Soil Moisture Retrieval Using Reflection Coefficients: Numerical Experiments

Ready access to small, inexpensive, unmanned aerial vehicles (UAVs) allows for small-scale electromagnetic propagation and scattering experiments using airborne antennas. We consider a low-power, copter-mounted transmit antenna radiating at frequencies on the order of a hundred megahertz, corresponding to wavelength on the order of meters, which at some horizontal distance generates a vertical interference pattern due to the interaction of a direct and ground-reflected wave. The depth of the interference fringes, which is directly related to the modulus of the soil reflection coefficient, can be measured by a second copter-mounted receiving antenna. By varying the geometry of the copter pair, and/or the transmitted signal frequency, the soil moisture profile up to a depth of a few meters can, in principle, be retrieved. In this article, we present the results of numerical experiments designed to evaluate the sensitivity of the angle- and frequency-dependence of the measured reflection coefficient to the soil moisture profile. Our simulations indicate that the retrieval errors are small suggesting that the technique is feasible.