## 3 Observation Quality  
In this section, we conducted an analysis of observation quality, focusing on the Signal-to-Noise Ratio (SNR) and multipath errors for four representative stations. The selected example for this analysis is the C46 satellite. Table 2 provides a summary of receiver types for the 120 selected stations. The receivers at these stations can be categorized into eight types, representing four different brands. To illustrate, we selected one representative station for each brand from the most commonly used receiver types. The SNR is a critical indicator for characterizing the signal strength and the receiver's ability to track signals. We analyzed the SNR for the four frequencies of BDS-3 from four representative stations with an elevation mask angle of 10\({}^{\circ}\). Figure 2 illustrates that SNR increases with the elevation angle, with the MET3 station exhibiting the best signal tracking performance. MET3 particularly excels in B2a, where the SNR can reach up to 60 dB-Hz. Moreover, Table 3 provides the mean SNR for the four BDS-3 frequencies at the selected stations. In Table 3, the mean SNR is approximately 3 dB-Hz stronger for a B2a signal than that for a B3I signal. We can conclude that the B2a signal demonstrates the highest SNR, followed by B3I, while B1C and B1I show comparable SNR values. \begin{table}
\begin{tabular}{l l} \hline \hline \multicolumn{1}{c}{**Items**} & \multicolumn{1}{c}{**Strategy**} \\ \hline Signals & BDS-3: B1I/B3I and B1C/B2a \\ Time span & DOY 274 to 304 in 2023 \\ Observation & Double-difference observations with ionosphere-free combination \\ Sampling interval & 180 s \\ Arc length & 24 h/72 h \\ Elevation mask & 3\({}^{\circ}\) \\ Geopotential & EGM 2008, 12 \(\times\) 12 degree [30] \\ N-body gravitation & DE421 ephemeris [31] \\ A priori reference frame & IGS20 [32] \\ Datum definition & Minimum constraint with no-net rotation (NNR) and no-net translation (NNT) aligning to IGS20 \\ Receiver antenna model & IGS20.atx [32] \\ Satellite antenna model & Phase center offsets (PCO) based on IGS20.atx and PCV are ignored [33] \\ Solar Radiation Pressure model & ECOM2 model with 7 parameters (DO, Y0, B0, BIC, B1S, D2C, D2S) [27] \\ Loading corrections & Ocean loading corrections: FES2014b [34] \\ Sub-daily ERP model & Desai-Sibois model [35] \\ Troposphere & A priori model: VMF3 grid [36], and zenith wet delay was estimated as piecewise constants with a temporal resolution of 2 h \\ Troposphere gradients & A priori model: Chen–Herring model [37], gradient parameters were estimated with a temporal resolution of 24 h \\ Earth orientation & A priori ERP: IERS EOP 20C04 [38], the UT1-UTC of first epoch was constrained to 0.00001 ms \\ Stochastic pulses & Apply the pulses with sigma of 10\({}^{-6}\) m/s in radial direction, 10\({}^{-5}\) m/s in along direction, and 10\({}^{-8}\) m/s in cross direction for the epochs of 12 h and 00 h [39] \\ Ambiguity & Fixed to integer \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Description of the processing scheme for ERP estimates. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**Receiver**} & \multicolumn{1}{c}{**Number of Stations**} & \multicolumn{1}{c}{**Typical Stations**} \\ \hline JAVAD TRE\_3 DELTA & 11 & MET3 \\ JAVAD TRE\_3 & 5 & - \\ LEICA GR50 & 10 & HOFN \\ LEICA GR30 & 2 & - \\ TRIMBLE ALLOY & 18 & MCHL \\ SEPT POLARX5 & 55 & VNDP \\ SEPT ASTERX4 & 3 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The summary of receiver types for typical stations. The multipath is one of obvious errors in GNSS observations. The multipath can be calculated as follows [40]:  
\[\text{MP}_{i}=P_{i}-\frac{f_{i}^{2}+f_{j}^{2}}{f_{i}^{2}-f_{j}^{2}}\lambda_{i} \varphi_{i}+\frac{2f_{j}^{2}}{f_{i}^{2}-f_{j}^{2}}\lambda_{j}\varphi_{j}-C_{i,j} \tag{1}\]  
where \(f_{i}\) and \(f_{j}\) (i \(\
eq\) j) denote the double frequencies. \(\text{MP}_{i}\) is the multipath of frequency i and \(P_{i}\) stands for the pseudorange measurement of frequency i. \(\lambda_{i}\) and \(\lambda_{j}\) represent the wavelengths.