# Global GNSS-RO Electron Density in the Lower Ionosphere
## 4 Discussions
### Comparisons with Ionosonde Data at Hermanus
GNSS-RO and ionosonde-derived \(N_{e}\) at each height within the \(f_{\rm min}\) altitude and \(h_{m}E\) for the 60 profiles analyzed in this comparison are displayed in Figure 15. Here, the ionosonde \(N_{e}\) profiles are interpolated at each GNSS-RO altitude for a direct comparison. While the R\({}^{2}\) and slope values for the linear regressions are nearly equal for the shifted and unshifted profiles, the y-intercept is closer to zero for the shifted profiles, closer to the 1:1 line that represents a perfect match between the two techniques. Overall, the densities show reasonable agreement with a larger variation for elevated densities. Figure 14: Example electron density profiles for the GNSS-RO and ionosonde measurements. The vertical line corresponds to the minimum return frequency observed by ionosondes, \(f_{\rm min}\). Plot (**a**) corresponds to a daytime profile, while (**b**) shows a nighttime profile. Histograms of the differences in density (\(N_{e,\text{GNSS-RO}}\)-\(N_{e,\text{ionosonde}}\)) as a function of altitude are displayed in Figure 16. The altitude bias in the unshifted profiles results in an underestimation of the electron densities by the GNSS-RO technique for the altitude range of roughly 90-110 km. However, when the ionosonde profiles are shifted in altitude, this underestimation bias is mostly removed, and the density differences are slightly less than zero for most altitudes. The mean absolute error (MAE) for the unshifted profiles is 1.9 \(\times\) 10\({}^{10}\) m\({}^{-3}\), while the shifted profiles' MAE is reduced to 1.4 \(\times\) 10\({}^{10}\) m\({}^{-3}\). Finally, the averaged daytime \(N_{e}\) for both techniques are displayed in Figure 17 for the shifted and unshifted profiles. Daytime profiles are constrained between 0800-1600 in LST, providing a total of 22 profiles for comparison. The altitude bias is obvious in the unshifted profiles and shifting the ionosonde profiles up in altitude helps to improve agreement. While the shifted ionosonde densities are larger than the RO-derived densities  
Figure 16: Histograms of the \(N_{e}\) difference (\(N_{e,\text{GNSS-RO}}\)-\(N_{e,\text{ionosonde}}\)) at each GNSS-RO altitude between the \(f_{\text{min}}\) altitude and \(h_{\text{m}}E\). Plot (**a**) shows the unshifted \(N_{e}\) differences, while (**b**) shows the shifted profile differences. The color bar corresponds to the number of measurements within each bin. Figure 15: Scatter plot of the electron densities predicted for each GNSS-RO altitude within the ionosonde \(f_{\text{min}}\) altitude and \(h_{\text{m}}E\). The ionosonde profiles are interpolated at each RO altitude for comparison. Unshifted profiles are compared in (**a**), while the ionosonde profiles shifted in altitude are displayed in (**b**). The linear regression parameters are also shown for both panels. on average, the mean \(N_{e}\) profiles agree within their estimated uncertainty at \(E\)-region altitudes. Encouraged by the preliminary comparison, we plan to carry out a further, larger validation study to include more ionosonde sites from different latitudes. The new GNSS-RO \(N_{e}\) data at the altitudes near the ionosonde \(f_{\rm min}\) may provide an alternative pathway for pinning the starting heights during ionogram inversion.