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Page 1
Study of Sensor Inter-calibrationUsing CLARREO
Jack Xiong, Jim Butler, and Steve Platnick NASA/GSFC, Greenbelt, MD 20771
with contributions fromMODIS Characterization Support Team (MCST), NASA/GSFC
CLARREO Workshop, 21-23 October 2008, Washington, DC
Outline
• Introduction– Applications of CLARREO– Requirements for CLARREO Observations
• Inter-calibration Approaches Using CLARREO– Lunar observations
• VIS/NIR/SWIR only
– Simultaneous nadir observations (SNO)– Ground observations (Dome C)
• Inter-comparison of Terra and Aqua MODIS calibration
• Sensitivity Study (future work)– RSR sensitivity study (using Schiamachy with LaRC)– Spectral and spatial sensitivity studies of Earth view
targets (Hyperion and AVIRIS)
• SummaryPage 2
Page 3
Introduction
• Applications of CLARREO
– Benchmark Observations
• Accurate, stable, SI traceable, spectrally resolved
– Inter-calibration for Other Sensors
• Consistent data records for long-term climate change
• Requirements for CLARREO Observations
– General Requirements
• Spectral; spatial; temporal; orbital
– Special Requirements
• Maneuver (pointing) capability
• Lunar observations
Inter-calibration Approaches Using CLARREO
• Lunar observations
– MODIS (T/A), SeaWiFS, Hyperion, VIRS/TRMM
• Simultaneous nadir observations (SNO)
– MODIS (T/A), AVHRR, AIRS, MISR, ASTR, Landsat, GLI,
VIRS/TRMM
• Ground observations
– Dome Concordia, Antarctica
– MODIS (T/A), AVHRR, AIRS, MISR, ASTR
– Inter-calibration of Terra and Aqua MODIS
Page 4
Collaboration with USGS (Moon), NOAA, JPL, JAXA, USGC (SNO/Dome C)
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Inter-calibration Using Lunar Observations
MODIS Lunar Observations:
via spacecraft maneuvers
at fixed phase angle
reference to a lunar model (USGS)
using integrated irradiance
/
/Terrr MODIS Modle
Aqua MODIS Modle
I IR
I I
Images form Aqua MODIS (band 1) Lunar Observations (Oct 04 – Jun 05)
Xiong and Sun (GRSL in press)
Advantages vs disadvantages:
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Inter-calibration Using SNO
Terra MODIS Aqua MODIS
AVHRR
SNOSNO
For RSB
1 2/ / /Terrr MODIS Ref-sensor t Aqua MODIS Ref-sensor tR r r r r
1 2Terrr MODIS Ref-sensor t Aqua MODIS Ref-sensor tT T T T T For TEB
Advantages vs disadvantages:
Page 7
Aqua MODIS SD Degradation (0.41 to 0.94m)
Any Impact on Calibration?
Inter-calibration Using a Ground Target
• Why using a ground target
– Validate on-board calibration; complement other cal/val approaches
– Monitor calibration long-term stability
– Support sensor inter-calibration
• Requirements for a ground “calibration” target
– Spectral and spatial uniformity and radiometric stability (minimum environmental impact)
– Site accessibility and data availability
– Ground measurements of radiometric traceability
Examples of Using Dome C for Inter-calibration
• Site Description
• Data Selection
• Methodology– Thermal emissive: reference to Automated Weather Station
(AWS) measurements
– Solar reflective: BRDF model based on ground measurements over Antarctic snow
• Results from MODIS– Recent presentation at SPIE Europe Remote Sensing (Xiong
et al. 2008)
• Future WorkPage 8
Page 9
Site Description
Dome ConcordiaAntarctica
• Located on Antarctic Plateau (75.1 S, 123.4 E)– One of the most homogeneous land surfaces
on earth in terms of surface temperature and emissivity.
• Uniformity over spatial scales typical of the ground footprint of satellite sensors
– High altitude (~3200 m) & minimal slope– Low snow accumulation rate– Extremely dry, cold & rarefied atmosphere
• Low fractional cloud coverage• Low atmospheric aerosol and water vapor
content
• Permanently manned Research Station now operational– AWS data available since 1995
• 10-minute averages of meteorological parameters (T, RH, WS, WD, P)
– Daily radiosonde measurements
• Frequent satellite overpass
CEOS Endorsed SiteNASA/NOAA/ESA Effort
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Data Selection
•MODIS Collection 5 Level 1B data
•Multiple MODIS observations each day (~8) at different angles of incidence.
