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

1. Ralph Snel, SSAG Calibration Subgroup and SQWG SADDU meeting June 16/17, 2008 Just how good is a L1 spectrum?An overview of SCIAMACHY calibration quality

2. Overview Introduction Overview of calibration status and quality of calibrated spectra Instrument Calibration Status Level 2 feedback Level 1 corrections applied Points of attention for calibration improvement Implementation of improvements Things to be aware of when using L1 data SQWG, SSAG Calibration Subgroup, and feedback to these groups

3. 1. Introduction Calibration equation: Si = I(λi) * T(λi) * Qi + SSi + DSi Si : recorded signal at pixel i I(λi) : incident radiation as function of wavelength T(λi) : optical transmission of instrument as function of wavelength Qi : detector quantum efficiency SSi : stray light signal at detector pixel i (depending on all signals in the channel) DSi : dark signal of detector pixel i

4. 2. Overview of calibration status and quality of calibrated spectra Instrument Calibration Status Level 2 feedback received Level 1 corrections applied

5. Instrument Calibration Status Absolute radiometric accuracy (polarised): Goal: 3-4 % Achieved: 3% (ch1), 4% (ch2), 3% (ch3), 2% (ch4), 6% (ch5), 4% (ch6) Degradation over time: 250 nm: 13 %/year, decelerating 300 nm: 9 %/year, constant 350 nm: 7 %/year, constant 400 nm: 4 %/year, accelerating 500 nm: 1 %/year, accelerating 700 nm: 0.5 %/year, accelerating 900 nm: 0.3 %/year, constant

6. Level 2 feedback received AAI: UV degradation, scan-angle dependent degradation BrO: Fit residual issue CH4: Pixel degradation CO: Pixel degradation, ice layer NO2: Slant column offset removed SO2: Spectral stray light, fit residuals O3: Spectral stray light, UV degradation Limb: Spatial stray light CO2: Odd-even fit residuals

7. Level 1 corrections applied Memory effect (channel 1-5): up to ~100 BU, not perfect Non-linearity (channel 6-8): up to ~200 BU, not perfect PMD cross-talk: residuals <0.6% Analog Offset: several thousand BU, residuals ~2-10 BU, good Orbital dark variation: 20 BU/s in channel 8, residuals ~10 BU/s Spectral stray light: few % depending on wavelength, residuals <1%, better for channels 1-2 Integration time correction (IR channels): ~2 ms, good Dead and Bad pixel flagging (IR channels): bad Polarisation correction: up to several tens %, quality under investigation Radiometric correction: quality few % Degradation correction: up to several tens %, quality scan-angle dependent, up to ~10%

8. 3. Points of attention for calibration improvement Radiometric calibration Degradation correction Polarisation calibration Dark signal correction Daily/geolocated dark signal variation channel 8 Bad and dead pixel treatment Solar reference spectra (ASM, ESM diffusers) Scan angle dependent effects Spectral stray light correction Spatial stray light correction

9. 4. Implementation of improvements Quality Working Group: operational processor SciaL1C tool: m-factors Prototype L0-1 processor at DLR-IMF NADC tools (SRON): patched L1b data, special correction tools Sciamachy Detector Monitoring Facility SDMF (SRON): in-house at SRON, development platform for new corrections, interfaces with NADC tools

10. 5. Things to be aware of when using L1 data (1) Calibration errors can be: time independent time dependent season dependent wavelength dependent polarisation dependent scan angle dependent scene dependent Instrument degradation: UV: mirror and diffuser contamination IR: ice layer IR: pixel degradation Channel overlaps: dichroic mirror shift Channel 2: detector material degradation (~350 nm)

11. 5. Things to be aware of when using L1 data (2) The calibration is not perfect! What worked before may not work in the future Some errors are introduced through the calibration (spectral features) Some errors may cancel or decrease when using: ESM diffuser or ASM diffuser Slit illumination differences may introduce features (up to ~10%) There are known polarisation equation errors in limb Errors may propagate very differently for different L2 products

12. 6. SQWG, SSAG Calibration Subgroup, and feedback to these groups (1) SCIAMACHY Quality Working Group SQWG: All parties are involved (IUP-UB, DLR-IMF, SRON, BIRA, KNMI) Responsibility is to the agencies (ESA, DLR, NIVR) Emphasis on operational processor and products SSAG Calibration Subgroup: All parties are involved Responsibility is to the SCIAMACHY Science Advisory Group Not limited to operational products More “anarchistic” approach Results may be implemented in SQWG context Large overlap with SQWG people and activities

13. 6. SQWG, SSAG Calibration Subgroup, and feedback to these groups (2) Feedback as L2 developer to SQWG and Cal Subgroup: Informal, personal contacts, etc Through SSAG to Calibration Subgroup To SQWG using Change Request procedure (contact: Heinrich Bovensmann) We appreciate any feedback!