# Inland Water Level Monitoring from Satellite Observations:
## 3 Review Results
### Satellite Missions for Inland Water Level Monitoring
#### 3.1.1 Radar Altimetry Satellite Missions
In contrast to Jason-2 and Jason-3, the altimeter of the Jason-1 mission was not operating in OL tracking mode [74]. So, the use of the Jason-1 mission is limited for the extraction of water surface elevation compared to most recent missions, such as Jason-3 and Sentinel-3 [57; 76; 77]. Also, Jason-3 demonstrates better performance compared to Jason-2 in mitigating the effects of the surrounding topography, thereby enhancing the accuracy of measurements over inland water bodies [72; 79; 80]. Furthermore, as a follow-up mission to Jason-3, Sentinel-6 satellite has the highest accuracy among CryoSat-2, Jason-3, and Sentinel-3 missions [71]. Sentinel-6 is superior to Sentinel-3 in temporal resolution thanks to its 10-day repeat cycle. However, the Sentinel-6 Michael Freilich (S6-MF) satellite was launched in November 2020 and as a result its altimetry data has not been widely used for the inland water level monitoring, but only in one publication. S6-MF sensor consists of a Poseidon-4 dual-frequency SAR altimeter. Poseidon-4 is designed to operate in an interleaved (open burst) mode, allowing for simultaneous operation of SAR and LRM acquisitions. As a result, the altimeter offers twice the number of observations compared to the Sentinel-3 altimeter, leading to a substantial enhancement in the domain of water level monitoring [81]. This mission is critical for maintaining the continuity of accurate measurements over inland water bodies. Furthermore, although the  
Figure 8: Number of applications per measurement mode of CryoSat-2. use of the S6-MF mission's data is limited, Refs. [71; 82] show the great potential of this state-of-the-art mission. In addition, ENVISAT and SARAL mission data have been used in the same six publications to assess the performance of historic missions in comparison to the newest altimetry missions for measuring inland water levels. Both missions have a 35-day repeat cycle. The ENVISAT mission was launched in March 2002 by ESA, and it has been inactive since 2012. Its payload included the advanced radar altimeter (RA-2) [83]. Also, SARAL was launched in February 2013 by CNES and the Indian Space Research Organization (ISRO) [84]. Its payload included the AltiKa radar altimeter, achieving higher accuracy performance than the previous radar altimeters [85; 86; 87]. Moreover, the SARAL mission is the second radar altimeter, after the Jason-2 mission, operating in OLTC. A comprehensive evaluation of seven different radar altimeters indicates that very good agreement was found between the most recent missions such as SARAL, Jason-3, and Sentinel-3 than for the older ones, such as ENVISAT and Jason-1/-2 [77]. The use of SARAL data provides more accurate inland water level measurements than using ENVISAT satellite's dataset, resulting from the use of the Ka-band with its smaller footprint and higher chirp frequency of 500 MHz [57; 72]. Also, a thorough evaluation of eight radar and 2 laser altimeters over Swiss lakes shows that the highest correlations between in situ measurements and altimetry missions are obtained with SARAL, Jason-3, and Sentinel-3 satellites. However, results obtained with the Sentinel-3 mission outperform the results obtained with any other mission [77]. Furthermore, among the satellite missions used in [76], SARAL gave the second-best performance and more accurate results than ENVISAT, while ENVISAT has slightly better lake water level estimates than Jason-1/-2, but its data missing rate is higher. However, among the eleven missions, the most recent Sentinel-3 has the best performance, while the accuracy of the SARAL mission was equivalent to the ENVISAT mission [79]. Moreover, in [58], the SARAL mission achieved higher accuracy measurements over Indian reservoirs than the ENVISAT satellite, and together with Sentinel-3 showed the highest accuracy.