## 4 Results and Validations
### Assessment of IGS-RT Orbit and Clock Products
For GLONASS satellites, the STD values of the difference between RT CLK01 clocks and the GFZ final clocks are 0.22 ns or so, which is the best among those RT GLONASS clocks. The STD values for CLK92 GLONASS clocks range from 0.3 to 0.4 ns, while those for GFZC2 and GFZD2 clocks are between 0.2 and 0.4 ns. Additionally, the accuracy of RT GLONASS satellite clocks provided by CLK81 is comparable to that by CLK92. The Galileo clocks derived from CLK92 provide a higher accuracy of between 0.1 and 0.25 ns than those of around 0.5 ns from GFZC2 and GFZD2. The STD value for BDS GEO satellite clocks from CLK92 can reach about 0.4 ns. For BDS IGSO and MEO satellites, a better clock accuracy of 0.2 to 0.6 ns can be obtained for GFZD2 compared to those for CLK92 and GFZC2. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**IGS-RT Service** & **TYPE** & **Along (cm)** & **Cross (cm)** & **Radial (cm)** \\ \hline \multirow{2}{*}{**CLK01**} & GPS & 4.00 & 2.12 & 1.34 \\  & GLONASS & 9.15 & 4.30 & 2.99 \\ \hline \multirow{2}{*}{**CLK81**} & GPS & 6.46 & 3.65 & 1.83 \\  & GLONASS & 8.70 & 6.06 & 2.83 \\ \hline \multirow{6}{*}{**CLK92**} & GPS & 6.54 & 6.34 & 3.00 \\  & GLONASS & 9.69 & 7.81 & 2.99 \\  & Galileo & 8.31 & 4.66 & 2.68 \\  & BDS GEO & 59.79 & 43.33 & 28.97 \\  & BDS IGSO & 50.22 & 24.74 & 16.58 \\  & BDS MEO & 17.13 & 9.68 & 4.48 \\ \hline \multirow{6}{*}{**GFZC2**} & GPS & 15.13 & 12.92 & 5.61 \\  & GLONASS & 19.50 & 13.67 & 5.85 \\ \cline{1-1}  & Galileo & 29.56 & 13.67 & 9.44 \\ \cline{1-1}  & BDS GEO & 65.83 & 12.90 & 14.63 \\ \cline{1-1}  & BDS IGSO & 22.02 & 40.96 & 25.28 \\ \cline{1-1}  & BDS MEO & 34.05 & 17.23 & 7.23 \\ \hline \multirow{6}{*}{**GFZD2**} & GPS & 10.56 & 9.77 & 3.89 \\ \cline{1-1}  & GLONASS & 13.68 & 9.57 & 4.04 \\ \cline{1-1}  & Galileo & 14.31 & 13.60 & 8.56 \\ \cline{1-1}  & BDS GEO & 60.37 & 15.70 & 14.47 \\ \cline{1-1}  & BDS IGSO & 24.98 & 27.54 & 10.48 \\ \cline{1-1}  & BDS MEO & 16.00 & 10.95 & 4.40 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The averaged RMS values of the differences between IGS-RT orbits and GFZ final orbits for GPS, GLONASS, Galileo, and BDS. Figure 5 illustrates the averaged STD values of the differences between the IGS-RT clocks and the reference clocks for each system. In general, the GPS clocks fit the best with the GFZ final clocks, while the GLONASS clocks reveal a slightly worse agreement. Compared to RT GLONASS clocks, the relatively lower clock accuracy for Galileo and BDS could be attributed to the currently uncompleted constellation, as well as the limited number of available tracking stations. However, the quality of the BDS and Galileo products is expected to be improved in the future with more satellites in orbit, a more densified tracking network, and the enhanced availability of more accurate parameters in the space segment [32]. Figure 4: STD values of differences between IGS-RT clocks and the GFZ final clocks for each satellite from GPS (**a**), GLONASS (**b**), BDS and Galileo (**c**). For GPS satellites, the RT clocks offered by CLK92 show the highest accuracy, while those by GFZC2 and GFZD2 present the worst accuracy. The averaged STD values of all GPS satellites are about 0.16, 0.20, 0.12, 0.26 and 0.24 ns for CLK01, CLK81, CLK92, GFZC2 and GFZD2, respectively. For the GLONASS satellites, the averaged STD values for CLK01 and CLK81 clocks are about 0.22 and 0.24 ns, respectively, while the GLONASS clocks provided by CLK92 reveal an averaged STD of 0.35 ns. The accuracies for both the GFZC2 and GFZD2 GLONASS clocks are about 0.29 ns. As for the Galileo satellites, the CLK92 clocks achieve a higher accuracy compared to GFZC2 and GFZD2. The averaged accuracies are about 0.22, 0.55, and 0.42 ns for the CLK92, GFZC2, and GFZD2 clocks, respectively. For the BDS satellites, the clocks derived from GFZD2 show the best accuracy in comparison with those from CLK92 and GFZC2. The averaged STD values of all BDS satellites are about 0.57, 0.56, and 0.44 ns for the CLK92, GFZC2, and GFZD2 clocks, respectively.