Vicarious Calibration Using Earth Targets
Xiangqian (Fred) Wu
Sensor Physics Branch
Satellite Meteorology and Climatology Division
Center for Satellite Applications and Research (STAR, formally ORA)
National Environmental Satellite, Data, and Information Service (NESDIS)
National Oceanic and Atmospheric Administration (NOAA)
With help from many colleagues
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Outline of the assignments:
 Overview of the technique
 Relation to ASIC3
 Present and planned capabilities
 Impediments to progress
 Recommendations
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Context
Outline of the assignments:
 Overview of the technique
 Relation to ASIC3
 Necessary conditions for ASIC3
 Common practices
 Present and planned capabilities
 Impediments to progress
 Recommendations
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Context
Some Necessary Conditions for ASIC3:
 Pre-launch
 Sensor be fully characterized
 Post-launch
Sensor performance be
 Continuously monitored
 Independently validated
 Deficiency/Anomaly be identified, resolved, documented, and
feedback for
• re-analysis of historical data
• development of future sensors
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Context
Some common avenues to ASIC3 (Integrated Cal/Val System):
 Verification of internal consistency of onboard calibration
 Cross calibration with reference radiances
 Dedicated sensor for on-orbit reference
 Terrestrial Targets
 Celestial targets: Moon, Star
 Cross calibration with measured radiances




Among any sensors, e.g., POES vs. GOES, operational vs. research
Same S/C (temporal, geometric), e.g., imager vs. sounder
Same series (spectral, spatial), e.g., POES vs. POES
SNO
 Cross calibration with simulated radiances
 NWP and CRTM
Weng
 Monitoring, archiving, and disseminating the results in near real time
throughout the sensor's mission life.
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Outline of the assignments:
 Overview of the technique
 Definition and Scope
 Instrumented and non-instrumented targets
 Four types of stable earth targets
 Relation to ASIC3
 Present and planned capabilities
 Impediments to progress
 Recommendations
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
 Calibration
 The extraction of signal from sensor’s measurements …
 … by means of reference signal
 Vicarious Calibration
 the reference is external to the sensor
At-aperture
radiance
Measurement
Artifact
Calibration
signal
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
 Calibration
 The extraction of signal from sensor’s measurements …
 … by means of reference signal
 Vicarious Calibration
 the reference is external to the sensor
 Scope of the discussion
 Limited to radiometric calibration
 Focused on VISNIR (METEOSAT also used it for IR)
 Stable earth targets
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Instrumented Earth Targets
 Lake Tahoe, Qinghai Lake: IR imaging instruments
Hook
Zhang et al
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Instrumented Earth Targets
 Lake Tahoe, Qinghai Lake: IR imaging instruments
 Railroad Valley (playa), Dunhuang (desert): VISNIR
Zhang et al
Thome
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Instrumented Earth Targets
 Lake Tahoe, Qinghai Lake: IR imaging instruments
 Railroad Valley (playa), Dunhuang (desert): VISNIR
 MOBY: ocean color
NOAA/NESDIS/ORA
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Instrumented Earth Targets




Lake Tahoe, Qinghai Lake: IR imaging instruments
Railroad Valley (playa), Dunhuang (desert): VISNIR
MOBY: ocean color
ARM Sites (SGP CART in particular): IR/MW
sounding instruments
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Instrumented Earth Targets




Lake Tahoe, Qinghai Lake: IR imaging instruments
Railroad Valley (playa), Dunhuang (desert): VISNIR
MOBY: ocean color
ARM Sites (SGP CART in particular): IR/MW
sounding instruments
Scripps
 Cal/Val Campaign with aircraft/ship
CIMSS
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Instrumented Earth Targets




Lake Tahoe, Qinghai Lake: IR imaging instruments
Railroad Valley (playa), Dunhuang (desert): VISNIR
MOBY: ocean color
ARM Sites (SGP CART in particular): IR/MW
sounding instruments
 Cal/Val Campaign with aircraft/ship
Stable Earth Targets




