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On-orbit Spectral Calibration of the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS)

On-orbit Spectral Calibration of the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS). David C. Tobin, Henry E. Revercomb, Robert O. Knuteson. USU/SDL CALCON 2003. University of Wisconsin Space Science and Engineering Center. Outline. Introduction to GIFTS

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On-orbit Spectral Calibration of the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS)

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  1. On-orbit Spectral Calibration of the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) David C. Tobin, Henry E. Revercomb, Robert O. Knuteson USU/SDL CALCON 2003 University of Wisconsin Space Science and Engineering Center

  2. Outline • Introduction to GIFTS • Optical design and spectral characteristics • Calibration requirements • Spectral calibration using Earth scene spectra • Example: ground based AERI spectra • Example: aircraft based Scanning-HIS spectra • Summary and future work

  3. Introduction to GIFTS

  4. GIFTS Measurement Concept Combine Advanced Measurement Technologies on a Geosynchronous Satellite to obtain 4-D Observations of the Atmosphere • Horizontal:Large detector arrays give near instantaneous wide 2-D geographical coverage • Vertical: Michelson interferometer (FTS) gives high spectral resolution that yields high vertical resolution • Temporal: Geosynchronous orbit allows high time resolution (i.e., motion observations)

  5. CrIS CrIS GIFTS 685 – 1129 1/cm 1650 - 2250 1/cm GOES Sounder CO CO2 N2O N2O CH4 H2O H2O CO2 O3 CO2 Spectral Coverage

  6. GIFTS Optical Design and Spectral Characteristics

  7. Expanded View of the Electro-optical Design

  8. On-Axis Wavenumber Scale • With constant Dx sampling, the wavenumber scale is determined by the effective laser frequency and the resulting interferogram sampling rate: • Optical Path Difference (OPD) scale: • dx = R/ nlaser X = (N/2) dx • Wavenumber scale: • dn = 1/(2X) = nlaser /(NR) • nscales with nlaser • Spectral calibration primarily deals with knowledge of the effectivenlaser

  9. Off-axis Effects • The beams of light reaching each FPA pixel pass through the interferometer at different angles, f. • With respect to the on-axis beam, the off-axis beams have slightly shorter OPDs: OPD(f) = OPD(0) cos(f). • For the GIFTS geometry, this causes two primary effects: a different (but correct) wavenumber scale for each pixel of the FPA, and small distortions in the Instrument Line Shape (ILS). • Integrating over a single FPA pixel and making small angle approximations, the GIFTS interferograms are represented as: where q is the mean off-axis angle for a given pixel and b is the half-angle subtended by a single pixel (~0.38 mrad).

  10. OPD(f)=OPD(0) cos(f) FTS axis X cos f M1 X X/2 M2 x1 x2 x1 x2 f OPD = x1 - x2 = X cos f OPD = x1 - x2 = X On axis beam Off axis beam after Fourier Transform Spectrometry, James W. Brault

  11. FPA Geometry FPA pixel i,j FPA interferometer telescope q2 = 97.4 mrad b q2/q1 = afocal ratio = 6.855 q(i,j) q1 = 14.2 mrad FPA pixel index j 512km 4km/pixel FPA pixel index i b = single pixel half angle = q2/2/128 = 0.38 mrad q(i,j) = off-axis angle to center of pixel = b[(2i-1)2 + (2j-1)2]1/2 Off-Axis Angle, q [mrad] Not to scale

  12. ILS Variations: very little ILS change over the detector array

  13. Self-apodization correction process • In expression for the measured interferogram, F(x), expand sinc function as a power series of (2pnxbq): • Compute perturbation terms and subtract from measured interferogram. • Similar process currently performed for AERI, HIS, S-HIS, NAST-I.

  14. Interferogram (counts) OPD (cm) GIFTS Off-Axis Interferogram Sampling Samples are triggered with the (on-axis) laser signal, but each IR beam/pixel experiences a different OPD according to its angle through the interferometer. But all sampled points lie on the same continuous interferogram.

