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Ground-based FTIR Update

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

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  1. Ground-based FTIR Update CAVIAR Meeting NPL, 29th September 2010 Tom Gardiner, Marc Coleman

  2. Summary • Aged mirror reflectivity measurements • ‘Smoothed’ calibration function • CAVIAR blackbody – uncertainty assessment and paper preparation

  3. 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.

  4. Updated reflectivity results

  5. 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.

  6. Raw Calibration Function (Phase 1 – broadband InSb measurement)

  7. 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.

  8. 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.

  9. Polynominal fit to smoothed calibration curve

  10. Residual between smoothed calibration curve and original function

  11. 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.

  12. CAVIAR Blackbody Uncertainty Table

  13. 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.

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