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Title
Mesures physiques et signatures en télédétection

633
850 900
lS data
850 900
for leaf
principal plane,
hese simulation
term of r.m.s.e.
not higher than
l better. Beside,
lendicular plane
any geometric
eflec tances. The
September. The
nodel on ASAS
es according to
aits, for the two
reproduce, with
il effects should
Wav«l*ngth
Figure 5 r.m.s.e of limited sampling inversion
results as a function of wavelength
- Cotton — Bare soil
Figure 6 SPOT reflectances on the 8 vs reflectances on Figure 7 Modeled versus measured reflectances over the
the 7 for the 3 sites sites, x September 7o September 8 6
6. Conclusion
In this study, we used three different data sets to test the possibility of retrieving surface bidirectional reflectance factor
from remotely sensed measurements. The first step was to use these bidirectional reflectance measurements to retrieve
parameters of several bidirectional reflectance models. The inversion procedure, aimed at minimizing the distance
between measured and simulated reflectances, appeared to be very efficient, and generally, there was no problem to make
the simulated reflectances look like the measured reflectances. The retrieval of surface or canopy characteristics, though
mere delicate, seemed also possible.
The input data set provided to the inversion was then limited to a single plane, to test the ability of the model to predict
reflectances in a different direction. This test showed that, if this operation seemed feasible when the sampled plane is the
solar principal plane, the retrieval of realistic parameters seemed difficult or impossible, with measurements off principal
plane. The last part of this study is concerned with the comparison between simulated and measured reflectances, using
two differents instruments, SPOT and ASAS, and appears most promising for this kind of application, especially when
considering inter calibration problems.
Acknowledgments
This work is a contribution to an EOS interdisciplinary Investigation (NAGW-2425). It was supported by the United
States Department of Agriculture, French “Ministère de la Recherche et de l'Espace”, “Progamme National de
Télédétection Spatiale” and “Centre National d'Etudes Spatiales”. We also acknowledge the helpful contributions and
comments of S. Moulin (LERTS-CNES/CNRS, TOULOUSE, FRANCE), K. Thome and D. Gellman (University of
Arizona Optical Sciences Center, TUCSON.AZ. USA).