Eumetsat GOME-2 Error Assessment Study
Interim Findings
Presentation by B.Kerridge
on behalf of Serco, RAL, IUP & SRON
GSAG, ESRIN, 11/12th April 2002
Structure
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4
Introduction
Approach
Baseline Error Budget
Interim Findings:
(a) Sampling Options for Band 1
(b) Spatial Aliassing
(c) Spectral Resolution & Slit-Function Shape
(d) RTM Assumptions & Earth Curvature
(e) Non-Lambertian Surface BRDF
(f) Cloud Obscuration & Horizontal RI Gradients
(g) Pointing & Geolocation
4 Summary & Further Work
1. Introduction
• GOME-2 Error Assessment Study commissioned by Eumetsat
• Scope:
– Identify through quantitative retrieval simulations factors which will
limit accuracies of trace gas columns and ozone profiles.
– Recommend operational settings and, if necessary, other action to
mitigate these.
• Consortium:
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Serco Europe Ltd (Prime Contractor)
RAL (Technical Co-ordinator, Ozone Profile Analysis)
IUP (Trace Gas Column Analysis, RTM Calculations)
SRON (Assessment of Instrumental Errors)
• Final Presentation:
– 25th June at Eumetsat
2. Approach
• Trace gas column / O3 profile algorithms as used by IUP / RAL
for GOME-1 flight data, with specific modifications for GOME-2.
• For O3 profiles, sensitivity to l-ranges and a priori explored.
• Simulations generally for:
• 12 geo-temporal scenarios with realistic tropospheric and
stratospheric trace gas profiles and solar geometry from orbit
propagator
• 2 surface albedos (0.05, 0.8)
• Variety of view angles (nadir, +/- 29o, +/- 45o (=1920km swath))
• Linear retrieval diagnostics analysed.
• Specified errors for GOME-2 quantified by linear mapping of
their spectral signatures for comparison with:
(a) Estimated Standard Deviations (ESD = sqrt(Sx) )
(b) Baseline Error Budget
3. Baseline Error Budget
Baseline error budgets based on GOME1 experience and using
GOME2 noise model
O3 Profile Errors:
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Radiometry: 2% of sun-normalised radiance
Polarisation correction: SRON prescription for GOME-2
Degradation of scan-mirror reflectance: quantified but neglected
Surface pressure: 10hPa
Temperature profile: error covariance matrix from IASI retrieval
Aerosol: LOWTRAN “high” - “background”
Trace Gas Column Errors:
– Photon noise & read-out noise, with Ring x-section fitted.
– Polarisation correction: error quantified but negligible.
– Largest error source caused by differential structures from diffuser plate
reflectivity, errors in trace gas columns exceeding >50% except for ozone
(see Richter and Wagner, 2001)
• Clouds, spectroscopy and other instrument effects not included.
4. Interim Findings
(a) Sampling Options for Band 1
• Baseline integration times: 12s for Band 1A; 0.1875s for Band
1B
 Single Band 1A pixel: 1920km x 80km
– Non-linear dependence of RT with view angle over +/- 45o
– RT errors at extreme view angles
– Horizontal variability of stratospheric O3 profile
• Flexibility to read out Band 1A at 1.5s (640km x 40km) and coadd
• Impact of increased read-out noise assessed, in frame of 1%
noise floor
• Co-adding 1.5s Band 1A pixels to 12s (640km x 320km) or 24s
(640km x 640km) yields similar esd to single 12s Band 1A pixel.
• Additional read-out noise insignificant also for co-addition of
0.1875s Band 1B pixels
(b) Spatial Aliasing
• Five Landsat ETM+ images (~180km x 180km at 1m resolution).
• Ensemble of >350 spatially-aliassed signatures calculated (via
l-dependent surface albedos) for each of the12 geo-temporal
scenarios.
• Ensemble min, max, mean (bias) and RMS examined.
• GOME-2 IFOV (0.29o ~4km on ground) effectively filters highfrequency structure (ie spatially-aliassed noise).
 Coarser resolution images (eg ATSR-2 1km x 1km) would suffice
for a future comprehensive study, and are calibrated over full
dynamic range.
