CALCON 2003 On-orbit Spectral Calibration of the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS)
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On-orbit Spectral Calibration of the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) David C. Tobin, Henry E. Revercomb, Robert O. Knuteson University of Wisconsin Space Science and Engineering Center USU/SDL CALCON 2003 CALCON 2003 GIFTS On-orbit Spectral Calibration 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 Slide 2 CALCON 2003 GIFTS On-orbit Spectral Calibration Introduction to GIFTS Slide 3 CALCON 2003 GIFTS On-orbit Spectral Calibration GIFTS Measurement Concept Combine Advanced Measurement Technologies a Geosynchronous Satellite to obtain Observations of the Atmosphere on 4-D • 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) Slide 4 CALCON 2003 GIFTS On-orbit Spectral Calibration Spectral Coverage CrIS CrIS 685 – 1129 1/cm 1650 - 2250 1/cm GIFTS GOES Sounder CO CO2 N2O N2O CH4 H2O H2O CO2 O3 CO2 Slide 5 CALCON 2003 GIFTS On-orbit Spectral Calibration GIFTS Optical Design and Spectral Characteristics Slide 6 CALCON 2003 GIFTS On-orbit Spectral Calibration Expanded View of the Electro-optical Design LWIR nlaser SWIR 1064nm, 9398 1/cm N 2048 4096 R 8 4 X, max OPD (N/2)R/nlaser= 0.87 cm Dv 1/(2X) = 0.57 1/cm Slide 7 CALCON 2003 GIFTS On-orbit Spectral Calibration 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) » n scales with nlaser • Spectral calibration primarily deals with knowledge of the effective nlaser Slide 8 CALCON 2003 GIFTS On-orbit Spectral Calibration 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: F x, dnN n e 2 b 2 i 2nx 1 2 sinc 2nxb 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). Slide 9 CALCON 2003 GIFTS On-orbit Spectral Calibration OPD(f)=OPD(0) cos(f) FTS axis M1 X cos f X X/2 M2 x2 x1 x2 f OPD = x1 - x2 = X On axis beam x1 OPD = x1 - x2 = X cos f Off axis beam after Fourier Transform Spectrometry, James W. Brault Slide 10 CALCON 2003 GIFTS On-orbit Spectral Calibration FPA Geometry FPA pixel i,j b = single pixel half angle = 2/2/128 = 0.38 mrad FPA interferometer telescope b 2 = 97.4 mrad (i,j) (i,j) = off-axis angle to center of pixel = b[(2i-1)2 + (2j-1)2]1/2 2/1 = afocal ratio = 6.855 1 = 14.2 mrad Not to scale FPA pixel index j Off-Axis Angle, [mrad] 512km 4km/pixel FPA pixel index i Slide 11 CALCON 2003 GIFTS On-orbit Spectral Calibration ILS Variations: F x, dnN n e 2 b 2 i 2nx 1 2 sinc 2nxb very little ILS change over the detector array Slide 12 CALCON 2003 GIFTS On-orbit Spectral Calibration Self-apodization correction process • In expression for the measured interferogram, F(x), expand sinc function as a power series of (2nxb): F x dnN n e i 2nx 2 2xb 3! dnN n n e 2 i 2nx 4 2xb 5! 4 i 2nx d n N n n e ... • Compute perturbation terms and subtract from measured interferogram. » Similar process currently performed for AERI, HIS, SHIS, NAST-I. Slide 13 CALCON 2003 GIFTS On-orbit Spectral Calibration GIFTS Off-Axis Interferogram Sampling 2 b 2 i 2nx 1 2 2 b 2 i 2nx 1 2 Interferogram (counts) F x, dnN n e F x, dnN n e sinc 2nx sinc 2nxb OPD (cm) 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. Slide 14 CALCON 2003 GIFTS On-orbit Spectral Calibration GIFTS Interferogram Data Cube Simulations interferogram point #1 (ZPD, OPD=0) Pixel # interferogram point #780 (CO2 resonance) Pixel # Pixel # 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 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. Slide 16 CALCON 2003 GIFTS On-orbit Spectral Calibration Calibration Requirements Slide 17 CALCON 2003 GIFTS On-orbit Spectral Calibration Calibration Allocation Tree Overall Calibration Spec Absolute: 1K (3 s) Radiometric Absolute: 0.95K (3 s) Reproducibility: 0.2K (3 s) over 24 hrs Laser Wavenumber Blackbody Emissivity & Temperature Instrument Temperature Stability Spatial Spectral Absolute: 510-6 (3 s) Stability: 110-6 (3 s) over 1 hr Instrument Line Shape Uncertainty Linearity Geometric Stability Uncalibrated Mirror Reflectance ROIC Cross-talk Pointing Mirror Polarization & Scattering Residual Radiance Noise Optical Cross-talk Aliasing Basic philosophy is to constrain the spectral calibration such that spectral errors do not contribute significantly to the total calibration budget Slide 18 CALCON 2003 GIFTS On-orbit Spectral Calibration Simulated Earth Scene Brightness Temperature Errors due to wavenumber scale uncertainties LW band Tb (K) SMW band Dn = n × Tb Diff (K) 