# Evaluation of GNSS-TEC Data-Driven IRI-2016 Model for Electron Density
## 3 Results
### Accuracy Analysis of Electron Density Profile
#### 3.1.2 Comparison with COSMIC Data  
The electron density of the upda-IRI-2016 output was evaluated in 2015 and 2019 using ionPrf product data from available COSMIC occultation observations within the China region. After performing the necessary quality control on the COSMIC data, the COSMIC electron density profiles within the China region were counted, and the IG-up values were interpolated according to the time and location of the COSMIC occultation event to drive the upda-IRI-2016 calculation of the electron density. Figures 5 and 6 present the available COSMIC electron density profiles for day 155 in 2015 and day 351 in 2019, respectively, compared to the electron density profiles output from IRI-2016 and upda-IRI-2016 driven by IG12 and IG-up, respectively. The black dotted, green solid, and red solid lines indicate the IEDs of COSMIC, IRI-2016, and upda-IRI-2016, respectively, and the headings show the time and geographic location (latitude and longitude, east and north, respectively) at which the occultation event occurred. Since an occultation event occurs with a drift in latitude and longitude, the latitude and longitude at the NmF2 tangent point are identified in this paper as the location where the occultation event occurred. As can be seen in Figures 5 and 6, the electron density in 2015 is much larger than that in 2019 at the same local time, which is due to the fact that the strength of the solar activity has a direct effect on the degree of ionization of the ionosphere. In high solar activity years, IRI-2016 underestimates the IED to some extent relative to the COSMIC data, while in low solar activity years it shows a trend of overestimation. Both IRI-2016 and upda-IRI-2016 output electron densities and the trend of the variations with altitude are in agreement with the COSMIC observations. However, the difference between the upda-IRI-2016 output electron density profiles and the COSMIC electron density profiles is smaller in both 2015 and 2019. In order to quantitatively analyze the information reflected in the above electron density contour plots, we counted the bottomside electron densities output by the model. Firstly, the precision of the electron density in 2015 and January 2019 (every day) is counted so that it can be compared and analyzed with the statistical results using ionosonde data as a reference. The COSMIC electron density profiles available in China for the whole year of 2015 (one day per week) are then counted and statistically analyzed for comparison with the bottomside electron density profiles output from IRI-2016 and upda-IRI-2016 driven by IG12 and IG-up, respectively. Although the number is limited, the distribution of dates is more balanced throughout the year. The statistical results of the bottomside electron density accuracy for 2015 and the first month of 2019 are shown in Table 2, from which it can be seen that the results are similar to those using ionosonde data as a reference. The errors of the bottomside electron density profiles output by upda-IRI-2016 are smaller than the errors of the bottomside electron density profiles output by IRI-2016 for both low and high solar activity years. The error of the bottomside electron density in the high solar activity years is larger than that in the low solar activity years. The bottomside electron density accuracy statistics for the whole year of 2015 are shown in Table 3, and the results for the whole year of 2015 are similar to those of the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Year**} & \multicolumn{2}{c}{**MAE (10\({}^{12}\) el/m\({}^{3}\))**} & \multirow{2}{*}{**PI**} & \multicolumn{2}{c}{**RMSE (10\({}^{12}\) el/m\({}^{3}\))**} & \multirow{2}{*}{**PI**} \\ \cline{2-2} \cline{5-6}  & **IRI-2016** & & & & **IRI-2016** & & **Upda-IRI-2016** \\ \hline
2015 & 0.1412 & 0.1042 & 26.2\% & 0.1636 & 0.1254 & 23.3\% \\
2019 & 0.0645 & 0.0516 & 19.9\% & 0.0750 & 0.0610 & 18.6\% \\ \hline \hline \end{tabular}
\end{table}