Microwave remote sensing of vegetation: Stochastic Lindenmayer systems, collective scattering effects, and neural network inversions.

摘要

In this thesis, we apply Stochastic Lindenmayer Systems (L-systems), based on which we construct tree-like structures. We then study wave scattering by trees. The trees are grown by using the Stochastic L-systems. The correlations of scattering by different branches are included by using their relative positions as governed by the growth procedure. The advantages of this method are that (1) the structure of trees is controlled by growth procedure and the calculation of the pair distribution functions and probability density function are not needed, and (2) the trees grown by Stochastic L-systems are quite realistic in appearance to natural trees.; We calculate the scattering amplitude from a layer of trees overlaying a flat ground by using coherent addition approximation and compare it to the independent scattering approximation as well as tree-independent scattering approximation. The coherent addition approximation takes into account the relative phase shifts between scatterers in a realization of trees. The tree-independent scattering approximation considers every tree as an independent scatterer. It is found that for C band, L band, and P band, the backscattering coefficients calculated by tree-independent scattering approximation are very close to those of coherent addition approximation. At C- band, L-band, and P-band, the distances between trees are still large compared with wavelength, the trees can still be treated as independent scatterers. However, we can observe increasing differences between the backscattering coefficients calculated by coherent addition approximation and independent scattering approximation when we shift the frequency from C band to L band to P band.; We use a discrete dipole approximation method to calculate the scattering from trees generated by Stochastic L-systems. The advantage of this approach is that the coherent mutual interactions between the branches are included. The validity of this discrete dipole approximation method is checked by performing the convergence tests, comparing with another moment method code for body of revolution based on the surface integral formulation, and reviewing the optical theorem. The results are compared with those of coherent addition approximation and independent scattering approximation. It is found that the coherent addition approximation gives good estimates to the co-polarized backscattering coefficients (both vv and hh). The differences are larger for the case of cross-polarized backscattering coefficients. It is also observed that the absorption coefficients of the horizontally polarization from independent scattering is not sensitive to change of the incident angle. The variation with incident angles is much larger for the vertically polarization case because the incident electric field vector changes with the incident angle. The difference between the discrete dipole approximation and the independent scattering approximation is due to the coherent mutual interactions among the branches within a tree. The mutual interaction creates a significant change of the internal field and the absorption can be several dB larger than that of the independent scattering case. (Abstract shortened by UMI.)