# A Short-Term Forecasting Method of Inter-Frequency Clock Bias for GPS Block IIF Satellites
## 3 Results
### Extraction of GPS IFCB  
In order to ensure the continuity of the IFCB sequence, 162 globally distributed MGEX continuous tracking stations were selected, and the distribution of these stations is shown in Figure 1. The daily observations were selected from DOY 112 to 141 in 2020, and all stations could receive three-frequency signal data with a sampling interval of 30 s. The satellite and receiver PCO/PCV were corrected using igs14.atx files released by IGS, and the phase windup was corrected using the empirical model [29]. For the absence of L5 band correction parameters at the receiver terminal in the antenna file, similar frequency parameters were used for approximate correction. The detailed processing configurations are presented on Table 1, which also applies to the performance evaluation of triple-frequency PPP. However, the G03, G04, G10, G18 and G25 were not involved in the analysis, because the G04 and G18 are IIA and IIR-A satellites, respectively, and the G03, G10 and G25 satellites have few data available for the current time period. To facilitate the analysis of IFCB time-varying characteristics within a single day, only the first 7 days' IFCB time series are given in Figure 2.  
The zero clock of the UTC was selected as the reference calendar to recover the daily IFCB, and the IFCB values of the zero clock were assumed to be 0. When a satellite at a station experienced cycle slip, the IFCB at the cycle slip calendar and the latter one calendar were no longer involved in the whole network adjustment calculation. If a satellite did not transmit a signal for a period of time for its own reasons, the moment that the satellite relaunched the signal was set to a reference calendar for the IFCB solution. It is noted that the current IFCB reference value is different from the reference value at the UTC zero clock, but it did not affect the PPP solution, because all benchmark deviations will be absorbed by the reference calendar. \begin{table}
\begin{tabular}{c c} \hline \hline
**Mode** & **Strategy and Value** \\ \hline Observations/frequencies & L1 + L2 + L5 \\ Orbits and clocks & Wuhan University \\ Ionosphere & Estimated-slant ionospheric delays (L1) \\ Zenith tropospheric delay & Estimated as random walker and the global mapping function was used \\ Cut-off & 15\({}^{\circ}\) for IFCB estimation and 10\({}^{\circ}\) for evaluation of triple-frequency PPP \\ Observation interval & 30 s \\ Satellite/receiver phase bias & Constant \\ Ambiguity & Constant/float \\ Estimation process & Kalman filter-forward \\ \hline \hline \end{tabular}
\end{table} 
Table 1: GNSS processing configurations for IFCB estimation and evaluation of triple-frequency PPP. Figure 1: Distribution of the MGEX stations. Figure 2: IFCB time series of the Block IIF satellites. when the ambiguity is initialized. As shown in Figure 2, the change laws of the IFCB time series for all GPS Block IIF satellites were more consistent, and they all embodied the periodic characteristics of approximate one sidereal day and approximate half one sidereal day, respectively, and the IFCBs had good repeatability between adjacent days, showing strong consistency. [PERSON] et al. expounded that the change regulation of amplitude was related to the Sun's elevation above the satellite's orbital plane [13]. Except for the G24 satellite, the change magnitude of the IFCB amplitude was approximately 12 cm, while the G24 satellite had a change magnitude of over 20 cm. This bias was difficult to accept for PPP, where the posterior residual was only at the centimeter level and, therefore, the IFCB must be corrected. In order to investigate the difference in the IFCB time series in different days with the time lag, the IFCB of the first day (DOY 112) was differentiated from the other day, respectively, and the residual sequence is shown with the example of G01 in Figure 3. It is clear that the residuals relative to the IFCB time series of the first day gradually increased over the time lag.