Tropospheric Infrared Mapping Spectrometers (TIMS) for ... · Cell Absorption Spectra vs Model 0.55...

Paper B3P3

Tropospheric Infrared Mapping Spectrometers (TIMS) for Improved Vertical Resolution Measurements of

Carbon Monoxide in the Lower Troposphere

A E Roche, J B Kumer, R L Rairden, J L MergenthalerLockheed Martin Advanced Technology Center, Palo Alto, CA

R. B.ChatfieldNASA Ames Research Center

R. Blatherwick, T. HawatDenver University

Our IIP 2006-2008 project* involves the design, performance, and application to atmospheric CO spectroscopy, of dual infrared grating spectrometers operating near 2.33 μm (Solar Reflective) and 4.68 μm (Thermal Emissive) regions. Both use 2-D detector arrays to provide simultaneous spectral and spatial information at ~0.25 and ~0.53 cm-1 resolution, respectively

Outline of This Presentation• Motivation and Approach• Spectrometer Design• Radiometric/Spectral Calibration and Performance Assessment• Ground-based Atmospheric Observations and Model Comparisons • Concept for Space Borne Instrument to Monitor Boundary Layer and Higher-Layer Tropospheric CO, and other air quality constituents

*Tropospheric Infrared Mapping Spectrometers (TIMS) for CO: PI J.B.Kumer, LMATC

MotivationGlobal air quality monitoring is a major theme for the next generation earth observing satellites. Two recent surveys* delineate needs and key measurement attributes:

Measurement of O3, CO, NOx, HCHO, SO2, VOCs and Aerosols

Sufficient Vertical Resolution (1- 3 km) to monitor the transport and chemical transformation of constituents from the boundary layer to the stratosphere, and identify geographical origin of pollutants and their contribution to regional air quality

High Horizontal resolution (2-5 km) and coverage, to pinpoint emission sources, conduct precise regional area transport tracking, and see through small cloud-free regions* Community Workshop on Air Quality Remote Sensing from Space; Defining an Optimum Observation Strategy. NCAR, Boulder CO, 2006

* Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond: National Research Council , 2007

Instrumental ApproachThe vertical resolution needed for boundary layer information for gases such as CO, O3, HCHO, CH4, with spectral features in the IR, implies high spectral resolution & high signal to noise.

These spectral/ spatial/radiometric requirements can be met by combining a large – format 2D array with a fixed grating:• Row pixels provide spectral information, column pixels provide simultaneous spatial information; integration time is maximized• The many small footprints along the slit and dispersive directions enable high spectral/horizontal resolution, and wide spatial swath

We focus here on tropospheric COAs shown later, a combination of emissive spectra near 4.68 μm and reflective spectra near 2.33 μm provides good boundary layer and higher layer information

Key instrument requirements for this combination are: Δν ≤ 0.5 cm-1 at 4.68 and ≤ 0.2 cm-1 at 2.33 μm NESR ≤ 2x10-10 w/cm2/sr/cm-1 at 4.6 μm, 5x10-10 at 2.3 μm

Solar Reflective Region (λc=2.33 μm) Emissive Region (λc=4.7 μm)

Potential for Near -Surface Carbon Monoxide MeasurementUsing Combined IR Emission and Solar Reflective Spectra

2.3 0.99CO: 6-22 km

4.3 0.97CO: 2-6 km

8.4 0.89CO: 0-2 km

Retrieval AπPrecision(%)

Parameter

2.5 0.99CO: 6-22 km

6.7 0.93CO: 2-6 km

24 0.11CO: 0-2 km

Retrieval AπPrecision(%)

Parameter

Daytime(Emissive+Reflective) PrecisionNightime (Emissive only) Precision

Spectrometer Design, Calibration, andPerformance

The Basic Technique: MWIR

Column 1λ=4.62 μ2163. cm-1

Column 1024λ=4.75 μ2105 cm-1

Total Array; Dispersion 58 cm-1Dispersion per Pixel: .06 cm-1

1024 columns

Echelle Grating87 g/mmUsed in 4th order

48.7 deg

IncidentRay

78KTemperature105Well Capacity (e)

