Interpretation of Microwave Signatures of Trees by a Combined Backscattering and Plant Structural Model

Abstract

The radar backscattering behaviour of trees is difEcult to investigate, firstly because measurement of vegetation parameters of trees is laborious, secondly because of the complex logistic associated with the use of radar measurement systems operating over a tree canopy. In this context, simulation studies performed with appropriate backscattering physical models can improve our understanding of observed microwave signatures of trees. This paper will present the methodology adopted to combine a tree growth model to a theoretical backscattering model. This methodology will be illustrated with data obtained during the MAESTR0'89 campaign in The Netherlands over poplar species showing different microwave signatures at P, L and Cbands. The methodology includes 3 main steps : 1) Poplar tree architectures are elaborated by a plant growth model which is based on a1 botanically accurate simulation of the functioning of buds. The plant model can be considered as a physical model with botanical laws. The general principle is to model the activity of buds at discretized times. Geometrical parameters such as the length and diameter of an internode or the branching angles are calculated according to specific stochastic laws. Because plant model is a numerical one, computer simulations are possible and lead to final 3-D images which are very faithful to the reality. Measurements performed in situ on poplar trees are used to adjust the simulated trees to real trees. 2) Geometrical parameters required as input in backscattering models are calculated firom the simulated 3D fdes. Vertical profiles of the pdf's in size and orientation and number density of the different tree components namely leaves, twigs and branches are determined. 3) Finally, the previous calculated geometrical parameters and other measured parameters are incorporated in the Karam and Fung's model (1991) to simulate the backscattering responses of poplar stands. Comparison with observed backscattering signal of different stands are made.