## 3 Experiment Setup  
Approximately, the 130 stations collected from IGS MGEX for the period day of year (DOY) from 001 to 030, 2021, are used to obtain GPS, BDS-3, and Galileo satellite mixed OSB. The distribution of the selected GNSS stations for OSB estimation can be found in Figure 1.  
The traditional phase time-variant ED OSB model is relatively simple. It can be calculated according to Equations (8)-(14), and its description will not be repeated again [33]. The undifferenced mixed OSB estimation processing strategies are illustrated in Table 1. The GPS L1/L2, BDS-3 B11/B3I, and Galileo E1/E5a are treated as the basic frequency, while GPS L5, BDS-3 BIC and B2a, and Galileo E5, E5b, and E6 are used to estimate satellite IFCB. The satellite orbits and clock offsets are fixed to the WHU final products, and the station coordinates are fixed to IGS weekly SINEX-file coordinates [3; 35]. The ZMC method for a constellation is used to eliminate rank deficiency between receiver and satellite IFCB. As for tropospheric delay, the dry parts are corrected by the modified Hopfield model based on the Global Pressure and Temperature 3 (GPT3) and Vienna Mapping Functions 3 (VMF3) models, while the wet parts are estimated by setting the parameter based on the wet part of VMF3 [36; 37]. The Phase Center Offset (PCO) and Phase Center Variations (PCVs) for GPS, BDS-3, and Galileo multi-frequency observations are corrected using the IGS antenna file (igs14.atx). In the bidirectional Kalman filter (forward and backward) of IFCB processing, the ambiguities are estimated as float constants, the ZWD is estimated as a random walk process, and the receiver clock is estimated as white noise. To avoid gross errors, and retain the IFCB's original characteristics, the satellite IFCB is estimated as a random walk process and the receiver IFCB is estimated as white noise. In the forward Kalman Filter of PPP processing, the processing strategy is basically the same as that of mixed OSB estimation. For more details for PPP processing strategies, please refer to [5; 38].  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Items** & **Strategies** \\ \hline Basic frequency pair & GPS L1/L2, BDS-3 B11/B3I, Galileo E1/E5a \\ Estimated frequency & GPS L5, BDS-3 BIC/B2a, Galileo E5b/E5/E6 \\ Elevation cutoff & 7 degrees \\ Weighting & Elevation weight [sin(elevation)] \\ Filter type & Bidirectional Kalman filter (forward + backward) \\ Satellite orbit and clock offsets & Fixed to WHU MGEX final precise products \\ Receiver coordinate & Fixed to IGS SINEX-file coordinates \\ Satellite IFCB & Estimated as random walk (\(10^{4}\) m\({}^{2}\)/s) [5] \\ Reference IFCB & Zero-mean condition \\ Receiver clock offsets & Estimated as white noise \\ Tropospheric delay & Dry part: modified Hopfield model \\  & Wet part: estimated as random walk (\(10^{-9}\) m\({}^{2}\)/s) \\ Ionospheric delay & Eliminated first order by IF observations \\ Satellite and receiver antenna & IGS MGEX values (igs14.atx) \\ Phase window effect & Corrected [39] \\ Relativistic effect & Corrected [40] \\ Earth rotation & Corrected [40] \\ Tide effect & Solid earth, pole, and ocean tide [40] \\ Ambiguity & Estimated as float \\ DCB & Parameter recombination \\ \hline \hline \end{tabular}
\end{table}
Table 1: Data processing strategy for undifferenced mixed OSB estimation.