Dark Current (e/s) at 78K, 0.5V Bias

<60 CDSRead Noise (e rms)

0.95 μShort Wave Cuton

5.2 μLong Wave cutoff

~55%QE (78K)

18.5 μPixel Pitch

1024x1024Format


~ 50Dark Current (e/s) at 78K, 0.5V Bias

<60 CDSRead Noise (e rms)


μLong Wave cutoff

QE (78K)

18.5 μPixel Pitch

1024x1024Format

HAWAII HgCdTe MBE Array

FPA

Spectral

Spatial

MWIR Spectrometer Optical Layout

Dewar Vacuum Shell36 cm LN2 Cold Plate

Radiationshields

Warm ObjectiveLens

View of MWIR Components From Above.

Grating

LittrowMirror

FPA

Camera Control

Power

ObjectiveLens

LN2 Dewar

Fully assembled MWIR Spectrometer with Electronics

36 cm

The Basic Technique: VSWIR

Column 200

4272 cm-1

Column 701λ=2.322 μ4307 cm-1

Total Array; Dispersion 35 cm-1Dispersion per Pixel: .03 cm-1

1024 columns

Echelle Grating31.6 g/mmUsed in 26th order

69.1 deg

IncidentRay


Dark Current (e/s) at 78K, 0.5V Bias

Read Noise (e rms)


2.5 μLong Wave cutoff

~74%QE (78K)

18.5 μPixel Pitch

1024x512Format


< 1Dark Current (e/s) at 78K, 0.5V Bias

< 25 CDSRead Noise (e rms)


μLong Wave cutoff

QE (78K)

18.5 μPixel Pitch

1024xFormat

HAWAII HgCdTe PACE Array

FPA

Spectral

Spatialλ=2.341 μ

HAWAII 1024x512HgCdTe array

Littrow Grating SpectrometerAssembly

Objective OpticsAssembly

LN2 Cold Plate

Dewar Window

ColdFilter

LN2 Dewar

VSWIR Spectrometer Layout

Completed VSWIR spectrometer channel, cover installed

ObjectiveOptics Barrel

Spectrometer Assembly

50 cm

LN2 Dewar

Fully Assembled VSWIR Spectrometer

TABLE 1-1 TIMS MODULE SPECTRAL PARAMETERS

Spectral Range Spectral

Resolution Aperture FOVMeasurement Products

Primary SecondaryMWIR 2105–2163 cm-1

4.75–4.62 μm0.53 cm-1 3.9 cm Along Slit: 10.4 deg

Pixel: 0.18 mradCO, H2O,

Clouds

VSWIR 4272–4307 cm-1

2.34–2.325μm0.25 cm-1 3.9 cm Along Slit: 4.9 deg

Pixel: 0.17 mradCO, H2O,

CH4, Clouds,

Instrument Optical Parameters and Measurement Products

0.8

0.84

0.88

0.92

0.96

1

4276428142864291429643014306

Cell Absorption Spectra vs Model

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

2110211521202125213021352140214521502155

TIMS 150torrMODEL 150t0rr/ 0.55 cm-1

4.65 micron 4.69 micron

4.7 μm MWIR Array

1024

1024

2.33 μm VSWIR Array

512

2155 2145 2135 2125 2115 Cm-1

Cm-1

CO Cell Absorption Spectra vs Model

645 torr

Spectral Calibration Using CO Cell Absorption

150 torr

D Δν~0.53 cm-1

Δν~0.25 cm-1

645 torr

TIMS 4.6 μm Radiometric Calibration vs Model

0

200

400

600

800

1000

1200

1400

1600

1800

0.E+001.E-072.E-073.E-074.E-075.E-076.E-077.E-078.E-07Radiance (W/cm2/sr/cm-1)

Cou

nts

0

50

100

150

200

250

300

350

Sour

ce T

empe

ratu

re (K

)

TemperatureQ3 Counts

Responsivity (W/cm2/sr/count)Measured: 5.03E-10Modelled: 4.46E-10

Model ( +/1 20% Systematic Error)