110-6 210-6 510-6 (1 sigma perturbations) wavenumber wavenumber • The absolute knowledge of the spectral calibration shall be known to better than 510-6 (3 sigma). • The stability of the spectral calibration shall be known to better than 110-6 (3 sigma) over 1 hour (Threshold), over 30 days (Objective). Slide 19 CALCON 2003 GIFTS On-orbit Spectral Calibration Spectral Calibration using Earth Scene Spectra Slide 20 CALCON 2003 GIFTS On-orbit Spectral Calibration • 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. Slide 21 CALCON 2003 GIFTS On-orbit Spectral Calibration Earth view spectral calibration example: ground based AERI spectra Slide 22 CALCON 2003 GIFTS On-orbit Spectral Calibration 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 Slide 23 CALCON 2003 GIFTS On-orbit Spectral Calibration 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 Slide 24 CALCON 2003 GIFTS On-orbit Spectral Calibration 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. sampling of the AERI spectra RMS residual RMS residual as a function of laser wavenumber perturbation for each case effective laser wavenumber offset (1/cm) wavenumber (1/cm) Slide 25 CALCON 2003 GIFTS On-orbit Spectral Calibration AERI laser wavenumber analysis Histograms, before and after Feb 2000 case number laser wavenumber offset (1/cm) laser wavenumber offset (1/cm) effective laser wavenumber offset versus case number • 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. Slide 26 CALCON 2003 GIFTS On-orbit Spectral Calibration Earth view spectral calibration example: aircraft based Scanning-HIS spectra Slide 27 CALCON 2003 GIFTS On-orbit Spectral Calibration Scanning High-resolution Interferometer Sounder • UW/BOMEM FTS spectroradiometer providing 1 cm-1 resolution IR spectra. • 632 nm (~15800 cm-1) HeNe laser. =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. Slide 28 CALCON 2003 GIFTS On-orbit Spectral Calibration Example Spectral Calibration: S-HIS Slide 29 CALCON 2003 GIFTS On-orbit Spectral Calibration SHIS spectral calibration, 730-740 1/cm, Nominal algorithm. laser wavenumber d = 0.00125 0.018 cm-1, ppm = 0.1 1.1 Slide 30 CALCON 2003 GIFTS On-orbit Spectral Calibration Scanning-HIS LW/MW and MW/SW Band Overlap LW HgCdTe band MW HgCdTe band LW/MW overlap Slide 31 SW InSb band MW/SW overlap CALCON 2003 GIFTS On-orbit Spectral Calibration Results for different SHIS Bands and spectral regions Band n1 - n2 (cm-1) note d nlaser (cm-1) d nlaser (ppm) 1 730 - 740 CO2 0.001 0.018 0.1 1.1 1 746 - 760 CO2 0.021 0.021 1.3 1.3 1 1170 - 1205 H2O, high noise 0.052 0.121 3.3 7.7 2 1170 - 1205 H2O 0.172 0.025 10.9 1.6 2 1560 - 1630 H2O 0.169 0.051 10.7 3.2 2 1785 - 1805 H2O, high noise 0.161 0.186 10.2 11.8 3 1785 - 1805 H2O, high noise 0.065 0.172 4.1 10.9 3 2050 - 2100 CO2, CO, H2O 0.036 0.032 2.3 2.0 3 2190 - 2200 N2O -0.010 0.042 -0.6 2.7 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 Slide 32 CALCON 2003 GIFTS On-orbit Spectral Calibration Summary and Future Work Slide 33 CALCON 2003 GIFTS On-orbit Spectral Calibration Summary • GIFTS ILS effects due to finite-field-of-view are small and correctable • Off-axis pixel spectral scale varies predictably from pixel-topixel. 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. Slide 34 CALCON 2003 GIFTS On-orbit Spectral Calibration 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. Slide 35 CALCON 2003 GIFTS On-orbit Spectral Calibration The End. Thank you Slide 36 CALCON 2003 GIFTS On-orbit Spectral Calibration Backup Material Slide 37 CALCON 2003 GIFTS On-orbit Spectral Calibration 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 Slide 38 CALCON 2003 GIFTS On-orbit Spectral Calibration Interferogram Sampling Analog Sampling SMW: LW: N 4N A/D Dt Normalization, Flat-fielding Coadd N 4N N 4N N N nlaser = 1E4/1.064mm = ~9398 1/cm SMW: (~1175-2350 cm-1) ~1.06x10-4 cm ~4.26x10-4 cm ~2.66x10-5 cm LW: (~587-1175 cm-1) ~8.51x10-4 cm Numerical Filter / Decimation N/4 = 4096 N/8 = 2048 Self-Apodization due to finite detector size 750 cm-1, i=1,j=1 750 cm-1, i=1,j=64 apodization 750 cm-1, i=64,j=64 1750 cm-1, i=1,j=64 1750 cm-1, i=64,j=64 OPD (cm) Limited to less than 1% Slide 40 CALCON 2003 GIFTS On-orbit Spectral Calibration Effect of Uncorrected ILS variations Rarely larger than 0.05 K without correction Slide 41 CALCON 2003 GIFTS On-orbit Spectral Calibration 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 Slide 42 CALCON 2003 GIFTS On-orbit Spectral Calibration
