## 5 Ice Height (Or Ice Sheet Height) Retrieval  
Ice height refers to the relative height of the ice sheet above the reference ellipsoid, which plays a crucial role in areas such as climate change, ocean resource development, and polar research. Ku band radar (such as Cryostat-2) [107] and laser altimeter (such as ICESat-2) play an important role in measuring sea ice height. In comparison, ICESat-2 can achieve a higher measurement accuracy than Cryostat-2 and can determine the surface height of ice sheets with a higher centimeter-level accuracy. However, orbit satellites with a single base radar or laser systems cannot fill the gaps in conventional repetitive orbits. Using dual-base GNSS-R measurements, data collected from low-Earth-orbit satellites may help to fill these scientific gaps, especially in measurements with small temporal and spatial resolutions [47]. Due to the short wavelength of GNSS carrier signals (\(\sim\)20-30 cm) and the relatively short chip length of their ranging codes (\(\sim\)30-300 m), more accurate sea ice height measurements can be obtained using carrier-phase information. [PERSON] et al. [23] conducted phase height measurements on sea ice using GNSS-R signals collected by TDS-1 and proposed the sensitivity of satellite GNSS-R carrier-phase observation to changes in sea ice height along the orbit. This study was the first to conduct satellite GNSS-R carrier-phase height measurements on sea ice at a relatively high elevation angle (\(>\)57\({}^{\circ}\)) under a 20 ms phase delay observation, obtaining a sea ice phase height measurement accuracy with an RMED of 4.7 cm in a 20 ms measurement. However, the lack of TDS-1 orbital position in this study leads to significant orbital errors in time delay measurement, which can have an impact on the retrieval results. Through this study, not only has the feasibility of this technology been verified in sea ice measurement, but it also provides a new method for monitoring sea ice changes and marine environments. In future research, the further analysis and verification of sea ice height retrieval under different thickness and elevation conditions will be conducted based on the availability of TDS-1 and a wider range of raw datasets or complex waveform collection in future spaceborne missions. In 2020, [PERSON] et al. [47] extracted height information from the initial grazing angle GNSS reflection event observed by the Spire satellite to determine the application and accuracy of spaceborne GNSS-R technology. The study showed that in areas covered with sea ice, the reflection of GNSS signals exhibited the strongest coherent signal, and the comparison between the estimated surface height at an unsmooth 50 Hz retrieval and the model sea surface showed a good consistency, with an RMS difference of about 3.9 cm. This work marks a preliminary attempt to use this new satellite technology for high-precision surface measurement. By analyzing the phase information of the GNSS signal received by the receiver carried by the Spire satellite, detailed information can be provided about surface features, and a focus on analyzing and studying the influencing factors of height retrieval in future research work is proposed. In 2021, [PERSON] and [PERSON] [48] used coherent GNSS reflection signals recorded by Spire Global Inc.'s LEMUR-2 low-Earth-orbit cube satellite to retrieve sea ice surface height and evaluated their carrier-phase measurement results. Three examples were used to evaluate the spaceborne GNSS-R sea ice height retrieval. The results showed that the overall trend of the retrieval results was consistent with ICESat-2 measurement results, and the study confirmed that the tropospheric delay model error was the main source of error in GNSS-R sea ice height measurement. In the future, effective methods for correcting tropospheric delay will be studied to improve the accuracy of spaceborne GNSS-R sea ice height measurement. In order to explore the potential of more spaceborne GNSS-R applications, [PERSON] et al.