# Inland Water Level Monitoring from Satellite Observations:
## 3 Review Results
### Satellite Missions for Inland Water Level Monitoring
#### 3.1.1 Radar Altimetry Satellite Missions
Furthermore, the ERS-2 mission's data have been used in the same publications in which ENVISAT and SARAL mission's data have been used for the evaluation of the performances of historic and operational altimeters for water level retrievals [57; 58; 72; 76; 77]. ERS-2, a follow-on mission of ERS-1, has a 35-day repeat cycle such as ENVISAT and SARAL missions. ERS-1 and ERS-2 were launched in July 1991 and April 1995 by ESA, and retired in March 2000 and September 2010, respectively. ERS-2 payload carried a radar altimeter (RA) operating at Ku-band [77]. The thorough evaluations of the performances showed that for larger waterbodies, higher RMSE, and lower R values than usual are obtained using ERS-2 and ENVISAT [57; 77]. Moreover, a comprehensive analysis of Indian reservoirs shows that SARAL and Sentinel-3A provide the highest accuracy of water level retrieval, followed by ENVISAT and then by ERS-2 [58]. In addition, it is indicated that classical LRM altimeters operating at Ku-band without OLTC (i.e., ERS-1, ERS-2, ENVISAT, and Jason-1) are highly affected by the hills and mountains in the surrounding inland water bodies. Due to the radar remaining fixed on the hills and mountains, there is a lack of data acquisition, while for ERS-2 and ENVISAT, the data loss rates over small lakes are slightly higher than those over large lakes. Also, Refs. [72; 77] provide an estimate of the altimeter biases for SARAL, ERS-2, and ENVISAT. On the other hand, for smaller inland water bodies, the ERS-1 and ERS-2 missions provide more accurate results than the T/P mission in contrast to large lakes, where T/P is more accurate thanks to its lower data missing rate and shorter repeat cycle [76]. T/P, the predecessor of the Jason series, has the same 10-day repeat cycle. T/P was launched in August 1992 and decommissioned in October 2005 [20]. In the past 3 decades a total of 20 altimetry satellites, radar and laser missions, have been launched with different altimeters on board. Between 1985 and 1990 Geosat was the sole radar altimetry mission in operation [19]. Geosat was launched in March 1985 by the US Navy, and retired in January 1990. Also, the successor of Geosat, the Geosat Follow-On (GFO) mission, was launched in February 1998 and decommissioned in October 2008. GFO retained the Geosat exact repeat mission's orbit with a repeat cycle of 17 days [88]. Both Geosat and GFO have an extremely high data missing rate of inland water level measurements hampering their usefulness for retrieving water levels [76]. Although contemporary radar altimetry missions provide more accurate estimates than Geosat, its measurements are still valuable since it was the sole radar altimeter from 1985 to 1990. Also, GFO is the least desirable choice for the lakes overpassed by GFO, ERS-2, and T/P in the same period. The comprehensive evaluation of eleven radar altimeters over different lakes in four countries shows that for the period prior to 2002, the order of selection priority should be ERS-2, ERS-1, and TOPEX/Poseidon. From 2002 to 2013, the order of selection priority should be ENVISAT, Jason-2, Jason-1, ERS-2, and GFO, while between 2013 and 2020, the order of selection priority should be Sentinel-3, SARAL, Jason-3, and Jason-2 [76]. Finally, in spite of the fact that the HY-2 mission has a total of four satellites (HY-2 A, B, C, and D), the number of publications associated with the HY-2 mission is rather low when compared to other contemporary missions. The thorough evaluation of seven ongoing altimeters for measuring inland water levels is the only one that includes information about HY-2B and HY-2C performance [71]. HY-2B and HY-2C were launched in October 2018 and in September 2020 respectively. HY-2B is equipped with dual-frequency altimeters (Ku and C bands), with a revisit period of 14 days in the early stage of the mission and 168 days at the end of the mission [89]. Also, the orbit revisit period of HY-2C is 10 days at the initial stage of the mission and it will become 400 days at the final stages [90].