an airborne version of POLDER has been built 
and several field experiments already 
performed (Deuzé et al., 1993, Bréon and 
Deschamps, 1993; Goloub et al., 1994). The 
airborne POLDER also acquired 
measurements during the HAPEX-Sahel 
experiment which was an international 
program focused on the soil-plant-atmosphere 
energy, water and carbon balances in the West 
African Sahel (Goutorbe et al., 1993; Prince et 
al., 1993). 
We herein present results of bidirectional 
polarised reflectance measurements acquired 
simultaneously with a field instrument, and 
from the airborne POLDER. Two surface 
reflectance models are proposed that 
reproduce accurately the measurements. We 
discuss our results in the context of remote 
sensing using polarised light. 
2- MEASUREMENTS AND PROCESSING 
2.a- Surface measurements 
An instrument, called REFPOL, has been 
developed in the "Laboratoire d'Optique 
Atmosphérique", Lille, France, for measuring 
the BRDF (Bidirectional Reflectance 
Distribution Function) and the BPDF 
(Bidirectional Polarisation Distribution 
Function) of surfaces in the field. It is 
equipped with 4 optical barrels, each of them 
carrying a different spectral filter. During the 
HAPEX-Sahel campaign, the 4 channels were 
centred at 450, 650, 850 and 1650 nm with 
passbands of 100 nm for the middle infrared 
band and 40 nm for the others. Polarizers 
rotate in front of the 4 detectors (3 photodiodes 
for the visible bands and one germanium for 
the near infrared band) and allow three 
successive measurements, with the polarizer 
turned by steps of 45°. 
The radiometer was mounted on a step-by- 
step electric engine which allows an 
inclination up to 75° on each side of nadir 
viewing. An inclinometer provided 
automatically the zenith orientation of the 
radiometer and this information was stored 
with the measurements. The total angular 
sampling time from -75° to +75° was of 120s. 
The apparatus (engine and radiometer) was 
mounted at the extremity of a 1.5m length 
boom, at the top of a seven meters high mast. 
The field of view (FOV) was approximately 16° 
(total) which yields a footprint of 2m diameter 
at nadir, so the boom allowed minimal 
contamination by the supporting mechanism. 
By rotating from +75° to -75°, the radiometer 
allowed to sample the angular signature of the 
radiance in a vertical plane defined by its 
azimuth angle relative to the sun. The angular 
signature of the reflectance is derived from 
measurements acquired with a varying surface 
cover as the radiometer rotates. Thus, a 
uniform surface is necessary to derive 
meaningful signatures. 
The digital counts are converted to radiance 
using calibration coefficients. The radiances 
are then normalised to the top of the 
atmosphere (TOA) solar irradiance. From the 
three reflectance measurements, the polarised 
reflectance is derived from: 
P p = ^[(Pi _ P2) 2 + (P2 ~ P3) 2 ] 
2.b- Airborne measurements 
The POLDER instrument optical design is 
based on the concept of a CCD matrix, a 
rotating filter wheel that caries spectral filters 
and polarizers, and a wide field of view lens 
(Deschamps et al., 1994). On the filter wheel, 9 
slots allow successive spectral measurements 
with various polarisation directions. The table 
below indicates the spectral bands that were 
selected for this campaign. 
Cent. Wavl. (nm) 
450 
570 
670 
865 
910 
Band Width (nm) 
20 
10 
10 
10 
20 
Polarisation 
Yes 
No 
No 
Yes 
No 
The bidimensional CCD matrix yield a 
multidirectional view of the area in one 
instantaneous shot. The POLDER field of view 
is 43° along track and 51° crosstrack. The field 
of view on the ground is, therefore, about 
twice the instrument altitude, or 8.2x10.9 km 2 . 
The CCD matrix is composed of 384x288 
pixels, which yields a spatial resolution of 
about 30 meters, and an angular resolution of 
approximately 0.1°. 
Three measurements are performed for the 
polarised channels with the polarizers turned 
by steps of 60°. From those three 
measurements, one deduce the polarised 
reflectance, the total reflectance, and the 
polarisation direction. 
3- SURFACE POLARISED REFLECTANCE 
MODEL 
The polarised reflectance of vegetation has 
been shown to be generated by specular 
scattering on the leaf surfaces. Following 
Rondeaux and Herman, if the leaf inclination 
distribution is isotropic, the polarised 
reflectance pp can be simply written as: