INSTITUTE OF BIOPHYSICS Secondary Seat of Pisa

Authors: Mohd Adha Abdul Majid, Nurul Hazrina Idris, Mohd Nadzri Md Reba, Vignudelli Stefano
Journal: Advances in Space Research
DOI: 10.1016/j.asr.2025.11.060
Abstract:
Standard unfocused UFSAR nadir-altimeter products with posting rate of 20 Hz provide water level heights with along-track spacing of 300 meters, which is far from being optimal for coastal zone and inland studies. The fully focused FFSAR processing, that allows much higher resolution and posting rate, shall resolve small scales interferences along-track and thus improve accuracy and precision of the geophysical estimates. This study looks at different water surfaces in rivers,lakes and coastal zone to investigate under which condition and processing scheme the accuracy of water level is higher with FFSAR rather than with UFSAR. We focus on Sentinel-3A,-3B and-6A missions during 2016-2023. Starting from the UFSAR products at 20 Hz posting rate, we investigate the optimal choice for the Level 1 to Level 2 processing algorithms and parameters. We find that posting rates of 80This paper evaluates the performance of Synthetic Aperture Radar (SAR) altimetry from Sentinel-3 for monitoring water surface elevation (WSE) in small reservoirs (<500 km2) in Malaysia. The study area focuses on Kenyir (326.4 km2), Temengor (137.3 km2) and Chenderoh (8.6 km2) reservoirs that have complex tropical landscapes and limited in-situ gauge coverage. Waveform classification reveals dominant (100%) quasi-specular returns (Class I) at Chenderoh, whereas Kenyir and Temengor show substantial proportions of multi-peak and complex waveforms, requiring enhanced retracking to mitigate land contamination. Six atmospheric and geophysical correction sets are tested, incorporating European Centre for Medium-Range Weather Forecasts (ECMWF) tropospheric models, ocean tide models including the Finite Element Solution 2014 (FES2014), Goddard Ocean Tide model version 4.10 (GOT v4.10), and the TOPEX/Poseidon 9th release ATLAS (TPXO9-ATLAS), as well as geoid models comprising the European Improved Gravity Model EIGEN6C4 (EIGEN6C4) and the Earth Gravitational Model 2008 (EGM2008). The best performing sets are Set 3 and Set 5 for Kenyir, Set 6 for Temengor, and Set 1 and Set 2 for Chenderoh. Correlations with gauge observations exceed 0.95 for all reservoirs (p<0.05). Root mean square error (RMSE) values are 49 cm (Kenyir), 50 cm (Temengor), and 80 cm (Chenderoh). Normalised RMSE highlights how relative performance depends on reservoir scale: Kenyir and Temengor achieve low relative errors (<7% of water-level range), while Chenderoh shows higher relative errors (39%) due to its narrow variability (∼2 m). These findings demonstrate the potential of Sentinel-3 SAR altimetry for monitoring small tropical reservoirs, while emphasising the importance of correction model choice and local reservoir characteristics.