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Hank Revercomb, Fred Best, Dave Tobin, Bob Knuteson, Joe Taylor
University of Wisconsin - MadisonSpace Science and Engineering Center (SSEC)
Calibration Status for the Infrared:HIRS, AIRS, IASI, CrIS, HES
Achieving Satellite Instrument Calibration for Climate Change (ASIC3) National Conference Center
16-18 May 2006
21 October panoramas
Topics A. Overview
B. HIRS/MODIS/GOES (filter radiometers)
C. AIRS on NASA Aqua (grating spectrometer), the 1st 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
Fundamental component of the Energy budget of the atmosphere
High Accuracy for establishing small trends is relatively easy to achieve, if spectral calibration is handled(Especially hot or cold Reference sources not needed)
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 IRbroadband)
VAS (1980-) –1st Geo Sounder (Spin-Scan)
GIFTS (2009 ?)
(30)
(1200) HES, GOES-R (2013-)
ITPR,VTPR (1972) / HIRS (1978-)
IASI / CrIS (2006-2009?)
AIRS (2002-)
IRIS / SIRS (1969-70) –1st Sounders
(1200)
(1200/2800)
GOES Sounder (1994-) – (3-Axis)
(1200)
(150-300)
(30)
(30)
Spectral Resolving Power (/ )Resolving Power @ 14 m
BLUE = Leo Purple = Geo Red = Aircraft
(2000) HIS (1986-1998)
Absolute Accuracy Requirements
0.1-0.3 K* 3-sigma is now achievable, but specifications are often an order of magnitude worse
Order 1 K* has been common for weather instruments, but they too would benefit from doing better
AIRS spec was 3% (~3K longwave, 1 K mid-shortwave at 300K), but < 0.2-0.3 K 3-sigma has been achieved
* brightness T at scene T
Calibration Performance Summary
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
High Spectral resolution will also offer greatly improved instrument-to-instrument consistency by allowing standardization of spectral sampling
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
InSbMCT
Mean Differences often larger than instrument
radiometric calibration errors,because of spectral differences
669-749 cm-1 2188-2600 cm-1
900
O3
802 N2O/ CO2WV
Channel #
(without accounting for different spectral characteristics)
HIRS to HIRS Differences
from Wang, Ciren, & Cao, 2005
Simulated Tb differencesfrom 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 Validationwith AIRS
29 August 2005, Tropical Atlanticfrom Wang, Ciren, & Cao, 2005
AIRS HIRS Ch5 HIRS-AIRS
HIRS-AIRSMean ~ 0.18 ru
~0.15 K
Individual HIRS calibrationuncertainties are much smaller than
HIRS-to-HIRS differences
wavenumber
MODIS Accuracy Assessment using AIRS
25 / 4.524 / 4.423 / 4.122 / 4.021 / 4.0
30 / 11.029 / 9.728 / 7.327 / 6.8
36 / 14.235 / 13.934 / 13.733 / 13.432 / 12.031 / 11.0
MODIS Band / wavelength(m)
Courtesy of MODIS Characterization Support Team
0.987 < < 0.9940.0014< < 0.0035
from T/V external BB comparison
T from 12 thermistors
AIRS Tb (K) AIRS minus MODIS (K)
Fantastic AIRS - MODIS Agreement for Band 22 (4.0m)!
