## 3 Coherence Detection and Inland Water Body Monitoring
### Retrieval of Inland Water Body (Or Lake Water Body) Surface Height
However, due to the lack of dual frequency, precise orbit determination, a high waveform delay resolution, and a sufficient calibration capability of instrument delay, CYGNSS has a lower measurement accuracy for inland water body height. In this study, the water surface height \(\Delta H\) compared to the WGS4 ellipsoid was calculated by the difference between the simulated bistatic delay \(\rho_{m}\) and observed bistatic delay \(\rho_{o}\), and the satellite elevation angle \(e\) at the specular reflection point (SP) was introduced. At the same time, the study further obtained the positive water surface height (water level) by using the EGM2008 geoid undulation [99], and the calculation formula is as follows. \[\Delta H=\frac{\rho_{m}-\rho_{o}}{2\sin e} \tag{3}\]  
\[H=\Delta H-N \tag{4}\]  
where \(N\) represents the undulation of the geoid relative to the ellipsoidal surface. Compared to L1 data, the raw IF data of spaceborne GNSS-R has more information, which can be overcome by special processing to overcome the shortcomings of L1 data. At the same time, the carrier phase can provide higher-accuracy measurements. [PERSON] et al. [24] demonstrated the use of spaceborne GNSS-R technology to extract the bistatic group delay and carrier-phase delay from quasi specular GNSS reflections using raw IF data from CYGNSS satellites. This study successfully measured the water level and surface topography of the Qinghai Lake as an example. The water level obtained from group delay observation was consistent with Cryosat-2 and in situ measurement results, achieving high-resolution (70 m) and high-precision (centimeter-level) measurements of the lake surface height. This study not only demonstrates the feasibility of using spaceborne GNSS-R raw IF data for water height monitoring, but also provides data support for subsequent research. However, due to receiver orbit errors and ionospheric correction residuals, there  
Figure 5: Schematic diagram of retrieval of inland water body height (in this figure, the black dashed line represents the simulated signal on the ellipsoid surface, \(\Delta H\) is the height of the water surface relative to the ellipsoid surface and \(e\) is the satellite elevation angle. Assuming that the observed and simulated reflected signals are parallel to each other, the path difference \(\Delta\rho/2\) between the observed and simulated reflected light rays can be represented by \(\Delta H\)) [98]. are still errors in group delay height measurements. In this study, the ellipsoidal height \(H_{e}\) of the water surface on the WGS84 ellipsoidal reference surface \(H_{ref}\) was calculated by the difference between the simulated bistatic delay \(\rho_{B}^{m}\) and the observed bistatic delay \(\rho_{B}^{o}\), introducing the incident angle \(i\) for calculation [2], as follows:  
\[H_{e}=H_{ref}+\frac{\rho_{B}^{m}-\rho_{B}^{o}}{2\cos i} \tag{5}\]  
In 2022, [PERSON] et al. [21] demonstrated the application prospects of spaceborne GNSS-R technology in the field of inland water body (lake water body) height retrieval by processing and analyzing the raw IF dataset of multi-task spaceborne GNSS-R, providing important references for subsequent research. Recently, [PERSON] et al. [97] estimated the lake water level in the Great Lakes region using dual-frequency (GPS L1 and L2) raw grazing angle GNSS-R data provided by the Spire satellite and SAR altimeter data provided by the Sentinel-3 satellite. The results indicated that when using a geoid model with accurate regions, observation accuracy could be improved, and the performance and complementarity of the two data sources in lake water level estimation were evaluated by comparing their results, proving that the comprehensive utilization of these two data sources can improve the accuracy and coverage of lake water level estimation. At the same time, the study pointed out the necessity of improving the geoid model to make these models more sensitive to GNSS-R technology.