Hank Revercomb, Fred Best, Dave Tobin, Bob Knuteson, Joe Taylor University of Wisconsin - Madison Space Science and Engineering Center (SSEC) Calibration Status for the Infrared: HIRS, AIRS, IASI, CrIS, HES Achieving Satellite Instrument Calibration for Climate Change (ASIC 3 ) National Conference Center May October panoramas

Topics A.Overview B.HIRS/MODIS/GOES (filter radiometers) C.AIRS on NASA Aqua (grating spectrometer), the 1 st modern high resolution sounder to fly D.Scanning HIS (aircraft FTS) E.AIRS - Scanning HIS Comparisons F.IASI & CrIS (future LEO operational FTS) G.Future GOES Sounders: GIFTS (FTS) & HES (FTS or Grating) H.Long-term climate records: approaches

A. Overview

The IR is a Key Climate Indicator u Fundamental component of the Energy budget of the atmosphere u High Accuracy for establishing small trends is relatively easy to achieve, if spectral calibration is handled (Especially hot or cold Reference sources not needed) u High information content to characterize complex climate changes is possible using spectrally resolved radiances

IR Sounders: Past, Present, and Future (Kory Priestley to cover Total IR broadband) VAS (1980-) –1 st Geo Sounder (Spin-Scan) GIFTS (2009 ?) (30) (1200)HES, GOES-R (2013-) ITPR,VTPR (1972) / HIRS (1978-) IASI / CrIS ( ?) AIRS (2002-) IRIS / SIRS ( ) –1 st Sounders (1200) (1200/2800) GOES Sounder (1994-) – (3-Axis) (1200) ( ) (30) Spectral Resolving Power ( /  )  Resolving 14  m BLUE = Leo Purple = Geo Red = Aircraft (2000)HIS ( )

Absolute Accuracy Requirements u K* 3-sigma is now achievable, but specifications are often an order of magnitude worse u Order 1 K* has been common for weather instruments, but they too would benefit from doing better u AIRS spec was 3% (~3K longwave, 1 K mid-shortwave at 300K), but < K 3-sigma has been achieved * brightness T at scene T

Calibration Performance Summary u New generation of high spectral resolution instruments offer significantly improved absolute calibration— 1 degree uncertainties replaced with concern over tenths of a degree Reasons include: –Fundamental advantage of high resolution (Goody and Haskins, J of Climate, 1998), plus accurate spectral sampling knowledge –Cavity onboard reference sources –Lower detector non-linearity from PV MCT in the longwave u High Spectral resolution will also offer greatly improved instrument-to-instrument consistency by allowing standardization of spectral sampling u But we can, and should, do even better for climate, especially by incorporating means for onboard verification of long-term accuracy (see Jim Anderson)

B. HIRS/MODIS/GOES (filter radiometers)

Simultaneous Nadir Overpass InSb MCT Mean Differences often larger than instrument radiometric calibration errors, because of spectral differences cm cm O3O3 802 N 2 O/ CO 2 WV Channel # (without accounting for different spectral characteristics)

HIRS to HIRS Differences from Wang, Ciren, & Cao, 2005 Simulated T b differences from SRF differences:  spectral differences are a primary contributor to HIRS-to-HIRS Diffs  challenge for climate record, even if the absolute cal is perfect Spectral Response Fns (SRFs) for HIRS on NOAA 16, 17, 18 ±2K

HIRS-18 Validation with AIRS 29 August 2005, Tropical Atlantic from Wang, Ciren, & Cao, 2005 AIRS HIRS Ch5 HIRS-AIRS HIRS-AIRS Mean ~ 0.18 ru ~0.15 K Individual HIRS calibration uncertainties are much smaller than HIRS-to-HIRS differences

wavenumber MODIS Accuracy Assessment using AIRS 25 / / / / / / / / / / / / / / / 11.0 MODIS Band / wavelength(  m)

Courtesy of MODIS Characterization Support Team <  < <   < from T/V external BB comparison T from 12 thermistors

AIRS Tb (K)AIRS minus MODIS (K) Fantastic AIRS - MODIS Agreement for Band 22 (4.0  m)! AIRS Histogram MODIS Uniform Scenes Selected Tobin, et al., 2006

MODIS Band 22 (4.0  m) AIRS-MODIS mean = K Little Dependence on Scene Temperature Little Dependence on X-track View Angle Little Dependence on Solar Zenith Angle Tobin, et al., 2006

Summary of AIRS-MODIS mean Tb differences Red=without accounting for convolution error Blue=accounting for convolution error with mean correction from standard atmospheres p-p Convolution Error (CE) Estimate mm Band Band Diff CE Diff Std N > 1K errors exist in some more opaque channels Tobin, et al., 2006

