## 4 Result and Discussion
### GLONASS PPP with Different IGS Acs Products  
In this subsection, the performances of the GLONASS PPPs with different IGS Acs products are investigated for different IFCB handling schemes. Note that the GLONASS PPP solutions without coordinate-fixed are indicated herein.  
Figure 14 shows the clock difference series of the two selected time-links using ESA and GFZ products in different schemes, respectively. The STD of the clock difference using the ESA and GFZ  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**The STD of One-Day Arc Solution (ns)**} & \multicolumn{2}{c}{**The STD of Ten-Day Arc Solution (ns)**} \\ \hline  & **(95\%)** & **(68\%)** & **(95\%)** & **(68\%)** \\ \hline IFCB0 & 0.304 & 0.074 & 0.411 & 0.305 \\ IFCB1 & 0.230 & 0.065 & 0.369 & 0.272 \\ IFCB2 & 0.211 & 0.064 & 0.368 & 0.236 \\ \hline \hline \end{tabular}
\end{table}
Table 7: The uncertainty (STD) of the GLONASS PPP solutions without fixed coordinates (ns).  
Figure 13: Pseudorange observation residuals of GLONASS PPP at AGGO and BRUX on DOY 227, 2017.  
products at the 95 and 68% levels in the one-day arc solutions and ten-day arc solutions, respectively, are presented in Tables 8 and 9. Taking Tables 7-9 together, it can be seen that the STD of the clock difference can be significantly reduced, especially for the ten-day arc solutions, when considering the GLONASS IFCBs. Interestingly, IFCB0 performs worst at the 95% and 68% levels, while IFCB2 performs best, even though different IFCBs processing strategies were adopted for different IGS ACs. This may be caused by the fact that when considering IFCBs at IGS ACs for each clock product that does not contain IFCBs, while IFCBs still exist in the pseudorange observations. This result means that IFCB estimation is necessary for the end user. Furthermore, estimating the IFCBs for each GLONASS satellite is the best choice. After applying the GFZ products, compared with IFCB0, the STD of IFCB2 is reduced by 23.47% from 0.294 to 0.203 ns, and by 14.88% from 0.430 to 0.366 ns, in the one-day arc solutions and ten-day arc solutions at the 95% level, respectively. At the 68% level, STD of IFCB2 is significantly reduced by 30.77% from 0.078 to 0.054 ns, and by 22.26% from 0.274 to 0.213 ns, for the one-day arc solutions and ten-day arc solutions, respectively.  
The STD of the clock difference using the ESA products at the 95 and 68% levels in the one-day arc solutions and ten-day arc solutions, respectively, are depicted in Table 8. Taking Tables 6, 8 and 9, it is of interest to note that different STD values of the clock difference are presented using different products. This difference can be attributed to the different accuracy of the precise orbits and clock for the IGS ACs. Similar to GLONASS PPP using the GFZ products, the IFCB2 schemes shows the best performance. Compared with IFCB0, the STD of IFCB2 is reduced by 38.78% from 0.343 to 0.210 ns, and by 32.95% from 0.516 to 0.346 ns in the one-day arc solutions and 10-day arc solutions at the 95% level, respectively. At the 68% level, STD of IFCB2 is obviously reduced by 11.29%, from 0.062 to 0.055 ns, and by 41.76%, from 0.376 to 0.219 ns for the one-day arc solutions and 10-day arc solutions, respectively.