Just how good is a L1 spectrum?Just how good is a L1 spectrum?An overview of SCIAMACHY calibration qualityAn overview of SCIAMACHY calibration quality

Ralph Snel, SSAG Calibration Subgroup and SQWGRalph Snel, SSAG Calibration Subgroup and SQWGSADDU meeting June 16/17, 2008SADDU meeting June 16/17, 2008

OverviewOverview

• IntroductionIntroduction• Overview of calibration status and quality of calibrated spectraOverview of calibration status and quality of calibrated spectra

• Instrument Calibration StatusInstrument Calibration Status• Level 2 feedbackLevel 2 feedback• Level 1 corrections appliedLevel 1 corrections applied

• Points of attention for calibration improvementPoints of attention for calibration improvement• Implementation of improvementsImplementation of improvements• Things to be aware of when using L1 dataThings to be aware of when using L1 data• SQWG, SSAG Calibration Subgroup, and feedback to these groupsSQWG, SSAG Calibration Subgroup, and feedback to these groups

1. Introduction1. Introduction

Calibration equation:Calibration equation:

SSii = I( = I(λλii) * T(λ) * T(λii) * Q) * Q

i i + SS+ SSi i + DS+ DS

ii

SSii : recorded signal at pixel i: recorded signal at pixel i

I(I(λλii) ) : incident radiation as function of wavelength: incident radiation as function of wavelength

T(λT(λii) ) : optical transmission of instrument as function of wavelength: optical transmission of instrument as function of wavelength

QQi i : detector quantum efficiency: detector quantum efficiency

SSSSi i : stray light signal at detector pixel i (depending on all signals : stray light signal at detector pixel i (depending on all signals in the channel)in the channel)

DSDSii : dark signal of detector pixel i: dark signal of detector pixel i

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2. Overview of calibration status and quality of 2. Overview of calibration status and quality of calibrated spectracalibrated spectra• Instrument Calibration StatusInstrument Calibration Status• Level 2 feedback receivedLevel 2 feedback received• Level 1 corrections appliedLevel 1 corrections applied

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Instrument Calibration StatusInstrument Calibration Status

Absolute radiometric accuracy (polarised): Absolute radiometric accuracy (polarised): • Goal: 3-4 %Goal: 3-4 %• Achieved: 3% (ch1), 4% (ch2), 3% (ch3), 2% (ch4), 6% (ch5), 4% Achieved: 3% (ch1), 4% (ch2), 3% (ch3), 2% (ch4), 6% (ch5), 4%

(ch6)(ch6)• Degradation over time:Degradation over time:

• 250 nm: 13 %/year, decelerating 250 nm: 13 %/year, decelerating • 300 nm: 9 %/year, constant300 nm: 9 %/year, constant• 350 nm: 7 %/year, constant350 nm: 7 %/year, constant• 400 nm: 4 %/year, accelerating400 nm: 4 %/year, accelerating• 500 nm: 1 %/year, accelerating500 nm: 1 %/year, accelerating• 700 nm: 0.5 %/year, accelerating700 nm: 0.5 %/year, accelerating• 900 nm: 0.3 %/year, constant900 nm: 0.3 %/year, constant

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Level 2 feedback receivedLevel 2 feedback received

• AAI:AAI: UV degradation, scan-angle dependent degradationUV degradation, scan-angle dependent degradation• BrO:BrO: Fit residual issueFit residual issue• CH4:CH4: Pixel degradationPixel degradation• CO:CO: Pixel degradation, ice layerPixel degradation, ice layer• NO2:NO2: Slant column offset removedSlant column offset removed• SO2:SO2: Spectral stray light, fit residualsSpectral stray light, fit residuals• O3:O3: Spectral stray light, UV degradationSpectral stray light, UV degradation• Limb:Limb: Spatial stray lightSpatial stray light• CO2:CO2: Odd-even fit residualsOdd-even fit residuals

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Level 1 corrections appliedLevel 1 corrections applied

• Memory effect (channel 1-5): up to ~100 BU, not perfectMemory effect (channel 1-5): up to ~100 BU, not perfect• Non-linearity (channel 6-8): up to ~200 BU, not perfectNon-linearity (channel 6-8): up to ~200 BU, not perfect• PMD cross-talk: residuals <0.6%PMD cross-talk: residuals <0.6%• Analog Offset: several thousand BU, residuals ~2-10 BU, goodAnalog Offset: several thousand BU, residuals ~2-10 BU, good• Orbital dark variation: 20 BU/s in channel 8, residuals ~10 BU/sOrbital dark variation: 20 BU/s in channel 8, residuals ~10 BU/s• Spectral stray light: few % depending on wavelength, residuals Spectral stray light: few % depending on wavelength, residuals

