## 2 Data Quality
### Carrier-to-Noise-Density Ratio  
The C/N0 is the ratio of the carrier signal to noise, which can reflect the signal strength of GNSS observations on the receiver side. The larger the C/N0, the smaller the noise and the better the signal quality, and vice versa. The C/N0 of every satellite at each epoch can be directly obtained from the observation files. In this study, the relationship between the C/N0 and the elevation of the GNSS satellite signal is investigated and analyzed. When obtaining the C/N0, the sampling interval of the observation data is set as 30 s and the elevation mask is set to 0\({}^{\circ}\). All C/N0 values within 5 degrees of elevation angle are grouped into one group, and then the average of C/N0 within each group of elevation angle is calculated [21].  
The average C/N0 corresponding to elevation for BDS-2 MEO, BDS-2 IGSO, BDS-3 MEO, BDS-3 IGSO, Galileo, GPS, GLONASS, and QZSS satellite is calculated, and one  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Longitude** & **Latitude** & **Receiver Type** & **Antenna Type** \\ \hline PIMO & 121.08 & 14.64 & JAVAD TRE\_G3 TH & & ASH701945C\_M \\ SIN1 & 103.68 & 1.34 & TRIMBLE NETR9 & LEIAR25.R3 \\ USUD & 138.36 & 36.13 & SEPT POLARX5 & AOAD/M\_T \\ YARR & 115.35 & \(-\)28.95 & SEPT POLARX5 & LEIAT504 \\ CEDU & 133.81 & \(-\)30.13 & SEPT POLARX5 & AOAD/M\_T \\ DARW & 131.13 & \(-\)11.16 & SEPT POLARX5 & JAVRINGANT\_DM \\ \hline \end{tabular}
\end{table}
Table 2: _Cont._  
Figure 1: Distribution of selected stations.  
typical satellite of each system was shown in Figure 2, respectively. For the BDS-2 MEO satellite, the C/N0 of the B1I signal for the C12 satellite is slightly worse than that of B3I and B2I; although the signal strength is different, the C/N0 variation of these three frequencies shows the same variation trend. The C/N0 of B1I, B3I, and B2I signals have comparable performance for all elevation angles in terms of BDS-2 IGSO satellites. If these three signals can be selected for users, the difference in C/N0 does not need to be considered. In terms of the BDS-3 satellites, the C/N0 of the B2b signal is worse than that of B1C, B1I, B2a, and B3I whether for the IGSO or MEO satellites. It can be clearly seen that the C/N0 of BDS-3 satellites is better than that of BDS-2, which may be that the Binary Offset carrier (BOC) and Quadrature Multiplexed Composite Binary Offset Carrier (QMBOC) signal design is applied to BDS-3 satellites. For MEO satellites, the C/N0 of BDS-3 is higher than that of BDS-2 with 1-2 dB-Hz; while these values are 2-3 dB-Hz for IGSO satellites. The C/N0 of the Galileo E5 signal shows the best performance among the five Galileo frequency bands. The C/N0 of the L2 Z-tracking signal channel for GPS satellites is poorer than that of other frequencies. Moreover, the C/N0 for the L1 C/A signal is slightly poorer than that of L1C, L2C, and L5 I + Q. The C/N0 of GLONASS G1 frequency is better than that of G2, which can be attributed to the lower frequency value of the G2 signal. The C/N0 values for five QZSS frequencies present a similar performance at the different elevation angles. In terms of the five systems: the GPS and Galileo satellites show the best performance, and the C/N0 value can reach 55 dB-Hz when the elevation is nearly 90 degrees, both the BDS and GLONASS are poorer than that of GPS and Galileo, and the QZSS presents the worst performance among five GNSS systems.  
Figure 2: The average C/N0 versus elevation angle of BDS-2/ BDS-3/GPS/QZSS/GLONASS/G Galileo from DOY 283 to 289, 2021.