## 4 Experiments and Analysis
### Performance Analysis for the Galileo System  
Actually, for most Galileo satellites, including the IOV (in-orbit validation) and FOC (full operational capability) types, have a satellite repeat period of about 10 days [25]. However, as for the E18 and E14 satellites, their satellite repeat period is about 20 days. This is mainly because that they were launched into incorrect orbits in 2014 and 2015, respectively [26; 27]. Therefore, in this experiment, the dataset collected during DOY:161-191, 2018 was used to assess the performance of the satellite repeat shift time of the three methods for Galileo satellites. Figure 10 demonstrates the results of the mean shift time of the three methods for most Galileo satellites, for which the satellite repeat period is about 10 days. From Figure 10, it is obvious that the mean shift time calculated by the three methods performs almost similarly for the same satellite. The maximum difference between the three methods is only 5 s, which appeared in E12 for BEM and CCM. In addition, it is clear that the shift time of the satellites is roughly divided into two categories, one around 2430 s and the other at about 2020 s. Therefore, it also indicates that the mean shift time of the different satellites is different, and the maximum difference reaches about 12 s between the E04 and E30 for CCM. Figure 8: The mean shift time for BDS MEO satellites calculated by the three methods during DOY: 161–181, 2018. Figure 9: The STD of the satellite repeat shift time for the three methods for each satellite during the period DOY:161–181, 2018. Similarly, Figure 11 presents the STD of the three methods to evaluate the fluctuation of the shift time for the satellite. Apparently, there is no significant difference in the STD among the three methods for the same satellite, and most of the STD values are higher than 4. However, as for the E11, E12, and E26 satellites, the fluctuation of the shift time is relatively stable compared to other satellites during this period. In addition, compared to the GPS and BDS systems as a whole, it is clear that the GAL satellite STD is almost greater than the GPS, BDS IGSO, and BDS MEO but is still smaller than the BDS GEO satellite. Since only two satellites have the special satellite repeat period, Table 1 presents the characteristics of the mean shift time and the corresponding STD of the three methods for each satellite. From Table 1, it can be seen that the shift time is different from the above satellites, and the mean shift time is more than double that of the normal orbital satellites. This is consistent with the theoretical derivation. Furthermore, it is obvious that the mean shift time for the same satellite is significantly different among the three methods, and this is different from other satellites. This phenomenon is perhaps due to the characteristic of the long repeat period, which leads to the error accumulation varying with the different methods. Moreover, from the STD of the three methods, it is also found that the fluctuation of the shift time performs higher than other satellite during this period. Numerically, the STDs of these methods are all greater than 30, which is markedly different from other system satellites. It may be mainly  
Figure 11: The STD of the satellite repeat shift time for the three methods for each satellite during the period DOY:161–191, 2018. Figure 10: The mean shift time for GAL(Galileo) satellites calculated by the three methods during DOY: 161–191, 2018.  
affected by three reasons. The first reason is that these two satellites are in the incorrect orbit due to the fault of the launching calculation; this may cause the satellite to be in an unstable state. The second reason is that these two satellites have longer satellite repeat periods, which is about 20 solar days. In addition, the satellite will repeat 37 times in the 20 solar days. Therefore, the longer satellite repeat period may lead to the accumulation of errors, resulting in a larger standard deviation of the shift time.