# Inland Water Level Monitoring from Satellite Observations:
## 3 Review Results
### Satellite Missions for Inland Water Level Monitoring
#### 3.1.2 Laser Altimetry Satellite Missions  
Existing altimetry satellites can be divided into radar and laser altimetry missions. Although radar altimetry is now commonly used to provide long-term monitoring of inland water levels in complement to or for replacing disappearing in situ networks of gauge stations, spaceborne laser altimeters have also been demonstrated to be an essential source of data. Compared to radar altimeters, laser altimeters have smaller footprints and higher sampling densities, making laser altimeters more suitable for small inland water bodies monitoring. As can be seen in Figure 7, ICESat-2 is the most extensively utilized laser altimetry mission. Its altimetry data are used in an equal number of research publications as the Jason-3 mission's data. ICESat-2, a follow-on to the ICESat-1 mission, was launched in September 2018 by NASA. ICESat-1 was launched by NASA in January 2003 and deactivated in August 2010 [91]. ICESat-2 carries the Advanced Topographic Laser Altimeter (ATLAS). ATLAS has a total of six laser beams, divided into three pairs, and each beam pair laser includes a strong energy beam and a weak energy beam [92]. In [93] is highlighted that the measurements by strong beam are more accurate than the weak beam. Furthermore, the key advancement of ICESat-2 is that it generates individual laser footprints of around 14 m diameter on the Earth's surface, with each footprint separated by only 70 cm in contrast to the ICESat mission with 70 m diameter laser footprints and 170 m along-track footprints [91]. As can be understood ICESat-2 has high spatial resolution and high density of observation. Thus, ICESat/ICESat-2 has a unique role in monitoring water level changes in most rivers and lakes, especially for small ones benefitting from their small footprints [93; 94; 95; 96]. However, the water levels from ICESat-2 are much more accurate than those from ICESat [97]. Also, comparing ICESat-2 with radar satellites, such as Jason-3 and Sentinel-3, is shown that ICESat-2 has the smallest variability in deviations between satellite-measured water level and the in-situ data. Moreover, the accuracy of the ICESat-2 mission is comparable to Sentinel-3 and Jason-3 satellites, but it has lower accuracy than S6-MF [71]. When available, ICESat-2 data show a strong potential for the monitoring of inland water bodies [77]. Furthermore, in [98; 99] it is highlighted that the use of night-time observations by the ICESat-2 mission achieved slightly higher accuracies than day-time measurements. On the other hand, the temporal resolution of the ICESat-2 satellite (91-day repeat cycle) is low and significantly lower than the radar altimeter satellites. Also, another basic limitation of ICESat and ICESat-2 laser altimeters is that clouds and aerosol have an impact on the quality of data because of attenuation and scattering effects [94]. Apart from the ICESat and ICESat-2 missions, the most recently launched spaceborne laser sensor is the Global Ecosystem Dynamics Investigation (GEDI). GEDI launched in December 2018 and has been acquiring data since April 2019 on board the International Space Station (ISS). GEDI is equipped with three identical lasers, and it generates laser footprints of 25 m in diameter on the Earth's surface, with 60 m along the track and 600 m across the track [100]. However, GEDI's main mission is to observe the forest canopy height and canopy vertical structure to characterize important carbon cycling. In [99; 101; 102] is indicated that the combination of GEDI and ICESat-2 mission data presents a valuable resource for hydrological and climatic studies, especially for frequent and accurate inland water dynamics monitoring. Thus, the combined dataset allows us to capture the seasonal, and annual dynamics of inland water levels. As these two missions are operating simultaneously can provide a mutual accuracy verification and increase the temporal density of measurements.