## 3 Characteristics of Xiaomi Mi 8 Raw GNSS Observations
### Carrier-to-Noise Density Ratio  
The C/N0 measured by the receiver is a diagnostic of both the signal power of tracked satellite and the noise density of the front-end [26,27]. A larger C/N0 indicates a stronger signal and a better observation quality for positioning; a lower C/N0 usually implicates a high probability of outliers and cycle slips.  
Figures 6 and 7 show normalized histograms, displaying the drop in C/N0 value between the survey-grade receiver Sepentrio PolaRx5, and Xiaomi Mi 8 for GPS (G), GLONASS (R), Galileo (E), BeiDou (C), and Quasi-Zenith Satellite System (QZSS) (J) on frequency L1/E1 and L5/E5, of the tracked satellites ranging in elevation from 15 to 90 degrees. The Sepent trio PolaRx5s connecting to a survey-grade antenna, which has a uniform quasi-hemispherical gain pattern, right-hand circular polarization, and a stable phase center, only has a \(\pm\)1.0 dBHz fluctuation in its C/N0 value on average relative to the reference station receiver. However, as for the linear polarization antenna embedded in Xiaomi Mi 8, the C/N0 reductions compared to the survey-grade antenna vary with respect to different navigation satellite systems and different frequencies. The average C/N0 drop is about 6, 10, 9, 11, and 7 dBHz for L1/E1 observations of the GPS, GLONASS, Galileo, BeiDou, and QZSS satellites, respectively; while the C/N0 drop in the normalized histograms for L5/E5 observations is quite different from that of L1/E1, with the main reduction from 5 to 15 dBHz and remaining relatively flat. This may be due to the imperfect antenna design of Xiaomi Mi 8 for capturing multi-frequency signals.  
In order to further analyze the characteristic of C/N0 against the elevations of satellites, average values of C/N0 within an elevation range of 1\({}^{\circ}\) are calculated for satellites G08/R06/E27/C09/J03, and shown in Figure 8 for Sepent trio PolaRx5s and Xiaomi Mi 8. The average C/N0 values of the survey-grade receiver increase relatively uniformly with the increase in elevation. However, the C/N0  
Figure 5: Sky plots and ADRS of observed GNSS satellites by Xiaomi Mi 8 in dynamitic test. (**a**) is for L1/E1 measurements. (**b**) is for L5/E5 measurements. The short red line is the ADRS field provided by an Android smartphone.  
values of Xiaomi Mi 8 sometimes have a sudden and drastic decrease as the elevation increases, especially in the low elevation region. Such rapid C/N0 variation is likely attributed to multipath effects due to poor multipath suppression of Xiaomi Mi 8. The C/N0 of Xiaomi Mi 8 with different frequency and different satellite is not significantly correlated to the satellite elevations. That is to say, the quality of Xiaomi Mi 8 data cannot be guaranteed even at high elevations. The conventional elevation-dependent weighting model is not appropriate for the Xiaomi Mi 8 data processing. The characteristic of pseudorange noise is discussed in the following section to find a suitable weighting model.  
Figure 6: Normalized histograms displaying the drop in C/N0 between a survey-grade antenna and Xiaomi Mi 8 antenna of L1/E1. The red histograms are the C/N0 drop between the two Septentrio receivers. The green histograms are the C/N0 drop between the Xiaomi Mi 8 and a Septentrio receiver. (**a**) is about GPS L1 measurements. (**b**) is about GLONASS L1 measurements. (**c**) is about Galileo E1 measurements. (**d**) is about BeiDou L1 measurements. (**e**) is about QZSS L1 measurements.  
Figure 8: Average C/N0 values against the satellite elevations for Septentrio and Xiaomi Mi 8 on L1/E1 and L5/E5. The C/N0 values within an elevation range of 1\({}^{\circ}\) are averaged.  
Figure 7: Normalized histograms displaying the drop in carrier-to-noise ratio between a survey-grade antenna and Xiaomi Mi 8 antenna of L5/E5. (**a**) is about GPS L5 measurements. (**b**) is about Galileo E5 measurements. (**c**) is about QZSS L5 measurements.