AIRS HistogramMODIS
Uniform ScenesSelected
Tobin, et al., 2006
MODIS Band 22 (4.0m)
AIRS-MODIS mean = -0.05 K Little Dependence on
Scene Temperature
Little Dependence onX-track View Angle
Little Dependence onSolar Zenith Angle
Tobin, et al., 2006
Summary of AIRS-MODIS mean Tb differences
Red=without accounting for convolution errorBlue=accounting for convolution error with mean correction from standard atmospheres
p-p Convolution Error (CE) Estimate
mBand
Band Diff CE Diff Std N 21 0.10 -0.01 0.09 0.23 187487 22 -0.05 -0.00 -0.05 0.10 210762 23 -0.05 0.19 0.14 0.16 244064 24 -0.23 0.00 -0.22 0.24 559547 25 -0.22 0.25 0.03 0.13 453068 27 1.62 -0.57 1.05 0.30 1044122 28 -0.19 0.67 0.48 0.25 1149593 30 0.51 -0.93 -0.41 0.26 172064 31 0.16 -0.13 0.03 0.12 322522 32 0.10 0.00 0.10 0.16 330994 33 -0.21 0.28 0.07 0.21 716940 34 -0.23 -0.11 -0.34 0.15 1089663 35 -0.78 0.21 -0.57 0.28 1318406 36 -0.99 0.12 -0.88 0.43 1980369
> 1K errors exist in some more opaque channels
Tobin, et al., 2006
No Shift MODIS shifted
Tb
diff
(K)
AIRS-MODIS: un-shifted, shifted SRF
SRF shift may explain MODIS Calibration ErrorsShifting 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
GE
O –
LE
O (
K)
GEOs relative to AVHRR
±2 K
Matt Gunshor, UW
Calculations are used to approximately account for spectral response differences
The large telescope neededfrom GEO presents othercalibration challenges
Oct ’05 May ‘06
Filter Radiometer Summary
Order 1 K calibration issues are not uncommon(e.g. relative GEO comparisons)
Spectral uncertainty is one probable cause for uncertainty exceeding 1 K (e.g. MODIS results from AIRS)
Lack of reproducible spectral sampling from instrument to instrument is also a key issue for long-term climate records
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
AIRS14 June 2002
Calculated
NASA Aqua
AIRS Onboard Blackbody:light trap cavity design—specular surface
nadir
emissivity, > 0.999T calibration ± 0.05 K
Primary T/V referenceemissivity, > 0.9999T knowledge ± 0.03 KPRT temperature sensors
ABB Bomem
AIRS: Other key factors
ILS knowledge: thermal/vac testing with FTS source, verified with gas cell tests
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)
Non-linearity*: <0.3% over much of spectrum, < ~1% peak; error assumed < 0.05 K after correction
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 CO2 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)
Performance Estimates 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, 1985-1998)
Longwave
Midwave
Shortwave
CO2
CO
N2O
H2O
H2OCH4/N2O
CO2
O3
CharacteristicsSpectral Coverage: 3-17 microns
Spectral Resolution: 0.5 cm-1
Resolving power: 1000-6000
Footprint Diam: 1.5 km @ 15 km
Cross-Track Scan: Programmable
including uplooking zenith view
Radiances for Radiative Transfer Temp & Water Vapor Retrievals Cloud Radiative Prop. Surface Emissivity & T Trace Gas Retrievals
Applications:
NASA WB57
Scanning-HIS Radiometric Calibration BudgetTABB= 227, THBB=310, 11/16/02 Proteus
Similar to AERI description in Best, et al., CALCON 2003
TABB = 260K, THBB = 310K TABB = 227K, THBB = 310K
LW
SW
MWSW
MWLW
TABB = 260K, THBB = 310K TABB = 227K, THBB = 310K
LW
SW
MWSW
MWLW
RSS ofErrors in THBB,TABB
TRfl
HBB, ABB
+ 10% of non-linearity
correction
3-sigmaTb error < 0.3 K
for Tb >220 K
Scanning HIS: Some key factors
ILS knowledge: fundamental instrument design (only weak dependence on geometry)
Spectral Calibration: atmosphere, stability maintained by onboard HeNe laser reference(no active temperature control required)
Non-linearity: < 2.5% longwave and midwave, negligible shortwave error < ~0.2 K after correction
Polarization: <0.05% (gold scene mirror)error < 0.04 K even uncorrected
Atmospheric Spectral Calibration: S-HIS
Atmospheric CO2 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 similaratmospheric spectral
calibration
E. AIRS - Scanning HIS Comparisons
Direct AIRS radiance validation
Mean differences generally <0.2 K with small standard deviations
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 D
egre
e
AIRS Validation with UW Scanning HIS
Direct S-HIS to AIRS comparison(without accounting for spectral & viewing differences)
AIRSSHIS
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: 2002.11.21 Gulf of Mexico Validation case: 2002.11.21
(AIR
Sob
s-A
IRS
calc
)-
(S
HIS
obs-
SH
ISca
lc)
(K)
“Comparison 2” (21 November 2002) Excluding channels strongly affected by atmosphere above ER2
AIRS-SHIS Summary: SW (2004.09.07) AIRS-SHIS Summary: SW (2004.09.07)
1st Direct SW Radiance ValidationExcellent 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
Instrument Line Shape knowledge: fundamental instrument design verified with laser sources in ground testing
