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Ground-based FTIR Update. CAVIAR Meeting NPL, 29 th September 2010 Tom Gardiner, Marc Coleman. Summary. Aged mirror reflectivity measurements ‘Smoothed’ calibration function CAVIAR blackbody – uncertainty assessment and paper preparation. Aged mirror reflectivity.

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ground based ftir update

Ground-based FTIR Update

CAVIAR Meeting

NPL, 29th September 2010

Tom Gardiner, Marc Coleman

summary
Summary
  • Aged mirror reflectivity measurements
  • ‘Smoothed’ calibration function
  • CAVIAR blackbody – uncertainty assessment and paper preparation
aged mirror reflectivity
Aged mirror reflectivity
  • Absolute reflectivity measurements of gold mirror used in solar tracker during Jungfrau campaign now underway.
  • Measurements under ambient conditions just completed.
  • Measurements under different humidity conditions underway.
updated calibration function
Updated calibration function
  • Latest mirror reflectivity measurements have confirmed transmission properties for both field campaigns and related calibration exercises.
  • Extrapolation outside of reflectivity measurement region has been revisited, and a (hopefully) improved estimate derived.
  • Updated calibrations functions now available that include these revisions, and integrate solar limb darkening effects.
issues with raw function
Issues with raw function
  • Calibration function should be a smoothly varying function across the the spectral range.
  • Raw function contains lots of high resolution information relating to laboratory water vapour and instrumental noise. This high frequency noise will feed into field results analysis.
  • Particularly the case in the interesting high frequency region
  • Raw function also not suited to re-sampling at different resolutions.
  • Smooth underlying function is visually obvious, but need an analytical method to extract it.
smoothing fitting procedure
Smoothing / fitting procedure
  • Filter raw spectrum using repeated application of a sharp gradient exclusion filter.
  • This removes the effects of the water lines and some high frequency noise.
  • Fit a high order polynominal to the remaining points.
  • This gives an analytical expression for the shape of the calibration curve, that can be applied to any frequency scale within the frequency limits of the original data.
  • Should be particularly useful when ratioing two calibration curves to determine scaling functions for different gains / apertures.
caviar blackbody
CAVIAR Blackbody
  • Now have a write up of the blackbody uncertainties.
  • Covers the various sources of uncertainty including :
    • Nextel coating reflectance
    • Cavity emissivity
    • Ambient reflection
    • Uniformity
    • Themometry
    • Cavity thermal gradient
    • Thermal gradient across coating
    • Formal traceability to the SI through AMBER facility
  • This will form the basis for calibration paper.
international intercomparison
International Intercomparison
  • CAVIAR blackbody used in first part of CEOS comparison of IR brightness temperature measurements in support of satellite validation : Laboratory and Ocean surface temperature comparison of radiation thermometers.
  • Took part in two stages
    • Stage 1 took place at NPL and involved laboratory measurements of participants’ blackbodies calibrated using the NPL reference transfer radiometer (AMBER), while participants’ radiometers were calibrated using the CAVIAR blackbody.
    • Stage 2 took place at Rosenstiel School of Marine and Atmospheric Science (RSMAS) and involved laboratory measurements of participants’ blackbodies calibrated using the NIST Thermal-Infrared Transfer radiometer (TXR), while participants’ radiometers were calibrated using the RSMAS and NIST water bath blackbodies.