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Mesures physiques et signatures en télédétection

This paper is to evaluate and compare the most used atmospheric correction methods for satellite thermal data
(Split Window methods and LOWTRAN radiative transfer code) and to propose a new method to improve the
atmospheric correction quality for 2 spectral channels' data. The studies are based on the high precision ground
brightness temperature measurements performed with a well-calibrated thermal camera and coincident
radiosounding data. Three satellite data were considered: ERS1-ATSR, NOAA11-AVHRR and LANDSAT5TM.
LANDSAT5TM. The results show that two of the ten tested Split Window methods provide satisfactory correspondence to
ground level measurements. The use of LOWTRAN 7 code has not given expected result with standart
radiosounding data, the difference between the ground measurements and the satellite data derived ground level
temperature remain about 2°C. The proposed new method is based on the LOWTRAN 7 transfer code and uses
the two channels’ measurements as input data. The results shown that the new method can improve significantly
the atmospheric correction precision for 2 channels' thermal measurements. The largest residual error is reduced
from 1.5°C (Split-Window methods) to 0.5°C.
Satellite thermal measurements are very important in applications such as energy balance and crop water stress
studies. These measurements are strongly affected by atmospheric effects, particularly by water vapour
absorption, and a correction is absolutely necessary for deriving surface thermal characteristics. Many
atmospheric correction methods are available (radiative transfer codes, simple formulations, Split Window
methods, etc.) But they are generally developed by theoretical or simulation analysis, and few comparisons and
validations have been achieved with accurate field ground truth measurements. This has complicated the task of
the users for making the suitable choice.
Otherwise, Split Window coefficients, generated by using a large number of atmospheric profiles, can give
satisfactory correction only when atmospheric conditions are not far from the average condition. In the other
cases, some large correction error can exist. However, the difference between the measurements in two spectral
channels used for the Split Window correction can provide some useful information about the real atmospheric
condition, it will be interesting to use this information to improve the atmospheric correction quality.
In this paper, we have used accurate field measurements performed on the "Crau" test site with the following
objectives: (1) test the most used Split-Window methods; (2) evaluate the correction accuracy performed by
Lowtran7 code with special and standard radiosounding data; and (3) try to develop a more accurate correction
The experiment has been conducted in the summer 1992 on the Crau test site . The data of three satellites were
used: ERS1-ATSR, NOAA11-AVHRR and LANDSAT5-TM. We have focused our field work on accurate
brightness temperature measurements with a calibrated thermal camera in the spectral band of the sensors and on
atmospheric radiosoundings at the time of satellite overpass.