<1%, better for channels 1-2<1%, better for channels 1-2• Integration time correction (IR channels): ~2 ms, goodIntegration time correction (IR channels): ~2 ms, good• Dead and Bad pixel flagging (IR channels): badDead and Bad pixel flagging (IR channels): bad• Polarisation correction: up to several tens %, quality under Polarisation correction: up to several tens %, quality under

investigationinvestigation• Radiometric correction: quality few %Radiometric correction: quality few %• Degradation correction: up to several tens %, quality scan-angle Degradation correction: up to several tens %, quality scan-angle

dependent, up to ~10%dependent, up to ~10%

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3. Points of attention for calibration improvement3. Points of attention for calibration improvement

• Radiometric calibrationRadiometric calibration• Degradation correctionDegradation correction• Polarisation calibrationPolarisation calibration• Dark signal correctionDark signal correction• Daily/geolocated dark signal variation channel 8Daily/geolocated dark signal variation channel 8• Bad and dead pixel treatmentBad and dead pixel treatment• Solar reference spectra (ASM, ESM diffusers)Solar reference spectra (ASM, ESM diffusers)• Scan angle dependent effectsScan angle dependent effects• Spectral stray light correctionSpectral stray light correction• Spatial stray light correctionSpatial stray light correction

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4. Implementation of improvements4. Implementation of improvements

• Quality Working Group: operational processorQuality Working Group: operational processor• SciaL1C tool: m-factorsSciaL1C tool: m-factors• Prototype L0-1 processor at DLR-IMFPrototype L0-1 processor at DLR-IMF• NADC tools (SRON): patched L1b data, special correction toolsNADC tools (SRON): patched L1b data, special correction tools• Sciamachy Detector Monitoring Facility SDMF (SRON): in-house at Sciamachy Detector Monitoring Facility SDMF (SRON): in-house at

SRON, development platform for new corrections, interfaces with SRON, development platform for new corrections, interfaces with NADC toolsNADC tools

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5. Things to be aware of when using L1 data (1)5. Things to be aware of when using L1 data (1)

Calibration errors can be:Calibration errors can be:• time independenttime independent• time dependenttime dependent• season dependentseason dependent• wavelength dependentwavelength dependent• polarisation dependentpolarisation dependent• scan angle dependentscan angle dependent• scene dependentscene dependent

Instrument degradation:Instrument degradation:• UV: mirror and diffuser contaminationUV: mirror and diffuser contamination• IR: ice layerIR: ice layer• IR: pixel degradationIR: pixel degradation• Channel overlaps: dichroic mirror shiftChannel overlaps: dichroic mirror shift• Channel 2: detector material degradation (~350 nm)Channel 2: detector material degradation (~350 nm)

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5. Things to be aware of when using L1 data (2)5. Things to be aware of when using L1 data (2)

• The calibration is not perfect!The calibration is not perfect!• What worked before may not work in the futureWhat worked before may not work in the future• Some errors are introduced through the calibration (spectral Some errors are introduced through the calibration (spectral

features)features)• Some errors may cancel or decrease when using:Some errors may cancel or decrease when using:

• ESM diffuser orESM diffuser or• ASM diffuserASM diffuser

• Slit illumination differences may introduce features (up to ~10%)Slit illumination differences may introduce features (up to ~10%)• There are known polarisation equation errors in limbThere are known polarisation equation errors in limb• Errors may propagate very differently for different L2 productsErrors may propagate very differently for different L2 products

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6. SQWG, SSAG Calibration Subgroup, and 6. SQWG, SSAG Calibration Subgroup, and feedback to these groups (1)feedback to these groups (1)

SCIAMACHY Quality Working Group SQWG:SCIAMACHY Quality Working Group SQWG:• All parties are involved (IUP-UB, DLR-IMF, SRON, BIRA, KNMI)All parties are involved (IUP-UB, DLR-IMF, SRON, BIRA, KNMI)• Responsibility is to the agencies (ESA, DLR, NIVR)Responsibility is to the agencies (ESA, DLR, NIVR)• Emphasis on Emphasis on operationaloperational processor and products processor and products

SSAG Calibration Subgroup:SSAG Calibration Subgroup:• All parties are involvedAll parties are involved• Responsibility is to the SCIAMACHY Science Advisory GroupResponsibility is to the SCIAMACHY Science Advisory Group• Not limited to operational productsNot limited to operational products• More “anarchistic” approachMore “anarchistic” approach• Results may be implemented in SQWG contextResults may be implemented in SQWG context• Large overlap with SQWG people and activitiesLarge overlap with SQWG people and activities

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6. SQWG, SSAG Calibration Subgroup, and 6. SQWG, SSAG Calibration Subgroup, and feedback to these groups (2)feedback to these groups (2)

Feedback as L2 developer to SQWG and Cal Subgroup:Feedback as L2 developer to SQWG and Cal Subgroup:• Informal, personal contacts, etcInformal, personal contacts, etc• Through SSAG to Calibration SubgroupThrough SSAG to Calibration Subgroup• To SQWG using Change Request procedure (contact: Heinrich To SQWG using Change Request procedure (contact: Heinrich

Bovensmann)Bovensmann)

We appreciate any feedback!We appreciate any feedback!