## 4 Results  
### Accuracy Analysis of Interpolation Model  
Using the same data processing strategy as that for the user, the atmospheric delay was extracted through the fixed ambiguity, and the difference between the extracted value and the interpolation was used to evaluate the internal coincidence accuracy. Figure 2 shows the residuals of ionospheric delay for scenario A. The common discontinuity of the three model epochs was caused by the lack of fixed-ambiguity common-view satellites at the reference stations. Compared with the other two models, USM had other discontinuous epochs. This was because there were less than four common-view GPS satellites with fixed ambiguity, and the interpolation coefficient could not be calculated. Most of the ionospheric delay residuals of the three models are within 0.1 m, and the residuals of the USM model exceed 0.1 m in some epochs. The ionospheric delay residual became significantly larger during 9:00-16:00 local time. The results for the three models were relatively similar for other scenarios and were not displayed. Table 2 shows the root mean square (RMS) and standard deviation (STD) values of the SD ionospheric residuals of all satellites for the three models. It can be seen that the ionospheric residuals of the three models increase with increasing distance between the user station and the reference station in scenarios A and B. The RMS values of the SD ionospheric delay residuals for DSM for the 41-km and 98-km reference networks were 1.4 cm and 3.2 cm, respectively. In the scenario in which the user station outside the network was 33 km away from the nearest reference station, the RMS and STD values of ionospheric residuals for DSM were 2.2 cm. The interpolation accuracy of DSM was slightly better than DIM and USM in both scenarios. As can be seen in Figure 3, the USM performed the worst in the B, and the interpolation accuracy of DSM is 34.7% higher than that of the USM. In other scenarios, the interpolation accuracy of USM is similar to that of DIM, the  
Figure 1: Distribution of stations. (**a**) The distribution of medium-scale reference networks; (**b**) the distribution of large-scale reference networks. The blue asterisks represent reference stations and red dots represent user stations. interpolation accuracy of DSM is 22.2%, 25.6%, 24.1%, and 22.2% higher than DIM in the four types, respectively. Compared with DIM, DSM fully considered the variation in the ionospheric delay of each satellite pair in different directions, so the interpolation accuracy was better than that of DIM. The USM equation was correlated, which may have led to a decline in the interpolation of atmospheric delay. According to the existing models, at least three reference stations are typically needed to interpolate the ionospheric delay. When three reference stations were used in scenario D, the interpolation accuracy of DIM and DSM did not change compared with when four reference stations were used, indicating that the server could still try to provide ionospheric corrections with just three reference stations. In contrast, USM could not provide service because the number of reference stations was insufficient. As with the RMS values, the STD values of DSM are also relatively small, which reflects the good stability of the interpolation results, as shown in Table 2. In addition, the RMS and STD values for DSM were equal for the different scenarios, indicating that there was no systematic error in the interpolation ionospheric delay residual of the model. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{8}{c}{**Scenario**} \\ \cline{2-10}
**Model** & \multicolumn{2}{c}{**A**} & \multicolumn{2}{c}{**B**} & \multicolumn{2}{c}{**C**} & \multicolumn{2}{c}{**D**} \\ \cline{2-10}  & **STD** & **RMS** & **STD** & **RMS** & **STD** & **RMS** & **STD** & **RMS** \\ \hline DIM & 1.8 & 1.8 & 4.3 & 4.3 & 2.9 & 2.9 & 1.8 & 1.8 \\ USM & 1.8 & 1.8 & 4.9 & 4.9 & 2.9 & 2.9 & - & - \\ DSM & 1.4 & 1.4 & 3.2 & 3.2 & 2.2 & 2.2 & 1.4 & 1.4 \\ \hline \hline \end{tabular}
\end{table}