2nd ESA EarthCARE Validation Workshop
25-28 May 2021 (online)

Geostationary Earth Radiation Budget(GERB)
Lessons learned

Jacqueline E Russell (GERB project scientist)
Imperial College London
1

Talk Outline• GERB measurements and onboard calibration• Onboard calibration, some strengths and some limitations• Unexpected signals and instrument / satellite effects• The power and difficulties of inter-instrument & in-orbit
comparisons and checks
2

GERB observations: outside view

3

GERB observations on the inside
Integrating sphere totrack SW

282 scan linesQuartz filter
turns TOTAL(0.3-100+µm)
into SW(0.3-4µm)

EarthView

Blackbody & deep space enable two point calibration
to determine gain for LW scenes
Multiplied by a ground calibration factor ‘B’ (ratio ofSW to LW responses) to determine SW gain
Solar integrating sphere onboard to enable SW gain
to be tracked

Telescope

Scan Mirror

Front EndElectronics

Internal
black-bodyused to gives
Scale andFold mirror
and detector remove offsets
Measurements ofspace obtained every
scan to give zero4GERB observations, from the inside outInternal black body is used to remove offsets andcalibrate the counts to SW and TOTAL filteredradiances
LW is obtained by subtraction, accounting for theeffect of the quartz filter

SW.

Total

Total-1/TSW

LW
CALIBRATIONPARAMETERS:Internal black body output,
Quartz filter transmission
Instrument spectral response
SW/TOTAL gain ratio (B)TOTAL (0.3 to beyond 100µm)
SW (0.3 to ~4µm) with the addition of quartz filter
Longwave is obtained by subtraction
Radiances are unfiltered (corrected for instrument spectralresponse) using spectral information from the SEVIRI imager
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GERB observation characteristics•

256 individually calibrated pixels•

Same telescope both TOTAL and SW by swapping in a quartz filter•

Offsets removed every 0.3s, Gain updated every 6 minutes•

Solar diffuser sphere to track ratio of response to SW vs LW sources•

5 mirrors (2 sides of scan mirror)•

Wide field of view•

Broad spectral range•

256 pixesl

4.0 to 100µm (100 to 2500cm-1)

TOT:0.32 µm - 100.0+ µm
SW: 0.32 µm - 4.0 µm
LW: 4.0 µm - 100.0+ µmAbsolute Accuracy: 1.0 %Signal/Noise:
1250 (SW) 400 (LW)
Dynamic Range:
0-380 (SW) 0-90 (LW) W m-2 sr-1
SPATIAL SAMPLING 44.6 ´ 39.3 km (NS ´ EW)TEMPORAL SAMPLING 15 minute SW and LW fluxes
CYCLE TIME Full Earth disc, both channels in 5 minutes 6

GERB onboard calibration
LW•

Black bodies provide very good tracking of response to longwave
scenes•Similar spectral distribution to longwave Earth scenes meanchanges/errors in spectral response partly compensated for•

Instrument gain more stable than expected (noise on gain can be
reduced by averaging)

4.0 to 100µm (100 to 2500cm-1)

SW

GERB SW 0.3 to 4µm•

Solar diffuser can track filter changes well & some differences•

Doesn’t give a similar response to the variety of Earth scenes•

Can’t diagnose spectral effects•

Isn’t an absolute source•

Changes with time•

Not sensitive enough to changes at blue/UV end7

Solar Diffuser Sphere: strengths & weaknesses
The integrating sphere observations can:Ø verify the stability of quartz filter transmission
Ø evidence differential changes to the two sides ofthe scan mirror
CAN NOTØ Provide absolute in-orbit calibration
Ø diagnose spectrally varying changes to theshortwave responseØ Track changes that occur equally to both sides ofthe scan mirror
Considerable degradation has been observed in thesphere’s reflectance at some wavelengths, making lessand less similar to Earth scenes and further
compromising its sensitivity to shorter wavelengths

Diode A 0.3-0.5µm
Relative change in diodes monitoring
sphere over 2005-2009. Periods ofboth diffuse (reflected from sphere)
and direct (to monitor dioderesponse) illumination shown
Diode output and sphere reflectanceis changing most significantly atshortest wavelengths

Differential aging of the two sides ofthe scan on the first GERB 2003-2015
mirror diagnosed by the CALMON due
to exposure in the park position.

