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Mesures physiques et signatures en télédétection

Dr Fred Prata
CSIRO, Division of Atmospheric Research, PMB 1 Mordialloc, Vic 3195, Australia.
Land surface temperature (LST) is an important quantity in the characterisation of
the bilateral energy exchanges between the surface and atmosphere and the surface and
sub-soil. Its measurement has proved to be extremely difficult because of the large hetero
geneity of natural terrain and because of the practical difficulties associated with making
a temperature measurement of the active surface.
Techniques for determining land surface temperatures from satellite-borne infrared
instruments will be discussed. These methods must take into account both the perturbing
effect of the atmosphere and surface emissivity effects. In particular, the angular effects
that arise because of viewing geometry are described and simple formulas are proposed
to account for these. The atmospheric opacity as well as being a strong function of
the water vapour amount also varies with viewing angle. Likewise, it is known from field
measurements and theoretical models that the emissivity of surfaces varies with view angle.
A third view angle effect arises through the geometrical structure of the vegetation on
the surface. These effects can be minimised by taking proper account of the atmospheric
behaviour, through the use of a radiative transfer model, by modelling the emissivity
dependence of the surface and by including corrections related to the geometrical structure
of the canopy. Some examples of angular effects are discussed here and illustrations of
these effects are provided from analyses of data from the Along Track Scanning Radiometer
and the airborne TIMS instrument. More research is required to fully understand the
effects of surface emissivity with viewing angle.
KEY WORDS: Land surface temperatures; Angular effects.
A satellite- or aircraft-borne infrared sensor viewing the earth’s surface senses radiation from
the earth and atmosphere along the line of sight. Ignoring scattering effects in the atmosphere
and assuming that there are no clouds within the field-of-view, the monochromatic radiation
at wavelength A received at the radiometer is,
R(\,0) = T.(\ } O)I,(\,0) + I a (\,6), (1)
where, I, is the emission from the surface, I a is emission from the atmosphere, t, is the total
atmospheric transmittance and 6 is the zenith view angle. The surface emission is related to
the radiative surface temperature, T, and emissivity, e, by,
J,(A,0) = e,(A, 6)B\[T,) + [1 - e.(A,0)]^, (2)
where B X [T] is the Planck function and F\ J. is the downward hemispheric monochromatic flux
from the sky. We have assumed that the surface is isotropic, in thermodynamic equilibrium
with the atmosphere, and have used Kirchoff’s law (conservation of energy) to express the
directional reflectivity of the surface in terms of the (directional) emissivity. While in practice
these conditions are never met, because the emissivity is high (reflectivity low) and the flux of