  15. Pixel # Pixel # Pixel # Pixel # GIFTS Interferogram Data Cube Simulations interferogram point #1 (ZPD, OPD=0) interferogram point #780 (CO2 resonance) In the spectral domain, produces apparent shifts of spectral features when plotted versus the on-axis wavenumber scale.

  16. Off-axis wavenumber scale normalization Puts spectra for all pixels onto a common wavenumber scale • Start with decimated interferograms (2048 pts LW, 4096 pts SMW) • Zero-pad interferograms by factor of 16. • FFT to get oversampled spectra. • Interpolate to standard wavenumber scale. • Can be performed before or after radiometric calibration.

  17. Calibration Requirements

  18. Overall Calibration Spec Absolute: £1K (3 s) Radiometric Absolute: £0.95K (3 s) Reproducibility: £0.2K (3 s) over 24 hrs Spatial Spectral Absolute: 510-6 (3 s) Stability: 110-6 (3 s) over 1 hr ROIC Cross-talk InstrumentLine ShapeUncertainty Optical Cross-talk LaserWavenumber Geometric Stability Blackbody Emissivity & Temperature Linearity Uncalibrated MirrorReflectance Pointing Mirror Polarization & Scattering Aliasing Instrument Temperature Stability Residual Radiance Noise Calibration Allocation Tree Basic philosophy is to constrain the spectral calibration such that spectral errors do not contribute significantly to the total calibration budget

  19. LW band SMW band Tb (K) Dn = n × 110-6 210-6 Tb Diff (K) 510-6 (1 sigma perturbations) wavenumber wavenumber Simulated Earth Scene Brightness Temperature Errors due to wavenumber scale uncertainties • The absolute knowledge of the spectral calibration shall be known to better than 510-6 (3 sigma). • The stability of the spectral calibration shall be known to better than 110-6 (3 sigma) over 1 hour (Threshold), over 30 days (Objective).

  20. Spectral Calibration using Earth Scene Spectra

  21. The short and long term geometric and laser frequency stability, and the resulting spectral calibration, of GIFTS are expected to be very good (better than requirement) by design. Argues for philosophy that spectral calibration is established on the ground with validation/monitoring on-orbit. • Spectral calibrations with Earth scene data will be used to monitor and remove any long term drifts as required. • Spectral positions of selected spectral features are known with high accuracy and are used to determine the spectral calibration. • For a given clear sky Earth spectrum, the effective laser wavenumber and resulting wavenumber scale of the observed spectrum is varied to produce best agreement with a calculated spectrum.

  22. Earth view spectral calibration example: ground based AERI spectra

  23. AERI: Atmospheric Emitted Radiance Interferometer • UW/BOMEM FTS spectro-radiometer providing 1 cm-1 resolution IR spectra. • 632 nm (~15800 cm-1) HeNe laser. q=0, b=23mrad. • The following presents analysis of AERI spectral calibration using 241 clear sky zenith spectra collected over a 3 year period at the SGP ARM site. sample longwave zenith viewing radiance spectrum

  24. 730-740 cm-1 region of Longwave spectrum • Fundamental n2 CO2 spectral line parameters known very well from laboratory work • Observed spectrum largely sensitive to lower troposphere atmospheric temperature

  25. sampling of the AERI spectra RMS residual as a function of laser wavenumber perturbation for each case RMS residual effective laser wavenumber offset (1/cm) wavenumber (1/cm) For each of the 241 cases, radiance spectra are computed using collocated radiosonde profiles, and the spectral calibration is performed. The optimal effective laser wavenumber is found when the RMS residual (observed minus calculated radiance) over the 730-740 1/cm region is minimized. Uncertainty in the final laser wavenumber is reported as 1 std. dev. over the ensemble.

  26. effective laser wavenumber offset versus case number Histograms, before and after Feb 2000 laser wavenumber offset (1/cm) case number laser wavenumber offset (1/cm) AERI laser wavenumber analysis • Earth view spectral calibration determines effective laser wavenumber to ~0.025 cm-1 / ~1.5 ppm (1 sigma) uncertainty. • ~0.65 cm-1 change in effective laser wavenumber with laser replacement in February 2000.

