Assessment of an Adaptive Subwaveform Coastal Retracker (ASCR) over global coastal oceans for SAR altimetry

Abstract

To improve the data availability of SAR mode altimeters in coastal zones, we propose a new Adaptive Subwaveform Coastal Retracker (ASCR) and include the empirical coastal retracker ITAS (Improved Threshold Adaptive Subwaveform) and the full-waveform coastal retracker MSCR (Modified SAMOSA+ Coastal Retracker) for comparison in this study. The Sentinel-3A/B altimeter data during the period between January 2020 and December 2022 are reprocessed by these coastal retrackers. The performance of the coastal retrackers, as well as the official Ocean retracker, is intensively assessed over global coastal oceans within 100 km to the coastline. The results show that the coastal retrackers outperform the official Ocean retracker in the 0–20 km distance band, with the data availability increased by up to 7 %, which is statistically significant considering the large amount of coastal sea level data. The ASCR retracker achieves overall better performance than both ITAS and MSCR retrackers. The power spectrum analysis further demonstrates that the noise level within 5 km to the coast is about 35 % higher than that offshore. Moreover, about 6 %–20 % of data loss is observed in the same distance band, which may be because the on-board altimeter fails to record the ocean returns reflected from the nadir sea surface. It is also found that the track-coastline angle is an important factor affecting the data quality of SAR-mode altimeters within the last 10 km to the coast. Finally, the crossover analysis and validation against tide gauges prove that the coastal retrackers can achieve better or at least comparable data quality with official Ocean retracker over global coastal oceans.