Assessment of retracked sea levels from Sentinel-3A Synthetic Aperture Radar (SAR) mode altimetry over the marginal seas at Southeast Asia

This paper presents the assessment of altimetric data from Sentinel-3A satellite operating in Synthetic Aperture Radar (SAR) mode for sea level research studies and applications over the largest archipelagos at Southeast Asia. Both qualitative and quantitative assessments are conducted by analysing the physical shapes of waveforms, comparing with quasi-independent geoidal height data and independent tide gauge measurements. The results identified the percentage of ocean like and non-ocean like waveforms are 91% and 9%, respectively. Off 9% of non-ocean like waveforms, the generic class is multi-peak (7%) followed by the quasi-specular waveforms (2%) observed near the coastline (< 10 km). Ocean like waveforms typically appear beyond 500 m from the coastline. When comparing with geoidal heights and tide gauge measurements, the performance of sea levels from several retrackers are assessed. The SAMOSA+ (SAR Altimetry Mode Studies and Applications) retracker outperforms other retrackers (i.e. sub-waveform and modified threshold retrackers with 30%, 20%, and 10%). That is, the standard deviation of differences against geoidal heights, and the temporal correlation against tide gauges are superior in most cases. In terms of root mean square error (RMSE), all retrackers are ranging with RMSE <= 20 cm in all cases. It can be concluded that in general, the SAMOSA+ retracked sea levels are accurate over the complicated regions at the Southeast Asia.