**5. Discussion**

The analysis provides valuable insights into how ionospheric parameters such as slant total electron content, relative ionospheric delay, Doppler shift, and peak electron density height vary in response to different conditions. These findings are crucial for optimizing the accuracy of space-borne GNSS-R applications, particularly in altimetry, aiding in the development of robust models, and enhancing the interpretation of data acquired through grazing GNSS-R configurations.

Under low-solar-activity conditions (F10.7 = 75), the resulting sTEC values from NEDM2020 and NeQuick 2 reveal that both models exhibit similar behavior across different scenarios. However, it is important to note that while an extensive evaluation of the models is not carried out in this study, differences in the sTEC computations and the relative total delay are observed. Significant differences of ~60 TECUs and up to 64% in relative ionospheric delay are observed in polar regions at very low elevation angles during daytime when comparing NEDM2020 and NeQuick 2. Under high-solar-activity conditions (F10.7 = 180), the relative differences can reach values up to 98%.

Grazing elevation angles, local time, regions, and solar activity emerge as the crucial factors determining ionospheric effects in GNSS-R. The observed elevation angle significantly influences the path traversed by GNSS signals through the ionosphere, while electron density variations rising from ionospheric diurnal cycles and geographical location contribute to fluctuations in the sTEC computation. The sTEC values exhibit a noticeable increase as the elevation angle decreases (very low to mid-low angles) in all regions during both daytime and nighttime.

Daytime events consistently result in higher sTEC values, larger relative ionospheric delay values, and higher Doppler shift magnitudes compared to nighttime events across all regions and elevation angles. The tropics region consistently displays the highest sTEC values across all elevation angle ranges, indicating the presence of higher electron densities. To provide a comprehensive synthesis of the study's findings based on the electron density retrievals from the NEDM2020 model, Tables 5 and 6 provide a summary of the results by presenting the median and standard deviation for each parameter outlined in the Section 4 for F10.7 = 75 and F10.7 = 180, respectively. The parameters provided by the summary tables are the relative ionospheric delay Δ*piono* in meters, absolute value of Doppler shift (| *f <sup>d</sup>*|) in Hertz, and peak electron density height (*Hm*) in kilometers.

**Table 5.** Overview of ionospheric parameters from the NEDM2020 sTEC computations during low solar activity (F10.7 = 75).


In general, as the F10.7 index increases, notable observations emerge: (1) There is a compensation effect, attributed to the direct signal contribution, leading to a decrease in the median level of relative ionospheric delay as elevation decreases, particularly at very low elevations. (2) The absolute Doppler shift exhibits a substantial increase in median values, scaling up to one order of magnitude, as elevation angles decrease to their lowest. (3) Notably, in tropical regions characterized by higher density peak heights, there is a more pronounced compensation by direct signal contribution in Δ*piono* at the lowest elevations, resulting in negative median delay values.

For a LEO GNSS-R mission employing the GPS L1 frequency, findings show that relative ionospheric delays can reach ~19 m during periods of LSA and ~70 m during HSA, equivalent to about 120 and 430 TECUs. The forthcoming ESA PRETTY mission will pioneer grazing altimetry at the L5 frequency, which, with its longer wavelength (~0.2548 m), is more sensitive to ionospheric group delays. Using 120 and 430 TECUs as benchmarks, relative ionospheric corrections of approximately 35 and 125 m can be expected for group delay altimetry during low and high solar activity.


**Table 6.** Overview of ionospheric parameters from the NEDM2020 sTEC computations during high solar activity (F10.7 = 180).
