Abstract Earth remote sensing using reflected GNSS signals is currently an emerging trend especially in ocean surface wind measurements. Unlike the existing scatterometer missions, GNSS-R uses L-Band navigation signals that can penetrate through clouds and rain. Rain may have a negligible impact on the transmitted signal in terms of path attenuation at this wavelength. However, there are other effects due to rain, such as changes in surface roughness and rain induced local winds, which can significantly alter the measurements. Currently, there is no observation-based characterization of all possible impacts of rain on radar forward scatter, which is the nature of operation of GNSS-R missions. In this study, we propose a 3-fold rain model which accounts for attenuation, surface effects of rain and rain induced local winds. We utilize the large dataset of measurements made by the CYGNSS mission to separate these different effects of rain. The attenuation model suggests that a total of at least 96% transmissivity exists at L-Band up to a rain rate of 30 mm/h. A perturbation model is used to characterize the other rain effects. It suggests that rain is accompanied by an overall reduction in the scattering cross-section of the ocean surface and, most importantly, this effect is observed only up to surface wind speeds of 15 m/s, beyond which the gravity capillary waves dominate the scattering in the quasi-specular direction. Observations also suggest that, at very low wind speeds, the lower bound on wavenumber of the portion of the surface roughness spectrum that influences the measurements deviates from the geometric optics approximation normally used. This work binds together several rain-related phenomena and enhances our overall understanding of rain effects on GNSS-R measurements.

中文翻译：

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降雨对 L 波段 GNSS-R 海面测量影响的表征

摘要 使用反射 GNSS 信号进行地球遥感是目前的一个新兴趋势，尤其是在海面风测量中。与现有的散射计任务不同，GNSS-R 使用可以穿透云和雨的 L 波段导航信号。就该波长的路径衰减而言，雨水对传输信号的影响可以忽略不计。然而，降雨还有其他影响，例如地表粗糙度的变化和降雨引起的局部风，这会显着改变测量结果。目前，没有基于观测的表征降雨对雷达前向散射的所有可能影响，这是 GNSS-R 任务运行的性质。在这项研究中，我们提出了一个 3 倍降雨模型，该模型考虑了衰减、雨和雨引起的局部风的表面效应。我们利用 CYGNSS 任务进行的大型测量数据集来区分雨的这些不同影响。衰减模型表明，在高达 30 毫米/小时的降雨速率下，L 波段至少存在 96% 的透射率。扰动模型用于表征其他降雨效应。这表明降雨伴随着海洋表面散射截面的整体减少，最重要的是，这种效应仅在表面风速达到 15 m/s 时才观察到，超过该风速时重力毛细波在散射中占主导地位在准镜面方向。观察结果还表明，在非常低的风速下，影响测量的表面粗糙度谱部分的波数下限偏离了通常使用的几何光学近似。