AIRS In-flight Spectral Calibration Steve Gaiser 1 Steve Gaiser, - - PowerPoint PPT Presentation

airs in flight spectral calibration steve gaiser
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AIRS In-flight Spectral Calibration Steve Gaiser 1 Steve Gaiser, - - PowerPoint PPT Presentation

AIRS In-flight Spectral Calibration Steve Gaiser 1 Steve Gaiser, AIRS in-orbit spectral calibration AIRS Science Team Meeting, May 2-3, 2000, Solvang, California page of 6 Introduction Spectral calibration is important Its a basic


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AIRS Science Team Meeting, May 2-3, 2000, Solvang, California

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Steve Gaiser, AIRS in-orbit spectral calibration page

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AIRS In-flight Spectral Calibration Steve Gaiser

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AIRS Science Team Meeting, May 2-3, 2000, Solvang, California

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Steve Gaiser, AIRS in-orbit spectral calibration page

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Introduction

  • Spectral calibration is important
  • It’s a basic need of any spectrometer.
  • It is used explicitly by the AIRS forward models.
  • 1% Dn

Dn centroid errors (a 1 mm focal plane position error) can cause radiometric errors of up to 0.4K.

  • Approach summary
  • Simulated radiances were created at multiple frequency sets (each

corresponding to a different shift of the focal plane), oversampling the AIRS detectors’ spacing.

  • Observed radiances are averaged, and narrow spectral bands (called

“features”) are correlated against the different simulated radiance

  • sets. A best fit shift is determined for each feature.
  • Individual feature shifts are combined to determine the best fit focal

plane position.

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AIRS Science Team Meeting, May 2-3, 2000, Solvang, California

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Steve Gaiser, AIRS in-orbit spectral calibration page

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Results Summary

  • Used Fishbein simulation of
  • Dec. 14, 2000 (240 granules)
  • No failures
  • Mean = -0.22 microns
  • Stddev = 0.25 microns
  • Requirements satisfied
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SLIDE 4

AIRS Science Team Meeting, May 2-3, 2000, Solvang, California

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Steve Gaiser, AIRS in-orbit spectral calibration page

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TGRS Paper Cross-references

  • Strow et al.
  • Frequency errors are channel dependent.
  • The nominal AIRS spectrometer model is within 0.0005 * Dn

Dn of the true frequencies for all channels.

  • Uncertainties in the array positions are the biggest source of error in

the AIRS spectrometer model.

  • Uncertainty in the SRF shapes (including fringes and other effects)

introduces an error in reference radiance spectra equivalent to a centroid error of less than 0.0002 * Dn.

  • McMillin et al.
  • Spectral stability for periods of the order a month are required for

tuning.

  • Fishbein et al.
  • Simulation based in part on NCEP forecasts for that day. (ref dropped;
  • ops!)
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AIRS Science Team Meeting, May 2-3, 2000, Solvang, California

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Steve Gaiser, AIRS in-orbit spectral calibration page

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Periodicity Analysis

  • Individual features show

power at orbital, semi-orbital, and quarter-orbital periods.

  • Semi-orbital variations

usually dominate.

  • 0.30 micron peak-to-peak

variation is best fit to dominant period for this day

Periodicity of one day’s shifts

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SLIDE 6

AIRS Science Team Meeting, May 2-3, 2000, Solvang, California

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Steve Gaiser, AIRS in-orbit spectral calibration page

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Conclusions

  • Based on simulation results, the current algorithm satisfies

requirements.

  • Additional improvements are possible if [when] real data

prove more difficult:

  • Include multiple climatologies (reference spectra for multiple

atmospheric profiles).

  • Weigh features according to the number of footprints averaged