# Inland Water Level Monitoring from Satellite Observations:
## 4 Discussion
The public agency websites and the hydrological databases, which provide inland water height data, used for the evaluation of satellite missions are presented in Tables A1 and A2 respectively. Furthermore, one of the main limitations of satellite missions to monitor inland water surface heights is the onboard tracking system. Land contamination, the main issue of altimetry missions over inland water bodies, is closely related except for the altimeter spatial resolution to the onboard retracking algorithms. This paper provides a comprehensive analysis of different algorithms used to retrieve inland water levels from different missions between 2018 and 2022. To retrieve water level data with higher accuracy, efforts are required to develop advanced retrackers to process the complex waveforms. Retrackers usually treat the first peak as the returned echo from the different targets. However, when there are inland water bodies with complex shapes or other bright targets nearby, this presumption may not be true. Thus, advanced retracking algorithms may be developed under certain use cases and conditions to detect the peak that corresponds to the correct target of interest [140]. From 2018 to 2022, 11 publications included novel or improved algorithms for inland water surface height monitoring, such as the ImpMWaPP [64]. Moreover, although Neural Networks (NN) have proven their efficiency for image processing and image classification tasks, the concept of NN is not yet completely clear what advantages can be derived from it in the field of altimetry [145]. This is quite obvious since only one publication used supervised learning to train a deep Convolutional NN for the extraction of water surface heights [140]. Therefore, it is recommended to explore the concept of NN in the field of altimetry further. Despite the recent advances in satellite altimetry missions, there is still much to be learned regarding inland water level changes. As it can be understood, in future missions the enhancement of spatial resolution and along-track sampling resolution of current missions should be also accompanied by an enhancement in temporal resolution. The development of state-of-the-art altimeters, such as SWOT altimeters with a 21-day period, is not enough to monitor applications such as flood events, or even weekly or more frequent dynamics of inland water bodies. Thus, altimeters should be accompanied by a larger altimetry constellation, including swath and conventional altimetry, and/or the use of an integrated altimeter and a Doppler Wave and Current Scatterometer such as SKIM. Future missions, such as SMall Altimetry Satellites for Hydrology (SMASH) [146], Sentinel-3C/-D/-NG Topo, and Sentinel-6 NG with higher spatial and temporal resolution will play a key role in a better understanding of the global water cycle and hence of inland water level monitoring. Many future altimetry missions are dedicated to inland waters hydrology significantly increasing the hydrology targets over rivers, with higher than 10 days revisit time. SMASH will be a constellation of 10 satellites, in the same orbit plane, providing daily measurement over each target and twice a day revisit for large watersheds. They will carry a single-frequency Ka radar altimeter. It will be able to provide measurements for rivers wider than 50 m and for lakes with areas larger than 100 m \(\times\) 100 m. Sentinel-3C/-D will follow in approximately 2025 and 2028, respectively, to ensure the continuity of the Sentinel-3A/-B missions. Sentinel-3 NG Topo aims to provide continuity of the Sentinel-3 mission, while increasing the quantity and quality of products and services. S3 NG-T incorporates both a nadir and a wide swath altimeter. S3 NG-T altimeters will be able to improve the spatial sampling of the SMASH constellation.