Scene Statistics
Scattering & Reflection
Ice Sheet (Greenland, Antarctica)
Desert (North Africa, Australia)
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Scene Statistics
 Representative works
• Brest and Rossow (1992)
• Tokunu and Itaya (1994)
• Crosby et al (2005)
 Reference
• Selected scenes, global or regional, all or clouds
 Assumptions
• Statistical characteristics of the selected scenes
are invariant in time
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Scene Statistics
SPAC E VIEW IN TEN SITIES
0
50
100
150
VISIBLE C H AN N EL IN TEN SITIES (C OU N TS)
Crosby et al 2005
200
400
450
FIT W ITH AN N U AL AN D SEMI-AN N U AL TER MS
FIT W ITH SOLAR C OR R EC TION
350
EAR TH VIEW IN TEN SITIES
Y(t) = INTENSITY WITH 5 MILLION COUNTS ABOVE
4*10^6
79% OF IN TEN SITIES AR E LESS TH AN 200
2*10^6
6*10^6
PLOT OF INTENSITIES WITH 5 MILLION COUNTS ABOVE
FOR GOES-10
0
FREQUENCY OF OCCURENCE
8*10^6
HISTOGRAM OF VISIBLE CHANNEL VALUES INTENSITIES
GOES-10 FOR JANUARY 3, 2000
2000
2001
2002
2003
TIME (YEAR )
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Scene Statistics
 Easy to implement
• No scene selection such as cloud/clear
 Fundamental flaw in assuming that
certain earth scenes are statistically
invariant in time
• Cannot detect climate change
• Often violated in shorter time scale
 Relative calibration only
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Scattering and Reflection
 Representative works
• Fraser and Kaufman (1986)
• Kaufman and Holben (1993)
• Vermote and Kaufman (1995)
 Reference
• Molecular scattering and reflection from sun glint or cloud
 Assumptions
• F&K: VIS measurements at certain angle is dominated by
molecular scattering, which is invariant in time and space
• K&H: Reflection from sun glint is spectrally invariant
• V&K:
– Spectral difference of aerosol scattering is known
– Reflection from cloud top is spectrally invariant
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
 Scattering and Reflection
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Scattering and Reflection
 Abundant targets
• Potential to have very large sample size
 Absolute calibration (subject to uncertainty)
 Rely on model
• Uncertainty in model input
• Sensitivity of model to input uncertainty
 Molecular scattering signal is weak (~4%)
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Ice Sheet
 Representative works
• Loeb 1997
• Tahnk and Coakley (2001)
 Reference
• Reflection from ice sheet over Greenland and
Antarctica
 Assumptions
• TOA reflectance is a quadratic function of solar
zenith angle
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Ice Sheet
Antarctica
Greenland
Channel 1
Channel 2
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Ice Sheet
 Strong signal (~1/3 of dynamic range)
 Correction involves solar zenith angle
• Advantage in the presence of orbit drift
 Relative calibration
 Difficulty in cloud detection (VIS or IR)
 Targets distribution – a lot of targets for
a short period of time
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Desert
 Representative works
• Staylor (1990)
• Rao and Chen (1996, 1999)
 Reference
• Reflection from selected desert sites
 Assumptions
• Surface characteristics is stable
• TOA reflectance is well understood
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Desert
From “Desert” by
Christoph Heidelauf
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Overview
Desert
 Strong signal
 Variety of signal
• Different desert
 Relative calibration
 Noise (H2O, dust/aerosol, O3)
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Capabilities
Outline of the assignments:
 Overview of the technique
 Relation to ASIC3
 Present and planned capabilities
 Operational calibration at NOAA/NESDIS
 Requirements for climate may differ from those for operations
 Impediments to progress
 Recommendations
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Capabilities
Since 1996
 Monthly update of AVHRR solar bands calibration coefficients
 Second Tuesday every month
 Disseminate the product
 Level 1B data stream
 Direct user notification
 Web (planned improvement)
 All actions archived
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Capabilities
Recent Improvements
Product monitoring
Sinusoidal Function
More checks for non-target pixels (cloud,
precipitation, dust)
Target homogeneity
Precision
 Current 5-10%
 Planned 3-5%
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Capabilities
Product Monitoring & Precision
N16 – N17 AVHRR during SNO: Mean ~40%; Difference < 2%
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Capabilities
Product Monitoring & Precision
 Standard Deviation
Ch. 1 Ch. 2 Ch. 3
1σ reflectance (%)
0.44
1.23
1.00
_ 3σ reflectance _
Mean reflectance
3.5
8.7
4.5
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Capabilities
Recent Improvements





Product monitoring
Sinusoidal Function
More checks for non-target pixels (cloud, precipitation, dust)
Target homogeneity
Precision
 Current 5-10%
 Planned 3-5%
 Accuracy: Uncertain