295

265

NESR (Radiance/√Hz)Measured: 7.5E-9Modelled: 6.3 ±2E-9

4.68 μm VSWIR Radiometric Calibration vs Model

0

1000

2000

3000

4000

5000

6000

0.E+00 2.E-07 4.E-07 6.E-07 8.E-07 1.E-06 1.E-06 1.E-06

Counts

Source Radiance (w/cm2/sr/cm-1)

Data

Model (± 20%)

546K521K496K

Model Measured Responsivity (w/cm2/sr/cm-1/count) 2.15 ±0.5 E-10 2.03 E-10NESR (w/cm2/sr/cm-1/√Hz) 5.5±2 E-9 4.4 E-9

2.33 μm VSWIR Radiometric Calibration vs Model

Ground Measurement Campaign at Denver University, May 2008

TIMS Co-Located with DU BrukerFourier Transform Spectrometer

May 2008 TIMS Measurement Campaign at Denver University

Objectives

Using TIMS 2.33 μm and 4.68 μm modules acquireradiometrically and spectrally calibrated solar absorption andsky emission spectra, fromwhich first order retrieval of atmospheric CO and other gases can be derived with a goal of DFS~ 2 (for CO)

Using TIMS 2.33 μm module, acquire spectra from terrainscattering as a simulation of nadir-looking Satellite data and to investigate effects of varying albedo on S/N

Concurrently with the TIMS measurements acquire Bruker FTSsolar absorption spectra at 2.33 and 4.68 μm, for use in TIMS spectral characterization and for correlative CO retrievals

MWIR Δν ~ 0.53 cm-1

VSWIR Δn ~ 0.27 cm-1

20 x 20 km footprint

• These represent what we’ll expect to see in the joint field tests with DU• The zero albedo case corresponds to the exclusive use of the MWIR• The addition of the VSWIR increases the total amount of vertical

information and extends it to higher altitudes

8.53 km

0.61 km

6.71 km

University of Denver, May 2008: MWIR 4.68 μm on deck

MWIR zenith mirror, shaded from direct sunlight. CO cell is in front of objective lens.

Remote control mirror rotates FOV intoBlack Body calibrator

MWIR dewar assembly mounted on cart. ( White cloth on top is the sun shade. )

VSWIR Heliostat

VSWIR viewing solar scattering surface in Bruker FTS laboratory

Solar Beam Relayed In to Laboratory From External Tracking Heliostat

The external optics for the VSWIR are more sensitive to temperature variations than the MWIR and it was operated inside the temperature-controlled laboratory

MWIR Flat Fielding and Radiometric Calibration: May 30

Cm-1

Cm-1

Cm-1

Responsivity Function

Raw Sky Signal

Flat Fielded/Calibrated Emission Spectrum

CO CO

H2O H2O

Video Image of ArrayViewing the Zenith Sky

212021252130213521402145215021552160

Wavenumber (CM-1)

0

20

40

60

80

100

120

140

160

Nw

/cm

2/sr

/cm

-1

TIMS MWIR Zenith Sky Radiance

FTIR 0.4 cm-1 Solar Absorption

H2OH2O

H2O

COCO

CO

COCO2

TIMS MWIR Emission Spectrum Overlaid with FTS Absorption Spectrum

H2O

CO

First Order Fit to MWIR Spectra*

*S. Desousa-Machado/L .Strow UMBC

VSWIR Flat Fielding and Radiometric Calibration: May 30 Data

Flat fielded/calibrated absorption spectrum

Video Image of VSWIR array observing diffusesolar scattering surface

CH4 CH4/H2O

Using a similar processTo that described for theMWIR for responsivity flat fielding, and using a shutterfor background subtraction

Spectrum Taken at this position

Wires over slit

428342874291429542994303

Wavenumber (CM-1)

0.1

0.15

0.2

0.25

0.3

0.35

Nw

/cm

2/sr

/cm

-1

TIMS VSWIR Solar Absorption

FTIR Solar Absorption

CH4

CH4CH4

CH4CH4

CH4

H2O

H2O/CH4

H2O/CH4

CO

CO CO

TIMS VSWIR Absorption Spectra Overlaid with FTS Spectra

FTIR Δν~ 0.2 cm-1

10 km20304050

Mt Evans

May 31 view from Denver University physics balcony

Mt Evans, 58 km

approx slit location4 degrees tall

VSWIR Mountain Viewing (1)

