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Mesures physiques et signatures en télédétection

excess of 200 of that for 5S. The comparison of the output from the 5S pseudo-code with that
produced using the 5S code indicate that the operational efficiency of the pseudo-code is
achieved without an appreciable loss in the accuracy of the result.
The observed differences between the predictions of the 5S model and those of the 5S pseudo-code
are associated with the separation of the computation of gaseous transmittance across the band
from the pre-computation phase for atmospheric parameters. In effect, a systematic variation
in gaseous transmittance across a band can produce a ramp effect, or spectral weighting, similar
to a spectra] response function. However, a spectral weighting related to absorption variation
across the band cannot be incorporated into the pre-computation phase because the levels of
absorption are related to the atmospheric path length which is itself a function of the illumination
and viewing geometry. Therefore, discrepanies between the model predictions increase as the
levels of spectral variation in the gaseous transmittance increase across a spectral band. This
is most pronounced in channel lb, then V2; channel VI and V3 have uniform levels of gaseous
transmittance across the spectral bands. This explains the variation in the percentage differences
indicated on table ( 1 ).
The proposed operational code could be applied to data from any satellite sensor if pre-computed
parameters are produced for the particular spectral band. Pre-computation of parameter sets
has already been done for SPOT HRV-2 bands 1, 2 & 3 and Landsat-5 TM bands 1 to 5 and
7 (for the same atmospheric and aerosol models as those used for ATSR-2), with gaseous
absorbing parameters at 5nm increments. The 5S code has to be modified to output these
pre-computation phase parameters.
Finally, an important feature of the atmospheric modelling provided by 5S or the 5S pseudo-code
which relates to the overall atmospheric correction procedure is that by using standard
atmospheric profiles and aerosol models, the variability in atmospheric optical properties in all
four spectral bands can be related solely to changes in the aerosol optical depth at the reference
wavelength x A (550run). Therefore within the overall atmospheric correction procedure, the
success will depend on the ability of the procedure to solve for this single parameter.
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