Modeling coherent scattering effects from a corn canopy at L-Band
Open AccessThis research was conducted to improve the understanding between microwave backscatter and the complex dynamics of an agricultural canopy, such as corn. In particular, how important are coherent scattering effects in a corn canopy at L-Band? To study this, a coherent scattering model was developed to compute L-Band backscatter from a corn canopy over a full growing season. The model consists of a layer of random medium over a lossy dielectric half-space having a rough interface. The random medium consists of three different scatterer types used to represent the corn stalk, leaf and cob. Each scatterer has prescribed orientation statistics and dielectric properties that were taken from field measurements over a corn canopy’s growing season. Concurrently, radar measurements from the same canopy was taken using a truck-mounted L-Band radar system, known as ComRad, developed and operated by George Washington University (GW) and NASA Goddard Space Flight Center (NASA/GSFC). A comparison between the measured and the modeled backscatter will show that coherent effects cannot be ignored at L-Band and highlights both the accuracy and robustness of the GW scattering model.In addition, a careful theoretical study of the scattering mechanisms that occur in the random medium model will be presented. From this study, enhanced backscatter, which is usually presented as a multiple scattering effect will be shown to occur under single scattering in the presence of a reflecting plane. The complex scattering phenomenon will be shown to be a simple summation of a direct (or volume) scatter and direct-reflected (or double-bounce) scatter. A graphical depiction of these scattering processes is given, which provides physical insight into a complicated problem. It will be shown that for both backscatter and specular scatter, an enhancement occurs. For like-polarization, this enhancement adds up to +3 dB additional power at backscatter, which is a well-documented phenomenon known as coherent or enhanced backscatter. A less-documented phenomenon shows up for the cross-polarized case where the enhancement can be negative, but also zero or positive, depending on the relationship of the incident angle to the Brewster angle, which was a novel contribution of this research.
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