## 3 Results  
### Individual Electron Density Profile Comparison  
In this section, we compare the electron density profiles (real heights) derived from GNSS-RO, ionosonde, and FIRI estimates. Initially, each individual GNSS-RO profile was compared with the corresponding ionosonde and FIRI profiles. The number of profiles used at each site is listed in Table 2. To focus on the D- and E-regions, EDPs are analyzed up to a maximum altitude of 130 km. An example of an individual profile comparison for each site can be seen in Figure 3, where the ionosonde profile is a blue dot-dash line, the GNSS-RO profile is a red dotted line, the FIRI profile is a solid cyan line, the \(fmin\) value from the ionogram is included as a vertical black dashed line, and the comparison altitudes are denoted by green markers on the ionosonde profile. These altitudes are the altitudes of the ionosonde profile that occur between the \(fmin\) altitude and the altitude of the E-region peak (\(hmE\)) and will be used to compare the ionosonde \(N_{e}\) at each given altitude to the corresponding GNSS-RO \(N_{e}\) value at that altitude. The provided GNSS-RO profiles have a 1 km vertical resolution [23], while ionosondes typically transmit with a frequency resolution of 100 kHz [7; 32] to obtain virtual heights that are inverted to provide EDPs with a 1 km vertical resolution using SAO Explorer. Since the RO altitudes and ionosonde real heights do not always align, the ionosonde EDPs are interpolated at the RO altitudes using a cubic spline (green markers in Figure 3). Among all of the individual profiles, there are two key features that are immediately apparent. First, the shape of the GNSS-RO profile in the E-region appears to be very similar to the ionosonde in this region; however, there is a separation of a few kilometers between the two. In the four examples above and nearly all of the individual profile comparisons, the GNSS-RO profile is at a slightly higher altitude at \(fmin\) than the ionosonde. The second key feature is the difference in the profile shapes below the ionosonde \(fmin\) where the RO profiles show a gradual decrease in electron density between the E- to D-region, where the ionosonde profiles drop rapidly to zero before reaching D-region altitudes. This difference will be discussed in more detail in Section 4. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Site** & **Hermanus** & **I-Cheon** & **Gakona** & **Fortaleza** & **Combined** \\ \hline Number of Profiles & 71 & 41 & 49 & 47 & 208 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The number of individual profiles compared for each site. Each GNSS-RO profile is compared against an ionosonde within 2\({}^{\circ}\) and 30 min of the profile time, and with an FIRI profile corresponding to the same local solar time and latitude. A total of 208 profiles were compared. Figure 3: Individual electron profile comparison examples. The GNSS-RO and ionosonde real height profiles have a similar shape in the E-region, but are separated in altitude. The CDAAC descriptors displayed here are (**a**) atmPhs_C004.2010.273.06.22.G17_2013.3520, (**b**) atmPhs_C002.2013.054.04.43.G22_2013.3520, (**c**) atmPhs_C005.2011.174.04.34.G20_2013.3520, and (**d**) atmPhs_C2E2.2020.262.09.28.R20_0001.0001. To account for a potential altitude bias between the GNSS-RO and ionosonde profile, the choice was made to shift one of the profiles so the profiles align at \(fmin\). Since there is some level of uncertainty in the altitude of both the GNSS-RO and the ionosonde profiles, particularly the starting altitude, the ionosonde profiles were shifted. Here, we shift the profiles strictly to compare the EDP shapes, and we do not imply that all ionosonde derived EDPs require a shift in altitude. A more detailed discussion regarding this altitude bias is provided in Section 4. The altitude shift consists of calculating the altitude difference between the two profiles at \(fmin\) and shifting the entire ionosonde profile by the difference, thus pinning the profiles together at \(fmin\).