# Inland Water Level Monitoring from Satellite Observations:
## 3 Review Results
### Waveform Retracking
#### 3.2.1 Novel and Improved Retracking Algorithms
The AMPDR retracker appears to provide the best and most robust results, having the lowest mean Std of all tracks, while using Sentinel-3 data obtained the lowest mean-RMSE (0.139 m) of all inland water bodies. It should be noted that AMPDR is called AMPDR when using a 50% threshold retracker, while using the OCOG retracker is called AMPDR. Also, it is proven that AMPDTR has the most inaccurate results for the Sentinel-3 and Jason-2/-3 missions, while AMPDOR has the lowest results for CryoSat-2 data. AMPDR provides the most stable results and it has the potential to monitor surface water heights of smallinland water bodies as well as to be applied to other altimeters. Moreover, having common study areas, seven lakes in the Tibetan Plateau, AMPDR using CryoSat-2 InSAR data outperform the ImpMWaPP retracker. Furthermore, in the same year, a new retracker called APD-PPT was presented using also CryoSat-2 data to monitor inland water level variations [73]. This new algorithm combines the adaptive peak detection (APD) with the PPT retracker since the sub-waveform is formed based on the APD-detected primary peak, and retracked by the PPT algorithm. However, the APD was originally developed to detect peaks of the LRM waveform [124; 125]. Also, Jason-2 LRM, in-situ gauge data, and five different retrackers (OCOG, PPT, WWMFW, MBP, ALES) were used to evaluate the performance of APD-PPT method using CryoSat-2 data. Both MBP and ALES have been successfully used to retrack LRM waveforms. As it is shown, APD-PPT retracker is the most appropriate for InSAR mode waveforms achieving the smallest mean-std of two lakes (0.186 m and 0.303 m), while Ice is the most accurate for LRM mode waveforms achieving the smallest mean-std (0.254 m). However, AMPDR is slightly more accurate than the APD-PPT method since the smaller the std, the better the retracking algorithm [23]. Furthermore, in 2022 a new retracking algorithm, named INPPTR was presented for InSAR waveforms to obtain stable water levels, such as in cases of ImpMWaPP, AMPDR, and APD-PPT retrackers [66]. INPPTR algorithm is based on NPPTR [22]. Also, for the evaluation of the accuracy of the novel INPPTR, the inland water levels were calculated by three more algorithms (NPPTR, NPPOR, MWaPP) and compared with in-situ data. In summary, the RMSE (0.17 cm-0.58 cm) and std (0.08 m-0.52 m) obtained by INPPTR are the lowest compared with the other retrackers, and therefore the accuracy of INPPTR is significantly higher than the other algorithms. Moreover, INPPTR can obtain retracking points from the reflected waveforms of complex terrains with high accuracy. Furthermore, from 2018 to 2022 two different modified multiple waveform persistent peak (MWaPP+) retrackers, both based on the MWaPP [23] method, were proposed to obtain accurate inland water levels. The first modified retracker uses the median of all available waveforms to form a new waveform and instead of the first sub-waveform, they extract the sub-waveform with the largest peak [126]. This retracker was evaluated for Sentinel-3 data. Also, the three other algorithms used to evaluate the modified retracker are the OCOG, SAMOSA+ [127], and primary peak COG (PPCOG) [119]. The comparison revealed that SAMOSA+, OCOG, and PPCOG are unable to handle multi-peak waveforms in contrast to modified MWaPP+, which obtains the highest accuracy in most cases. In the majority of virtual stations MWaPP+ retracker obtains really small RMSE values, between 0.13 m and 0.6 m, but in some other stations, RMSE values range from 0.9 m to a few meters. Also, it is important to obtain accurate results that MWaPP+ is less sensitive to off-nadir bright targets. Moreover, a few months later another modified MWaPP+ retracker was adopted for CryoSat-2 InSAR and Sentinel-3 data [65]. Also, for Jason-2/-3 data products the ALES+ retracker was used to extract retracking heights. For the evaluation of the performance of the modified retracker were used also the OCOG, NPPTR, NPPTR [0.5], NPPTR [0.8], NPPOR, and MWaPP.