# Characterizing Ionospheric Effects on GNSS Reflectometry at Grazing Angles from Space
## 4 Results
### Relative Ionospheric Group Delay Analysis
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**LSA (F10.7 = 75)**} & \multicolumn{3}{c}{**HSA (F10.7 = 180)**} \\ \hline  & **Ele.:** & **Ele.:** & **Ele.:** & **Ele.:** & **Ele.:** & **Ele.:** & **Ele.:** \\  & **5\({}^{\circ}\)–10\({}^{\circ}\)** & **10\({}^{\circ}\)–20\({}^{\circ}\)** & **20\({}^{\circ}\)–30\({}^{\circ}\)** & **5\({}^{\circ}\)–10\({}^{\circ}\)** & **10\({}^{\circ}\)–20\({}^{\circ}\)** & **20\({}^{\circ}\)–30\({}^{\circ}\)** \\ \hline North & & & & & & \\ DT & 30\% & 28\% & 26\% & 59\% & 17\% & 15\% \\ NT & 64\% & 49\% & 46\% & 98\% & 38\% & 28\% \\ \hline Tropics & & & & & & \\ DT & 48\% & 17\% & 17\% & 27\% & 16\% & 14\% \\ NT & 58\% & 35\% & 38\% & 88\% & 76\% & 48\% \\ \hline South & & & & & & \\ DT & 19\% & 21\% & 23\% & 41\% & 21\% & 24\% \\ NT & 64\% & 53\% & 51\% & 66\% & 56\% & 51\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mean relative difference in the relative ionospheric delay between NEDM2020 and NeQuick 2 during high and low solar activity. Figure 9: Relative ionospheric delay in the distinct regions and grazing angle elevation ranges along with local time variations. The sTEC outcomes obtained from the NEDM2020 model, utilized as the reference model in this study, form the basis for the following analysis. Figure 10 illustrates the ionospheric delay distribution during low solar activity, categorized by elevation angles, regions, and local time distinguishing between daytime and nighttime. At low and mid-low elevation angles, the contribution of each ray to the delay remains relatively similar in magnitude, resulting in positive values for the relative ionospheric delay. Overall, during daytime events, the \(\Delta_{p_{\text{iano}}}\) is on average 120% greater compared to nighttime events. During HSA periods (F10.7 = 180), the relative ionospheric delay range can increase by up to 200% with respect to low-solar-activity periods, as seen in Figure 11. In low- and mid-low-elevation scenarios, the distribution of \(\Delta_{p_{\text{iano}}}\) behaves similarly to LSA but with higher magnitude values. Notably, in the tropics region at very low elevations, the distribution is more widespread, with relative delays primarily consisting of negative values. This highlights the higher influence of the direct ray on \(\Delta_{p_{\text{iano}}}\) compared to low-solar-activity periods. Figure 11: Distribution of relative ionospheric delay depending on elevation, daytime (DT), and nighttime (NT) using NEDM2020 sTEC retrievals with F10.7 = 180. Figure 10: Distribution of relative ionospheric delay depending on elevation, daytime (DT), and nighttime (NT) using NEDM2020 sTEC retrievals with F10.7 = 75. Group Delay Altimetry and Ionospheric Delay Uncertainty Analysis  
As a single-frequency GNSS-R mission, PRETTY relies on ionospheric correction models to ensure precise sea surface height measurements, introducing a level of model uncertainty in the correction process. Figure 12 presents the altimetric uncertainty at grazing elevation angles. Figures 10 and 11 depict the distribution of the relative ionospheric delay, showing a noticeable diurnal cycle effect where daytime observations exhibit higher relative ionospheric delays compared to nighttime observations. This diurnal variation is also reflected in the sea surface height uncertainties. Furthermore, it is evident that ionospheric uncertainties have a significantly greater impact on sea height retrievals in the Tropics region, where the general level of ionization is higher. In this geographical area, we observe a higher altimetric uncertainty dispersion, particularly in the mid-low elevation angle regime (during daytime, 0.22 m mean and 4.08 m std), where the combined delay of the incident and reflected rays surpasses that of the direct ray. Consequently, this leads to higher relative delays and, by extension, a more pronounced impact on GNSS-R altimetric retrievals within this specific elevation range and region.