## 4 Result and Discussion
### Glonass PPP for Different End Users  
In this part, the solutions of GLONASS PPP, neglecting the type of receiver and antenna, are presented by using CODE products. Furthermore, the coordinates of receiver of time-keeping laboratories are known values, while the coordinates of time users are unknown. Hence, the influence of IFCBs on GLONASS PPP-based time transfer for different end users (with/without fixing coordinates) are compared. Figures 7 and 8 display the STD series of one-day arc solutions and ten-day arc solutions, respectively, in different schemes with coordinate-fixed over all the tests. We can see that the STDs of the one-day arc solutions are mainly less than 0.2 ns, while a part of the solutions is poor. This result is, unexpectedly, the facts listed as follows. Figure 9 depicts the clock differences of AGGO-PT11 and BRUX-PT11 at DOY 235, 2017. Obviously, a clock discontinuity may be present at DOY 235 on two time-links. We address this issue by considering a representative example (depicted in Figure 10), which lost part of the observations depicted in the red box. The ambiguities will be re-estimated in the second arc. Hence, the solutions of GLONASS PPP present a clock discontinuity, while the GPS PPP shows a good performance due to the greater visibility of satellites. One the other hand, the STD of the 10-day solutions ranged from 0.1-0.4 ns. We further find that some results are greater than 0.4 ns, such as the TSK2-PT11 and STK2-PT11 time-links. We surmise that this may be related to the Geometric Dilution of Precision (GDOP). [PERSON] et al. [13] displayed the average global GDOP of GLONASS with an elevation cut-off angle of 7\({}^{\circ}\). The results demonstrated that the GLONASS GDOP is analogous in the middle- and high-latitude regions than that in the low-latitude regions. Combined with Figures 7 and 8, we can conclude that the day-boundary discontinuities are still a problem for GLONASS PPP-based time transfer. Meanwhile, the STDs of the clock difference were determined with one-day arc solutions and ten-day arc solutions in Table 6. We can observe that the 0.075 and 0.280 ns were obtained for the IFCB0 solutions at the 68% level in one-day arc solutions and ten-day arc solutions, respectively, while 0.300 and 0.403 ns at the 95% level, respectively. Interestingly, it is found that the STDs of the clock difference between the 68% and 95% levels for one-day solutions are slightly different. The difference can be attributed to the fact that the clock jumps due to the observation  
Figure 6: The improvement percentage of the GLONASS PPP with two handle schemes on some stations selected from the stations equipped with identical receivers (**top**) and mixed receivers (**bottom**). interruption. After considering IFCBs in GLONASS-only PPP, time transfer performance is improved compared to that of IFCB0. We further support this finding as shown in Table 6, which demonstrates that IFCB1 performs better than IFCB0, while IFCB3 performs best among the processing schemes. Taken together, uncertainty (STD) of IFCB2 is significantly improved by 30.0% from 0.300 to 0.211 ns and by 10.9% from 0.403 to 0.359 ns for the one-day arc solutions and ten-day solutions, respectively, at the 95% level. At the 68% level, the STD of the clock difference is significantly reduced, by 17.3% from 0.075 to 0.062 ns and by 20.0% from 0.28 to 0.224 ns for the one-day arc solutions and 10-day solutions, respectively. Moreover, we further find that the improvement in STD at the 95% level is more obvious than that at the 68% level for one-day arc solutions. As stated before, one can conclude that a good performance for GLONASS PPP can be achieved under the observation interruption when considering the IFCBs. Figure 10: Satellite visibility of GPS (**top**) and GLONASS (**bottom**) at PT11 on DOY 235, 2017. Figure 9: The clock differences of the one-day arc solutions at AGGO-PT11 and BRUX-PT11 at DOY 235, 2017, in three handling schemes.