## 5 Ice Height (Or Ice Sheet Height) Retrieval
[21] explored the processing methods, data products, and potential applications of raw IF datasets for multi-mission spaceborne GNSS-R. In this study, they demonstrated the potential of spaceborne GNSS-R technology in sea ice height retrieval and monitoring by developing and optimizing processing algorithms. On this basis, [PERSON] et al. [54] used, for the first time, 17 Spire Global Inc. CubeSat constellations with dual-frequency GNSS-R (GG-R) carrier-phase observation data under grazing geometry to retrieve the ice cover heights in Antarctica and Greenland, and systematically evaluated the retrieval results. The relative heights were adjusted and verified using a Digital Elevation Model (DEM). The study showed that the root mean square error (RMSE) of the two regions, Antarctica and Greenland, was 1.7 m. The GG-R method can supplement the shortcomings of existing satellite altitude measurement products in covering high-latitude areas, providing a promising alternative solution. However, the relatively weak GNSS-R signal and complex bottom morphology at the edges of ice sheets result in low signal coherence, making it difficult for GNSS-R altimetry to sample coastal areas. In future research, it is possible to collect raw IF signals in coastal transition areas and perform relevant processing to solve this problem. This study uses WGS84 as a reference ellipsoid and calculates the relative height \(\delta h\) above the reference ellipsoid by measuring the difference between the direct signal path \(TR\) from the transmitter to the receiver and the distance \(T\hat{X}R\) between the reflected signal from the transmitter to the \(SP\) and then back to the receiver, without measuring the total ionospheric phase \(\Delta\Phi^{RD}_{iono-frac}\) and the elevation angle \(\alpha\) of the \(SP\). The calculation formula is as follows [54]:  
\[\delta h=\frac{T\hat{X}R-TR-\Delta\Phi^{RD}_{iono-frac}}{2 sin\alpha} \tag{7}\]  
where \(\Delta^{RD}\) corresponds to the difference between the total phase measurement values of the direct signal and the reflected signal. Table 5 summarizes the methods for retrieving ice height (or ice sheet height) using spaceborne GNSS-R raw IF signals in the relevant literature, and compares the accuracy of these different methods. The use of spaceborne GNSS-R raw IF data has shown great development prospects in measuring sea ice height. It is not limited by seasonal and weather conditions and can achieve the continuous monitoring of sea ice throughout the year, providing the possibility for long-term research on sea ice changes. This is of great significance for understanding the seasonal and interannual changes in sea ice. At the same time, it can also provide high-resolution sea ice measurement data to capture small changes in sea ice surface. Spaceborne GNSS-R technology can also be extended beyond the observation range of ICESat-2 and Cryosat-2 to monitor the height of the Antarctic ice sheet, providing valuable data support for high-precision sea ice height measurement. However, spaceborne GNSS-R technology also faces some challenges in measuring sea ice height, especially in the edge areas of ice sheets. Due to low signal coherence, it is difficult to increase the sampling frequency above 50 Hz in coastal areas. In addition, the tropospheric delay model, as the main source of error in sea ice height measurement, will also have a significant impact on the accuracy of sea ice height measurement. In response to these existing issues, future research can improve GNSS-R measurements by collecting raw IF signals in coastal transition areas. This can increase the number of delay taps and reduce chip spacing, thus better describing the returned waveforms and ultimately evaluating whether it is possible to accurately measure the height of these coastal areas, especially glaciers [54]. In addition, exploring more accurate tropospheric delay correction methods is also an important means to improve the accuracy of sea ice height measurement [48].