IASI Level 0 and 1 processing
algorithms description
B. Tournier
D. Blumstein
F. Cayla
ITSC-XII – 27/2-5/3/2002 – Melbourne
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Fourier transform spectroscopy
 Physical measurement is not a spectrum
 Lots of transformations are necessary to retrieve the spectrum
 Interferogram

1
I ( x)   S ( )1  cos2x  d
2 
 raw spectrum computation
TF ( I ) :  S ( ) 
1

 I ( x )e
2 ix
dx

 calibrations ...
Michelson interferometer
ITSC-XII – 27/2-5/3/2002 – Melbourne
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Radiometric calibration
 Non linearity correction
 Radiometric calibration done in the complex space using
 cold calibration view SCS
 hot calibration view SBB
 First step done on board
 Post-calibration on ground
(level 1 processing)
Radiometric calibration
ITSC-XII – 27/2-5/3/2002 – Melbourne
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Field of View effects and spectral properties
 Basic fact
 Wave number  at an angle q from the axis of the instrument
 Wave number  cos q on the axis
give the same raw spectrum (first order only)
 Consequences
 angle between pixel and interferometric axis must be taken into account in the
spectral calibration (off axis pixel)
 broadening and deformation of the Instrument Spectral Response Function
(ISRF) due to the extension of the IASI pixel (approx. 0.8 degree wide)
 non uniform distribution of radiance in the field of view of the instrument
must be taken into account at user level
ITSC-XII – 27/2-5/3/2002 – Melbourne
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Instrument spectral response function
 Level 1b ISRF
 is not the same for the 4 IASI pixels
 is dependant of the wave number
 depends on a lot of possibly varying instrument characteristics
 Level 1c ISRF removes this complexity (is uniq and constant)
IASI Level 1c Instrument Spectral Response Function
4
2,00E+00
3
2
645 cm-1
2000 cm-1
2700 cm-1
1
-3
-2
-1
1,00E+00
5,00E-01
0,00E+00
-2,00E+00
0
-4
ISRF Shape Variation
1,50E+00
0
1
2
3
-1,50E+00
-1,00E+00
-5,00E-01
0,00E+00
5,00E-01
1,00E+00
1,50E+00
2,00E+00
4
-5,00E-01
ISRF Definition Domain Width in cm-1
-1
Level 1b ISRF
ITSC-XII – 27/2-5/3/2002 – Melbourne
Level 1c ISRF
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Level 1 processing architecture
IASI scientific telemetry
IIS images (64x64 pix.)
coded spectra
Spectra
decoding
IIS images
radiometric
calibration
30 images
/ line
AVHRR images
4 x 30 spectra / line (8 sec.)
interferometric
axis estimation
1 (Y,Z)
/ line
spectral
functions
interpolation
spectra l
calibration
function
radiometric
post calibration
Spectra 1A
Resampling
Spectra 1B
apodisation function
AVHRR / IIS
coregistration
AVHRR
radiances
analysis
Apodisation
Spectra 1C
(4 x 30 / line)
sounding products
annotations
ITSC-XII – 27/2-5/3/2002 – Melbourne
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Input to level 1 processing
 System parameters are computed off-line by the IASI Technical
Expertise Center (TEC) based in Toulouse
 Parameters used by the level 1 processing are mainly
 Spectral data base (ISRF, apodisation functions, spectral calibration functions)
 Input for radiometric post-calibration (reflectivity of scan mirror, emissivity of
reference black body)
 Spectrum coding/decoding tables
 Geometry information for localisation of IASI products
 Co-registration Imager/Sounder information
 Input for NRT spectral calibration algorithm
 Limits of spectral bands
 NB: Noise and noise covariance matrix are provided for higher level
processing
ITSC-XII – 27/2-5/3/2002 – Melbourne
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