## 4 Results and Validations
### ZTD Validation with the Final Tropospheric Products
Besides, better ZTD estimates can be achieved in the combined GPS/GLONASS and GPS/GLONASS/Galileo/BDS solutions, compared to the GPS-only solution. As mentioned previously, the GFZC2 provides the best RT ZTD results for all the three solutions in both fixing coordinate and kinematic modes. In the fixing coordinate mode, the RMS value for the GPS-only solution is about 7.9 mm, while the values are about 7.5 mm and 7.3 mm for the combined GPS/GLONASS and the combined GPS/GLONASS/Galileo/BDS solutions, respectively. However, the RMS values for the three solutions are much larger in the kinematic mode, which are about 16.5, 15.0, and 15.1 mm, respectively. Figure 11 presents the averaged RMS of the RT ZTD differences for all stations during the whole period (DOY 84-101, 2017) in both the fixing coordinate and the kinematic modes. Generally, the RT ZTDs retrieved in the fixing coordinate mode show much higher accuracy compared to those in the kinematic mode and the advantage of multi-GNSS is also obvious in this case. Compared to the other IGS-RT service, the RT ZTD of GFZC2 shows the highest accuracy for each solution in both the fixing coordinate and the kinematic modes. The averaged RMS values of GFZC2 ZTDs are about 6.50, 5.04, and 5.06 mm in the fixing coordinate mode for the GPS-only, the combined GPS/GLONASS, and the combined GPS/GLONASS/Galileo/BDS solutions and about 14.73, 13.57, and 13.74 mm in the kinematic mode. Consequently, the improvement in the accuracy of the real-time estimated ZTDs can reach about 22.2% due to introducing multi-GNSS observables compared to the GPS-only solution. For the multi-GNSS real-time retrieving tropospheric delays, the ZTD accuracy in the fixing coordinate mode can be improved by a factor of approximately 2.7 in comparison to that in the kinematic mode. The RMS values of the CLK81 ZTD estimates for the GPS-only and the combined GPS/GLONASS solutions are about 13.61 and 11.39 mm, respectively, in the fixing coordinate mode, and about 33.50 and 25.57 mm, respectively, in the kinematic mode. In the combined GPS/GLONASS/Galileo/BDS solution, the RMS values for the GFZD2, CLK92 and GFZC2 RT ZTDs are about 9.46, 6.16, and 5.06 mm, respectively, in the fixing coordinate mode, and about 20.32, 14.39, and 13.74 mm, respectively, in the kinematic mode. The related RMS statistics are listed in Table 5.  
Figure 10: RMS values of the ZTD differences from the GPS-only (“G”), the combined GPS/GLONASS (“G/R”), and the combined GPS/GLONASS/Galileo/BDS (“G/R/E/C”) solutions in the fixing coordinate (**a**) and the kinematic (**b**) modes with respect to the USNO final tropospheric products at station ONS1 on DOY 092, 2017. Figure 11: Averaged RMS and STD values of the RT ZTD differences for all stations during the whole period (DOY 84-101, 2017) in both the fixing coordinate (**a**) and the kinematic modes (**b**). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**IGS Service**} & \multirow{2}{*}{**Solution**} & \multicolumn{2}{c}{**Fix Coordinate Mode**} & \multicolumn{2}{c}{**Kinematic Mode**} \\ \cline{3-6}  & & **RMS** & **STD** & **RMS** & **STD** \\ \hline \multirow{2}{*}{**CLK01**} & G & 9.80 & 5.78 & 18.21 & 18.00 \\  & G/R & 8.92 & 4.83 & 14.31 & 13.66 \\ \hline \multirow{2}{*}{**CLK81**} & G & 13.61 & 12.73 & 33.50 & 32.97 \\  & G/R & 11.39 & 10.44 & 25.57 & 25.21 \\ \hline \multirow{2}{*}{**CLK92**} & G & 7.70 & 4.85 & 17.94 & 17.13 \\  & G/R & 6.18 & 3.46 & 14.64 & 13.01 \\  & G/R/E/C & 6.16 & 3.45 & 14.39 & 12.70 \\ \hline \multirow{2}{*}{**GFZC2**} & G & 6.50 & 5.32 & 14.73 & 14.11 \\  & G/R & 5.04 & 3.94 & 13.57 & 13.45 \\ \cline{1-1}  & G/R/E/C & 5.06 & 3.97 & 13.74 & 12.60 \\ \hline \multirow{2}{*}{**GFZD2**} & G & 10.81 & 10.84 & 23.40 & 23.41 \\  & G/R & 9.67 & 9.69 & 20.40 & 21.42 \\ \cline{1-1}  & G/R/E/C & 9.46 & 9.48 & 20.32 & 21.32 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Statistical results of the RT ZTD differences for the fixing coordinate and the kinematic modes (unit: mm).