# Improved Short-Term Clock Prediction Method for Real-Time Positioning
## 3 Experiment Results
### Experiment Using Real-Time Clock Production  
The time span of real-time clock corrections is set from day 201 to 206 in 2015 with a sampling interval of 30 s to further verify the availability of the improved model in practical real-time clock application. For the solution strategy, readers can refer to [PERSON] et al. [23]. The difference from IGS final products is that fitting and prediction using a raw clock correction sequence directly are not ideal due to the impact of the clock basis. To eliminate the reference effect in the prediction, the stable G01 satellite atomic clock is selected as the reference clock to obtain the difference in sequence between G01 and the other satellites. The new improved model is then used to perform a prediction. Figure 7 shows the different signal peaks in real-time linear prediction residuals, and the amplitude spectrum of 4-h prediction is similar to that of 12-h prediction. Therefore, the prediction strategy is changed to extract the long-term trend more effectively, and the 12-h, 6-h, and 4-h periodic terms are selected (\(k\) = 48 h).  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{6}{c}{**Improvement Percentage of the New 3-h Model with IGS Product (\%)**} \\ \hline
**IIF** & **1 h** & **2 h** & **3 h** & **IIR-M** & **1 h** & **2 h** & **3 h** \\ \hline G01 & 50.88 & 39.58 & 28.66 & G05 & 21.37 & 15.90 & \(-\)1.44 \\ G03 & 63.12 & 49.27 & 38.92 & G07 & 17.20 & 4.84 & \(-\)0.19 \\ G06 & 41.06 & 21.88 & 0.97 & G12 & 50.55 & 51.20 & 50.51 \\ G09 & 31.38 & 16.25 & 1.68 & G15 & 15.73 & 3.75 & \(-\)10.59 \\ G10 & 78.55 & 72.53 & 70.03 & G17 & 26.72 & 24.15 & 21.38 \\ G25 & 60.75 & 50.31 & 37.20 & G19 & 18.93 & 9.59 & 3.29 \\ G26 & 66.75 & 60.41 & 55.23 & G31 & 22.08 & 16.86 & 11.87 \\ G27 & 25.26 & 25.56 & 18.30 & & & & \\ G30 & 58.89 & 45.83 & 27.12 & & & & \\ G32 & 50.29 & 36.85 & 37.39 & & & & \\ \hline
**IIR** & **1 h** & **2 h** & **3 h** & **IIR** & **1 h** & **2 h** & **3 h** \\ \hline G02 & 31.70 & 27.10 & 28.11 & G19 & 18.93 & 9.59 & 3.29 \\ G11 & 76.98 & 75.03 & 73.52 & G20 & 4.92 & \(-\)6.74 & \(-\)20.96 \\ G13 & \(-\)12.47 & \(-\)15.77 & \(-\)19.04 & G21 & 25.42 & 18.77 & \(-\)2.99 \\ G14 & 8.15 & \(-\)1.42 & \(-\)8.80 & G22 & 18.84 & 10.93 & 2.05 \\ G16 & 37.18 & 34.94 & 16.96 & G23 & \(-\)0.81 & \(-\)12.78 & \(-\)24.10 \\ G18 & 20.58 & 7.06 & \(-\)4.09 & G28 & 50.49 & 46.02 & 44.36 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Corresponding improvement percentage of the new model.  
\(N\) and \(m\) take 1, 2, and 3 h. The prediction results are evaluated with the IGS final clock, in which the clock reference has been eliminated. Based on the analysis of the IGS precise clock, we use the 3-h prediction scheme.  
The performance of 1-h, 2-h, and 3-h prediction using the improved model and 3-h observation is shown in Table 5. G08, G10 and G24 carry cesium AFS and G32 belongs to the Block IIA, so all of them are not calculated in Table 5. The accuracy of 1-h, 2-h, and 3-h prediction is less than 0.4, 0.5, and 0.8 ns, respectively. A remarkable finding is that the overall performance of Block IIF remains better than those of other types of satellites.  
Reference bias does not reflect clock accuracy. As long as the clock correction of the reference clock does not affect the accuracy of the satellite position, which is better than 1 ms, the system error of the clock can be completely absorbed by receivers in the user position without affecting the final positioning accuracy. The reference bias of the G01 clock can thus be absorbed into the receivers' clock error and exerts no significant effects on the final positioning accuracy [14]. Therefore, the improved model can be used in the real-time application.