## 2 Precise Orbit Positioning Processing Approach
### Ambiguity Fixing of BeiDouObservations  
The BDS satellite-induced code bias is identified in its code observations. Although it has a minor impact on positioning since the code measurements are assigned a much lower weight compared to the carrier phase measurements, the precision and consistence of the WL ambiguity derived from the HMW combination observations are severely affected. The variation of this bias is elevation dependent and can be corrected with empirical correction model for Inclined Geo-synchronous Orbit (IGSO) and Medium altitude Earth Orbit (MEO) satellites [18] and also for GEO satellites [19]. Due to the slight difference in satellite elevation for regional networks, such code bias is almost identical for all receivers and can be absorbed by the WL UPDs at the satellite side [6]. However, when applied in a global network in this study, they can neither be eliminated by DD nor assimilated into the UPDs. Therefore, the code biases of BDS satellites should be corrected before ambiguity fixing. The impact of the correction is shown in Figure 1. We can see that the BDS code observation residuals after correcting the satellite-induced code bias show a typical symbol of elevation-dependent: the higher the elevation angles, the smaller the residuals. Figure 1: The RMS values of IF code residuals of PPP and POP solutions. For each system, the code residuals of all involved satellites are lumped together to calculate the RMS values. The Beidou code observations before and after corrected with the satellite-induced code bias are shown in the right two subplots. Besides the satellite-induced code bias, the poor orbit quality of BDS GEO satellites will also affect the estimation of the float ambiguity as well as the kinematic position significantly. The geometric orbit error \(d\rho_{r}^{s}\) for BDS GEO satellites can be expressed as:  
\[d\rho_{r}^{s}=(\alpha_{r}^{s},\beta_{r}^{s},\delta_{r}^{s})\cdot(dx^{s},dy^{s}, dz^{s}) \tag{5}\]  
where \((\alpha_{r}^{s},\beta_{r}^{s},\delta_{r}^{s})\) is the direct cosine unit vector at the direction from the receiver \(r\) to the satellite \(s\), \((dx^{s},dy^{s},dz^{s})\) is the orbit error in radial, cross and along directions. [PERSON] et al. [6] and [PERSON] et al. [7] both use a regional network to force the orbit error to be absorbed by the NL UPDs. However, it is not practical for a global network as the unit vector is different between GEO satellites and global receivers. For PPP ambiguity fixing, the coordinates of reference stations, orbits and clocks are fixed; the orbit errors will be mostly absorbed by the IF ambiguities. Since orbit bias is changing with time, its effect is different on the estimated IF ambiguities depending on the continuous tracking time of each ambiguity besides the station location. This will, consequently, result in inaccurate UPD estimates and, finally, fixing the performance of ambiguity fixing at rover stations. However, the temporal varying orbit error can more likely be assimilated into the satellite clock estimates rather than the float ambiguities. Therefore, for POP, the orbit error will be mainly absorbed by satellite clock parameters rather than the ambiguities, which will lead to a better ambiguity fixing and positioning performance. In a multi-GNSS analysis, the satellite clock offsets of GPS, GLONASS, Galileo and Beidou may be biased with the constant since the satellite code hardware bias is absorbed by the satellite clock [20]. According to our experimental analysis, when applying such multi-GNSS clock products, the constant bias will be mostly assimilated into the code observation residuals. Here, take the observation data of the MGEX station CAS1 for analysis. The PPP and POP are processed with IF pseudorange and carrier phase measurements, and only the code observation residuals are analyzed. For GPS, even though different satellites have different elevation angles, they generally show a symbol of elevation-dependence: the higher the elevation angles, the smaller the residuals.