Observations of river water levels from nadir radar altimeters are currently limited by orbital spacing and by the winding nature of rivers, making them inadequate for monitoring the spatial–temporal dynamics of rivers. In this study, we developed an automatic off-nadir processing method to estimate water surface elevation and river slope in cross-track direction using Sentinel-3A/-3B and Sentinel-6A Fully-Focused SAR (FFSAR) processed data. Once the off-nadir reflected echoes are recorded in the range window, a slant range correction is applied to the retracked range to obtain the water level. From both the satellite altitude and the position of range window, we find that the maximum cross-track distance usable in this method is 6.6 km for Sentinel-3A/-3B and 9.3 km for Sentinel-6A. The validation of off-nadir water surface elevations (WSEs) in three rivers (Rhine, Danube, and Oder) against in-situ data, shows an accuracy in terms of standard deviation of difference (STDD) between 0.04 and 0.09 m for Sentinel-3A/-3B and Sentinel-6A. Moreover, the water surface slopes estimated from off-nadir processing, compared to slopes derived from in-situ data, show an accuracy, in terms of STDD, varying from 0.7 cm/km to 1.3 cm/km. Comparison of the river profiles over a 60-km river channel from off-nadir and wide-swath altimetry data, shows a STDD of 0.14 m for Sentinel-6 and 0.19 m for Sentinel-3B respectively. This study confirms the effectiveness of off-nadir processing in improving the accuracy of river surface elevation and slope measurements.

中文翻译：

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通过高度计测量非最低点河流水位和坡度全聚焦 SAR 模式


目前，最低点雷达高度计对河流水位的观测受到轨道间距和河流蜿蜒性质的限制，使其不足以监测河流的时空动态。在本研究中，我们开发了一种自动偏离最低点处理方法，使用 Sentinel-3A/-3B 和 Sentinel-6A 全聚焦 SAR （FFSAR） 处理数据估计交叉航迹方向的水面高程和河流坡度。在范围窗口中记录到非像底点反射的回波后，将对重新跟踪的范围应用倾斜范围校正以获得水位。从卫星高度和距离窗口位置来看，该方法中可用的最大交叉航迹距离对于 Sentinel-3A/-3B 为 6.6 km，对于 Sentinel-6A 为 9.3 km。根据原位数据对三条河流（莱茵河、多瑙河和奥得河）中非最低点水面高程 （WSE） 的验证表明，Sentinel-3A/-3B 和 Sentinel-6A 的差值标准差 （STDD） 精度在 0.04 和 0.09 m 之间。此外，与从原位数据得出的坡度相比，从最低点处理估计的水面坡度显示出 STDD 的精度，从 0.7 厘米/公里到 1.3 厘米/公里不等。从最低点和宽幅测高数据中比较 60 公里河道上的河流剖面，显示 Sentinel-6 和 Sentinel-3B 的 STDD 分别为 0.14 m 和 0.19 m。这项研究证实了偏离最低点处理在提高河流表面高程和坡度测量精度方面的有效性。