The suitability analysis of soil moisture retrieval using GNSS-R technology

Abstract

Soil moisture measurements are important to hydrology, climatology, and agriculture. Global Navigation Satellite System reflectometry (GNSS-R) is a new and powerful tool for sensing soil moisture. Different methods have been proposed for the reception and processing of reflected signals, such as multiantenna and single-antenna patterns. Considerable efforts have been made on ground-based and airborne observations. Meanwhile, a number of space-borne missions have also been implemented. During the suitability analysis of soil moisture remote sensing via GNSS-R technique, three key factors have to be determined-the specular reflection point, spatial pixel size, and sensing depth in the soil-regardless of the extraction methods of the reflected signals or the types of the observation platform. However, no relatively comprehensive explanations are available in the current literature. Theoretical analysis and formula derivation are conducted to systematically and quantitatively determine the extent of soil detection in three dimensions from the abovementioned aspects. First, the geographical position of the specular reflection point on the WGS84 ellipsoid surface is determined with an iterative algorithm. The effect of elevation between the real ground surface and the ellipsoid is discussed. Second, the first Fresnel zone is defined as the spatial pixel of the GNSS-R technique. The size of the pixel is regarded as the spatial resolution based on the concept of the equi-delay ellipse. Thirdly, the penetration distance of GNSS signals in soil is expressed. The concept of sensing depth is proposed on penetration distance and is based on the theory of microwave remote sensing. This concept makes soil moisture detection more describable and comparable. To further analyze the feasibility of soil moisture remote sensing with GNSS-R, the results of two application scenarios are shown: (1) A ground-based GPS measurement was performed in Marshall, Colorado, US, from the Plate Boundary Observatory. This measurement corresponds to the single-antenna pattern. The relative location of the specular reflection points, average area of the first Fresnel ellipse clusters, and sensing depth of the time-series soil moisture are analyzed. (2) An airborne GNSS-R experiment was conducted in Zhengzhou to obtain soil moisture content. This measurement corresponds to the multiantenna pattern. The spatial distribution of estimated soil moisture with a certain resolution based on flight tracks and relevant sensing depth are manifested. The results of the ground-based GPS measurement and airborne GNSS-R experiment are reasonable and convincing. Given that this study determined the extent of detected soil in three dimensions, this work provides theoretical basis for the precision evaluation of retrieved soil moisture. BeiDou mainly differs from GPS in the carrier frequency for remote sensing using GNSS reflected signals. Therefore, the results of this study provide references for future development of the BeiDou-R technique in China.