Abstract The L-band (1.41 GHz) passive microwave remote sensing technique is the approach used by the first satellites dedicated to soil moisture measurement, the European Space Agency's (ESA) Soil Moisture and Ocean Salinity (SMOS), and the Soil Moisture Active Passive (SMAP) mission developed by the National Aeronautics and Space Administration (NASA). These satellites aim to provide global soil moisture maps for the top ~5 cm layer of soil with an accuracy better than 0.04 m3/m3. However, with a passive microwave observing resolution of ~40 km, non-soil targets such as surface rock may possibly confound the brightness temperature observations and degrade the accuracy of retrievals for many SMOS and SMAP pixels across the world. Since the microwave contribution of rock is not well accounted for in current soil moisture retrieval algorithms, simply ignoring its existence may be detrimental to the performance of resultant soil moisture products. Using a combination of model simulations and airborne field campaign data from central Australia, this study has determined that a rock cover fraction threshold of up to 0.4 can be tolerated before the 0.04 m3/m3 soil moisture target accuracy is potentially exceeded under extreme dry or wet conditions. However, this threshold reduces to 0.2 when assessed in terms of a brightness temperature impact >4 K. These rock fraction thresholds have subsequently been applied to the Ecoclimap rock cover map, identifying the SMOS and SMAP pixels globally that are likely to be adversely affected if rock is unaccounted for. The results show that approximately ~3.3% of all SMOS and SMAP pixels may have brightness temperature impacts exceeding 4 K from surface rock, with Asia being the most affected, having ~6.0% affected pixels. These values reduce to ~1.5% of SMOS and SMAP pixels globally, and ~3.1% for Asia, when assessed in terms of soil moisture errors expected to possibly exceed 0.04 m3/m3 when not accounting for surface rock.

中文翻译：

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L波段被动微波观测对土壤水分反演的表层岩石效应

摘要 L 波段 (1.41 GHz) 无源微波遥感技术是第一批专门用于土壤水分测量的卫星、欧洲航天局 (ESA) 的土壤水分和海洋盐度 (SMOS) 以及土壤水分有源无源卫星使用的方法。 (SMAP) 任务由美国国家航空航天局 (NASA) 开发。这些卫星旨在提供精度高于 0.04 m3/m3 的顶部 ~5 cm 土壤层的全球土壤湿度图。然而，由于被动微波观测分辨率约为 40 公里，地表岩石等非土壤目标可能会混淆亮温观测并降低全球许多 SMOS 和 SMAP 像素的反演精度。由于目前的土壤水分反演算法没有很好地考虑岩石的微波贡献，简单地忽略它的存在可能对所得土壤水分产品的性能有害。使用模型模拟和来自澳大利亚中部的机载现场活动数据的组合，该研究确定，在极端干燥或潮湿条件下可能超过 0.04 m3/m3 土壤水分目标精度之前，可以容忍高达 0.4 的岩石覆盖率阈值使适应。然而，当根据亮度温度影响大于 4 K 进行评估时，该阈值降低到 0.2。这些岩石分数阈值随后被应用于 Ecoclimap 岩石覆盖图，在全球范围内识别可能受到不利影响的 SMOS 和 SMAP 像素，如果岩石下落不明。结果表明，大约~3。3% 的 SMOS 和 SMAP 像素可能会受到来自地表岩石的超过 4 K 的亮温影响，其中亚洲受影响最大，大约有 6.0% 的像素受到影响。这些值在全球范围内减少到 SMOS 和 SMAP 像素的约 1.5%，在亚洲减少到约 3.1%，当根据土壤湿度误差进行评估时，如果不考虑表层岩石，预计可能超过 0.04 m3/m3。