Spectral Calibration: atmosphere stability maintained by onboard 1.54 m diode laser reference with < 1ppm validated over 14 days*
Non-linearity: < 1% longwave, negligible mid- and short-wave error < ~0.15 K after correction*stability verifiable in orbit from out of band features
Polarization: <0.05% (gold scene mirror with overcoating)error < 0.04 K even uncorrected*
* Denis Blumstein and Thierry Phulpin, cnes
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
ILS knowledge: fundamental instrument design (only weak dependence on geometry), verified with laser source and gas cell tests
Spectral Calibration: atmosphere &/or onboard Ne source; stability maintained by T control of onboard 1.55 m diode laser reference
Non-linearity: <0.1% longwave & negligible elsewhere error < ~0.07 K even uncorrected stability verifiable in orbit from out of band features
Polarization: <0.05% (gold scene mirror)error < 0.04 K even uncorrected
CrIS Observed and Calculated Instrument Line Shape (ILS)
CO2 laser source, Center, Edge & Corner Pixels of 3x3 array (laser = 775.18765 nm, CO2laser = 942.383333 cm-1, dx=-0.7mrad, dy=0.1 mrad)
Pure sincCenter FOV5Edge FOV4
Corner FOV1
Center FOV
Edge FOV
Corner FOV
centroid (cm-1)
Obs 942.367 942.195 942.034
Calc 942.366 942.195 942.034
FWHM (cm-1)
Obs 0.747 0.757 0.767
Calc 0.751 0.759 0.767
Lfoot
Obs 0.358 0.329 0.313
Calc 0.347 0.328 0.313
RfootObs 0.347 0.326 0.311
Calc 0.345 0.329 0.313
Calculated
Observed
FTS design expectations confirmed
Observed and Calculated ILSs for Run1, FOVs 5, 4, and 1
laser = 775.18765 nm, CO2laser = 942.383333 cm-1, dx=-0.7mrad, dy=0.1 mrad
Pure sincCenter FOV5Edge FOV4
Corner FOV1
Calculated
Observed
G. Future GOES Sounders: GIFTS (FTS) &
HES (FTS or Grating)
GIFTS internal calibration concept HES specifications Added value for cross-calibration of
IR sensors on other platforms
4-d Digital Camera: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 SpectrometerGeostationary Imaging Fourier Transform SpectrometerNew Technology for Atmospheric Temperature, Moisture, Chemistry, &
Winds
GIFTS Sensor Module Technologies
GIFTS: Wrapped up for Thermal Vacuum Testing at SDL
Test Facilities where GIFTSis undergoing Thermal Vacuum testing
“MIC2” Multi-function IR Calibrator
GIFTS Chamber
Space Dynamics Lab, Utah State University
Internal Blackbody References
Blackbody apertureBlackbody aperture
Boundary Boundary of area of area seen by IR seen by IR detectorsdetectors
Red outline is flip-in Red outline is flip-in mirror seen through mirror seen through covercover
Flip-in mirror coverFlip-in mirror cover
Vis Flood SourceVis Flood Source
Specification Estimate
GIFTS Absolute Calibration-Longwave(3-sigma brightness T error at scene T)
0.00
0.20
0.40
0.60
0.80
1.00
1.20
190 210 230 250 270 290 310
Scene Temperature [K]
Bri
gh
tness T
em
pera
ture
Err
or
[K]
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)
0.00
0.20
0.40
0.60
0.80
1.00
1.20
190 210 230 250 270 290 310
Scene Temperature [K]
Bri
gh
tness T
em
pera
ture
Err
or
[K]
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 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)
Common requirements for FTS and grating: Strong attempt to limit requirements to those perceived to be achievable by both approaches.
Spectral coverage trades under consideration: Options for spectral coverage are being explored to minimize complexity, risk and cost (and performance)
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
Still using 1 K specification(although, specified in terms of a brightness T error at 300K)
We can, should, and probably will do better
Spectral calibration uncertainties are intended to not significantly inflate absolute calibration errors
Spectral Calibration Knowledge
Channel Centers need to be known very accurately (< 3 ppm)The goal should be less than 1 ppm
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
5 ppm
3 ppm
1 ppm
5 ppm
3 ppm
1 ppm
5 ppm
3 ppm
1 ppm
5 ppm
3 ppm
1 ppm
Sinc ILS
Gausian ILS
Wavenumber (cm-1)
Bri
gh
tnes
s T
emp
erat
ure
(K
)
5 ppm
3 ppm
1 ppm
5 ppm
3 ppm
1 ppm
5 ppm
3 ppm
1 ppm
5 ppm
3 ppm
1 ppm
Sinc ILS
Gausian ILS
Wavenumber (cm-1)
Bri
gh
tnes
s T
emp
erat
ure
(K
)Tb 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
GEO High Spectral Resolution as a transfer standard
General ThesisWe 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)
-0.10
-0.05
0.00
0.05
0.10
0.15
0.20
190 210 230 250 270 290 310
Scene Temperature [K]
Bri
gh
tness T
Err
or
[K]
dTh=0.056 deh=0.00072 des=0.0005 dTs=5dTtel=0.1 dCh=0.04 dCs=0.04
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