## 2 Data Quality Analysis 1 (DQA1): Satellite Position Difference When Ephemeris Updates  
Ephemeris data is more precise than almanac data. Each satellite broadcasts its own ephemeris data. Before user positioning, ephemeris data for each satellite in view must be downloaded first. The information in the ephemeris data helps users to calculate the precise location, inclination and size of the satellite orbit. After that, the user can use the time information to calculate the satellite position on this orbit. However, the satellite position calculated from ephemeris data is not the true location of the satellite [10]. Due to the fact that an elliptical orbit cannot reflect the real dynamics of the satellite, satellite positions calculated from ephemeris data still have some errors. When one ephemeris updates its own satellite position, the new orbit parameters will create a new satellite orbit. If the satellite moves in a stable manner on its expected orbit, and the orbit parameters in the ephemeris data are provided correctly, the satellite position difference between the new and original ephemeris data will be under a certain value. As shown in Figure 2, there still is a slight difference because the error between the true location and different orbit _i.e._, the new and old ephemeris orbit is not the same. In this section, we use the true GPS and BDS SIS to acquire the ephemeris data and verify the satellite position difference calculated from each new and original ephemeris when the ephemeris updates. Before calculating the positioning result for each ephemeris data, the ephemeris is classified. The flag in Table 2 presents the state of each ephemeris flag. Flag 0 means the satellite did not have any ephemeris before. This satellite is a new incoming satellite, and it does not have any old ephemeris  
Figure 2: Schematic diagram of DQA1. to provide a comparison. Flag 1 means this satellite already has an old ephemeris and that it is currently broadcasting a new ephemeris. Generally, the ephemeris data updates for a certain period. In the case of the GPS, the ephemeris updates every two hours, and in the case of the BDS, the ephemeris updates every hour. If one satellite is influenced and deviates from the original orbit, the ephemeris will update more quickly than its specified period. Flag 2 mark the ephemeris data for which the update time is over the predetermined time (GPS: 2 h, BDS: 1 h), and this flag also marks an unhealthy satellite. If one ephemeris flag is 2, this satellite ephemeris will be removed, and the next ephemeris will be waited for, which will be marked as flag 0. In this section, we only analyze cases of flag 1 ephemeris. The results are presented for each ephemeris update under normal conditions. Another important quality index to assess the open service positioning accuracy of a satellite navigation system is the user range accuracy (URA). The URA definition is given in both the GPS interface control document (ICD) [2] and BDS ICD [4], and URA is a parameter in the ephemeris to provide a conservative RMS estimate of the user range error (URE) in the associated navigation data for the transmitting satellite. URA includes all errors for which the Space and Control Segments are responsible. That is, the URA presents the statistical result for the satellite ephemeris and clock errors, and it does not include the errors after the signal transmitted from the satellite (_i.e._, the ionospheric, tropospheric and receiver errors). The comparison of the broadcasted URA values and the corresponding DQA1 results is given at the end of this section. The calculation of the satellite position follows the GPS ICD and the BDS ICD. Figure 3 demonstrates the GPS satellite position difference for each satellite when the ephemeris updates (flag = 1). The analysis results are calculated for one week from 15 July to 22 July 2013. The horizontal axis presents each satellite number, and the vertical axis shows the position difference value.