# Inland Water Level Monitoring from Satellite Observations:
## 4 Discussion
S6-MF is superior to Sentinel-3 in temporal resolution and capturing water level changes of inland water bodies, but worse in global spatial resolution due to its orbit configuration. Moreover, by adopting the OLTC, the latest altimetry missions, S6-MF and SWOT, are expected to significantly increase the valid observation rates as well as the data availability over inland water bodies. Furthermore, data only acquired by Sentinel-3 and CryoSat-2 missions were used consistently throughout the five-year period covered by this paper. CryoSat-2 emerges as the second most widely used mission since its data were used in 12 different publications, highlighting the significance of this mission for inland water level monitoring. Researchers use all different modes of the Cyosat-2 mission, LRM, SAR, and InSAR, to evaluate inland water levels showing that for very large lakes LRM and SAR modes can achieve the same RMS [62]. However, for small inland water bodies, the quality depends also on the retracking scenario. Also, applying the Fully-Focused SAR algorithm to CryoSat-2 data permits the retrieval of inland water levels of narrow rivers smaller than 300 m in width, with a precision between 4 and 11 cm [69]. The future mission Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL), builds upon the CryoSat-2, S6-MF, SARAL, and SWOT missions and it will carry, for the first time, a dual-frequency (Ku-Ka) SAR radar altimeter having the capability for FF-SAR processing so as to further enhance the along-track sampling resolution [142]. Also, in spite of the fact that the HY-2 mission consists already of four satellites (HY-2 A, B, C, and D), data only from HY-2B and HY-2C missions are used to evaluate its performance over inland water bodies in one documentation [71]. The upcoming launch in 2024 of the HY-2E altimetry mission will provide valuable measurements over continental water surfaces and thus will open a new era of scientific advances for inland water level monitoring. Also, the last launch of an altimetry mission in December of 2022, named SWOT [25], is the first mission specifically dedicated to the monitoring of continental waters. Its Ka-band radar Interferometer is extremely useful for comprehensive global monitoring of water bodies. The recent launch of the advanced SWOT mission shows already a tendency of research interest [143; 144]. SWOT, as a swath-based SAR altimetry mission, provides high along-track and cross-track resolution. Therefore, it is capable of producing images of high-resolution water surface elevation measurements, hence surface water slope, that is necessary for estimating streamflow. Apart from the radar altimeters, laser altimetry missions, e.g., ICESat and ICESat-2, and GEDI instruments, have been used for inland water level monitoring. Laser altimeters exhibit a high growth trend, while ICESat-2 is the most extensively utilized laser altimetry mission. In 2022, its data were used in seven studies for the extraction of inland water levels, a number substantially higher than any other mission's application. It is shown that laser altimeters have a unique role in monitoring water level changes in most rivers and lakes, especially for small ones, benefitting from their higher sampling densities and much smaller footprint than radar altimeters [93; 94; 95; 96]. However, a significant limitation to their use is the temporal resolution, e.g., 91-day repeat cycle for ICESat-2 satellite, which is significantly lower than in radar altimeters. Also, another basic limitation of laser altimeter is that clouds and aerosols have an impact on the quality of data because of attenuation and scattering effects [94]. Furthermore, although satellite altimetry missions are mainly used to monitor inland water levels, there are also different techniques, such as GNSS-R [106], and gravimetry [105] which also provide accurate water level measurements, but are less accurate than altimetry missions.