faces" thematic unit prefigures
the setting-up of a European
cal parameters (CSP). The goal is to pro-
vide varied users with satellite products
that meet their specific needs. With this
aim, POSTEL has achieved thematic and
computing skills: this double competence
allows both to develop operational job-
strings from algorithms specified by
Expertise Centres, and to validate scien-
tifically the products thus generated.
string of the POLDER project which
reproduces the biophysical properties of
continental ecosystems from surface bi-
directional reflectances. The data pro-
cessing architecture of this jobstring was
first reorganised to meet industrial quali-
ty criteria, then the whole system was
made operational before being delivered
to the POLDER Production Centre of the
CNES. Today, the activity consists in
maintaining a computer and algorithmic
follow-up, and in assessing the quality of
biophysical parameters before distribu-
ting them to users. Such actions are per-
formed in co-operation with scientific
laboratories (LSCE, LOA) and compa-
nies (NOVELTIS) partners of the project
that develop its algorithms. Lastly,
MEDIAS-France acts as an interface
with the users and meet their specific
demands.
data are the first components of the cata-
logue of POSTEL products. They include
directional albedos, NDVI (Normalised
Difference Vegetation Index), LAI (Leaf
Area Index), FVC (Fraction of
Vegetation Cover), FAPAR (Fraction of
Absorbed Photosynthetically Active
Radiation), with a 6-km/10-day spatio-
temporal resolution. The Bi-directional
Reflectance Distribution Function
(BRDF) is another product specific to
will find them on-line at the POSTEL
site:
POLDER/produits/
users. For instance, the BRDF database
concerns those involved in measurement
physics (e.g. the CSIRO and the
University of Boston), that develop more
and more effective radiative transfer
models. Albedos, LAI, FVC and FAPAR
are essential to model water, energy and
carbon exchanges at the ground-vegeta-
tion-atmosphere interface. They are
aimed at scientists and at institutes deve-
loping operational methods in hydrology,
meteorology, climatology, carbon cycle,
land use changes, food security and agri-
cultural production.
directions of light and observation. The
main BRDF features are a peak of
reflectance for back-scattering when the
solar and viewing angles coincide, and
a minimum reflectance for forward-
scattering. 22594 BRDFs collected by
POLDER-1 from November 1996 to
June 1997 have been gathered. They pro-
anisotropy of the main continental
biomes and their natural variability. They
are consequently an exceptional product
for many environmental studies, and a
remarkable tool to test the performances
of radiative transfer models.
of zenithal and azimuthal viewing direc-
tions. They are derived from the inver-
sion of a BRDF linear semi-empirical
model on the spectral bi-directional
reflectances measured during the 30 days
of synthesis period. The short-wave
directional albedo represents the wide
band (250nm, 2500nm). It is computed
directional albedos at 443nm,
670nm, 765nm and 865nm. The
NDVI derived from the spectral
directional albedos at 670nm and
865nm is thus corrected from angu-
lar effects. Its temporal evolution is
then linked only to changes in the
state of surface.
while the FVC quantifies the share
of surface covered with vegetation.
These parameters derive from the
inversion of a model of radiative
transfer by neural network. The daily
with a vegetation index computed in the
optimum angular configuration to mi-
nimise ground contribution.
their area, their transitions and their spa-
tial heterogeneity. The temporal evolu-
tion of parameters at a 10-day resolution
shows the natural seasonal variations, the
consequences of extreme climatic events
and the man-induced damage suffered by
ecosystems.