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