Remote Assessment of Soil Electrical Conductivity Using a P-Band Amplitude-Phase Interferometer Deployed on Two UAVs

To implement precision farming technology in agricultural crop production at all stages of plant vegetation, it is necessary to promptly determine the physical parameters of soil from its surface to the depth of the root system with a minimum resolution element area of 2–4 m2. The Global Navigation Satellite System (GNSS) reflectometry methods widely used for these purposes are affected by the roughness of the probed surface and the presence of vegetation on it, do not provide penetration of the radio signal to the required depth, and do not allow carrying out measurements at a specified point in the field in a short time. The proposed study uses a bistatic P-band radar system placed on unmanned aerial vehicles (UAVs). The features of the formation of interference signals with vertical and horizontal polarizations are considered under the condition that the receiving antenna is on the border of the radiative (Fresnel) and far-field region of the transmitter. The factors influencing the accuracy of measuring the Brewster angle by the phase method are shown using a three-layer model of radio wave propagation. To eliminate the possible ambiguity in Brewster angle measurements caused by oscillations in the phase shift of interference waves with vertical and horizontal polarizations and to increase the accuracy of soil electrical conductivity (EC) assessment, it is proposed to combine measurements of the phase differences and ratios of the acting amplitudes of interference waves with different polarizations. It is shown that the proposed system can be used for zoning of agricultural soils based on their EC.