## 3 Results
### \(foE\) and \(hmE\) Comparison  
The E-region electron density peak, \(foE\), and corresponding altitude, \(hmE\), for GNSS-RO and ionosonde profiles were compared next. These values were extracted directly from the ionograms and were estimated for each corresponding GNSS-RO profile through visual inspection of the EDPs. For this comparison, only the \(hmE\) from the unshifted ionosonde profiles was used. Due to the smoothing nature of the optimal estimator algorithm and the one kilometer vertical resolution, some of the GNSS-RO profiles did not have a clear E-region peak (to be discussed in more detail in Section 4). In these cases, the profile was excluded from the \(foE\) and \(hmE\) comparison. The number of profiles used for each site and for the combined comparison is shown in Table 4.  
GNSS-RO and ionogram \(foE\) values are displayed in Figure 10 for all four sites. Due to a large percentage of GNSS-RO profiles at I-Cheon without a clear E-region peak, there were not enough profiles for a statistically significant comparison. While the figures for I-Cheon are included, the discussion will focus on Hermanus, Gakona, and Fortaleza. At each site, the \(foE\) between the two profile methods are generally spread around the 1:1 line with slopes near one. The average \(foE\) of the GNSS-RO profiles is slightly lower than the ionosonde profiles at Hermanus and Gakona and is virtually the same at Fortaleza (Table 5). There is relatively large variability in \(foE\) for both the GNSS-RO and ionosondes, which leads to lower R\({}^{2}\) values despite the linear fit having nearly the same slope at all sites. Figure 9: Average EDPs for GNSS-RO measurements and FIRI predictions for each site showing general agreement in the D- to E-transition region. The average density was calculated in 1 km increments and is shown with standard deviation uncertainties, shown by shading for both profiles. Note that the electron density scale varies between sites. A cumulative distribution function (CDF) histogram for the difference between the values was calculated and is shown in Figure 11. The \(foE\) difference was calculated by subtracting the ionosonde \(foE\) from the GNSS-RO \(foE\), and the CDF histograms produced similar results for all sites. Hermanus has a slight negative skew at one standard deviation, while Gakona has a slight positive skew and Fortaleza is nearly symmetric about the mean. Hermanus and Gakona had three and two profiles with an \(foE\) difference greater than \(1\) MHz, respectively. These sites also have a slightly negative average difference, meaning that the \(foE\) difference is smaller than the \(foE\) difference. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Site** & **Hermanus** & **I-Cheon** & **Gakona** & **Fortaleza** & **Combined** \\ \hline Total Profiles & 71 & 41 & 49 & 47 & 208 \\ \(foE\) and \(hmE\) Profiles & 55 & 14 & 26 & 36 & 131 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The number of profiles used for \(foE\) and \(hmE\) comparison for each site. Many profiles had to be removed, particularly at I-Cheon and Gakona due to GNSS-RO profiles without a clear E-region peak. Figure 10: A comparison of GNSS-RO and ionogram \(foE\) measurements. The black line is the 1:1 line, and the red line is the linear fit. Most of the slopes are near one, with the exception of I-Cheon which contained the fewest RO profiles with a clear E-region peak. the ionosonde tends to measure a slightly larger \(N_{e}\) at the E-region peak. For all sites, most profiles have an \(foE\) difference within \(\pm 0.5\) MHz. The \(hmE\) comparison was performed in the same manner as \(foE\), with the \(hmE\) values shown in Figure 12. As before, there is not enough data for a statistically significant comparison at I-Cheon, so the figures for I-Cheon are included, but the discussion will focus on Hermanus, Gakona, and Fortaleza. The GNSS-RO \(hmE\) generally do not match well with the ionosonde values.