## 4 Results
### Characteristics of the Mixed OSB  
The mixed OSB obtained from Figure 2 contains pseudorange and phase time-variant OSB. The zero-mean constraint of satellite mixed OSB obtained from the two schemes may not be consistent, which introduces systematic errors in comparing the results. In order to better illustrate the satellite mixed OSB situation for two schemes, it is necessary to convert all the mixed OSB to the same reference datum. To simplify the description, Scheme 1 and Scheme 2 represent the mixed OSB obtained from the ED GFIF model and undifferenced model, respectively. To analyze the characteristics at each frequency, Figures 3-5 show the time series of the mixed OSB for GPS L5, BDS-3 B1C, B2a, and Galileo E5b, E5, and E6 signals on DOY 011 in 2021. The time resolution is 30 s. Figure 6 shows the corresponding mixed OSB amplitudes, in which x-axes depict the PRN of each satellite (for clarity and simplicity, specific numbers are not labeled). It is not difficult to find that the OSB for GPS BLOCK-IIF and BDS-3 signals have obvious amplitudes, while the GPS BLOCK-IIA, BLOCK-IIR, and Galileo OSB amplitudes are relatively small. The amplitude of GPS BLOCK-IIF L5 OSB is in the decimeter range, while the amplitude of BDS-2 B2I and BDS-3 B1C and B2a OSB is in the centimeter range. The OSB for Galileo E5b, Galileo E5, and Galileo E6 is around 1\(\sim\)2 cm. The OSB for BDS-3 B1C and BDS-3 B2a is around 1\(\sim\)3 cm. We all know that the accuracy of phase observation values is about 0.3 cm, so the IFCB correction should be considered in PPP. By comparing the OSB of Scheme 1 and Scheme 2, it can be seen that the trend of their changes is basically the same. The results further proved that the new undifferenced method is reasonable. But, undeniably, there are some differences in some satellites between the two schemes, especially G18, G23, C44, C45, and C46. There are four reasons for this phenomenon: (1) Scheme 1 assumes that the phase time-variant OSB of the same satellite at different stations has good consistency and the receiver phase time-variant OSB can be combined into satellite phase time-variant OSB. As for Scheme 2, the zero-mean condition for all satellites is introduced to separate the receiver and satellite OSB, which avoids the negative of the receiver on satellite OSB estimation. (2) Due to the use of ZMC in Scheme 2, all OSBs obtained from Scheme 2 will absorb the datum for a satellite constellation, while Scheme 1 will not. In other words, the OSBs obtained from Scheme 2 are similar to estimating satellite clock offsets, which will absorb the OSB for the reference satellite. Therefore, there are differences between the two schemes. (3) The number of BDS-3 C44/C45/C46 observations is relatively small, which affects the estimation accuracy [5]. (4) From a theoretical analysis of the formula, some parameters of the undifferenced model have certain correlations. The undifferenced model requires satellite orbit, satellite clock offsets, PCO, and PCV to achieve the estimation of OSB. Inevitably, the accuracy of these products will affect the estimation of undifferenced OSB. Therefore, the results with the new method suffer from irrational systematic errors. However, the undifferenced OSB model has better coupling with orbit and clock errors. The corresponding self-consistency between the satellite products is stronger, which will be beneficial for PPP. Those systematic errors introduced by precise products can be eliminated through the ED method. Therefore, the ED method can reflect the noise level. To further compare the advantages and disadvantages of the two schemes, Figures 7-9 depict the corresponding mixed ED OSB for GPS L5, BDS-3 B1C, B2a, and Galileo E5b, E5, and E6 signals. It is not hard to observe that the ED OSB of Scheme 2 is better than that of Scheme 1 in terms of stability and accuracy. The noise of Scheme 1 is significantly higher than that of Scheme 2.