## 2 Data Quality Analysis 1 (DQA1): Satellite Position Difference When Ephemeris Updates
The red point in Figure 3 demonstrates the position difference for every satellite. The blue circle and blue bar represent the mean and the standard deviation value for each satellite, respectively. The Maxima of the update difference is 2.01 m. The total satellite mean value is 0.50 m, and the standard deviation is 0.30 m.  
In order to make a comparison with the GPS, the ephemeris update results for the BDS are shown in Figure 4, and the statistical results are detailed in Table 3 for each satellite. In general, the BDS satellite position update difference is smaller than that of the GPS because the update rate of the BDS satellites is faster than that of the GPS satellites. Each ephemeris data will update within one hour for the BDS satellite. The GEO satellites exhibit the best satellite positioning results regardless whether or not the mean value and the standard deviation are generally better than those of the GPS. However, the IGSO (PRN 10) and MEO (PRN 11 and 12) exhibit dramatic differences during a normal ephemeris update. The maximum of the ephemeris update difference is 54.278 m. This sud \begin{table}
\begin{tabular}{c c} \hline
**Flag** & **Receive State** \\ \hline
**0** & New data incoming \\
**1** & Data update \\
**2** & Expired/Unhealthy data \\ \hline \end{tabular}
\end{table} 
Table 2: The state of the SIS ephemeris data. position will lead to an increase in user positioning error. This information illustrates that the orbit prediction for the BDS is not as stable as that of the GPS for the BDS IGSO and MEO satellites. After analyzing the URA index values in the ephemeris for the same week (_i.e._, 15 July to 22 July 2013) the URA index values show different results than that of DQA1. The GPS result is shown in Figure 5. The vertical axis indicates the URA index value, and the horizontal axis is time. The different colors in the figure are for the different satellite results. For GPS, the URA index values vary from 0 to 2, and the corresponding URA values are from 0.0 to 4.85 m (1\(\circ\)) [2]. In general, the broadcasted GPS URA index values are stable and have few variations. On the other hand, the broadcasted BDS URA index values are shown in Figure 6. In the collected dataset for this specific week, the BDS URA index values are very stable at 0, that is, three constellations (GEO, IGSO and MEO) of BDS URA values are all within range of 0.0 to 2.40 m [4]. Based on the URA results, BDS satellites could provide stable  
Figure 4: BDS DQA1: Satellite position difference between new and original ephemeris. Figure 3: GPS DQA1: Satellite position differences between new and original ephemeris. positioning service to users with sufficient accuracy. Though the URA values are one-sigma (1o) statistical results, the objective of the work presented here is to show that there are some rare irregularities in the broadcasted BDS ephemeris and almanac data that might be potential threat to the positioning services with more stringent requirements, for instance, the performance based navigation (PBN) for aviation. If there are sudden changes in satellite position and both the new and original ephemerides are healthy, and the update period between the two ephemerides is within one hour and the URA index values are the same, then there would be no way to confirm the consistency of the ephemeris data, unless the GNSS receiver uses the DQA1 test proposed in this research to verify each satellite position and remove the large change in the satellite position due to the new ephemeris update. Moreover, if users want to improve positioning accuracy under this circumstance, a precise orbit determination is needed as suggested in [11]. Figure 5: The broadcasted GPS URA index values for one week.