625 
One model is designed for bare soils and the 
other for vegetation canopies. They both 
assume that the polarised reflectance is 
generated by single specular reflection over 
isotropically distributed facets (bare soil) or 
leaves (vegetation). The two physically based 
models need only one variable parameter; the 
refractive index of the reflecting medium. 
We found a very good agreement between 
the model predictions and the surface 
measurements, collected in the field over 
various surface coverages and solar angles. In 
particular the observations confirm that, for 
similar geometry, the polarised reflectance 
generated by the bare soil is much larger than 
that generated by the vegetation. The largest 
polarised reflectances observed over bare soils 
are on the order of 0.12, whereas they never 
exceed 0.03 over the vegetation. The model 
accurately reproduce the polarised reflectance 
angular signature for zenith angles up to about 
55°. For larger zenith viewing angles, the 
approximations concerning the mutual 
shading of reflecting elements fail, and the 
observations are smaller than the model 
predictions. 
At aircraft level, the polarised radiance 
generated by the surface is ambiguously 
mixed with that resulting from aerosol and 
molecular scattering. Moreover, atmospheric 
scattering reduces the signal from the surface. 
Thus, the measurements are difficult to 
interpret in terms of surface reflectance. 
Besides, since the surface polarised reflectance 
depends mostly on the vegetation cover, our 
results suggest that its information content is 
small. There is a negative correlation between 
the NDVI and the polarised reflectance which 
confirms that bare soils generate more 
polarisation than vegetation. 
On the other hand polarised reflectance may 
be used for aerosol remote sensing. This is 
particularly true since the surface models 
shown in this paper allow one to predict the 
surface polarised reflectance. However, the 
airborne measurements acquired during the 
Hapex-Sahel campaign suggest that desertic 
aerosols yield little polarisation. In fact, as the 
aerosol optical thickness increases, the increase 
in polarised radiance generated by aerosol 
scattering is compensated by the decrease of 
that resulting from other processes (masking 
effect). 
ACKNOWLEDGEMENT 
Both the REFPOL and the POLDER 
instruments have been designed, build, 
serviced and operated by the Laboratoire 
d'Optique Atmosphérique (LOA), Lille, 
France, with a sponsorship from the Centre 
National d'Etudes Spatiales (CNES) and 
Centre National de la Recherche Scientifique 
(CNRS). We acknowledge the work of many 
individuals from this laboratory who made 
this study possible. 
REFERENCES 
Bréon, F.M., D. Tanré, P. Lecomte, and M. 
Herman; 1994: Bare Soils and Vegetation 
Polarized Reflectance: Measurements and 
Models. IEEE Trans. Geosc. Rem. Sens. 
Submitted. 
Bréon, F.M. and P.Y. Deschamps; 1993: Optical 
and Physical parameter retrieval from 
POLDER measurements over the ocean 
using an analytical model. Rem. Sens. 
Environ., 43,193-209. 
Coulson, K.L.; 1966: Effect of reflections 
properties of natural surfaces in aerial 
reconnaissance. Appl. Optics, 5,905. 
Curran, P.J., 1978:,A Photographic Method for 
Recording of Polarized Visible Light for 
Soil Surface Moisture Indications. Rem. 
Sens. Env., 7,305-322. 
Curran, P.J., 1981: The Relationship Between 
Polarized Visible Light and Vegetation 
Amount. Rem. Sens. Environ., 11,87-92. 
Curran, P.J., 1982: Polarized visible light as an 
aid to vegetation classification. Rem. Sens. 
Environ., 12,491-499. 
Deschamps, P.Y., F.M. Bréon, A. Bricaud, J.C. 
Buriez, M. Herman, M. Leroy, A. Podaire, 
G. Sèze; 1994: The POLDER mission: 
Instrument characteristics and scientific 
objectives. IEEE Trans. Geosc. Rem. Sens. 
Accepted 
Deuzé, J.L., F.M. Bréon, J.L. Roujean, P.Y. 
Deschamps, C. Devaux, M. Herman, and A. 
Podaire; 1993: Analysis of the POLDER 
(POLarization and Directionality of Earth's 
Reflectances) Airborne Instrument 
observations over Land Surfaces. Rem. Sens. 
Env., 45,137-154. 
Egan, W.G.; 1968: Aircraft polarimetric and 
photometric observations. Proc. 5th Int. 
Symp. Rem. Sens, of Env., Univ. of Michigan, 
Ann Arbor, pp 169-189. 
Egan, W.G.; 1970: Optical Stokes Parameters 
for Farm Crops Identification. Rem. Sens. 
Env., 1,165-180. 
Fitch, B.W., R.L. Walraven, and D.E. Bradley; 
1984: Polarization of Light reflected from 
grain crops during the heading growth 
stage, Rem. Sens. Env., 15,263-268.