## 3 Results and Discussion
### Satellite IFCB Characteristic  
As by-products of clock estimation with multi-frequency signals, the satellite and receiver IFCBs are principally linear combinations of corresponding hardware delays. With the code hardware delays corrected at the satellite end, the resulting satellite IFCBs between the third signal B1C or B2a and the datum signals B1I and B3I are thought to be with a small deviation from zero. The satellite IFCBs between the B1C/B2a and B1I/B3I signals are presented in Figures 6 and 7 for the UC models with multi-frequency signals. The satellite IFCBs are generally less than 0.2 m, which confirms the inference above. While not explicitly shown here, the satellite IFCB differences are well below 1.0 cm between both models. Leaving out the convergence part, no obvious periodical signals are observed for the resulting satellite IFCBs between the B1C and B1I/B3I signals. However, apparent nearly diurnal variations with peak-valley values reaching 3.0 cm are found for the satellite IFCBs between the B2a and B1I/B3I signals. Recalling Equation (6) for multi-frequency data processing, we speculate that one of the main contributions is the inaccuracy of satellite and receiver PCO corrections applied for the B2a signal.  
To further analyze satellite IFCB between the B2a and B1I/B3I signals, the IFCB estimation is conducted with the GFIF combination. Considering the handling of satellite and receiver PCO corrections, three different strategies, including both satellite and receiver PCO-corrected (as a benchmark), only receiver PCO-corrected and both satellite and receiver PCO-uncorrected, are adopted to obtain the satellite IFCB. As shown in Figure 8, the generated satellite IFCB with satellite and receiver PCO-corrected are very similar to those in Figure 7. This kind of similarity can be fully explained with the underlying equivalence of the UC and GFIF models. When it comes to the satellite IFCB with only receiver PCO-corrected in Figure 9, no clear differences can be observed compared to the benchmark ones. However, as shown in Figure 10, further ignoring the receiver PCO in the GFIF model leads to obviously more stable satellite IFCB estimations, which suggests the aforementioned temporal variations are mainly originated from incorrect PCO corrections at the receiver end. Moreover, the reduction of nearly diurnal signals in satellite IFCB with PCOs uncorrected suggests that the GFIF combinations of satellite and receiver PCO corrections should be close to, if not exactly equal to zero in Equation (9). In general, the temporal stability of satellite IFCB based on the GFIF model confirms the consistency between the B2a and B1I/B3I signals.  
While the satellite IFCB shown in Figure 10 is already much more stable than the original ones in Figure 8, the underlying basis is that the GFIF combinations of PCO corrections can be fully ignored. However, the assumption is hardly valid in theory and the residual small variations for some satellites in Figure 10 confirm its failure. To further investigate the residual signals in the satellite IFCB, we here further use the PCO information in the ig8R3.atx file to conduct IFCB estimations with the GFIF model. The resulting satellite IFCB is shown in Figure 11. Because of the relatively smaller number of stations used (about half of that for Figures 8-10), there exists a re-convergence on 6 April for one single satellite and the other satellites experience tiny jumps due to the zero-mean constellation constraint. Except for the re-convergence, the temporal variations are generally disappeared for all satellites compared to Figure 10, which clearly demonstrates the consistency between the B2a and B1I/B3I signals. From the above analysis, we can  
Figure 7: Real-time satellite IFCB for B2a and B1I/B3I signals, the top one denotes results with B1I/B3I/B2a triple-frequency UC model, the bottom one denotes results with B1I/B3I/B1C/B2a quad-frequency UC model.