Radiometric Calibration of the ASTER Thermal Infrared (TIR) Subsystem

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Radiometric Calibration of the ASTER Thermal
Infrared (TIR) Subsystem

• Simon J. Hook
• NASA/JPL

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Outline

• ASTER TIR subsystem.
• On-board calibration (OBC) of TIR subsystem.
• In-flight validation of TIR subsystem.
• Consequence of instrument drift on higher level
  data products.
• Conclusions/Future work

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TIR Subsystem
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TIR Subsystem Characteristics
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TIR Baseline Performance Requirements
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On Board Calibration of TIR

• Short term calibration performed for every strip
  with blackbody set to nominal value.
• Long term calibration performed by periodically
  looking at blackbody a 4 temperatures between 270
  K and 340 K. Temperatures are 270 K, 300 K, 320
  K and 340 K. To date 50 long term calibrations
  have been performed.

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Radiometric Database Status
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C1 of online RCC DB
Band 12
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Calibration error in using online RCC DB
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In-Flight Validation of ASTER Data at the Lake
Tahoe CA/NV Automated Validation Site
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2000-09-20-D
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Measurements

• Offshore
• bulk temperature, skin temperature, air
  temperature, wind speed, wind direction, relative
  humidity, net radiation.
• Onshore
• air temperature, wind speed, wind direction,
  relative humidity, short and longwave radiation
  (up and down), sky imager, aerosols, total column
  water.

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Bulk temperature measurements
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Laboratory Calibration Radiometer

• NIST designed cone in a 44 liter temperature
  controlled bath. Stability at 25 C /- 0.0007 C
  (7008-IR)
• Thermistor standard probe with an accuracy
  specification of 0.0015 C over 0-60 C and
  stability/yr of 0.005 C. (Model 5643-R)
• Readout system with an accuracy of 0.0025 C at
  25 C and resolution of 0.0001 (Chub E4)
• Secondary standard PRT.

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Laboratory Calibration of Radiometer
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Cross Comparison of Radiometers at Miami
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Cross Comparison of Radiometers at Miami
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Means and standard deviations of the estimated
skin SST differences between pairs of radiometers
for the entire cruise period, and for each half
of the cruise
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Data Reduction Methodology For Radiance at
Sensor Validation

• Extract the bulk temperatures.
• Extract the radiometric temperature.
• Correct the radiometric temperature to skin
  kinetic temperature.
• Propagate the skin temperature to the satellite
  using a radiative transfer model and interpolated
  atmospheric profile.
• Convolve the propagated at-sensor radiance to the
  instrument response function to obtain the
  Vicarious Radiance (VR).
• Extract the image radiance derived using the On
  Board calibrator (OBC).
• Compare and contrast the OBC and VR Radiance
  values.

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Sensitivity Analysis for Correction of
Radiometric Temperature to Skin Temperature
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Skin Effect
Water surface
After Minnett et al. 2000
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Variations in the bulk-, skin-, air-temperatures
and wind speed on June 7, 2001 at L. Tahoe
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Consequence of Calibration Drift on Products

• AST08 Surface temperature
• AST05 Surface emissivity

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Summary/Future

• Calibration of the TIR subsystem is changing with
  time.
• Rate of change is currently increasing.
• Radiometric coefficients are not updated with
  sufficient frequency, introducing artifacts into
  data.
• Artifacts exceed calibration requirement.
• Consequence of calibration drift depends on
  product.
• ASTER team currently working on a procedure to
  correct the drift.