AbstractuAmicrowave scattering model has been developed for layered vegetation based on an iteratlve solution of the radiative transfer equation up to the second order to account for multiple scattering within the canopy and between the ground and the canopy'. The model is designed to operate over a wide frequency range for both deciduous and coniferous forest and to account for th_ branch size distribution, leaf orientation distribution, and branch orientation distribution for each size. The canopy is modeled as a two-layered medium above a rough interface. The upper layer is the crown containing leaves, stems, and branches. The lower layer is the trunk region modeled as randomly positioned cylinders with a preferred orientation distribution above an irregular soil surface. Comparisons of this model with measurements from deciduous and coniferous forests show good agreements at several frequencies for both ilke and cross polarizations. Major features of the model needed to realize the agreement include allowance for (1) branch size distribution, (2) second-order effects, and (3) tree component models valid over a wide range of frequencies.

For an incident plane wave, the field inside a thin scatterer (disk and needle) is estimated by the generalized Rayleigh-Gans (GRG) approximation. This leads to a scattering amplitude tensor equal to that obtained via the Rayleigh approximation (dipole term) with a modifying function. For a finite length cylinder the inner field is estimated by the corresponding field for the same cylinder of infinite length. The effects of different approaches in estimating the field inside the scatterer on the backscattering cross section are illustrated numerically for a circular disk, a needle, and a finite length cylinder as a function of the wave number and the incidence angle. Finally, the modeling predictions are compared with measurements.

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