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Mesures physiques et signatures en télédétection

Thanks to its optical design, the airborne POLDER instrument allows a multidirectional measurement of a
surface target reflectance. During the “La Crau 91” experiment, the radiometer has been flying over a study area
of 10x10 km composed of various cultivated fields. The measurements have been projected on the surface on a
regular grid of resolution 100 m. For each grid point, up to 50 directional measurements provide a good
description of the surface directional signature. These signatures are discussed for a few selected targets
representative of the area. The measurements show typical features of the reflectance angular signature,
including the “hot spot” and specular reflection.
A simple directional model is then applied to the measurements. It provides, for each of the 10 4 grid points, a
normalized reflectance corrected for the angular effects, together with two parameters which gives a quantitative
assessment of the directional signature. The paper discusses these parameters and the ability of the model to
reproduce the measurements. To our knowledge, it is the first time that reflectance angular signature are
measured at the regional scale, a key step to the operational processing of POLDER measurements at the global
KEY WORDS: Surface reflectance, airborne measurements, directional model, hot spot, POLDER
Optical remote sensing of the surface is a very efficient tool for an assesment of land use, vegetation
coverage, vegetation health and primary productivity. However, a number of difficulties make difficult the
quantitative use of satellite measurements. A major difficulty results from atmospheric effects on the
reflectance, in particular due to aerosol scattering. Another difficulty for the use of spectral reflectances is their
anisotropy: A daily coverage of the Earth from a limited number of instruments, such as the AVHRR series,
requires the measurements to be acquired at various viewing angles, up to about 60°. The reflectance anisotropy
makes impossible the direct comparison of measurements acquired at nadir viewing with those acquired at large
viewing angles. On the other hand, the reflectance angular signature may be a new source of information, in
addition to the spectral signature, for a quantitative monitoring of the surface.
For these reasons, there has been a large interest in the past years on the reflectance directional signature of
natural sufaces, either vegetation or bare soil. Directional effects have been identified on satellite data time
series (Gutman, 1987; Roujean et al„ 1992a). Directional signature measurements have also been acquired in
the field (Kriebel, 1978; Kimes, 1983; Kimes et al„ 1985) and in the laboratory (Coulson, 1968). Similarly,
several models have been designed to reproduce and predict the observed reflectance signatures (Otterman and
Weiss, 1984; Pinty et al., 1989; Verstraete et al., 1990; Roujean et al., 1993). The POLDER instrument, either
airborne or spacebome, (Deschamps et al., 1994) has been designed for the measurement of directional effects
on the reflectance. Its concept permits to measure the reflectance of a single surface target from several
directions as the instrument goes along its path. POLDER will be launched in 1996 on the Japanese ADEOS
platform. An airborne version of the instrument has been flying for several years (Br6on and Deschamps, 1993;
Deuzdetal., 1993).
During the “La Crau 90” campaign (Deuz6 et al. 1993), a first attempt to derive directional signatures from
airborne POLDER data was made. Meaningful directional signatures were derived from the measurements for a
number of surface vegetation covers that were typical of the area. However, due to a lack of geometric accuracy
in the measurements, only surface targets that were homogeneous on a rather large scale (more than a hundred
meters) could be studied in terms of directional signatures. Moreover, even the selected targets showed some
unexpected noise. These setbacks were corrected for the following campaign that took place in 1991. This
paper presents the La Crau 91 POLDER campaign and an analysis of the measurements in terms of surface
directional signature. Thanks to the data improved quality, the analysis can be performed on a large number of
surface grid points, which is a large improvement to the limited set of target analyzed previously, and a key step
to the global scale that will be studied with spacebome POLDER.