Trace Gas Columns
• Fitting windows small, so errors insensitive to low-frequency
structure.
• Max errors (0.02% O , 2% NO , 1% BrO, 10% OClO); below the
O3 Profiles
• Bands 1 & 2 handled as separate steps.
• Band 1 sees low-frequency structure only at longest
wavelengths.
• Errors <esd for Band 1 range 265-307nm, but sometimes
exceed esd when Band 1 range extended to 265-314nm (nb
albedo extremes).
 Reduction in Band 1B detector read-out time desirable.
• Errors arise from non-linear RT dependence on surface albedo
plus scene inhomogeneity (ie not eliminated for negligible readout time).
• Only small interval (~100 of 1024 detector pixels) of Band 2
used, and 2nd order polynomial fitted to log(sun-normalised
radiance).
 Band 2 retrieval insensitive to low-frequency structure from
(c) Spectral Resolution & Slit-Function Shape
O3 Profiles
Slit-function FWHM:
• Trade-off between increasing esd and decreasing
undersampling error as Band 2 FWHM is increased
(redistributing but conserving photons).
• On this criterion, increase from 2-3px (0.24-0.36nm) would be
beneficial, however knowledge of shape would then be more
critical.
Slit-function Shape:
• Gaussian shape assumed in Band 2 FM for joint retrieval of:
– Slit FWHM and wavelength registration from direct-sun
spectra
• Analysis (OG) for EQM indicates non-Gaussian and asymmetric
shape
• Shape for defocused case broader and more wavelength
dependent, although closer to Gaussian, than for focused case.
• Measurements of direct-sun and sun-normalised spectra
synthesised for “true” shapes (focused, defocused, Gaussian)
using high-res solar spectrum, adding Ring to backscattered
spectrum before convolution.
• Gaussian used in retrieval scheme FM.
• Two simulations w.r.t. undersampling:
- l-shift (direct-sun - backscattered) retrieved (as GOME-1)
- l-shift not retrieved but mapped, after interpolation & regridding.
• Errors on retrieved O3 small when “true” shape really is
Gaussian (ie FHWM of Gaussian recovered by retrieval), but
• Errors larger for defocus (wider) case than for focus case
– because, although smaller in amplitude, the spectral signature due
to erroneous shape is more correlated with structure of O3 Huggins
Band.
• O3 errors due to incorrect shape can exceed 100% in
troposphere.
• Sign, magnitude and height-dependence of this error are similar
to bias found for GOME-1 (RAL data) from comparison with
>2,000 sondes.
• Characterisation of shape with onboard line lamp will be limited
by:
– Absence of suitable lamp lines between 306 and 333nm
– Discrete sampling of narrow lines by broad detector pixels
 Characterisation of Band 2 l-dependent slit-function shape at
Trace Gas Columns
• High res FTS absorption x-sections used.
• Simulation of FWHM increase due to defocusing from 2px to
5px (0.24-0.6nm Band 2 & 0.5-1.25nm Band 3).
 Esds for O3 (<1%) and BrO (<60%) increased by only factor
~1.1.
• NO2 esd (<20% at 2px) increased by factor ~1.24 at 3px.
• Undersampling errors small for O3 (<0.5%) and NO2 (<2%), but
substantial for BrO (<100%).
• Assessment of slit opening (increased photon flux):
– Error improvement by 30 percent by doubling slit width
– Desirable to reduce integration time <0.1875s, hence ground pixel
size, to avoid possible saturation
 Slit opening improves SNR and avoids undersampling problems
(d) RTM Assumptions & Earth Curvature
• Assumption: fully-spherical RTM too computationally expensive
for operational processing.
• Calculations by CDI RTM in pseudo-spherical and fully-spherical
modes differenced and linearly-mapped.
O3 Profiles
• CDI (with GOME FM x-sections) also used to calculate K’s.
• Pseudo-spherical approximation (as implemented in CDI) can
cause substantial errors at all altitudes (ie in ss- as well as msdomain).
• Largest errors at largest SZAs, as expected.
