GFZD2 outperform those for the other three services (i.e., CLK01, CLK81, and CLK92) in each solution. Based on the GFZC2 products, the averaged initialization time of RT ZTDs can reach about 522 s (8.7 min), 492 s (8.2 min), and 480 s (8.0 min) in the GPS-only, dual-system (GPS/GLONASS), and the four-system (GPS/GLONASS/Galileo/BDS) solutions, respectively.