## 2 GNSS Signal Characteristics
### Pseudorange Residual  
The raw code observation equation is written as:  
\[P_{i}=\rho+cdt_{r}-cdt^{s}+T+\frac{f_{1}^{2}}{f_{i}^{2}}I+B_{P_{i}}^{r}-B_{P_{i} }^{s}+\varepsilon_{P_{i}} \tag{5}\]  
where \(\rho\) is the geometric distance between satellite and receiver, \(c\) is the speed of light in vacuum, \(cdt_{r}\) is the receiver clock error, \(cdt^{s}\) is the satellite clock error, \(T\) is the tropospheric delay, and the other symbols are the same as before. The coordinates of SYS2 were taken as known, and the products such as precise satellite orbit and clock, final ionospheric TEC grid, etc., were provided by the IGS data center. The effects of earth tide, relativity, and Sagnac are modeled and corrected sufficiently, and the code hardware delay is ignored. Moreover, the pseudorange residuals of GNSS satellites are computed with the prior information, and the results are not affected by the parameter estimation based on Kalman filter. Therefore, the receiver clock error is computed by least squares estimation in single point positioning (SPP). The pseudorange residuals of GPS, BDS, and GLONASS are shown in Figure 5, and it should be noted that \(\varepsilon_{P_{i}}\) contains the multipath error.  
Figure 4: Difference \(D_{i}\) of observed GPS, BDS, and GLONASS satellites in static data collected by the Huawei Mate30 and geodetic receiver. (**a**) Huawei Mate30 difference \(D_{i}\) of GPS; (**b**) Huawei Mate30 difference \(D_{i}\) of BDS; (**c**) Huawei Mate30 difference \(D_{i}\) of GLONASS; and (**d**) geodetic receiver difference \(D_{i}\) of GPS, BDS, and GLONASS.  
In Figure 5, the statistical results show that the RMS value of pseudorange residuals of GPS, BDS, and GLONASS are 3.942, 3.196, and 8.063 m, respectively. [PERSON] et al. (2017) computed the RMS of GPS, BDS, and GLONASS pseudorange residuals at approximately 6-7, 3.09, and 12-13 m, with single difference (SD) for the Google Nexus9, Huawei P10, and Samsung S8, while the pseudorange error of surveying receiver was ignored. For most smartphones, the RMS of GLONASS pseudorange residuals may be lower than that of GPS and BDS. Moreover, with the increase of \(C\)/\(N_{0}\), the two-sigma (95% confidence  
Figure 5: Pseudorange residuals of observed GPS, BDS, and GLONASS satellites in static data collected by the Huawei Mate30. (**a**) Pseudorange residual of GPS; (**b**) pseudorange residual range of GPS; (**c**) pseudorange residual of BDS; (**d**) pseudorange residual range of BDS; (**e**) pseudorange residual of GLONASS; and (**f**) pseudorange residual range of GLONASS. The value after the satellite pseudo random noise (PRN) number is the RMS of pseudorange residuals.  
level) range of GPS P1 pseudorange residual decreases from the initial 17.727 m (C/N\({}_{0}\leq\) 30) to 8.428 m (40 \(<\) C/N\({}_{0}\)); the range of BDS decreases from 20.157 m (C/N\({}_{0}\leq\) 30) to 9.341 m (40 \(<\) C/N\({}_{0}\)) and the range of GLONASS decreases from 30.105 m (C/N\({}_{0}\leq\) 30) to 13.314 m (40 \(<\) C/N\({}_{0}\)). Meanwhile, the C/N\({}_{0}\) value of the GPS L5 band is generally lower than that of L1 band. There are only 11 GPS L5 observations with a value of C/N\({}_{0}\) above 35 dB-Hz among the 100 min, which are so few that there is no need to draw them in Figure 5b. The average C/N\({}_{0}\) values of L1 band of GPS PRN 10, PRN 25, and PRN 32 are 8.59, 12.18, and 9.46 dB-Hz higher than that of L5, respectively.  
The RMS of GPS, BDS, and GLONASS pseudorange residuals with each C/N\({}_{0}\) range is shown in Table 1. As the C/N\({}_{0}\) value increased, the RMS of GPS, BDS, and GLONASS pseudorange residuals all decreased significantly. The number of GPS observations with a C/N\({}_{0}\) value of L5 above 35 dB-Hz is so few that we did not include their statistics in Table 1. However, the RMS of BDS pseudorange residuals increases when the C/N\({}_{0}\) value of BDS is above 40 dB-Hz, which is caused by the satellites with a low elevation, such as PRN 35.