No ShiftMODIS shifted Tb diff (K) AIRS-MODIS: un-shifted, shifted SRF SRF shift may explain MODIS Calibration Errors Shifting MODIS Band 35 (13.9  m) by 0.8 cm -1 Works to Remove Mean bias and Scene Tb Dependence Spectral uncertainty appears to dominate the uncertainty

GEO/LEO Intercalibration Comparing NOAA-15 AVHRR to a global constellation of Geostationary Imagers GEO – LEO (K) GEOs relative to AVHRR ±2 K Matt Gunshor, UW Calculations are used to approximately account for spectral response differences The large telescope needed from GEO presents other calibration challenges Oct ’05 May ‘06

Filter Radiometer Summary u Order 1 K calibration issues are not uncommon (e.g. relative GEO comparisons) u Spectral uncertainty is one probable cause for uncertainty exceeding 1 K (e.g. MODIS results from AIRS) u Lack of reproducible spectral sampling from instrument to instrument is also a key issue for long-term climate records u New instruments with Nyquist spectral sampling and broad spectral coverage will greatly reduce this uncertainty

C. AIRS on NASA Aqua (grating spectrometer), the 1st modern high resolution sounder to fly  Demonstrates key advantages of high spectral resolution for calibration accuracy

AIRS 4 May 2002 Launch AIRS 14 June 2002 Calculated NASA Aqua

AIRS Onboard Blackbody: light trap cavity design—specular surface nadir emissivity,  > T calibration ± 0.05 K Primary T/V reference emissivity,  > T knowledge ± 0.03 K PRT temperature sensors ABB Bomem

AIRS: Other key factors u ILS knowledge: thermal/vac testing with FTS source, verified with gas cell tests u Spectral Calibration: atmosphere (rough in-flight check via parylene source) stability maintained by T control of whole spectrometer/aft optics assembly (2.2%  shift/K) u Non-linearity*: <0.3% over much of spectrum, < ~1% peak; error assumed < 0.05 K after correction u Polarization*: ±worst case 0.4K (9 & 15  m); error assumed < 0.07 K after correction *from Pagano et al, ITWG, 2003

peak-to-peak seasonal changes (about 5 ppm) cause brightness temperature differences of about ~0.5 K p-p in 15  m CO 2 band (Caused by instrument T changes of < 0.25K) But these changes are detectable, and can be corrected with the AIRS Nyquist sampling Larrabee Strow, et al.

D. Scanning HIS (aircraft FTS) u Performance Estimates u Key calibration considerations Aircraft instrument offers preview of future operational instruments and validation of AIRS and future instrument calibration accuracy

UW Scanning HIS: 1998-Present (HIS: High-resolution Interferometer Sounder, ) Longwave Midwave Shortwave CO 2 CO N2ON2O H2OH2O H2OH2O CH 4 /N 2 O CO 2 O3O3 Characteristics Spectral Coverage: 3-17 microns Spectral Resolution: 0.5 cm -1 Resolving power: Footprint Diam: km Cross-Track Scan: Programmable including uplooking zenith view u Radiances for Radiative Transfer u Temp & Water Vapor Retrievals u Cloud Radiative Prop. u Surface Emissivity & T u Trace Gas Retrievals Applications: NASA WB57

Scanning-HIS Radiometric Calibration Budget TABB= 227, THBB=310, 11/16/02 Proteus Similar to AERI description in Best, et al., CALCON 2003 RSS of Errors in T HBB, T ABB T Rfl  HBB,  ABB + 10% of non-linearity correction 3-sigma Tb error 220 K

Scanning HIS: Some key factors u ILS knowledge: fundamental instrument design (only weak dependence on geometry) u Spectral Calibration: atmosphere, stability maintained by onboard HeNe laser reference (no active temperature control required) u Non-linearity: < 2.5% longwave and midwave, negligible shortwave error < ~0.2 K after correction u Polarization: <0.05% (gold scene mirror) error < 0.04 K even uncorrected

Atmospheric Spectral Calibration: S-HIS Atmospheric CO 2 lines Wavenumber Scale chosen to minimize difference Estimated accuracy =1.2 ppm (1 sigma) With many samples, the 3-sigma accuracy is < 1 ppm AIRS does similar atmospheric spectral calibration

E. AIRS - Scanning HIS Comparisons  Direct AIRS radiance validation u Mean differences generally <0.2 K with small standard deviations u Demonstrates aircraft capability for highly accurate validation of S/C obs