0.5

0.6

0.7

0.8

0.9

0 100 200 300 400 500 600 700 800

nearer mountain distant snow

Spectral profiles viewing front range and distant snowcaps

VSWIR image data, flat-field approximated

VSWIR Mountain viewing (2)

0.0E+00

2.0E-08

4.0E-08

6.0E-08

8.0E-08

1.0E-07

1.2E-07

1.4E-07

4276428142864291429643014306

Nearer Mountainmodel including Ch4, H2O & CO

First order Modeling of VSWIR Mountain Spectra

Concept for Application of TIMS Technology to Space-Borne Air

Quality Monitoring from GEO

Separate Arrays

Collimated Region

Dichroic Beam splitters inserted in collimatedBundle

Re Imaging Optics and Filters

Single Grating,Different orders

Band 1 Band 2

TIMS 2.33 mm Design provides an accessible collimated bundleto facilitate beam-splitting to multiplechannels

Spectrometer

Spectral region (cm-1) λ/Δλ Target Constituents

a S1 b

4277 to 4313 [~2.33 μm]

19500 CO Profile; CH4 & H2O columns

3036 to 3058 [~3.28 μm]

19000 O3 Column

a

S2 b c

2760 to 2840 [~3.58 μm]

8500 HCHO column; O3 H2O, CH4, N2O column

2112 to 2160 [ 4.68 μm]

8000 CO Profile (In combination with S1-1) Some H2O and O3 vertical information

1035 to 1069 [9.51 μm]

7500 O3 Partial Columns

Expanded Capability TIMS for a GEO Mission

Primary Tropospheric Science Products

Aggregated Footprint

Retrieved Precision

CO Profile

12x12 km Layer

0-2 km: 10%

2-6 km 6%

6-22 km 4%

HCHO Column 2.6x1015 mole/cm2

O3 Profile

Layer

0-6 km 5%

6-12 km 3%

12-22 km 2%

CH4, H2O, N2O Columns Several %

6x6km to 12x12 km

6x6km to 12x12 km

6x6km to 12x12 km

4.86 projected degrees (PD)

9.48 PD

11.79 PD

9.48 PD

4.86 projected degrees (PD)

9.48 PD

11.79 PD

9.48 PD

S2

S1

Two MirrorScanner

TelescopePrimaryMirror

TIMSSpectrometersEarth

Scene

Secondary

1Px=3 km

1 EW Step=6 km

3072 km1024 Px

Projected ground footprintsFrom Geo Orbit

1024 spectral Pxν

Slit Projection Dispersive direction

3 E-W Scan Blocks Covers 50N to 45S of North And South America in 1 hr, including calibration , as required by the GEO-CAPE

Spatial/Temporal Coverage for a GEO-TIMS Instrument Concept

Applications of TIMS-IIP Technology to Space Borne Air Quality Monitoring

Global Atmospheric Composition Mission (GACM)[Team members include JPL/NASA GSFC, KNMI, LMATC]

Boundary-Layer Ozone Measurements from GEO[Team members include NASA ARC/GSFC, LMATC]

TIMS Technology is included in proposed payloads for several Air-Qualitymission studies:

Summary and Conclusions

• Demonstrated High Spectral Resolution, Low-Noise Capability of Coupled Grating-Large Format 2-D Arrays in the SWIR and MWIR

• Ground-Based atmospheric absorption and emission spectra showwell isolated lines of CO, H2O, CH4, amenable to retrieving all species

Analysis of May campaign data underway, expect to retrieve 2-layer CO information, showing the advantage of combined solar-reflective and thermal emission measurements for low–troposphere sounding

Developing engineering concepts for space-borne application

• The total vertical information is substantially improved from ~ 2.32 to 2.91 for instrument goal spectral resolutions







2.7 km

+ 8.5 km7.53km

2 km

Benefit of Higher Spectral Resolution on DFS for Space View

Backup

A

B

Sensor

VSWIR comparison of nadir and upwards looking viewing upper panel vertical shows vertical pathslower panel shows slant paths

