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Proceedings of the Symposium on Global and Environmental Monitoring

solar zenith angle of the block of pixels being
Unlike channels 1 and 2 , which receive energy
from the sun reflected from the earth-atmosphere
complex, channels 4 and 5 are sensitive to the
radiation emitted by the earth at long infra-red
wavelengths. The equivalent brightness
temperatures of channels 3, 4 and 5 are
calculated by first applying the calibration
coefficients provided by the AVHRR and then
inverting the Planck function. The intensity of
the response in channel 3 is partly due to
reflectance of solar energy and partly due to the
radiation of the earth in that portion of the
spectrum. The portion due to reflectance is
estimated by using the channel 4 brightness
Because the atmosphere does not scatter and
absorb all wavelengths equally, the radiation
reaching the sensor has a spectrum different from
that which would have been received in the
absence of an atmosphere. The radiance measured
by the orbital instrument is therefore only
partially dependent on the reflectance of the
target (Tanr6 e t al. 1986). As a result of
atmospheric scatter a substantial portion of the
radiation reaching the sensor is contributed by
reflection of the sun's radiation within the
atmosphere. The target is illuminated not only
by direct sunlight, but also by skylight, and a
non-negligible part of the energy reaching the
sensor has been scattered more than once in its
double passage through the atmosphere.
The algorithms used in the pre-processing
software will as far as possible estimate surface
geophysical parameters (bi-directional
reflectance factors of channels 1 and 2 and
brightness temperatures of the surface)
independently of atmospheric properties.
The physics of the radiative transfer through the
atmosphere are well documented, and form the
basis of several models providing atmospheric
correction. However, most such models involve
recursion to account for multiple scattering or
ray- or photon-tracing and are too cumbersome to
incorporate into the operational preprocessing of
AVHRR data, whose main advantage is its high
temporal frequency, and hence the potential
timeliness of the data.
Most of the effects of the atmosphere can
nevertheless be corrected operationally, and to
this aim the LOA, under the Tecnodata contract,
is to optimise its "Simulation of the Satellite
Signal in the Solar Spectrum", or "5S" model
(Tanre et a l . 1986) for incorporation in the pre
processing software.
The model, as its name suggests, was originally
designed to use ground and atmospheric
characteristics to model the signal received at
the satellite, and not to derive ground
characteristics from the satellite signal.
However, it is based on a set of analytic
expressions of the optical properties of the
atmosphere, each of which is reversible, given
the inherent symmetry and reversibility of the
passage of light through any transparent medium.
For the purposes of atmospheric correction of
AVHRR data, the model has therefore been re
organised to derive the reflectance of the
surface from
the signal received at
The original 5S model is described in detail
(with FORTRAN listings of the code) in Tanre e t
al. ( 1 986 ) .
nature of the atmospheric
Water vapour. Channel 2 suffers markedly
from absorption by water vapour. This leads in
particular to considerable sensitivity of the
NOVI to the water vapour content of the
atmosphere. In order to correct for this effect
it is necessary to know or to estimate the water
vapour content at the location at which the
radiometric data are being collected. The
European Centre for Medium Range Weather
Forecasts at Reading have archives (dating from
1/1/1980) of data derived from a model of
atmospheric conditions which provide an estimate
of relative humidity and air temperature at
midday at 7 barometric altitudes for cells in a
2.5°x2.5° grid. The European Community is
covered by about 200 of these cells. At present
it is not known whether the JRC will have access
to these data in real-time for inclusion in the
pre-processing chain, or whether it will be
necessary to assume seasonal and regional mean
conditions in water vapour content.
Aerosols. The software will not attempt to
correct the data for atmospheric aerosols, since
on the scale of Western Europe, aerosol loadings
vary locally and unpredictably with time.
Insufficient data exist on their distribution and
particle size for any kind of routine mapping,
and it is therefore not possible to eliminate
their effect on a continental scale and on an
operational basis using present techniques. This
is to be regretted since for channel 1 and 2 of
the AVHRR the contribution of aerosols to noise
in the signal is often more important than that
of any other atmospheric source.
Surface characteristics. The 5S model used to
derive the algorithm for the atmospheric
correction assumes that the surface is spatially
non-uniform, but that it is a Lambertian
reflector. This is clearly incorrect, but we
lack the necessary knowledge of the bi
directional reflectance of surfaces in Europe at
the AVHRR scale. We hope that in the mid to long
term we will possess this information as one of
the results of a major collaborative effort
between the JRC and the University College
Look-up tables. The Joint Research Centre
requires an algorithm that functions efficiently
and rapidly to remove the major calculable
effects of the atmosphere on channel 1 and
channel 2 data. The need for a rapid algorithm
implies the use of a formula coupled with look-up
tables that are pre-loaded with the results of
any calculation that can be carried out
beforehand. Thus, for example, the software will
set up sun angle and view angle tables which it
can index when the geometry of the orbit is
known .
Auxiliary files are set up
to current estimates of water
for rapid
vapour and
oxygen concentrations,