• Errors small for nadir-view, and much larger at +/-45o than at +/29o
 Correction scheme required for outer pixels of 1920km swath
Trace Gas Columns
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Errors on O3, NO2 and BrO slant-columns negligible (<1%), provided
solar geometry for ground rather than TOA is used in plan-parallel
atmosphere
NOTE: RTM errors will end up as AMF error rather than slant column
errors
 Does not require hardware changes, improvements possible by using
most appropriate RTM assumptions compromising processing speed
and accuracy
(e) Non-Lambertian Surface BRDF
• Spectra calculated by CDI in fully-spherical mode using angulardependent surface BRDFs and their Lambertian equivalents for:
– dark land, bright land, ocean & snow (April 55oN) & sunglint (5oS)
O3 Profiles
• Quasi non-linear simulation: gross deviations in BRDF
accommodated through surface albedo retrieval, as per GOME1 scheme.
• O3 errors from Lambertian assumption <5%, except >30% for
sunglint.
• Sunglint occurs in eastward views, peaking near 960km swath
edge.
• It will affect a substantial fraction of data in tropics south of
equator. (Peak intensity and affected area depend on surface
Trace Gas Columns
• Difference in slant column fits from BRDF and from semihemispherically integrated albedo provides error estimate
• Errors on O3, NO2 and BrO slant columns for most surface types
<1%
• Errors due to sun-glint can be up to 3% for all trace gases
• NOTE: This is again an AMF error rather than slant column
error
 Small error, improvement possible by retrieving Lambertian equivalent
albedo directly from the spectra
(f) Cloud Obscuration & Horizontal RI Gradients
• Issue: extent to which cloud obscuration or horizontal gradients
in refractive index would be more serious for 1920km than
960km swath.
Cloud:
• ATSR-2 forward view (55o) ~ GOME-2 extreme (45o) for 1920km
• ATSR-2 statistics analysed for GOME-1 80x40km px for one
year
 Forward/nadir differences not substantial, even for occurrence of
totally cloud-free scenes (12% vs 14%).
Horizontal Gradient in RI:
• LOS path-lengths calculated for non-refracting and refracting
(w/wo 0.14K/km gradient 0-40km height) atmosphere with raytracing model
(g) Pointing & Geolocation
• Errors as built taken from EPS Geolocation & Co-registration
Budget
– Nadir: 1.6km along-track, 1.2km across-track
– 1920km swath edge: 3.1km along-track, 3.6km across-track
• Direct impact on viewing geometry quantified
Trace Gas Columns
• Errors <1% and generally ~0.5%
O3 Profiles
• Across-track errors generally <2% (even at 1920km swath edge)
• Along-track errors <<1%
 Direct impact of pointing errors at these levels is negligible cf
others.
5. Summary and Further Work
1. Band 1 Sampling:
– Integrate for 1.5s (1A) / 0.1875s (1B) and co-add; retain 1A/1B
boundary.
2. Spatial Aliassing:
– Errors on trace gas columns not significant
– Errors on O3 profiles ~ esds for Band 1 limit of 307nm, > esds for
314nm.
 Reduction in read-out time desirable.
3. Pointing Errors: negligible direct impact
4. Spectral Resolution & Slit-Function Shape:
– FWHM increase from 2-3px would reduce sensitivity to
undersampling, but would increase sensitivity to errors in
knowledge of shape.
– Accurate knowledge of shape in Band 2 <350nm vital for O3
profiles.
 Pre-flight measurements required, since onboard line-lamp not
adequate.
5. Swath-width:
– Cloud obscuration & horizontal RI gradients not significant factors.
– Errors from pseudo-spherical approximation negligible for trace gas
columns but large for O3 profiles in outer pixels of 1920km swath.
 Correction scheme needed if CPU time too great for fullyspherical RTM.
– Errors from Lambertian BRDF approximation negligible for trace
gas columns and <5% for O3 profiles except for sunglint, where
they are large. Sunglint more pervasive for 1920km swath, but not
decisively.
Further study required to:
(a) Address identified issues in greater depth (eg swath, slit-shape).
(b) Address issues not covered by this study (eg spectroscopy, diffuser
spectral structures, DOAS/AMF assumptions, possible
improvements in hardware for 3rd generation GOME).