## 5 Conclusions  
A comparison of the bottom-up approach for generating electron density (\(N_{e}\)) profiles in the D- and E-region ionosphere created by [22] and refined by [23] was performed. GNSS radio occultation (GNSS-RO) profiles were compared against ionograms at four sites around the world when the occultation was within \(2^{\circ}\) of the ionosonde location and within 30 min of a sounding that clearly measured the E-region and its peak. Ionograms were hand-scaled to ensure quality and soundings with sporadic-E (\(E_{s}\)) were removed from the comparison. The ionosonde sites covered all three the latitudinal regions; Fortaleza,  
Figure 21: Calculated \(\mathrm{d}z/\mathrm{d}f_{p}\) for the shifted GNSS-RO and ionosonde profiles which show elevated ionosonde derivatives at higher altitudes near \(hmE\). Brazil, in the equatorial region; Hermanus, South Africa, and I-Cheon, South Korea, in the mid-latitudes; and Gakona, Alaska, at high-latitudes. In addition to ionosonde derived profiles, FIRI profiles were also generated for comparison. This comparison was primarily concerned with the E-region with a focus on the region between the minimum frequency measured by the ionogram (\(fmin\)) and the height of the E-region peak (\(hnmE\)). From the comparison, the GNSS-RO and ionosonde EDPs were shown to have similar shapes in the region of interest, but the ionosonde profiles tended to be slightly lower in altitude. There is uncertainty in the EDP altitudes for both the ionosonde and GNSS-RO profiles, thus the altitude difference at \(fmin\) was calculated and the profiles were shifted to focus on the profile shapes. The shifted EDPs show generally good agreement. The FIRI EDPs did not align well with the GNSS-RO profile in the region of interest. However, they have a nearly identical shape at lower altitudes below \(fmin\), where the ionosonde follows a parabolic shape to zero \(N_{e}\) based on an assumed shape of the E-region. This agreement between the RO profiles and FIRI below \(fmin\) is evidence that the GNSS-RO profiles are capable of measuring the D- to E-transition, which is not possible from ionosondes. Ionosonde E-region peak frequencies (\(foE\)) and altitudes (\(hmE\)) were also compared with RO profiles showing clear E-region peaks. The \(foE\) values produced a moderate R\({}^{2}\) values, although there is large variability in the values. At all sites, the magnitude of the \(foE\) difference between the profiles is within 0.5 MHz. The \(hmE\) comparison showed an interesting trend with the GNSS-RO profiles; the ionosonde profiles had \(hmE\) values evenly distributed from about 95-120 km, depending on the site. The GNSS-RO profiles, however, contained nearly all of the \(hmE\) values in a much smaller altitude range between 105 and 115 km. As a result, there was a low R\({}^{2}\) value between the datasets, and the GNSS-RO \(hmE\) tends to be a few kilometers higher in altitude. Electron density profiles were compared for both shifted and unshifted profiles using altitudes between \(fmin\) and \(hmE\). Because the ionosonde profiles tend to be lower in altitude, the \(N_{e}\) at any given altitude tends to be higher, which was evident for the unshifted profiles. For the shifted profiles, the \(N_{e}\) values showed a stronger R\({}^{2}\) with a lower mean average error. The ionosonde \(N_{e}\) was slightly larger on average than the GNSS-RO \(N_{e}\). Finally, virtual heights were calculated from the GNSS-RO profiles using a numerical ray tracer for comparison with direct ionosonde observations. Many of the GNSS-RO profiles showed reasonable agreement with the ionosonde observations near \(fmin\), but there was an overall positive bias in the RO observations that could be due to low \(foE\) estimates or elevated \(hmE\) estimates. Interestingly, repeating the ray tracer comparison using ionosonde electron density profiles showed a negative altitude bias with respect to the direct observations, which may be due to the assumption of a quasi-parabolic layer shape.