T (CALMON ) ={[V (CALlight) - V (ibb)]- [V (CALdark ) - V (ibb)]}{[V (CALlight) - V (ibb)]- [V (CALdark ) - V (ibb)]}'
sw

sw

sw

sw

tot

tot

tot

tot

8

Expect: Unexpected effects & Stray signals
Some early surprises in early
results that required changes tooperation and process:geolocation & straylight

Geolocation errors May & June 2003

9

Continue to expect: Unexpected effects & change
Some effects that only occur or
become apparent over time, that
require calibration updates
and reprocessing like differential
aging of the mirror, mispositioningof the quartz filter and aging in
the instrument response10Inter-instrument comparisons:Powerful & difficult
Untangling pixel & scene
dependence of differences
requires a lot of matches tountangle differencesCERES-GERB SW filtered radiance comparison

1.04

CERES/GERB radiance ratio

1.02

10.98

0.96

0.94

0.92

0.9AllOcean

Dark Veg

Bright Veg

Dark Desert

Bright Desert

Cloudy11

GERB/CERES comparisons

Spatial matching canintroduce systematiceffects that look likescene dependent
effect, and untangling
processing vs
instrument bias iscomplex

GERB-2/FM3(Ed3a)GERB /CERES reflected solar radiance
Ocean:

0.931+/-0.009=[0.922-0.941]

Dark veg.:

0.951+/-0.006=[0.944-0.957]

Bright veg.: 0.969+/-0.011=[0.958-0.980]
Dark desert:

1.001+/-0.010=[0.991-1.011]

Bright desert:

0.980+/-0.006=[0.974-0.986]

Clouds:0.959+/-0.009=[0.950-0.968]

12

Seeing things change:
GERB/CERES comparisons over time
To diagnose change during
2004 to 2007 GERB 2 / CERES
instrument life using
comparisons, the problems are
similar but even more involved

Use processingclassification ofscene toanalyze

13Quantify the change:
GERB / CERES comparisons over time

GOME spectral darkening

Difference between instrument
and the rate of change in thedifference varies with spectral
distribution of energy in thescene
14Vicarious comparison can lead to traceable updates
GERB 1 & 2 lunar
observations
Applying full calibration
knowledge would alter LW <0.3%, but would give a 5% offset
in SW
NPL re-checked SW cal source
absolute level

Peak LW and TOTAL radiances agree to with 0.5 and 0.2% respectively
Disk integrated LW and TOTAL radiances within, 1.8 and 1.6% respectively or towithin 1% if disk edge defined by threshold on SW radianceSsimilarly day/night consistency
checks between narrowbandbroad band and GERB observed
thermal radiances led to an error
in the quartz filter calibration
parameter normalisation being
discovered

Peak SW agrees to within 1.3% and disk integrated SW to within 0.3%
15

Summary• Instruments and satellites don’t behave as expected•

Flexibility and revisiting processing may be necessary• Inter-instrument in-orbit comparisons•

Extremely useful although not definitive, differences in processing and footprint can lead to systematics,
whilst temporal and angular matches are more likely to be noise• Tracking SW aging is not straightforward•

Expect change and allow for adaptations in processing to deal with these• Ground to orbit calibration transfer is difficult (particularly in the SW)•But in orbit tests for consistency can be sufficient to indicate that ground calibration analysis needs to berevisited• Vicarious targets and qualities checks are important throughout the mission life•

Useful to evaluating processing and providing inter instrument comparison for ideal cases, identifying
systematic changes and gross offsets
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Lessons• There will be things you don’t expect•

Communication with instrument team likely to be needed to understand and solve some issues•

Expect to need to update processing to cope and revisit your plans• Some errors and effects only get highlighted by use or time•

Maintain communication with instrument team and enable baseline values to be revisited• There will be changes to the instrument over time you have to deal with to maintaindata quality•

Expect to have to keep checking, and update your processing and revisit your plans• Some artefacts you never expects can occur years into a mission•Continuous quality monitoring is important and updates to processing and reprocessing will likely beneeded17

Lessons
THOUGHOUT THE MISSION LIFE (and probably after)
Maintain communication with instrument Engineers
Retain ability to adapt processing
Expect to have to update and reprocess data

Also think about how you communicate issues anduncertainties to users best (and then tell me)

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