gomos level 1b processor improvements · 2018-05-15 · atmospheric chemistry validation of envisat...
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![Page 1: GOMOS level 1b processor improvements · 2018-05-15 · Atmospheric Chemistry Validation of ENVISAT – ESRIN - 3-7 May 2004 Page 9 Level 1b processor improvements Background correction](https://reader034.vdocument.in/reader034/viewer/2022042100/5e7c5b9efab3aa0cba6d0bc0/html5/thumbnails/1.jpg)
Page 1Atmospheric Chemistry Validation of ENVISAT – ESRIN - 3-7 May 2004
GOMOS level 1b processorimprovements
Gilbert BarrotACRI-ST
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Page 2Atmospheric Chemistry Validation of ENVISAT – ESRIN - 3-7 May 2004
GOMOS measurement principle
Occultation of stars
30-40 per orbit150.000 per year
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GOMOS data processing
GOMOS measurements(star+limb)
from high to low altitudes Atmospheric transmissionsError bars
Photometer signalGeolocation, Datation
Line densities of speciesLocal densities of species
Error bars, HRTP
Level 2
Level 1b
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GOMOS level 1b tasks
• Computes the atmospheric transmission andassociated error bars at all tangent pointaltitudes
• Provide the geolocalisation and thedatation of the observations
• Clean the GOMOS measurements fromenvironmental and instrumental perturbations
need the data and an orbital model/data
need calibration tables and algorithm parameters
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Page 5Atmospheric Chemistry Validation of ENVISAT – ESRIN - 3-7 May 2004
Instrumental perturbations
Offset
Electronic gainNon-linearity
Dark charge
RTS
Wavelength assignment
Clean spectrum
Level 0 data
Star 029mv=1.67
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Dark charge + RTS
Clean spectrum
Level 0 data
Star 106mv=2.65
RTS+noise
Instrumental perturbations
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GOMOS level 1b processing status
Current version is defined by level 1b DPM 6.1
Products format defined by IODD 6.0
Prototype GOPR 6.0aIPF processor 5.00 (prev. July)
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Level 1b processing improvements
Three sources of improvements
1. Processor improvements (algorithms/parameters)2. Instrument knowledge improvement (calibration)3. New data in the output products
No important modification sinceprevious issue (DPM 5.4 – GOPR 5.4d – IPF 4.x)
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Leve
l1b
pro
cess
or impr
ovem
ents Background correction (parameters only)
exponential interpolation between bandsno background correction in full dark limb
PRNU correction (algorithm+calibration)star spectrum location on CCD defined by LUT
Reference star spectrum validity (algo+parameters)no possible negative values
Atmosphere thickness (parameters only)atmosphere width set to 160 km
Ray tracing (parameters)convergence of ray tracing set to 0.1 µrad
Cosmic rays (algorithm+parameters)thresholds function of gain setting
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Page 10Atmospheric Chemistry Validation of ENVISAT – ESRIN - 3-7 May 2004Inst
rumen
t kn
owledg
e im
prov
emen
ts
Spectrometer B PRNUfor a better flat-field correction
Star spectrum location on the CCD arrayalso for a better flat-field correction
Reflectivity LUT upgradefor a better transmission computation
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New
out
put
data
Illumination conditionssun zenith angles at satellite and tangent pointsun azimuth angle at the tangent pointsun coordinatesillumination condition (PCDillum)classification dark, twilight/straylight, daylightGeometrical information
occultation obliquityLimb signal in the central band
ratio U/C indicates the percentage of limblight mixed to the star signal
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Level 1b processing improvementsImpact on LMA minimisation (level 2) below 50 km
Tmeasured versus Tmodel
Level 1b 5.4Chi-2 > 3
Level 1b 6.0Chi-2 < 2
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Level 1b processing improvements
Impact on Ozonesmall impactbetween 20and 40 km
noisy below20 km becausestill use level 2issue 5.4d forcomparison
O3 local density∆∆∆∆<0.5%
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Level 1b processing improvements
Impact on Ozonebetter catch ofthe O3 secondarypeak at altitude80 to 100 km
O3 secondarypeak
White curve is 6.0
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Impact on O2(SPB1)
Level 1b processing improvements
O2 local densities∆∆∆∆ < 1 % (5.4 vs 6.0)
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Impact on H2Obetter flat-fieldcorrection
Level 1b processing improvements
H2O transmission∆∆∆∆ > 3 % (5.4 vs 6.0)
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Impact on H2O
Level 1b processing improvements
H2O local densities∆∆∆∆ > 15 % (5.4 vs 6.0)
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Still some work to do…
Bright limb missioncleaning the starsignal from limblight Central band signal
Shot-noise error oflimb band ≈≈≈≈ 10% of thestar signal at 30 kmFor Star 3 (mv=-0.05)Shot noise error >100%at 30 km for Star 105(mv=2.64)
Star 3
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Still some work to do…
Bright limb mission Star signal is not properlycorrectly… transmission is unrealistic
Estimated transmissionStar signal after limb correction
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Still some work to do…Reflectivity correction in elevation
How to characterise it ?
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Still some work to do…
Spectrometer B PRNU (H2O species)PRNU characterised a posteriori from transmission spectra ?
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Still some work to do…
SP-FP discrepancydifferences between SP spectral average andFP temporal average in the same spectral band
Correction of the residual RTS signaluse the transmission model as a « reference »in the level 2 processing
LSF larger than target bandwill imply a modification of the band settingre-calibration needed
and for more details… read the GOMOS data disclaimer
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Conclusion
Dark limb cases (algorithms+parameters+calibration) are correctly handled by GOMOS level 1b processing (6.0a)
Bright limb mission must be improved
Level 1b processing and calibration status