Radar response of periodic vegetation canopies

Vector radiative transfer theory is used to model the scattered intensity from a layer of randomly oriented particles over a periodic rough surface. To account for the periodic nature of row-structured vegetation, the number density of particles within the layer is assumed to be varying periodically in the horizontal direction. Using Fourier series expansions and orthogonality properties, the radiative transfer equation is solved for the transformation matrix relating the incident and scattered intensities, from which the backscattering coefficient of the layer can be computed for any incidence direction and polarization configuration. The experimental component of this investigation consisted of radar observations at 1–5,4–75, and 9–5 GHz made by a truck-mounted system for a field of corn under three conditions: (a) full, which means that the corn plants were in their natural state, (b) defoliated, which was accomplished by stripping off the leaves and removing them, thereby leaving behind only bare vertical stalks, and (c) bare soil, which corresponds to the soil surface after having removed the stalks. The soil surface is modelled as a composite consisting of a deterministic periodic component and a random roughness component. A two-scale polarimetric scattering model is formulated and used to compare with the experimental observations. Excellent agreement between theory and measurements is realized as a function of both incidence and azimuth angles at all three microwave frequencies. The canopy model was then applied to the corn canopy under the two other conditions: stalks alone and full canopy. The model results were compared with radar backscatter measurements made for each of three look directions, including perpendicular and parallel to the row direction and at 45° relative to the row direction. For the stalk canopy, it was observed that the quasi-periodic arrangement of the stalks within the row enhances the backscatter at L-band when looking perpendicular to the row direction, which is attributed to a coherent-scattering effect associated with the stalks. A heuristic approach is used to model the quasi-periodic structure of the stalks by deriving a coherency factor which multiplies the first-order radiative solution for randomly located stalks. A similar coherency factor was also introduced for the leaves of the full canopy. The modified model was found to provide good agreement with experimental observations at L-band for all polarizations and at all look directions.