Vegetation Sensing Using GPS Interferometric Reflectometry: Experimental Results With a Horizontally Polarized Antenna

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Measurement of vegetation state is required both for modeling and for satellite data validation. GPS Interferometric Reflectometry (GPS-IR) is a passive microwave remote sensing technique that has been applied to several applications, such as sensing soil moisture, snow depth, ocean roughness and wind speed, and vegetation growth. Previous studies proposed that geodetic GPS data could be used to measure vegetation biophysical parameters, for example, vegetation height and vegetation water content (VWC). However, the measurement range and accuracy are limited because the geodetic GPS antenna is designed to suppress reflections. In this paper, the antenna design for vegetation sensing via GPS-IR is discussed. Then, a customized dipole antenna is developed specifically for vegetation sensing. A field experiment is conducted at Boulder, CO, USA to evaluate the performance of the proposed antenna. The GPS metric, which is the normalized signal-to-noise ratio (SNR) amplitude ${A_{{\rm{norm}}}}$ , shows a linear relationship with in situ measurements of VWC over a range of 0—6 kg/m2, which is much greater than the range from the geodetic data (0—1.0 kg/m 2). The sensitivity of ${A_{{\rm{norm}}}}$ to VWC change is −0.11 (W/W)/(kg/m2), and the average measurement error of VWC is ∼12.4%. Thus, the horizontally polarized antenna is suitable for vegetation monitoring in most environments, including agricultural settings. The effects of soil moisture on GPS SNR amplitude are also documented, but these are minor compared to the decrease in amplitude resulting from vegetation growth.