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Mesures physiques et signatures en télédétection

Polarized Reflectance Angular Signatures
from Surface and Airborne Measurements
François-Marie Bréon 1 , and Didier Tanré 2
1: Laboratoire de Modélisation du Climat et de l'Environnement
CEA/DSM/LMCE; 91191 Gif sur Yvette, France
2: Laboratoire d'Optique Atmosphérique
USTL, Bat P5; 59655 Villeneuve d'Ascq, France
Concomitant surface and airborne
measurements of the bidirectional polarised
reflectance have been acquired during the
Hapex-Sahel experiment. Two analytical
models are suggested for the surface; one for
bare soil and the other for the vegetation.
They both consider that polarised radiance is
generated at the surface by single specular
reflection. The comparison of model estimates
with surface measurements confirms this
hypothesis. The measurements verify that
bare soils generate much more polarised
reflectance than vegetation canopies do. For a
given surface, the polarised reflectance
depends mostly on the scattering angle (the
angle between the sun and view direction), but
also on the solar and view zenith angles.
At aircraft level, surface polarised
reflectance is ambiguously mixed with that
generated by molecular and aerosol scattering.
However, a signal from the surface can be
evidenced. During the experiment, we found
that the measurements depend little on the
aerosol optical thickness. When aerosol
optical thickness increases, the additional
polarised radiance generated by aerosol
scattering is partly compensated by a
reduction of that produced by other processes
(masking by aerosol diffusion).
KEY WORDS: Surface reflectance, airborne
measurements, polarisation, model,
POLDER, directional signatures
Polarised reflectance measurements of
natural surfaces have been initiated by
Coulson (1966) in the 60’s. Since then, there
have been several attempts to correlate the
polarised light reflected by surfaces to their
biophysical properties. The polarisation was
said to be related to the surface roughness
(Wolff, 1975) and to the size of reflecting
elements (Egan, 1970). Some authors tried to
correlate the polarisation to soil moisture
(Egan, 1968; Curran, 1978) or to vegetation
biomass (Curran, 1981). It was also said that it
could be used for a better classification of
surface cover (Curran, 1982; Egan, 1970; Fitch
et al., 1984) and for monitoring the vegetation
canopy state as well (Vanderbilt and de
Venecia, 1988; Vanderbilt et al. 1985). A review
of early attempts to use polarisation for surface
remote sensing is given in Tamalge and
Curran (1986). Theoretical studies to
understand the nature and to modelize the
polarisation from Earth surfaces were also
performed. The polarisation was recognised to
be generated by specular reflection at the
surface of reflecting elements such as leaves
(Vanderbilt and Grant; 1985), rocks or sand
grains (Grant, 1987). Rondeaux and Herman
(1991) developed a physical model for
vegetation canopies and they showed that the
inversion of the model against field
measurements allows the retrieval of the
canopy leaf angular distribution and so the
estimation of the vegetation state. The
polarisation properties of natural targets have
also been investigated in the laboratory using
an incandescent lamp (Woessner and Hapke,
1987) or a polarised laser beam (Gibbs et al.,
1993) as a source.
The POLDER instrument (Polarisation and
Directionality of the Earth Reflectance;
Deschamps et al., 1994), to be launched in 1996
on the Japanese ADEOS platform, will
measure the polarisation of Earth Reflectances.
It will be the first attempt of using polarisation
measurements for global Earth monitoring.
A major concern for the use of polarised
light over land surfaces is the capability to
discriminate between polarisation generated in
the atmosphere and that generated by the
surface. So, for a good interpretation of the
satellite polarisation measurements, a data
base of polarised reflectances for several
ecosystems and different atmospheric
conditions is needed. To achieve this objective,