0- ABSTRACT 
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 
scale. 
KEY WORDS: Surface reflectance, airborne measurements, directional model, hot spot, POLDER 
1- INTRODUCTION 
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.