# Inland Water Level Monitoring from Satellite Observations:
## 3 Review Results
### Satellite Missions for Inland Water Level Monitoring
#### 3.1.1 Radar Altimetry Satellite Missions
Although CryoSat-2 has a lower temporal resolution over a fixed Virtual Station compared to Sentinel-3, it provides measurements made upstream and downstream of a specific location that could be assimilated in a model. Also, Figure 8 depicts the number of applications per measurement mode, i.e., SAR, LRM, and InSAR among the twelve publications in which CryoSat-2 data was used. As can be seen, there is no apparent preference by researchers for a specific measurement mode of the CryoSat-2 mission. Among the twelve publications, researchers utilized all measurement modes to evaluate the water level derived by CryoSat-2 over inland water bodies in two of them [62; 63]. Based on their analysis, the quality of water level measurements depends on the measurement modes, shape, and size of inland bodies but also on the retracking algorithms analyzed in the following subsection. For very large lakes, the accuracy achieved between the different measurement modes is the same, in contrast to the small inland water bodies, where SAR and InSAR measurement modes achieve more accurate measurements. Moreover, in four publications researchers utilized only InSAR mode either to test the retracking performance of different algorithms [64; 65; 66] or to evaluate the performance of different satellite missions [67]. Furthermore, the quality of inland water level measurements is tested using only SAR mode [68; 69], while in [70] some study regions were observed in SAR mode, and some other lakes were covered in LRM. In addition, in [71; 72] LRM is utilized to evaluate the performance of different missions for measuring inland water levels, whereas both LRM and InSAR modes are used to test different retracking algorithms [73] showing that CryoSat-2 InSAR mode data can be used to monitor small inland water bodies that have not been measured by other altimetry missions. Based on these publications, the CryoSat-2 mission can deliver useful water level observations over inland water bodies worldwide, as even the conventional LRM mode of the altimeter delivers good results. However, the appropriate retracking algorithms are able to greatly improve the accuracy of CryoSat-2 water levels having a major impact on inland water level monitoring. Moreover, comparing data from CryoSat-2 and Sentinel-3, the most widely used missions, it is shown that Sentinel-3 demonstrates superior overall performance. Sentinel-3, thanks to OLTC and its orbit configuration, is superior to CryoSat-2 in capturing water level changes and temporal resolution [65; 67; 71; 72]. Except for the Sentinel-3 and CryoSat-2 missions, the 10-day-revisiting Jason series are substantially used for monitoring the inland water level. Jason-1, Jason-2, and Jason-3 provide observations with a 10-day repeat cycle. This orbit has been used since 1992 by the T/P mission. Jason-1 satellite was launched in December 2001 as a cooperation between the Centre National d'Etudes Spatiales (CNES) and National Aeronautics and Space Administration (NASA). However, Jason-1 was decommissioned in June 2013 [74]. Furthermore, the Jason-2 mission was launched in June 2008 by a cooperation between NASA, CNES, the European Organization for the Exploitation of Meteorological Satellites (EUMESTAT), and the National Oceanic and Atmospheric Administration (NOAA). In addition, Jason-2 was deactivated in October 2019. Its payload was composed of the Poseidon-3 radar altimeter [21]. Jason-3 was launched in January 2016. Its payload consists of the Poseidon-3B radar altimeter, which is operating in OL tracking mode over land surfaces [57]. Comparisons conducted among the Jason missions revealed that water level changes obtained by Jason-2 and Jason-3 missions, thanks to OLTC, are more accurate than the Jason-1 missions. Also, the correlation between water level estimates acquired by Jason-2 and Jason-3 missions is higher than the correlation between measurements obtained by Jason-1 and Jason-2 [57; 58; 75; 76; 77; 78].