# Inland Water Level Monitoring from Satellite Observations:
## 4 Discussion  
The development of services to provide inland water level time series from satellite missions is getting more attention than ever before, which is motivated by the urgent need for global water cycle monitoring. Due to the absence of perspective for improving in situ networks worldwide, space-based remote sensing could be considered a unique technique that provides up-to-date observations with global coverage and homogeneous accuracy. The existence of a high quantity of operational altimetry satellites, and the prospect of cutting-edge missions, such as HY-2E, S3-NG, CRISTAL, as well as SWOT, a swath interferometric altimeter providing high spatial resolution and river slope, intensify the use of altimetry for inland surface water monitoring, and therefore for global water cycle monitoring. Although satellite radar altimetry was primarily designed for ocean monitoring, altimetry data has proven extremely important for inland water studies. Altimetry missions have been used for inland water surface height and hydrological cycle monitoring in the last three decades. In particular, since the launch of the T/P and Envisat missions, monitoring of lakes and rivers has been the goal of research and the quantity of altimetry research publications has substantially increased. However, the use of altimetry missions for inland water level monitoring was facilitated by the advent of OLTC, first implemented by the Jason-2 mission [21], and SAR mode developments, first used in the CryoSat-2 mission [22; 23], and was confirmed in 2016 with the launch of Sentinel-3A mission, which was the first mission operated both in OLTC and SAR mode nearly globally [24]. In this work, we addressed the role of spaceborne remote sensing in estimating inland water surface heights, a key component of the water cycle, and of land surface hydrology. We reviewed the studies, over the last five years, between 2018 and 2022, which described the advances made by satellites, and hence by altimeters to characterize the dynamics of inland water level. The quality of water level determination depends on the measurement modes of each mission, the retracking algorithm, as well as the shape and size of inland water bodies. Thus, due to the fact that each mission has its own characteristics, the publications associated with specific missions were examined in order to depict the trend of each mission in inland water level monitoring. Also, publications including radar altimetry data were charted based on the retracking algorithms, whether they are novel and improved  
Figure 10: Number of applications per continent and type. or existing and known ones, applied to the waveforms deemed to be from inland water bodies [48]. Proper algorithms are important to be used to retrack the waveforms since extracted ranges over inland water bodies are contaminated due to the magnitude of the radar footprint. This research shows that data from the Sentinel-3 mission, in SAR mode, were used in nearly half of the publications included in the current review, 20 out of 48, establishing this mission as the dominant for monitoring inland water levels. The OLTC feature significantly increased the data availability, while the SAR mode substantially increased along-track the spatial resolution. Sentinel-3, SAR mode, also delivers significantly higher precision measurements (std around 15 cm) than the previous generation of altimeters, such as ERS-2, Envisat, T/P, and Jason-1, while it provides also higher accuracy inland water level time series than current altimeters, such as CryoSat-2, and Jason-3. Also, both Jason-3 and Sentinel-3, perform better than the previous generations of altimeters in terms of data availability. However, in [71], the only publication where S6-MF mission's data was used, is shown that S6-MF as a follow-up mission to Jason-3, has the highest accuracy among CryoSat-2, Jason-3, and Sentinel-3 missions.