GNSS-R (Global Navigation Satellite System-Reflectometry) has been demonstrated to\nbe a new and powerful tool to sense soil moisture in recent years. Multi-antenna pattern and\nsingle-antenna pattern have been proposed regarding how to receive and process reflected signals.\nGreat efforts have been made concerning ground-based and air-borne observations. Meanwhile,\na number of satellite-based missions have also been implemented. For the in-depth study of soil\nmoisture remote sensing by the technique of GNSS-R, regardless of the extraction methods of the\nreflected signals or the types of the observation platform, three key issues have to be determined:\nThe specular reflection point, the spatial resolution and the detection depth in the soil. However,\nin current literatures, there are no comprehensive explanations of the above three key issues.\nThis paper conducts theoretical analysis and formula derivation, aiming to systematically and\nquantitatively determine the extent of soil moisture being detected in three dimensions from the\nabove-mentioned aspects. To further explain how the three factors behave in the specific application,\nthe results of two application scenarios are shown: (1) a ground-based GPS measurement in\nMarshall, Colorado, US from the Plate Boundary Observatory, corresponding to single-antenna\npattern. The relative location of the specular reflection points, the average area of the First Fresnel\nEllipse Clusters and the sensing depth of the time-series soil moisture are analyzed, and (2) an\naviation experiment conducted in Zhengzhou to retrieve soil moisture content, corresponding\nto the multi-antenna pattern. The spatial distribution of soil moisture estimation with a certain\nresolution based on the flight tracks and the relevant sensing depth are manifested. For remote\nsensing using GNSS reflected signals, BeiDou is different from GPS mainly in the carrier frequency.\nTherefore, the results of this study can provide references for Chinaâ??s future development of the\nBeiDou-R technique.
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