## 4 Data Quality Analysis 3 (DQA3): Applicable Period for Ephemeris  
According to the official description, the GPS satellite updates its new ephemeris data every two hours and the BDS satellite updates every hour. Because of the weakness of ephemeris, the use periods are less than those for the almanac data. The ephemeris can be used for only a few hours. If one satellite dose not receives a new ephemeris, the original ephemeris should be removed after a certain time, or the expired ephemeris data will generate the wrong satellite position and clock correction for the user. In this section, the same ephemeris is used for six hours in order to compare the satellite positioning results with the updated ephemeris, as shown in Figure 10.  
For one user, each satellite is in receivable space at different times. The first requirement is to select the period for each satellite SIS data that continues for six hours. As a result, all the GPS and BDS satellites can be received for over six hours. Figure 11 presents all the GPS satellites using their own ephemeris for six hours. The different colors stand for different satellites. The updated ephemeris is used to calculate the true location of each satellite. In Figure 11, the slight jump at the two hours point is due to the new ephemeris difference as discussed in Section 2.1.1. However, after four hours, every GPS satellite position difference is dramatically increased. The maximum difference after six hours is 94.9 m, and the minimum is less than 10 m. As a result, if the GPS ephemeris does not update when it need to, the available period of the ephemeris is about four hours.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{GEO} \\ \hline PRN & 1 & 2 & 3 & 4 & 5 \\ Mean & \(-\)0.02 & \(-\)0.03 & 0.00 & 0.05 & 0.03 \\ std & 0.24 & 0.38 & 0.11 & 0.11 & 0.19 \\ \hline \multicolumn{5}{c}{IGSO} \\ \hline PRN & 6 & 7 & 8 & 9 & 10 \\ Mean & \(-\)0.01 & \(-\)0.05 & \(-\)0.03 & 0.04 & 0.00 \\ std & 0.37 & 0.41 & 0.27 & 0.27 & 0.26 \\ \hline \multicolumn{5}{c}{MEO} \\ \hline PRN & 11 & 12 & 13 & 14 & X \\ Mean & 0.17 & 0.20 & \(-\)0.03 & 0.06 & X \\ std & 0.59 & 0.41 & 1.30 & 0.64 & X \\ \hline \hline \end{tabular}
\end{table}
Table 4: Statistical results for BDS DQA2.  
Figure 10: Schematic diagram of DQA3.  
The DQA3 result for the BDS is shown in Figure 12. Compare with the GPS satellites, all the BDS satellite position errors rise after three hours. The maximum difference after the six hours is 221.5 m. This error is greater than that of the GPS due to the shorter expired time of the ephemeris. The available period of the BDS ephemeris is defined in this paper as being three hours. This result is an hour faster than the GPS ephemeris. This also explains the why the BDS ephemeris must be updated every hour. It can be concluded from the ephemeris data comparison that the time and orbit prediction system of the BDS is not as mature as that of the GPS. The BDS ephemeris data needs an integrity system to validate a new incoming ephemeris. The user positioning result will be smoother  
Figure 11: GPS DQA3: The same ephemeris used for 6 h.  
Figure 12: BDS DQA3: The same ephemeris used for 6 h.  
after the new ephemeris has been tested first. On the other hand, the results show that the control of the BDS satellite (by the ground control station in China) still has room for improvement.