Bases physiques de la télédétection et sa problématique

Basic physical principales and problematical aspects of teledetection

Professeur Groupe de Recherches en Télédétection Radiométrique Université Louis Pasteur, Strasbourg

Abstract

The physical principles of teledetection and their application to various fields related to surface water and moisture are discussed. In the first part of the report, the general principle of active and passive teledetection is described, with emphasis on the various electromagnetic radiation quantities requiring measurement. It is then explained why electromagnetic radiation conveys interesting information regarding the ground surface and immediate subsoil. The reason is stated to be that surface luminance as given by expression (1) depends explicitly on the optical parameters of the interface (directional spectral emissivity and bidirectional spectral reflectivity), and also on surface temperature. It is shown how these parameters depend on the properties and humidity of the medium (Fig. 3). The importance of dynamic study of surface temperature (i.e. Earth's response to solar excitation and a measure of the energy balance at ground level) is also demonstrated. The main parameters to be considered are summarised in Table l, which shows one of the difficulties of teledetection, i.e. that many parameters have to be taken into account but few are measured (I_λp output signals). This unfortunately is not the only difficulty. It is shown how both atmospheric effects and the radiometer itself interfere with surface luminance emission (expression (2)). Corrective methods whereby the data are reversed are described. A passing reference is made to the inherent difficulty of the considered measurement method, resulting rom the difference between the measurement scales at ground level and from the satellite, which is amplified by the difference between the respective scanning areas. It is shown by examples that not all the problems referred to need necessarily be solved for effective use of teledetection methods.