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.

中文翻译：

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全球沿海海洋自适应子波形沿海再跟踪器 （ASCR） 的 SAR 测高评估


为了提高沿海地区 SAR 模式高度计的数据可用性，我们提出了一种新的自适应子波形沿海再跟踪器 （ASCR），并包括经验沿海再跟踪器 ITAS （改进的阈值自适应子波形） 和全波形沿海再跟踪器 MSCR （改进的 SAMOSA+ 沿海再跟踪器） 用于本研究的比较。2020 年 1 月至 2022 年 12 月期间的 Sentinel-3A/B 高度计数据由这些沿海再跟踪器进行再处理。沿海再跟踪器以及官方海洋再跟踪器的性能在距海岸线 100 公里以内的全球沿海海洋上进行了密集评估。结果表明，沿海再跟踪器在 0-20 公里距离范围内的性能优于官方的海洋再跟踪器，数据可用性提高了 7%，考虑到大量的沿海海平面数据，这在统计上是显着的。ASCR retracker 的整体性能优于 ITAS 和 MSCR retracker。功率谱分析进一步表明，距离海岸 5 公里以内的噪音水平比海上高约 35%。此外，在同一距离带内观察到大约 6 %–20 % 的数据丢失，这可能是因为机载高度计未能记录从最低点海面反射的海洋回波。研究还发现，航迹-海岸线角度是影响距海岸最近 10 km 内 SAR 模式高度计数据质量的重要因素。最后，针对潮汐仪的交叉分析和验证证明，沿海再跟踪器可以获得与全球沿海海洋官方海洋再跟踪器更好或至少相当的数据质量。