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

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 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

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 3 Introduction to GIFTS

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 4 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) GIFTS Measurement Concept

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 5 Spectral Coverage CrIS CO 2 O3O3 N2ON2O CO N2ON2O CO 2 H2OH2O H2OH2O CH 4 CrIS GIFTS GOES Sounder 685 – /cm /cm

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 6 GIFTS Optical Design and Spectral Characteristics

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 7 Expanded View of the Electro-optical Design LWIRSWIR laser 1064nm, /cm N R 84 X, max OPD (N/2)R/ laser = 0.87 cm Dv 1/(2X) = /cm

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 8 With constant  x sampling, the wavenumber scale is determined by the effective laser frequency and the resulting interferogram sampling rate: –Optical Path Difference (OPD) scale: dx = R/ laser X = (N/2) dx –Wavenumber scale: d = 1/(2X) = laser /(NR) » scales with laser Spectral calibration primarily deals with knowledge of the effective laser On-Axis Wavenumber Scale

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 9 The beams of light reaching each FPA pixel pass through the interferometer at different angles,  With respect to the on-axis beam, the off-axis beams have slightly shorter OPDs: OPD(  ) = OPD(0) cos(  ). 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  is the mean off-axis angle for a given pixel and b is the half-angle subtended by a single pixel (~0.38 mrad). Off-axis Effects

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 10 OPD(  )=OPD(0) cos(  ) after Fourier Transform Spectrometry, James W. Brault M2M2 M1M1 On axis beam Off axis beam X/2 FTS axis  x2x2 x1x1 x2x2 x1x1 X cos  OPD = x 1 - x 2 = X cos  OPD = x 1 - x 2 = X X

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 11 FPA Geometry FPA interferometer telescope  1 = 14.2 mrad  2 = 97.4 mrad    1 = afocal ratio =  km 4km/pixel b = single pixel half angle =  2 /2/128 = 0.38 mrad  (i,j) = off-axis angle to center of pixel = b[(2i-1)2 + (2j-1)2] 1/2 Off-Axis Angle,  mrad  Not to scale  (i,j) b FPA pixel i,j FPA pixel index i FPA pixel index j

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 12 ILS Variations: very little ILS change over the detector array

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 13 In expression for the measured interferogram, F(x), expand sinc function as a power series of (2  xb  ): Compute perturbation terms and subtract from measured interferogram. »Similar process currently performed for AERI, HIS, S- HIS, NAST-I. Self-apodization correction process

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 14 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. OPD (cm) Interferogram (counts)

CALCON 2003 GIFTS On-orbit Spectral Calibration GIFTS Interferogram Data Cube Simulations interferogram point #1 (ZPD, OPD=0) interferogram point #780 (CO 2 resonance) Pixel # In the spectral domain, produces apparent shifts of spectral features when plotted versus the on-axis wavenumber scale.

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 16 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. Off-axis wavenumber scale normalization

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 17 Calibration Requirements

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 18 Calibration Allocation Tree Overall Calibration Spec Absolute:  1K (3  ) Spatial Blackbody Emissivity & Temperature Linearity Instrument Temperature Stability Uncalibrated Mirror Reflectance Residual Radiance Noise Pointing Mirror Polarization & Scattering Instrument Line Shape Uncertainty Geometric Stability ROIC Cross-talk Optical Cross-talk Laser Wavenumber Aliasing Spectral Absolute: 5  (3  ) Stability: 1  (3  ) over 1 hr Radiometric Absolute:  0.95K (3  ) Reproducibility:  0.2K (3  ) over 24 hrs Basic philosophy is to constrain the spectral calibration such that spectral errors do not contribute significantly to the total calibration budget

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 19 Simulated Earth Scene Brightness Temperature Errors due to wavenumber scale uncertainties T b Diff (K) wavenumber T b (K) wavenumber SMW band LW band  = × 1    (1 sigma perturbations) The absolute knowledge of the spectral calibration shall be known to better than 5  (3 sigma). The stability of the spectral calibration shall be known to better than 1  (3 sigma) over 1 hour (Threshold), over 30 days (Objective).

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 20 Spectral Calibration using Earth Scene Spectra

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 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.

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 22 Earth view spectral calibration example: ground based AERI spectra

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 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.  =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

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide cm -1 region of Longwave spectrum Fundamental 2 CO 2 spectral line parameters known very well from laboratory work Observed spectrum largely sensitive to lower troposphere atmospheric temperature

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 25 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 /cm region is minimized. Uncertainty in the final laser wavenumber is reported as 1 std. dev. over the ensemble. sampling of the AERI spectra wavenumber (1/cm) effective laser wavenumber offset (1/cm) RMS residual RMS residual as a function of laser wavenumber perturbation for each case

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 26 AERI laser wavenumber analysis case number laser wavenumber offset (1/cm) effective laser wavenumber offset versus case number Histograms, before and after Feb 2000 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.

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 27 Earth view spectral calibration example: aircraft based Scanning-HIS spectra

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 28 Scanning High-resolution Interferometer Sounder sample nadir viewing brightness temperature spectrum UW/BOMEM FTS spectro- radiometer providing 1 cm -1 resolution IR spectra. 632 nm (~15800 cm -1 ) HeNe laser.  =0, b=20mrad. The following presents analysis of SHIS spectral calibration using 82 clear sky nadir spectra 8 to 12 km altitude over a 3 week period at the SGP ARM site.

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 29 Example Spectral Calibration: S-HIS

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 30 SHIS spectral calibration, /cm, Nominal algorithm. laser wavenumber  =  cm -1, ppm = 0.1  1.1

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 31 Scanning-HIS LW/MW and MW/SW Band Overlap LW HgCdTe bandMW HgCdTe band SW InSb band LW/MW overlap MW/SW overlap

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 32 Band (cm -1 ) note  laser (cm -1 )  laser (ppm) CO   CO   H 2 O, high noise   H2OH2O   H2OH2O   H 2 O, high noise   H 2 O, high noise   CO 2, CO, H 2 O   N2ON2O   2.7 Results for different SHIS Bands and spectral regions starting laser = 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 scales with laser laser differs by band even though SHIS detectors share same field stop and aft optics

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 33 Summary and Future Work

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 34 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 cm -1. Summary

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 35 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. On-going/Future Work

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 36 The End. Thank you

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 37 Backup Material

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 38 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 GIFTS: 2-D Detector Array Technology

~4.26x10 -4 cm ~8.51x10 -4 cm ~1.06x10 -4 cm ~2.66x10 -5 cm SMW: (~ cm -1 ) LW: (~ cm -1 ) A/D Analog Sampling  t Normalization, Flat-fielding Co- add Numerical Filter / Decimation laser = 1E4/1.064  m = ~9398 1/cm SMW: N LW: 4N N 4N N 4N NNNN N/4 = 4096 N/8 = 2048 Interferogram Sampling

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 40 Self-Apodization due to finite detector size Limited to less than 1% OPD (cm) apodization 1750 cm -1, i=64,j= cm -1, i=1,j= cm -1, i=64,j= cm -1, i=1,j= cm -1, i=1,j=1

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 41 Effect of Uncorrected ILS variations Rarely larger than 0.05 K without correction

CALCON 2003 GIFTS On-orbit Spectral Calibration Slide 42 laser wavenumber  =  cm -1, ppm = 0.3  1.3 SHIS spectral calibration, /cm, Nominal algorithm but using mean calculation as reference for all cases.