## 5 GNSS Positioning Performance of Xiaomi Mi 8
### Static Test Analysis
As to the dual-frequency positioning results with GPS, Galileo, and QZSS measurements, Figure 15 shows that the time differenced (TD) filter algorithm with a dual-frequency ionosphere-free combination positioning error RMS is 1.71 m, 2.04 m, and 1.63 m in E, N, and U directions, respectively; the SPP positioning error is 5.17 m, 5.32 m, and 8.67 m, respectively. These positioning errors are much larger than that of DF-GEJ results given by single-frequency positioning mode, as shown in Table 4. Clearly, the dual-frequency ionosphere-free combined measurements greatly amplified the pseudorange noise, and the positioning performance degraded. Thus, the ionosphere-free combination pseudorange is not suitable for GNSS positioning with raw data from Xiaomi Mi 8. When conducting single-frequency positioning with L1/E1 measurements or dual-frequency positioning with the uncombined L1/E1 and L5/E5 measurements, interestingly, whether using single-frequency or dual-frequency GNSS observations, the GEJ combination has the best positioning accuracy. When the BeiDou observations are included, the positioning accuracy decreases, and the horizontal error RMS is about 2 m. As the time differenced filter algorithm can smooth the noise of pseudorange observations, its positioning accuracy is mainly affected by the broadcast ephemeris error, satellite position dilution of precision (PDOP), and ionospheric delay error. According to the error analysis of the broadcast ephemeris of various navigation systems by [PERSON] et al. (2017); [PERSON] et al. (2017), the near-static observation geometry for BeiDou GEO satellites results in a degraded orbit determination accuracy in comparison with Inclined GeoSynchronous Orbit (IGSO) and Medium Earth Orbit (MEO) satellites. Its along-track component of GEO satellite orbits is the least well determined, with errors up to tens of meters, and typical signal-in-space range errors reaching 3 m. In addition, Figure 2 shows that, under static conditions, four GEO satellites are observed. Therefore, it is reasonable to conclude that the positioning error increment is due to the inclusion of the BeiDou GEO data. To verify this point, we re-processed the same data but excluded BeiDou GEO satellites. The results are shown in the fifth row of Figure 14 and seventh row of Figure 15. It is clear that the horizontal error RMS decreases rapidly. In addition, as illustrated in Figure 2, the carrier phase observations of the BeiDou GEO satellites are interrupted much more often than those of other satellites. Therefore, when there are sufficient visible satellites, it is suggested that, for Xiaomi Mi 8, Beidou GEO satellites should not be used. Comparing the TD filter results of C/N0-dependent weighting with that of the SISRE value based weighting in Table 4, it can be seen that, the positioning accuracy of the SISRE value-based weighting is worse than that of the C/N0-dependent weighting model. The pseudorange noise is the major one in all error sources, and much larger than the errors of GNSS satellite orbits and clocks. A reasonable weighting model should be set based on the noise level of pseudorange observations, as the pseudorange noise is clearly related to the C/N0 as shown in Figure 9. Therefore, the C/N0-dependent weighting model is more suitable.