ITSC-XII – 27/2-5/3/2002 – Melbourne 1

IASI Level 0 and 1 processing algorithms description

B. TournierD. BlumsteinF. Cayla

ITSC-XII – 27/2-5/3/2002 – Melbourne 2

Fourier transform spectroscopy

Physical measurement is not a spectrum Lots of transformations are necessary to retrieve the

spectrum

Interferogram

raw spectrum computation

calibrations ...Michelson interferometer

dxSxI

2cos1)(21)(

dxexISITF xi

21 )()(:)(

ITSC-XII – 27/2-5/3/2002 – Melbourne 3

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 4

Field of View effects and spectral properties

Basic fact Wave number at an angle from the axis of the instrument Wave number cos 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

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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)

-1

0

1

2

3

4

-4 -3 -2 -1 0 1 2 3 4

645 cm-12000 cm-12700 cm-1

IASI Level 1c Instrument Spectral Response Function

-5,00E-01

0,00E+00

5,00E-01

1,00E+00

1,50E+00

2,00E+00

-2,00E+00 -1,50E+00 -1,00E+00 -5,00E-01 0,00E+00 5,00E-01 1,00E+00 1,50E+00 2,00E+00

ISRF Definition Domain Width in cm-1

ISRF

Sha

pe V

ariat

ion

Level 1b ISRF Level 1c ISRF

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Level 1 processing architecture

AVHRR / IIScoregistration

AVHRR images

IIS imagesradiometriccalibration

Spectradecoding

interferometric axis estimation

radiometric post calibration

AVHRRradiancesanalysis

spectral functions

interpolationResampling

Apodisation

IASI scientif ic telemetry

Spectra 1A

Spectra 1Bapodisation function

spectra l calibration

function

4 x 30 spectra / line (8 sec.)

1 (Y,Z)/ line

30 images/ line

sounding productsannotations

Spectra 1C(4 x 30 / line)

IIS images (64x64 pix.) coded spectra

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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