MSI Level 1 Product validation needs
Rene Preusker and Jürgen Fischer

EarthCARE Status – May 2021
2nd ESA EarthCARE Validation Workshop
25-28 May 2021 (online)
ESA
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2nd ESA EarthCARE Validation Workshop | 25-28/05/2021 | Slide 1

MSI – Optical Bench

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MSI - SNR and NEDTESA UNCLASSIFIED - For Official Use

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MSI – VNS (visible/nearIR/shortwaveIR) Calibration• Two radiance reference points for the VNS:
1) observation of a diffuser that can be rotated into the field of view and2) instrument response with the aperture closed.
• Two pairs of diffusers (QVD) = 4 diffuser = (VNS+SWIR2) x (perDay+perMonth) to monitor
degradation (tbc)
è VNS is calibrated against reflectance standard (not radiance)!
• South Polar Region (@terminator = shadowed earth, Sun in the line of sight)• Dark calibration @ night side of orbit. (validity to be tested during commissioning!)

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2nd ESA EarthCARE Validation Workshop | 25-28/05/2021 | Slide 4

MSI – TIR Calibration (in short)• Warm blackbody +reference body + Cold deep space• once per day (orbit?)• Blackbody has been calibrated pre-flight against an NIST traceable source from NPL.

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What needs to be validated (inflight) in order tomake L1 fit for purpose?
1. absolute radiometric and relative radiometric, linearity
2. straylight (spectral, spatial)
3. spectral position (idealy the rsr)
4. FoV and Orbital dependencies of 1-3.
5. (Not discussed here: Pointing / georeferencing / co-registration)

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Inflight absolute and relative radiometric using invariant
targets

Chander et al 2013
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Inflight absolute and relative radiometric
Actually, there is no clear cut between the validation of a calibration a calibration (vicarious) itself and absolute calibration
monitoring
1.

2.

Invariant Calibration Targets: Precise RTM simulations, ideally accompanied with ground based atmospherecharacterisation. Use of established sites allows inter-satellite comparisons. If done correctly, it is very complex!
Can not be made without parallel precise spectral characterisation•Rayleigh Calibration oceanic sites, offglint geometry, no clouds, only background aerosol. Applicable only forwavelengths/geometry with significant Rayleigh signal : VIS 1.

•DCC (Deep Convective Clouds): nearly perfect solar diffusers (adequate for inter-band: relative radiometriccalibration). Worldwide distributed, mainly +- 10°.

•LWC (liquid water clouds, trade wind zones): good for intermediate intensities, non-linearities vs bright•

Sun Glint Reflectance follows refractive index: Good for inter-band, but MSI is slightly tilted to off-glint:-(•Moon In Orbit manoeuvre to view the moon. ROLO or LIME (ESA) as reference. Only few times a year, only few pixel(moon is small!) . Can be used for straylight too! Good for stability/degradation monitoring!

•SST: relatively stable, monitored by many ground based and satellite missions. Needs good RTM (water vapour)•(Pseudo) Invariant Calibration Sites (PICS): mostly deserts, DOME-C…

SNO (Simultaneous Nadir Observations): Inter-satellite. Geos and HR sensors can be used as transfer (but they may
have only one nadir!). RSR must be as similar as possible (or RTM based transfer). Spatial variability should be low.
Eventually, radiometric consistency (closures) between instruments on one platform (BBR ßàMSI) should be used

• ESA
StatisticalIntercomparisons(look also on higher moments of pdf’s ), Double-Differenceswith anintermediatereference
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2nd ESA EarthCARE Validation
Workshop| 25-28/05/2021| Slide 8

RayleighChander et
al 2013
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Chander et
al 2013
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Straylight using steep gradients (OLCI example)

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Straylight, using steep gradients (OLCI example)

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2nd ESA EarthCARE Validation Workshop | 25-28/05/2021 | Slide 12Straylight, using steep gradients (OLCI example)

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Straylight using MoonESA UNCLASSIFIED - For Official Use

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Straylight using Moon

Color scale is linear,
centered on zero (deepspace value) and limited tostray-light radiance levels
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Diffusor BRDF

Manoeuvres can/shall be used to characterize theBRDF of diffuser too! Diffusor is the source for VNS
calibration. If BRDF is not characterized, annual
modifications will be introduced!

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Solar constant constant?

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12 years

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1 month: 03/2017

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An other month: 05/2017

Summary solar:
Uncertainty is probably in the order of 0.2-1 ‰. Single
events up to 3 ‰.
(Variation could be larger in the blue, and smaller in NIR)ESA UNCLASSIFIED - For Official Use

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Validation of spectral position / rsr
As for radiometric, there is no clear cut between inflight characterisation, monitoring and validation. Precise spectral
characterisation is pre-requisite for vicarious radiometric characterisation.

Standard procedure: Convolution of a high resolution reference spectra withassumed RSR. The reference spectral must belong to strong spectral gradients,
otherwise there is no spectral sensitivity, and variations cannot be
disentangled from radiometric variations! Also, emphasis on different spatialresolution, if different instruments are used!
Reference spectra:•SNO‘s with HR data like FLEX (VIS), Sciamachy, IASI(2) TIR, ......

•Strong spectral atmospheric, surface or solar features (hardly applicable for MSI)ESA UNCLASSIFIED - For Official Use

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FoV and Orbital dependencies

This is the cherry on the cake. Actually everything must be stratified with respect toviewing geometry and orbit! MSI is particularly difficult, since we already know, thatMSI has huge spectral variations over small FoV. Further there may be an
dependence of thermal stray-light, dark current, thermal elastic modifications onorbit (illumination/heating/cooling…)

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MSI – Spectral Response Functions
VIS

SWIR1

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NIR

SWIR2

MSI VNS spectral response functions fora) VIS, b) NIR, c) SWIR1 and d) SWIR2channel. Grey areas indicate responseslower than -0.01, thick white lines
indicate nadir view, thin white lines
indicate nadir view +/- 50 pixel(~approx. +/-25 km) and dashed whitelines indicate viewing angles/acrosstrack pixel that exhibit the minimum
and maximum of central wavelength
within -70 pixel (~approx. -35 km; pinkline) and +230 pixel (~approx. 115 km;pink line) around nadir.
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Lessons learned from previous missions
1. Effort and budget should be allocated to instrument pre-launch characterization and knowledgetransfer. Some instrument features observed during operations are extremely difficult to be characterized
and to be disentangled from each-other in-flight!! House-keeping data can support instrument
characterisation (we will never use 99.5 % of the HK data, but we don’t want to miss any!)
2. “… the calibration is a never-ending process and a dedicated effort should be allocatedthroughout the full mission lifetime (and beyond), this entails the need for regular
reprocessing campaigns for the continuous improvements of the level 1 dataset“
3. Multi-sensor radiometric inter-comparison over Pseudo-Invariant Calibration Sites (PICS) is an invaluable
source, however, the uncertainty of the methods requires meticulous work to incorporate, BRDF,atmospheric, spatial averaging and spectral effects. (RadCalNet, spectral HR sensors to establish transfer
functions, community agreed RTM, protocols, …) It is always a cooperative approach: (CEOS, GSICS)
4. Orbital maneuverer provide view on moon (ROLO, LIME) for stability monitoring and diffusor BRDF
investigations
5. “… the calibration is a never-ending process and a dedicated effort should be allocatedthroughout the full mission lifetime (and beyond), …
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