8 AIRS FOVs used in the following comparisons (shown in MODIS 12 micron image) 1 Degree AIRS Validation with UW Scanning HIS

Direct S-HIS to AIRS comparison (without accounting for spectral & viewing differences) AIRS SHIS 8 AIRS FOVs, 448 SHIS FOVs, PC filtering

“comparison 0” 8 AIRS FOVs, 448 SHIS FOVs, PC filtering S-HIS Spectrum Nyquist sampled without gaps

Gulf of Mexico Validation case:

(AIRSobs-AIRScalc)- (SHISobs-SHIScalc) (K) “Comparison 2” (21 November 2002) Excluding channels strongly affected by atmosphere above ER2

AIRS-SHIS Summary: SW ( ) 1 st Direct SW Radiance Validation Excellent agreement for night-time comparison from Adriex in Italy

F. IASI & CrIS (future LEO operational FTS)  Operational extension of AIRS will be very useful for climate applications

IASI on Metop 17 July 2006 launch scheduled

IASI: Other key factors u Instrument Line Shape knowledge: fundamental instrument design verified with laser sources in ground testing u Spectral Calibration: atmosphere stability maintained by onboard 1.54  m diode laser reference with < 1ppm validated over 14 days* u Non-linearity: < 1% longwave, negligible mid- and short-wave error < ~0.15 K after correction* stability verifiable in orbit from out of band features u Polarization: <0.05% (gold scene mirror with overcoating) error < 0.04 K even uncorrected* * Denis Blumstein and Thierry Phulpin, cnes

IASI Radiometric Calibration : Interscan and Interpixel radiometric performance Principle Design Requirements Sub systems Performances Development plan Interscan radiometric performance (Spec. 0.1 K) HBB at 294 K, Pixel 1 Interpixel radiometric performance (Spec. 0.1 K) HBB at 294 K, Pixel 1 ± 0.1 K Interscan: Incidence Angles (deg): , -50, , Interpixel: Spectral residuals for 4 detectors/band overlaid

CrIS : AIRS Successor for NPOESS, will be equally good, or better 1.Overall Calibration Spec: <0.4 K ( Design specs: < 0.45%, LW, 0.58% MW, 0.77% SW) - Actual performance will significantly exceed specification, especially after incorporating planned NIST measurements of reference blackbodies - Non-linearity very small - Polarization effects very small 2.Spectral Calibration: Instrument Line Shape (ILS) extremely well known and stable from first principles

CrIS: Other key factors u ILS knowledge: fundamental instrument design (only weak dependence on geometry), verified with laser source and gas cell tests u Spectral Calibration: atmosphere &/or onboard Ne source; stability maintained by T control of onboard 1.55  m diode laser reference u Non-linearity: <0.1% longwave & negligible elsewhere error < ~0.07 K even uncorrected stability verifiable in orbit from out of band features u Polarization: <0.05% (gold scene mirror) error < 0.04 K even uncorrected

CrIS Observed and Calculated Instrument Line Shape (ILS) CO 2 laser source, Center, Edge & Corner Pixels of 3x3 array ( laser = nm, CO2laser = cm -1, dx=-0.7mrad, dy=0.1 mrad) Pure sinc Center FOV5 Edge FOV4 Corner FOV1 Center FOV Edge FOV Corner FOV centroid (cm -1 ) Obs Calc FWHM (cm -1 ) Obs Calc Lfoot Obs Calc Rfoot Obs Calc Calculated Observed FTS design expectations confirmed

Observed and Calculated ILSs for Run1, FOVs 5, 4, and 1 laser = nm, CO2laser = cm -1, dx=-0.7mrad, dy=0.1 mrad Pure sinc Center FOV5 Edge FOV4 Corner FOV1 Calculated Observed

G. Future GOES Sounders: GIFTS (FTS) & HES (FTS or Grating)  GIFTS internal calibration concept u HES specifications u Added value for cross-calibration of IR sensors on other platforms

4-d Digital Camera: Horizontal: Horizontal: Large area format Focal Plane detector Arrays Vertical: Vertical: Fourier Transform Spectrometer Time: Time: Geostationary Satellite “GIFTS” Geostationary Imaging Fourier Transform Spectrometer New Technology for Atmospheric Temperature, Moisture, Chemistry, & Winds

GIFTS Sensor Module Technologies

GIFTS : Wrapped up for Thermal Vacuum Testing at SDL

Test Facilities where GIFTS is undergoing Thermal Vacuum testing “MIC2” Multi-function IR Calibrator GIFTS Chamber Space Dynamics Lab, Utah State University

