Bistatic system for radar sensing of soil moisture

Abstract

The article is devoted to the development of a method for determining the moisture content of the upper layers of agricultural lands on the basis of mathematical modeling of a radar signal reflected from the soil. For the radar study of the moisture content of agricultural lands, it is proposed to use remote sensing using relatively inexpensive unmanned aerial vehicles (UAVs) of a multi-rotor type (a bistatic system consisting of a radar transmitter and a radar receiver placed on two UAVs). At the same time, it is necessary to ensure a high probability of correct determination of soil moisture to a depth exceeding the groundwater level. In remote sensing, the reflection coefficient from the air-soil interface depends on the dielectric properties of the top soil layer and the polarization of the sounding signal. When modeling the reflected signal, it is advisable to represent the earth's surface as a flat-layered structure, the upper part of which will be represented by vegetation and heterogeneities followed by the area with a relatively constant dielectric constant value to the groundwater level and then the area with a linearly increasing conductivity. Studies have shown that the Brewster effect in soils manifests itself when the probe signal is perpendicularly polarized. When the soil surface is irradiated with a perpendicularly polarized radar signal at Brewster's angle, the required penetration depth of the probing signal is ensured, at which the reflected signal will contain enough information about the percentage of moisture in the upper soil layers. The information contained in changes in the amplitude and phase of the reflected radio signal is related to the structure and dielectric properties of the upper soil layers. On the basis of this information, rational agro-technical measures can be justified, both operational (watering, fertilization) and long-term (reclamation, drainage).

Algorithm for the Joint Flight of Two Uncrewed Aerial Vehicles Constituting a Bistatic Radar System for the Soil Remote Sensing

“…The Brewster's angle is determined from the expression for the reflection coefficient for a vertically polarized electromagnetic wave (Bazhenov et al, 2021) (Equation 1):…”

“…As is known, when a plane electromagnetic wave is incident on the interface between two dielectric media, the fraction of reflected energy is determined by the reflection coefficient R. Thus, the purpose of the research is to create conditions under which the reflection coefficient from the air-surface interface will tend to zero (R→0), and the energy of the incident wave will penetrate the soil to form a reflected signal from the inner layers. To satisfy this condition, Bazhenov et al (2021) propose to use the effect of total refraction, i.e., carry out surface irradiation at Brewster's angle, while the remote radar sensing system should be placed on two crewless aerial vehicles (UAVs) and implemented as bistatic. Chandra & Tanzi (2018) performed mathematical modeling of subsurface layers' radar sounding to search for and rescue people during earthquakes and other global disasters using "oblique" surface irradiation at angles close to Brewster's angles.…”