•Only near-nadir overpasses used (nadir track within +/- 50 km of Dome C). One granule every 2-3 days.
•20x20 pixel average centered on Dome C
•No cloud screening applied for TEB. All granules used.
•Uniformity screening applied for RSB to eliminate any granules showing greater than 2% non-uniformity in reflectance over the 20x20 pixel area
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Methodology and Approach (TEB)
AWS surface temperature measurements are used as a proxy to track any trends in the relative bias between MODIS Terra & Aqua.
TMODIS = BTMODIS – TAWS
Relative Bias = TTerra – TAqua
Relative Bias calculated for each MODIS band and only for days with measurements from both Terra & Aqua.
Applications to other sensors: relative spectral response (RSR), spatial resolution (ground footprint)
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Methodology and Approach (RSB)
A BRDF model developed by Warren et al. (JGR 1998) based on near-surface reflectance measurements over the Antarctic snow
R (θ,ψ,φ) = c1 + c2cos(π- φ)+ c3cos[2(π- φ)]
c1, c2 and c3 are functions of cos(θ) and cos(ψ)
c1 = a0 + a1[1 - cos(ψ)], c2 = a2[1 - cos(ψ)], c3 = a3[1 - cos(ψ)]
ai = b0i + b1i cos(θ) + b2i cos2(θ) (i = 0, 1, and 2)
where θ is the incident solar zenith angle, ψ is the viewing zenith angle, and φ is the relative azimuth angle.
A ratio of the observed reflectance factor r to modeled reflectance factor R is calculated by Δr = r / R
Spectral BRDF of Antarctic snow from Hudson, Warren et al (JGR 2006)
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Sensor (11 and 12 m) and AWS Observations
Terra: Black diamonds; Aqua: Blue squares
Good correlation between sensor and AWS observations(focusing long-term behavior, not individual observations)
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Long-term draft (<10mK) for bands 31 and 32; high quality on-board TEB calibration
Excellent calibration consistency (11m: 0.025±2.984K; 12m: 0.013±3.010K)
No obvious temperature dependent bias (<20mK)
Relative Bias = TTerra – TAqua (time)
TMODIS = BTMODIS – TAWS
Terra: Black diamonds; Aqua: Blue squares
11m 12m
Relative Bias = TTerra – TAqua (temperature)
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-4
-3
-2
-1
0
1
2
3
4
180 190 200 210 220 230 240 250 260
MODIS B31 BT (K)
B
T (
MO
DIS
- A
IRS
)
Aqua MODIS 'Test' Collection at Dome C
One orbit – June 20, 2006(near nadir footprints)
Dome C data (near nadir)
190K – 330K
Inter-comparison of Aqua MODIS and AIRS at 11 m
(using Dome C observations)
Old version
New version
Page 16
Sensor (0.65 and 0.86 m) Observations
strong correlation between sensor observations and solar zenith angle
Page 17
Sensor (0.65 and 0.86 m) Observations versus Modeled Values
Averaged fitting residual: 1.3 – 1.9%
Model parameters derived using sensor first-year observations
Sensitivity Study (future work)
• RSR Sensitivity Study
– Extend from our previous study reported at
April/May CLARREO workshop (e.g., preferred inter-
calibration scene types vs. spectral band)
– Work with LaRC using Schiamachy data
• Spectral and Spatial Sensitivity Studies
– Hyperion observations, Dome C and other targets
• 0.4 to 2.5m, 30m IFOV (@705km altitude)
– AVIRIS observations
• 0.4 to 2.5m, 1 mrad IFOV
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Summary
• Sensor Inter-calibration Using CLARREO– Improve current sensor inter-calibration approaches with
highly accurate, stable, and spectrally resolved observations
– Resolve calibration differences or establish calibration consistency among sensors with on-orbit SI traceable measurements
• Ground Target Characterization Using CLARREO – Extend consistent data records, using observations from
previous, current, and future missions/sensors, for studies of long-term climate changes
– Dome C site can be used to track sensor long-term stability and calibration consistency among sensors (challenges in VIS/NIR/SWIW)
• Other Approaches– Lunar observations; SNO
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