# A Short-Term Forecasting Method of Inter-Frequency Clock Bias for GPS Block IIF Satellites
## 4 Discussion
As shown in the Figure 10, due to the smaller magnitude of IFCB and the larger positioning errors in the convening stage, the positioning accuracy of the three triple-frequency PPP cases is at the same level in the first 25 min. By analyzing the convergence time, it can be seen that without IFCB correction, the solution time required for the coordinate errors of PPP to reach 10 cm in the east, north and vertical directions was approximately 36, 15 and 48 min, respectively. If the forecasted IFCB was used, the corresponding three settlement time were approximately 26, 13 and 42 min, respectively. If measured IFCB was used, the corresponding three settlement times were approximately 24, 12 and 40 min respectively. The results show that the IFCB bias has a great influence on the convergence time of GPS three-frequency uncombined PPP, and when the IFCB correction is applied, the convergence speed can be significantly improved. It is worth noting that the convergence time was defined as the time required for the coordinate residuals to converge within 10 cm and remain stable in the subsequent epochs. In addition, compared with no IFCB correction, the IFCB prediction method proposed in this paper can still significantly improve the positioning performance of three-frequency uncombined PPP, especially in the eastern direction. The main reason may be that the satellite moves from north to south, and the dilution of precision values in the eastern direction are worse than those in the northern direction. For the kinematic PPP solution, the coordinate parameters of the station were estimated by epoch as white noise, which can better reflect the performance of the established  
Figure 10: Average convergence time series of static PPP for east (E), north (N), up (U) and three dimensional (3D) directions. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Solution Strategy** & **E** & **N** & **U** \\ \hline Un-IFCB & 1.46 & 0.92 & 2.83 \\ Pred-IFCB & 0.98 & 0.79 & 2.34 \\ With-IFCB & 0.92 & 0.78 & 2.29 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mean coordinate residuals of triple-frequency un-combined PPP of three cases (cm). PPP model [36]. As shown in Figure 11, compared with no IFCB correction, the positioning accuracy of GPS satellites in the three directions significantly improved when the forecasted IFCB and the measured IFCB were used. Statistics show that when the measured IFCB correction was used, the average RMS of kinematic PPP residual of all stations were approximately 1.75, 1.73 and 5.14 cm in east, north and vertical direction, respectively, and the positioning accuracy in horizontal direction and vertical direction improved by approximately 51.0% and 41.5% compared with no IFCB correction. When the forecast IFCB correction was used, the average RMS of the kinematic PPP residual was approximately 1.88, 1.81 and 5.50 cm, respectively, the positioning accuracy in horizontal direction and vertical direction improved by approximately 50.2% and 39.7% compared with no IFCB correction. It can be seen that IFCB has a significant impact on GPS kinematic PPP, especially in the east direction. Moreover, the improvement of kinematic positioning performance by using measured IFCB and forecast IFCB is basically the same. It should be noted that in the calculation of coordinate residual RMS in this paper, the coordinate residual in the PPP convergence period was excluded, and the residual data were collected from 2 h.