  27. Earth view spectral calibration example: aircraft based Scanning-HIS spectra

  28. Scanning High-resolution Interferometer Sounder • UW/BOMEM FTS spectro-radiometer providing 1 cm-1 resolution IR spectra. • 632 nm (~15800 cm-1) HeNe laser. q=0, b=20mrad. sample nadir viewing brightness temperature spectrum • The following presents analysis of SHIS spectral calibration using 82 clear sky nadir spectra collected @ 8 to 12 km altitude over a 3 week period at the SGP ARM site.

  29. Example Spectral Calibration: S-HIS

  30. SHIS spectral calibration, 730-740 1/cm, Nominal algorithm. laser wavenumber d = 0.00125  0.018 cm-1, ppm = 0.1  1.1

  31. Scanning-HIS LW/MW and MW/SW Band Overlap LW HgCdTe band MW HgCdTe band SW InSb band LW/MW overlap MW/SW overlap

  32. Results for different SHIS Bands and spectral regions starting nlaser = 15799.706 cm-1 • Accuracy degrades with increasing measurement noise and decreasing spectral contrast in a given spectral range. Spectral contrast varies with atmospheric conditions. • Consistent results for various spectral ranges within bands confirms n scales with nlaser • nlaser differs by band even though SHIS detectors share same field stop and aft optics

  33. Summary and Future Work

  34. Summary • GIFTS ILS effects due to finite-field-of-view are small and correctable • Off-axis pixel spectral scale varies predictably from pixel-to-pixel. Renormalization via interpolation in ground processing provides all spectra on a common wavenumber scale. • Good laser and geometric performance of GIFTS design argues for spectral calibration established on ground and validated/monitored on-orbit using Earth views. • Analysis of AERI and Scanning-HIS datasets suggests that long term variations in GIFTS spectral calibration can be determined/monitored successfully on-orbit using Earth view spectra. Ensemble 1 standard deviation of the Earth view spectral calibration results are ~1.5ppm (AERI) and ~1.1ppm (S-HIS) using 730-740 cm-1.

  35. On-going/Future Work • Comprehensive error analysis of AERI and Scanning-HIS spectral calibration results. • Ground-up estimate of uncertainty in calculated spectral scale at GIFTS spectral resolution for various spectral ranges and atmospheric conditions. • Extend Scanning-HIS analysis to include larger range of observed spectra/profiles. • Develop practical plan for performing GIFTS on-orbit spectral calibration/monitoring.

  36. The End. Thank you

  37. Backup Material

  38. GIFTS: 2-D Detector Array Technology • Two 128 x 128 pixel IR detector arrays with 4 km footprint size • One 512 x 512 pixel visible detector array with 1 km footprint size • Views 512 km x 512 km region with all three arrays in ~10 seconds • Each 10 second observation period provides 16,384 spectra and retrievals

  39. Analog Sampling A/D Dt Normalization, Flat-fielding Co-add Numerical Filter / Decimation SMW: N LW: 4N N 4N N 4N N N N/4 = 4096 N/8 = 2048 SMW: (~1175-2350 cm-1) LW: (~587-1175 cm-1) ~1.06x10-4 cm ~4.26x10-4 cm ~2.66x10-5 cm ~8.51x10-4 cm Interferogram Sampling nlaser = 1E4/1.064mm = ~9398 1/cm

  40. 750 cm-1, i=1,j=1 750 cm-1, i=1,j=64 750 cm-1, i=64,j=64 apodization 1750 cm-1, i=1,j=64 1750 cm-1, i=64,j=64 OPD (cm) Self-Apodization due to finite detector size Limited to less than 1%

  41. Effect of Uncorrected ILS variations Rarely larger than 0.05 K without correction

  42. SHIS spectral calibration, 730-740 1/cm, Nominal algorithm but using mean calculation as reference for all cases. laser wavenumber d = 0.00489  0.021 cm-1, ppm = 0.3  1.3

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