Landscape freeze–thaw (F/T) state parameters are an integral part of research on terrestrial hydrological processes, vegetation growth dynamics, and terrestrial–atmospheric trace gas exchange. Therefore, the development of large-scale, continuous, and rapid F/T observation records is essential. This study proposes a scheme to effectively retrieve quasi-global daily soil F/T states from spaceborne Global Navigation Satellite System Reflectometry (GNSS-R) observations. This method takes into account the GNSS-R surface reflectivity and its influence factor. Specifically, the method employed surface reflectivity derived from Cyclone Global Navigation Satellite System (CYGNSS) data from 2018 to 2020, along with vegetation opacity data from Soil Moisture Active Passive (SMAP) as input layers in random forests for F/T retrieval. This approach provides F/T results on a quasi-global scale, does not rely on continuous F/T observations, and employs the least ancillary data. To validate the retrieval results, SMAP F/T data over the same period and temperature data from more than 100 ground stations were also used as reference. Results showed that the F/T retrieval accuracy of CYGNSS was 95.5% and 82.0% with respect to the SMAP data and data from independent in situ stations, respectively. Given the high temporal coverage of the CYGNSS F/T results, the temporal coverage improvement is 23.3% compared with the actual SMAP observations on a daily scale. Furthermore, as gap-filling data complemented SMAP, CYGNSS data can effectively improve the SMAP F/T sample interval (accuracy improvement of 26.0%). More importantly, the F/T accuracy of the CYGNSS and SMAP combination based on monthly performance is 85.1%, i.e., 20.5% better than that of the existing SMAP daily F/T product. Overall, the quasi-global daily F/T results obtained from CYGNSS and SMAP measurements can complement existing databases with well-defined accuracy.

中文翻译：

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来自星载 GNSS-R 技术和 SMAP 辐射计测量的高时间分辨率准全球景观土壤冻融图

景观冻融（F/T）状态参数是陆地水文过程、植被生长动态和陆地-大气微量气体交换研究的一个组成部分。因此，发展大规模、连续、快速的F/T观测记录至关重要。本研究提出了一种从星载全球导航卫星系统反射测量（GNSS-R）观测中有效检索准全球日常土壤 F/T 状态的方案。该方法考虑了GNSS-R表面反射率及其影响因素。具体来说，该方法采用从 2018 年至 2020 年旋风全球导航卫星系统 (CYGNSS) 数据导出的表面反射率，以及来自土壤湿度主动被动 (SMAP) 的植被不透明度数据作为随机森林中用于 F/T 检索的输入层。这种方法提供了准全球范围内的 F/T 结果，不依赖于连续的 F/T 观测，并且使用最少的辅助数据。为了验证反演结果，还参考了同期SMAP F/T数据和100多个地面站的温度数据。结果表明，CYGNSS对SMAP数据和独立现场站数据的F/T反演精度分别为95.5%和82.0%。鉴于 CYGNSS F/T 结果的高时间覆盖率，与每日规模的实际 SMAP 观测相比，时间覆盖率提高了 23.3%。此外，由于填空数据对SMAP进行了补充，CYGNSS数据可以有效提高SMAP F/T采样间隔（精度提高26.0%）。更重要的是，CYGNSS和SMAP组合基于月度性能的F/T精度为85.1%，比现有SMAP日F/T产品提高了20.5%。总体而言，从 CYGNSS 和 SMAP 测量获得的准全球每日 F/T 结果可以以明确的精度补充现有数据库。