## 7 Conclusions and Future Work  
This study collected actual SIS GPS and BDS data to present a detailed comparative study of GPS and BDS navigation data. The GPS ephemeris and almanac data were treated as a baseline by which to evaluate the BDS ephemeris and almanac data. The five DQAs proposed in this paper successfully captured the anomalies resulting from the use of BDS navigation data to calculate the satellite information. Specifically, the DQA1 detected that BDS IGSO and MEO have a larger position difference than that of the GPS when ephemeris updates. BDS users could use the DQA1 mechanism to test a new ephemeris and remove the irregular ephemeris data in order to have a stable satellite position solution. Compared to the GPS, the DQA2 showed that all of the BDS satellites have larger clock differences when the ephemeris updates. That is, the BDS ephemeris clock corrections accuracy  
\begin{table}
\begin{tabular}{c c c c c} \hline  & \multicolumn{4}{c}{GPS} \\ \hline
**MAX** & **PRN** & **Vel.** & **E.A.** & **Orbit** \\ \hline
1 st date & 3 & 0.35 & 0.003 & 2079.03 \\
1 st week & 26 & 2.63 & 0.052 & 23,328.54 \\
2 nd week & 26 & 6.80 & 0.125 & 60,979.13 \\
3 rd week & 26 & 12.48 & 0.302 & 107,782.35 \\
4 th week & 32 & 6.80 & 0.103 & 60,190.15 \\
5 th week & 26 & 18.71 & 0.427 & 161,347.99 \\ \hline \end{tabular}
\begin{tabular}{c c c c} \hline
**MAX** & **PRN** & **Vel.** & **E.A.** & **Orbit** \\ \hline
1 st date & 5 & 0.20 & 0.003 & 4017.31 \\
1 st week & 3 & 0.37 & 0.030 & 52,818.54 \\
2 nd week & 3 & 1.70 & 0.115 & 134,449.08 \\
3 rd week & 1 & 2.01 & 0.327 & 293,921.55 \\
4 th week & 1 & 3.21 & 0.342 & 302,857.73 \\
5 th week & 1 & 2.75 & 0.292 & 328,923.79 \\ \hline \end{tabular}
\begin{tabular}{c c c c} \hline
**BDS IGSO** \\ \hline
**MAX** & **PRN** & **Vel.** & **E.A.** & **Orbit** \\ \hline
1 st date & 10 & 0.23 & 0.003 & 3388.02 \\
1 st week & 7 & 4.90 & 0.013 & 11,788,168.81 \\
2 nd week & 10 & 21.86 & 0.297 & 327,414.07 \\
3 rd week & 10 & 80.69 & 1.015 & 1,201,839.57 \\
4 th week & 10 & 108.70 & 2.693 & 1,697,328.75 \\
5 th week & 10 & 147.73 & 2.633 & 2,188,705.08 \\ \hline \end{tabular}
\begin{tabular}{c c c c} \hline
**BDS MEO** \\ \hline
**MAX** & **PRN** & **Vel.** & **E.A.** & **Orbit** \\ \hline
1 st date & 14 & 0.10 & 0.002 & 800.21 \\
1 st week & 12 & 1.84 & 0.033 & 14,524.20 \\
2 nd week & 12 & 2.58 & 0.044 & 19,293.95 \\
3 rd week & 12 & 5.25 & 0.071 & 39,728.10 \\
4 th week & 11 & 5.31 & \(-\)0.077 & 38,446.30 \\
5 th week & 12 & 10.50 & 0.189 & 81,091.40 \\ \hline \end{tabular}
\end{table}
Table 7: DQA5 statistic result.  
is insufficient. BDS users are recommended to have an additional estimation scheme for the BDS satellite clock correction. The DQA3 estimates the applicable period of the ephemeris data, and for the same working accuracy level, the GPS ephemeris could be used for 4 h, and the BDS ephemeris could be used for 3 h. According to these results, if one BDS satellite does not receive a new ephemeris update, then a BDS user should remove the old ephemeris data after 3 h. The DQA4 verified that the satellite position differences between the almanac and ephemeris for both systems are of almost the same quality. Finally, the DQA5 validated the applicable almanac data periods for both systems. As shown in the results, the GPS almanac could be applied for over 5 weeks. On the other hand, the BDS IGSO elevation angle estimates failed after the third week.  
In conclusion, the BDS can provide positioning service as good as that of the GPS most of the time. However, sometimes the BDS will have irregular ephemeris and almanac data information, and BDS users will not be able to discover these anomalies using a conventional GNSS receiver. Importantly, the probability and magnitude of unusual BDS ephemeris and almanac data events are larger than those of the GPS. Thus, if one would like to integrate the GPS and BDS for navigation and guidance, then additional navigation data quality analyses as proposed in this paper are needed.