MWIR comparison of nadir and upwards looking viewing upper panel vertical shows vertical pathslower panel shows slant paths

A

B

Sensor

Illustration of the approach for evaluation of sensitivity and performance parameters (2 of 2)

Pixel statistics were measured across 100 consecutive images at each of seven blackbody source temperatures: 20 C, 100 C, 150 C, 200 C, 240 C, 270 C, and 300 C. Average values are plotted vsvariance for a single pixel, for a 100x100 pixel subarray, and for the entire 512x1024 pixel image. The slope provides a statistical measure of detector electrons per count.

Statistics of 100 consecutive images,series at various blackbody source tempertures

0

2000

4000

6000

8000

10000

12000

14000

0 500 1000 1500 2000 2500 3000standard deviation squared

sign

al, c

ount

s

full image averages

subarray averages

single pixel (452, 451)

slope = 6.2

Dec07 – jan08 slides

Instrument noise is dominated by temperature model uncertaintyThe VSWIR (α=0.10) is necessary to get info near the surface; The TIMS space borne CO DFS* of ~ 2.33 compares with ~ 1.5 for MOPITT, AIRS,TES, using only the MWIR, and ~1.0 for SCIAMACHY using only VSWIREven the ground-based TIMS can achieve DFS ~ 2.2




Nightime Only (4.68 μm; α=0) Day Time (4.68/2.33 μm; α=0.1)

2.7 km

8.5 km

1 km

6 km

1 km

*Degrees of Freedom for Signal

Sky Spectral Radiance Calibration. When viewing the sky, the observed signal counts SCi on a pixel Pxi is given by: Sky counts =SCi=[SNi]x[Wi]x[R] + [MNi]x[Wi]x[(1-R)] +ICi where:

• SNi is the sky spectral radiance on Pxi, at νi cm-1 and is the quantity we wish to derive; it is in watts/cm2/sr/cm-1

• Wi is the instrument end-end responsivity for Pxi at νI cm-1 and is determined independently • R is the known reflectivity of the sky view mirror, 1-R its emissivity • MNi is the sky view mirror radiance on Pxi at νI cm-1; it’s a function of temperature (TMN) and emissivity • ICi is the total instrument “background”counts due to radiance from all sources other than the sky

radiance, including any internal radiance leaks. To derive SNi , we place an extended blackbody of known temperature (TB) and emissivity (eB) directly in the instrument field of view and observe counts BCi on Pxi given by: Blackbody Radiance Counts = BCi=[BNi]x[(eB)]x[Wi] +ICi where:

• BNi is the blackbody source radiance on Pxi at νi, cm-1 temperature TB, emissivity eB

The sky radiance is then derived by subtracting the observed sky-view counts from the observed blackbody source counts to give: SNi ={ [BNi]x[(eB)] - [MNi]x[(1-R)] – [BCi - SCi]/[Wi]}/R This process subtracts the instrument background counts and flat-fields the response to sky-view radiance.




• The addition of 4.3 μm spectrometer reduces noise due to temperature uncertainty and improves the retrieval to the extent that 3 plus pieces of vertical information are obtained

0.21

0.26

0.31

0.36

0.41

4280.04285.04290.04295.04300.0

aidans FF 10am30may 08

scaled bruker 0.270 wv#

0.23

0.28

0.33

0.38

0.43

4280.04285.04290.04295.04300.0

aidans FF 10am30may 08scaled bruker 0.162 wv#

TIMS Scaled Bruker 0.27 cm-1

TIMS Scaled Bruker 0.16 cm-1

Cm-1

Preliminary Analysis of VSWIR Solar AbsorptionSpectra; Data acquiredMay 30, 2008 in Denver

VSWIR overlaid by Brukerrunning at 0.27 cm-1 FWHM

VSWIR overlaid by Brukerrunning at 0.16 cm-1 FWHM

CH4

CH4 CH4

H2O/CH4

CO

Tropospheric Infrared Mapping Spectrometers (TIMS) for ... · Cell Absorption Spectra vs Model 0.55...

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