# Inland Water Level Monitoring from Satellite Observations:
## 3 Review Results
### Waveform Retracking
#### 3.2.2 Official Retracking Algorithms
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Publication** & **Satellite Mission** & **Data** & **Retracker** & **Validation Data** & **Accuracy Metrics** \\ \hline \multirow{3}{*}{[126]} & \multirow{3}{*}{Sentinel-3A} & \multirow{3}{*}{L1b} & \multirow{3}{*}{MWaPP+} & Retrackers (OCOG, & \multirow{3}{*}{RMSE = 0.13–2.08 m} \\  & & & & SAMOSA+, PPCOG, & \\  & & & MWaPP+ & MWaPP) and In-situ & & \\  & & & & & gauge data & \\ \hline \multirow{3}{*}{[65]} & \multirow{3}{*}{CryoSat-2 (InSAR), Jason-2/-3, Sentinel-3A} & \multirow{3}{*}{InSAR L1b, S-GDR, LAN L2} & \multirow{3}{*}{another modified MWaPP+} & Retrackers (OCOG, & \multirow{3}{*}{RMSE = 0.13–2.08 m} \\  & & & & NPPTR, NPPTR & \\  & & Jason-2/-3, & & [0.5], NPPTR [0.8], & & mean-RMSEs = 0.08–0.16 m, & \\  & & & LAN L2 & MWaPP+ & NPPOR, MwaPP, & std = 0.25–0.30 m, R \(>\) 0.79, & \\  & & & & In-situ gauge and & & \\  & & & & Hydroweb product & & \\ \hline \multirow{3}{*}{[128]} & \multirow{3}{*}{Sentinel-3} & \multirow{3}{*}{L1b} & \multirow{3}{*}{L2 Ocean retracked, non waveform portion selection method} & L2 Ocean retracked, In-situ gauge, Retrackers (Threshold, OCOG, & \multirow{3}{*}{m} \\  & & & & two-step SAR & \\  & & & physical-based) & & \\ \hline \multirow{3}{*}{[130]} & \multirow{3}{*}{Sentinel-3} & \multirow{3}{*}{L2} & \multirow{3}{*}{Bimodal correction algorithm} & In-situ gauge, Retrackers (Ice-sheet, & \multirow{3}{*}{r = 0.93, RMSE = 0.05 m} \\  & & & algorithm & SAMOSA-3, OCOG, & \\  & & & Sea-Ice) & & \\ \hline \multirow{3}{*}{[132]} & \multirow{3}{*}{Sentinel-3} & \multirow{3}{*}{L1b} & Novel retracker based on numerical simulations & In-situ gauge data, OCOG & \multirow{3}{*}{ubRMSE = 0.03–0.07 m and biased-RMSE = 0.10–0.13 m} \\  & & & & & QCOG & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont._the combination of data from all the radar altimeters significantly improved the extracted water levels. Moreover, among the eleven radar altimetry missions used in [76], the Sentinel-3 mission has the best performance using empirical/model-free retrackers (e.g., OCOG). As can be seen, all retrackers of each mission have similarly good performance over large inland water bodies, apart from the ERS-1 sea ice retracker. On the other hand, over small inland water bodies, the empirical retrackers, such as the Ice1 (OCOG) of SARAL, ENVISAT, ERS-1/-2, and Jason series missions, obtain more accurate results than the physical/model-based algorithms, such as the MLE4 retracker of Jason missions, the ocean algorithm of SARAL, ENVISAT, ERS-1/-2 missions or the non-model-based sea ice retracker. Also, the OCOG retracker produced the lowest mean RMSEs and the lowest mean data loss rate, while the SAMOSA-2 algorithm achieves slightly greater RMSE and lower bias than OCOG. However, it would be really interesting to see the results comparing the OCOG retracker with the SAMOSA+, which is customized for inland water. Following these two algorithms of Sentinel-3, the OCOG algorithm of SARAL obtained the second-best performance among the eleven radar altimetry missions. In addition, using the Ice-1 retracker the ENVISAT had slightly better performance than Jason-1/-2 missions. However, due to the 35-day repeat cycle of ENVISAT and its higher data loss rate compared to the Jason-1/-2 missions, Jason missions (with a 10-day repeat cycle) offered more frequent and continuous estimates of water levels compared to ENVISAT. As a result, the empirical retrackers are recommended for inland water level monitoring in combination with the Sentinel-3 mission, which gave the most accurate results. Also, in [79] OCOG retracker of Sentinel-3B outperforms the other altimeters (Envisat, SARAL, Jason-2/-3) as well as Sentinel-3A with a maximum RMSE of 0.21 m over small reservoirs in the Brazilian semiarid region. In [115], research was conducted to determine the absolute bias of Sentinel-3 and Jason-3 altimeters as well as the accuracy of these two altimeters. It is highlighted that the accuracy of Sentinel-3A with OCOG retracker was the best in terms of RMS.