**6. Conclusions**

In this paper, we have analyzed ionospheric effects in GNSS-R at grazing angles. This study encompasses the characterization of slant total electron content, relative ionospheric delay, the influence of ionospheric correction model uncertainties on GNSS-R group delay altimetry retrievals, the Doppler effect, and peak electron density height changes. Various factors have been considered such as satellite geometry, latitude-dependent regions, temporal variations, and solar activity.

When analyzing the results during LSA (low solar activity) and HSA (high solar activity), it becomes evident that as the elevation decreases into the grazing regime below 20◦, the median relative ionospheric delay decreases due to the compensation from the direct signal contribution. However, it is important to note that the standard deviation of the delay, especially in terms of the Doppler shift, undergoes a substantial increase. This behavior poses a significant challenge for the model-based correction of ionospheric delay in GNSS reflectometry altimetry at grazing elevation angles.

While model uncertainties do affect group delay sea height estimates it is important to highlight that these effects are not uniform across all GNSS-R observations. Coherent phase observations, for instance, offer a remarkable level of precision, down to the centimeter scale. Along reflection tracks characterized by consistent ionospheric bias, relative altimetry at a centimeter precision level can be achieved. This means that even in the presence of ionospheric delay bias, LEO space-borne GNSS-R systems, as reported in [26], can still provide precise results in the altimetric inversion.

Total electron content, a crucial ionospheric parameter, exhibits complex variations spanning diurnal, monthly, seasonal, and 11-year solar cycles. Extended temporal coverage is essential for deciphering these patterns, especially in dynamic regions allowing analysis of seasonal trends. This study highlights the importance of spatially extended data, particularly in tropical areas with substantial ionospheric variability. Such data is key to comprehending ionospheric parameter evolution across different time scales and regions, influenced by factors like solar activity and geomagnetic storms.

GNSS-R (global navigation satellite system reflectometry) stands as a valuable and complementary remote sensing tool in ionospheric studies, effectively addressing areas not covered by alternative methods. This capacity offers significant contributions to the modeling, prediction, and comprehension of ionospheric effects.

**Author Contributions:** Conceptualization: M.M., M.S. and G.S.; methodology: M.M., M.S. and G.S.; software: M.M. and M.S.; data resources: M.M. and M.S.; writing—original draft preparation: M.M.; writing—review and editing: M.S., G.S., M.H. and J.W.; visualization, M.M.; supervision, M.S., J.W. and M.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the German Aerospace Center, Institute for Solar-Terrestrial Physics (DLR-SO).

**Data Availability Statement:** Data are available under the authorization of Spire Global Inc.

**Acknowledgments:** The authors would like to thank Spire Global for providing real orbit from Lemur-2 constellation LEO satellites.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**


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