Earlier (Aircraft via NOAA-9): 37.8% for Channel 1
Lately (MODIS, supported by ATSR and MISR): ~41%
Reconcile the difference
Impact on users
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Capabilities
Planned for near future
 Account for water vapor variation
Refl=α+βt+A*cos(ωt+φ0)
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
Heidinger
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Vicarious Calibration Using Earth Targets
Capabilities
Planned for near future
 Account for water vapor variation
 Other model of target BRDF
Refl=α+βt+A*cos(ωt+φ0)
cosρ=cosθcosθ0 + sinθsinθ0cos(azm)
θ≈0 → cosρ≈cosθ0
Stable orbit → θ0~t
• Are θ and θ0 reciprocal?
• Is Refl linear function of cosρ?
• Account for orbit drift
(METEOSAT)?
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Impediments
Outline of the assignments:




Overview of the technique
Relation to ASIC3
Present and planned capabilities
Impediments to progress
 Target characterization
• Reference value
• Diurnal/Annual variation
• Atmospheric effect (water vapor, aerosol, O3)
 Sensor characterization
 Multiple good targets
 Recommendations
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Impediments
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Impediments
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Impediments
Sensor Characterization
 Spectral Response Function
•
•
•
•
Inadequately specified
Many unknowns
Effort to archive all online …
… and quantify their uncertainty
 Radiometric Calibration
• Pre-launch calibration procedure
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Impediments
Perfect target generates the same signal …
 At different time
 On synoptic (“weather”), seasonal (vegetation), and inter-annual (El Niño) scales
 High altitude (less water vapor and dust variation)
 From different parts
 Sensor’s IFOV, navigation error, cloud detection
 In different spectral band
 Difference/Uncertainty in SRF of sensors
 To different directions (sun/sensor geometry)
 Flat
 Low latitude
 Near the upper limit of sensor’s dynamic range
 To increase S/N ratio and
 To reduce uncertainty when extrapolated
 Not contaminated (by clouds)
 More for GEO
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Recommendations
Outline of the assignments:
 Overview of the technique
 Relation to asic3
 Present and planned capabilities
 Impediments to progress
 Recommendations
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Vicarious Calibration Using Earth Targets
Recommendations
 Continue the effort in vicarious calibration using stable earth targets
 No other way to measure and calibrate the climate in the past
 Collaboration




International (sites, sensors, creativity)
GEO and LEO
Research and operation
Producer and user. Find a good application.
•
•
•
•
NDVI
Aerosol
Cryosphere
Radiation budget
 Learn the lesson
 Channel alignment (GOES-R ABI with VIIRS)
 Better specifications
 Operation overlap
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Backup
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Backup
Channel Index
Pixel Size (m)
Central l (nm)
Dl (nm)
ABI
VIIRS
ABI
VIIRS
ABI
VIIRS
ABI
VIIRS
VIIRS?
1
M3
1000
750
470
488
40
20
No
2
I1
500
375
640
640
100
80
No
3
I2
1000
375
860
865
40
39
Yes
4
M9
2000
750
1380
1378
30
15
Yes
5
I3
1000
375
1610
1610
60
60
Yes
6
M11
2000
750
2260
2250
50
50
Yes
Aligned With
Some were modified to be aligned with
VIIRS
Some differences remain
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Backup
Channel 1 Reflectance of a Corn Field Relative to That of NOAA-14
(due to differences among sensors' spectral response functions)
1.00
N07
N09
N11
0.95
N16
N15
Ratio to A204 (NOAA-14)
N17
0.90
0.85
0.80
0.75
1
2
3
4
5
6
05/21, 06/06, 06/13, 06/19, 06/27, and 07/03 in 2002
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Backup
Channel 2 Reflectance of a Corn Field Relative to That of NOAA-14
(due to differences among sensors' spectral response functions)
1.07
N07
1.06
N09
N11
1.05
N16
N15
N17
Ratio to A204 (NOAA-14)
1.04
1.03
1.02
1.01
1.00
0.99
0.98
0.97
1
2
3
4
5
6
05/21, 06/06, 06/13, 06/19, 06/27, and 07/03 in 2002
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Backup
NDVI of a Corn Field Relative to NOAA-14 Estimate
(due to differences among sensors' spectral response functions)
1.16
N07
1.14
N09
N11
N16
1.12
N15
Ratio to A204 (NOAA-14)
N17
1.10
1.08
1.06
1.04
1.02
1.00
0.98
1
2
3
4
5
6
05/21, 06/06, 06/13, 06/19, 06/27, and 07/03 in 2002
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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Backup
Achieving Satellite Instrument Calibration for Climate Change, May 17, 2006, Lansdowne, VA
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