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Mesures physiques et signatures en télédétection

M.Herman®and V. Vanderbilt®
(1) Laboratoire d'Optique Atmosphérique. URA 713, Université de Lille,. Villeneuve d'Ascq, France
(2) NASA Ames Research Center, MS 242-4, Moffet Field , California, USA
The polarization of the sunlight scattered by atmospheric aerosols or cloud droplets and reflected from ground surfaces
or plant canopies may convey much information when used for remote sensing purposes. The typical polarization
features of aerosols, cloud droplets and plant canopies, as observed by ground based and airborne sensors, are
investigated, looking especially for those invariant properties amenable to description by simple models when
possible. We address the question of polarization measurements from space. We examine what should be the interest
of such measurements for remote sensing purposes, and test their faisability by using results obtained during field
campaigns of the airborne POLDER instrument.
Mots clef: Polarization-Plant canopies-Aerosols-Modeling-Remote sensing
Reflections by ground surface and scattering in the atmosphere are the mechanisms by which the incident natural solar
radiation is partially polarized. The most efficient polarizing mechanisms, specular reflection and scattering by
molecules, are able to totally polarize natural incident light, if it is reflected by a surface at the Brewster angle or
scattered at an angle of 90° by molecular dipoles. These mechanisms are responsible for the large polarizations
exhibited by the glitter pattern on natural water surfaces, and for the polarization of the skylight observed at right to
the sun. Although less efficient, scattering by atmospheric aerosols and cloud droplets and diffuse reflection from
rough ground surfaces and plant canopies are other polarizing sources. The polarization of the scattered sunlight,
therefore, may convey much information when used for remote sensing purposes. Here we consider some aspects of
this problem.
Many observations, since the late 18th century, have been made of the polarization of the terrestrial skylight, specially
by Arago, Babinet and Brewster. Because molecular scattering was soon shown to be the dominant mechanism
polarizing skylight, polarization then was used chiefly by astronomers, who sought to obtain information about the
atmospheres and ground properties of other planets and satellites. Extended polarization studies of the Martian and
lunar surfaces and of the Venus and Jupiter upper atmospheres were thus conducted from telescopic observations and,
later on, from fly by or in orbit sounder data (by Lyot 1929, Dollfus 1961, Gehrels et al. 1980 for example). The
most impressive result was probably the work of of Hansen and Arking (1971) and Hansen and Hovenier (1974) who
were able to demonstrate, from the analysis of some 50 years of measurements of the polarized light reflected by
Venus, that the upper haze layer of this planet consisted of hydrated sulfuric acid particles.
Concerning the Earth, polarization studies were mainly turned toward Earth surfaces and largely devoted to the
polarization properties of bare soils, desertic sites (e.g. by Coulson et al. 1965, Egan 1985), vegetative covers (e.g.
Vanderbilt et al. 1990). To a lesser extent, the polarization of the atmosphere was observed from ground based
measurements (e.g. B.Herman, Deuze et al. 1988). And, occasionally, the polarization features of clouds or of the
earth-atmosphere system were observed by airborne or balloon-borne devices (Rao 1969, Coffeen and Hansen 1972,
Deuze et al 1989). An extensive review of these experiments is given by Coulson, 1988. However, polarization
sensors were never launched aboard earth-orbiting satellites. Such observations were long ago suggested. But
promizing sensor designs, althought developed (e.g. from airborne measurements, by Coffeen and Hansen 1972), have
not been pursued through to launch of a satellite. The only measurements of the polarization of the earth-atmosphere
system were provided by photographs of polarized light taken during several American space shuttle missions, the first
occuring in 1984 (Coulson et al. 1986, Egan 1991). As the polarization of light is easily measured, it seems
regrettable that so many Earth observing devices have measured for so long so many photons, never looking at their
polarization state. However, the situation should change in the near future as present plans are to collect polarization
measurements using two satellite sensors, the EOSP instrument to be flown as a part of the EOS mission of NASA
and POLDER which will be flown during the ADEOS mission of NASDA. These planned experiments should
provide the first extensive, global polarization data set, potentially filling this information gap.
We will consider here the polarization characteristics of targets, in the ground-atmosphere system, that may be of
■merest for remote sensing purposes: plant canopies, aerosols and clouds. We will show typical polarization features
of these targets, as observed by ground based and airborne sensors, looking especially for those invariant properties