Vicarious%20Calibration%20Using%20Earth%20Targets

1
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

2
Vicarious Calibration Using Earth Targets

• Outline of the assignments
• Overview of the technique
• Relation to ASIC3
• Present and planned capabilities
• Impediments to progress
• Recommendations

3
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

4
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

5
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
• Monitoring, archiving, and disseminating the
  results in near real time throughout the sensor's
  mission life.

Weng
6
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

7
Vicarious Calibration Using Earth Targets
Overview

• Calibration
• The extraction of signal from sensors
  measurements
• by means of reference signal
• Vicarious Calibration
• the reference is external to the sensor

Measurement
At-aperture radiance
Artifact
Calibration signal
8
Vicarious Calibration Using Earth Targets
Overview

• Calibration
• The extraction of signal from sensors
  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

9
Vicarious Calibration Using Earth Targets
Overview

• Instrumented Earth Targets
• Lake Tahoe, Qinghai Lake IR imaging instruments

Hook
Zhang et al
10
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
11
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
12
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

13
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

Scripps
CIMSS
14
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)

15
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

16
Vicarious Calibration Using Earth Targets
Overview

• Scene Statistics

Crosby et al 2005
17
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

18
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
• FK VIS measurements at certain angle is
  dominated by molecular scattering, which is
  invariant in time and space
• KH Reflection from sun glint is spectrally
  invariant
• VK
• Spectral difference of aerosol scattering is
  known
• Reflection from cloud top is spectrally invariant

19
Vicarious Calibration Using Earth Targets
Overview

• Scattering and Reflection

20
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)

21
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

22
Vicarious Calibration Using Earth Targets
Overview

• Ice Sheet

Antarctica
Greenland
Channel 1
Channel 2
23
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

24
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

25
Vicarious Calibration Using Earth Targets
Overview

• Desert

From Desert by Christoph Heidelauf
26
Vicarious Calibration Using Earth Targets
Overview

• Desert
• Strong signal
• Variety of signal
• Different desert
• Relative calibration
• Noise (H2O, dust/aerosol, O3)

27
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

28
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

29
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

30
Vicarious Calibration Using Earth Targets
Capabilities

• Product Monitoring Precision

N16 N17 AVHRR during SNO Mean 40 Difference
lt 2
31
Vicarious Calibration Using Earth Targets
Capabilities

• Product Monitoring Precision
• Standard Deviation

Ch. 1 Ch. 2 Ch. 3
1s reflectance () 0.44 1.23 1.00
_ 3s reflectance _ Mean reflectance 3.5 8.7 4.5
32
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

33
Vicarious Calibration Using Earth Targets
Capabilities

• Planned for near future
• Account for water vapor variation

ReflaßtAcos(?tf0)
Heidinger
34
Vicarious Calibration Using Earth Targets
Capabilities

• Planned for near future
• Account for water vapor variation
• Other model of target BRDF

ReflaßtAcos(?tf0)
cos?cos?cos?0 sin?sin?0cos(azm) ?0 ?
cos?cos?0 Stable orbit ? ?0t

• Are ? and ?0 reciprocal?
• Is Refl linear function of cos??
• Account for orbit drift (METEOSAT)?

35
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

36
Vicarious Calibration Using Earth Targets
Impediments
37
Vicarious Calibration Using Earth Targets
Impediments
38
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

39
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
• Sensors 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 sensors dynamic range
• To increase S/N ratio and
• To reduce uncertainty when extrapolated
• Not contaminated (by clouds)
• More for GEO

40
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

41
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

42
Backup
43
Backup
Channel Index Channel Index Pixel Size (m) Pixel Size (m) Central l (nm) Central l (nm) Dl (nm) Dl (nm) Aligned With
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

• Some were modified to be aligned with VIIRS
• Some differences remain

44
Backup
45
Backup
46
Backup
47
Backup