CONICET | Buscador de Institutos y Recursos Humanos

Soil moisture (SM) is a key biophysical variable that controls the partition of both mass (rain, runoff) and energy (evapotranspiration) fluxes on the soil and therefore is important for the monitoring of many hydro-geochemical cycles. From the agricultural point of view, it effectively limits crop productivity. Therefore, the generation of regional-scale soil moisture maps is a critical task that can only be accomplished systematically using remote sensing techniques.One promising topic is to estimate surface soil moisture using the signal reflected from the soil in the microwave regime, using the well known relation between dielectric constant and soil moisture. In this framework, lluminators Of Opportunity (IOO) present several operative advantages to estimate soil moisture in the framework of small satellites. First, IOO bistatic radar configurations do not require a dedicated transmitter, which significantly reduces implementation costs. Also, there is evidence that soil bistatic scattering function is more sensitive to soil moisture than soil backscattering.Traditional SM retrieval techniques based on IOO relies on the computation of the bistatic scattering coefficient of the soil, which is related to soil moisture through soil dielectric constant. However, for IOO characterized by sufficiently long pulses, the Interference Pattern Technique (IPT) can be used. This technique consists on measuring the angular power fluctuations of the interference between the direct and the reflected electric fields as the platform or the IOO moves. This happens because the two signals (direct and reflected) arrive to the antenna within the same pulse and therefore are coherently added.Since most IOO emits at circular polarization, the technique is based on measuring the vertically polarized component of the received signal. This ?vertically polarized interference pattern? is analyzed in order to find a minimum (?notch?), which according to theory should correspond to the Brewster angle, which is itself related to surface dielectric constant and soil moisture. However, the relation between the angle in which the notch is found and the presence of the notch itself is based on a simplified analysis of soil specular scattering, by assuming a plane interface (Fresnel).In this paper, we implemented the 2nd order Small Perturbation Model (SPM) for the system geometry, to simulate vertically polarized interference pattern expected in the antenna as a function of both geometric and soil properties (dielectric constant and roughness). In the analysis, we show that the SPM predicts the notch, and its expected monotonic behavior as a function of soil dielectric constant. However the notch amplitude depends strongly on soil roughness, and it disappears for rough soils (s > 2 cm). Experimental results obtained using an antenna over a tripod and GPS satellites shows that SPM theoretical predictions present a reasonable agreement with the observations.The results of this work can be used to add another dimension to traditional SM retrieval techniques based on IOOs, since this approach does not relies on the comparison of amplitudes, but on the interference between the direct and reflected signal. It is important to note that this technique cannot be used as is with GNSS (GPS) systems, since pulse duration is too short for orbital applications. However, other IOOs could be used instead to develop an application based on this technique.