tailieunhanh - Geoscience and Remote Sensing New Achievement_2

Our planet is nowadays continuously monitored by powerful remote sensors operating in wide portions of the electromagnetic spectrum. Our capability of acquiring detailed information on the environment has been revolutionized by revealing its inner structure, morphology and dynamical changes. The way we now observe and study the evolution of the Earth’s status has even radically influenced our perception and conception of the world we live in. | 16 Optical and Infrared Modeling Abdelaziz Kallel Tartu Observatory Estonia 1. Introduction In order to understand the relationships between the vegetation features namely amount and structure and the amount of sunlight reflected in the visible and near- to middle-infrared spectral domains many empirical methods based on various vegetation indices . NDVI EVI Kallel et al. 2007 and physical approach namely based on radiative transfer RT theory have been developed. In RT two model types can be distinguished i one-Dimensional 1-D models providing a semi analytical expression of the Bidirectional Reflectance Distribution Function BRDF of canopy architecture its scattering parameters and scene geometry Gobron et al. 1997 Verhoef 1984 1998 ii 3-D models based on Monte Carlo simulations of a large number of photons randomly propagating through a canopy Gastellu-Etchegorry et al. 1996 Lewis 1999 North 1996 . Compared to 1-D models such 3-D methods allow to take into account canopy heterogeneity with high accuracy. However they suffer from long running times making their inversion difficult. The RT theory was first proposed by Chandrasekhar 1950 to study radiation scattering in conventional . rotationally invariant media. Such an assumption could be sufficient to model for example light scattering in the atmosphere but appears rudimentary for modeling the reflectance of leaves or shoots in a vegetation canopy. To extend the formulation to such a case many models are proposed. Among the 1-D model one can cite SAIL Ver-hoef 1984 that is among the most widely used in case of turbid null size components crops canopies. The SAIL model allows to derive a non-isotropic BRDF considering two diffuse fluxes upward downward flux to model the multiple scattering of the radiant flux by the vegetation elements. These fluxes are assumed to be semi-isotropic which is only an approximation that lead to reflectance underestimation Pinty et al. 2004 . As a solution Verhoef 1998 .