# Inland Water Level Monitoring from Satellite Observations:
## 3 Review Results
### Satellite Missions for Inland Water Level Monitoring
#### 3.1.1 Radar Altimetry Satellite Missions  
The Sentinel-3 mission has exhibited a high growth trend since Sentinel-3 was the first mission operated both in SAR mode and in OLTC over almost all Earth surfaces. The data from the Sentinel-3 mission was utilized in nearly half of the publications included in the current scoping review paper, 20 out of 48, establishing this mission as the dominant for monitoring inland water levels. Sentinel-3A and Sentinel-3B satellites were launched by the European Space Agency (ESA) in February 2016 and April 2018 respectively. The satellite ground track repeats in a 27-day cycle [54]. Moreover, Sentinel-3 satellites can be operated either in Closed-Loop (CL) or in Open-Loop (OL) tracking mode. In OL tracking mode, also called Diode/Digital Elevation Model (DEM), the altimeter uses an elevation prior uploaded onboard the altimeter as a function of longitude and latitude to position its receiving window. This information is provided by the OLTC [55]. In 2008, the first OLTC was implemented in the POSEIDON-3 altimeter on the Jason-2 satellite as an experimental mode [21]. Also, until 2010, all altimeters onboard satellites utilized a type of altimeter known as conventional pulse-limited Low-Resolution Mode (LRM). In 2010, Synthetic Aperture Radar (SAR) was implemented in the CryoSat-2 mission, which was launched by the ESA. The CryoSat-2 mission offered the opportunity to showcase the advantages of SAR mode or \"Delay-Doppler altimeter\" over the conventional pulse-limited LRM for various applications and also over inland waters [23; 56]. In addition, the SRAL instrument of Sentinel-3 operates in both modes: LRM and SAR mode. The LRM is a backup mode working as a conventional pulse-limited radar altimeter. In contrast, in the SAR mode, it employs SAR technology inherited from CryoSat-2 to increase the  
Figure 7: Number of applications per altimetry mission and year, from 2018 to 2022.  
along-track sampling resolution and elevation measurement accuracy. In SAR mode, the altimeter has a higher along-track resolution of 300 m compared to several km in LRM [54]. Out of the twenty publications, only two of them utilized Sentinel-3 data in LRM mode to assess the performance of the Sentinel-3 mission in retrieving water levels [57; 58]. In both documents, it is highlighted that with the higher resolution of SAR mode, it is possible to obtain inland water surface elevations from smaller water bodies than with LRM, as well as to provide estimations with higher accuracy. As depicted in Figure 7, data only from the Sentinel-3A and CryoSat-2 missions were utilized consistently throughout the five-year period covered by this paper (2018-2022). Therefore, CryoSat-2 emerges as the second most widely used mission since its data was utilized in 12 different publications, highlighting the significance of this mission for inland water level monitoring. SIRAL carried by CryoSat-2 is the first space-borne altimeter featuring delayed Doppler technology providing an orbital footprint of 300 m, and its reference ellipsoid in WGS84 [23; 59]. Thus, CryoSat-2 ushered in a new era in radar altimetry. CryoSat-2 offers the opportunity to make comparisons of SAR and conventional altimeters by a multi-mode altimeter, since depending on the geographical mask it has three measurement modes, namely LRM, SAR, and Interferometric SAR (InSAR) [60]. Moreover, depending on the sensor mode SIRAL has different footprint sizes on the ground. The width of the footprint is several km for LRM, depending on the roughness of the terrain, while both SAR and InSAR modes have -300 m along-track resolution. Furthermore, in contrast to the 27-day repeat cycle of Sentinel-3, CryoSat-2 has a long repetitive cycle (369 days) [61]. Thus, CryoSat-2 crosses a river many times during the long repeat cycle, albeit at different locations. CryoSat-2 offers a very dense spatial sampling pattern [60].