Internal Blackbody References Blackbody aperture Boundary of area seen by IR detectors Red outline is flip-in mirror seen through cover Flip-in mirror cover Vis Flood Source Specification Estimate

GIFTS Absolute Calibration-Longwave (3-sigma brightness T error at scene T) GIFTS External BB Original Requirement (excluding uncertainty of Non-linearity & Polarization correction) Longwave Band (800 cm -1 ) Tc=255, Th=290, Ts=240, Tt=230, Tm=220

GIFTS Absolute Calibration-Shortwave (3-sigma brightness T error at scene T) GIFTS External BB Original Requirement (excluding uncertainty of non-linearity & Polarization correction ) Shortwave Band (1800 cm -1 ) Tc=255, Th=290, Ts=240, Tt=230, Tm=220

C. HES Sounder Status u Three industries are competing to build HES: Ball, BAE, and ITT each have $20 M contracts to chose between an FTS and a grating approach and to perform an advanced phase A design (my description) u Common requirements for FTS and grating: Strong attempt to limit requirements to those perceived to be achievable by both approaches. u Spectral coverage trades under consideration: Options for spectral coverage are being explored to minimize complexity, risk and cost (and performance) u Process is just past mid-way: A delta-Mid Term Review is planned for mid-May A winner is expected to be chosen next year

HES Absolute Calibration Requirement u Still using  1 K specification (although, specified in terms of a brightness T error at 300K) u We can, should, and probably will do better u Spectral calibration uncertainties are intended to not significantly inflate absolute calibration errors

Spectral Calibration Knowledge u Channel Centers need to be known very accurately (< 3 ppm) The goal should be less than 1 ppm u This is tighter than originally required of AIRS and CrIS (1% of  = /1200 implies 8 ppm), although both can meet the tighter goal

3.) Spectral Calibration: Long-wave,  =0.625 cm -1 T b errors for labeled spectral shift error in ppm Note that 5 ppm is equivalent to 0.6 % of  at 750 cm -1 Also, note that the larger errors for the sinc ILS are consistent with its larger absorption line amplitudes and sounding sensitivity Recommend a < 1 ppm goal (0.1% of  for sounding bands)

H. Long-term climate records: approaches  Dedicated mission to establish an IR benchmark u GEO High Spectral Resolution as a transfer standard

General Thesis We should start collecting the best practical climate reference observations that can be continued for decades the time is right to augment planned observing systems with a satellite to provide reference IR spectral data to accurately document current and evolving climate conditions

Why improve accuracy?  Time to unequivocally quantify climate change is proportional to uncertainty (30 years can go to 10 years by reducing uncertainty from 0.3 to 0.1 K)  We can do better, if it has the priority  Results need to be unassailable to have societal impact: should employ in-orbit validation of critical reference blackbody stability

Augmentation required for climate monitoring 1.Better time of day coverage: Add a satellite in a non-sun-synchronous orbit 2.More complete spectral coverage: Add far IR coverage to get better information on cloud phase, including mixed-phase clouds 3.Highest possible radiometric and spectral accuracy: Optimize the design for accuracy and augment in-flight verification measurements

Basic Mission for providing an IR Spectral Measurement Standard  Far IR spectral coverage (to 50 or 100 microns) to capture most of the emitted energy and unique information on cirrus clouds  Orbit providing local time coverage and crossing operational sun-synchronous orbits to allow inter- comparisons (e.g. 90-degree polar)  Fourier Transform approach with stable laser reference providing an accurate spectral standard  Dual instruments to detect any unexpected drifts  Overall calibration uncertainty (3-sigma) < 0.1 K  On-orbit verification for key radiometric properties (e.g. Blackbody T, , linearity)

Arrhenius Calibration Uncertainty 3-sigma, UW blackbody characteristics, 800 cm -1 RSS Goal = 0.1

Conclusions (the potential) Dramatic improvements in current research and future operational satellite IR measurements have much to offer climate applications Coupling reasonably high spectral resolution with broad spectral coverage makes it possible to achieve very high accuracy with high information content These new capabilities offer the potential to unify the entire international complement of IR observations from different instruments and platforms

Conclusions (but, we need to…) Endorse this climate role and put special emphasis on making new instruments as accurate as they can be to realize the potential of technological investments already made Maintain a careful validation program for establishing the best possible direct radiance check of long-term accuracy-- specifically, continuing to use aircraft- or balloon-borne instruments that are periodically checked directly with NIST Commit to a simple, new IR mission that will provide an ongoing backbone for the climate observing system This mission will greatly enhance the value of upcoming operational systems for climate, by filling in spatial and diurnal sampling gaps and by acting as a benchmark with improved ties to fundamental standards in-flight