2005 March 15Oxford Spring School 1 The EOS AURA Tropospheric Emission Spectrometer (TES) 1:...

2005 March 15 Oxford Spring School 1

The EOS AURA Tropospheric Emission Spectrometer (TES)1: Principles & Techniques

Reinhard Beer

Science DivisionThe Jet Propulsion Laboratory

California Institute of TechnologyPasadena, CA 91109, USA

[email protected]


Recommended Texts

“Remote Sensing by Fourier Transform Spectrometry”, Reinhard Beer, John Wiley & Sons Inc., New York (1992)

“The Fast Fourier Transform”, E. Oran Brigham, Prentice-Hall Inc., New Jersey (1974)

Both texts are somewhat dated, but still serve as good introductions.


Global measurements of tropospheric ozone and its precursors from TES combined with in-situ data and model predictions will address the following key questions:

How is the increasing ozone abundance in the troposphere affecting

The TES Experiment

- air quality on a global scale?

- climate change?

- oxidizing reactions that “cleanse” the atmosphere?


Product NameRequired

Sensitivity*

Level 1A Interferograms N LLevel 1B Spectral Radiances N LAtmospheric Temperature Profile N L 0.5KSurface Skin Temperature N 0.5KLand Surface Emissivity** N 0.01

Ozone (O3 )VMR Profile N L 1 - 20 ppbv

Water Vapor (H2O) VMR Profile N L 1 - 200 ppmv

Carbon Monoxide (CO) VMR Profile N L 3 - 6 ppbv

Methane (CH4) VMR Profile N L 14 ppbv

Nitrogen Dioxide (NO2) VMR Profile L 15 - 25 pptv

Nitric Acid (HNO3) VMR Profile L 1 - 10 pptv

Nitrous Oxide (N2O) VMR Profile N L Control***

*** Tropospheric concentration known

Product Source

* Sensitivity range maps to expected concentration range** Water emissivities are known and are therefore input , not output, parameters

TES Standard ProductsN = Nadir, L = Limb Viewing


The TES Instrument

TES is an infrared imaging Fourier Transform Spectrometer (FTS) operating in the spectral range 650 – 3050 cm-1 (roughly 3.3 – 15.4 m).

It features 4 1x16 element optically-conjugated focal plane arrays each optimized for a different spectral region and operating at a temperature of 63K using mechanical refrigerators. In addition, each focal plane is equipped with interchangeable cooled filters that limit the instantaneous spectral bandwidths to about 250 cm-1. This provides needed control over the instrument thermal background and reduces the data rate.

Except for two external mirrors (part of the pointing system), the entire optical path is radiatively-cooled to about 165K, further to reduce the instrument background.

A particular feature of TES is that it can, unlike any other spaceborne FTS instrument, observe both downwards and sideways to the limb.


Earth

Atmosphere


First Principles


The Principle of Superpositionelectromagnetic amplitudes are additive

Imagine 2 wavetrains of the same amplitude and frequency:

Phase in Wavelengths

0 1 2 3 4

Am

pli

tud

e

+A

0

-A


Let the wavetrains be a A x1 2 co s( ) and a A x2 2

2 co s( )

Upon addition, we get a a a A x x A x x x x 1 2 1 2 1 2 1 2

2 2 2[co s( co s( ] co s( { } ) co s( { } )

Setting x x1 2

and dropping the subscripts we end with

a A x 2 2 2co s( co s( { / } )

which represents a wavetrain of the same frequency but phase shifted by /2 (second cosine term) and an amplitude (first cosine term) that depends on the phase difference. Observe that the resultant amplitude can vary between 2A and zero.


Spectrometric Units

The units most commonly used in spectrometry are the nanometre (1 nm = 10-9 metres), the micrometre (1 m = 10-6 metres) and the reciprocal centimetre (cm-1).

For reasons that will become clear, it is this latter unit that is most commonly used in infrared spectrometry.

CONVERSION FACTORS:

1 cm-1 = (Frequency in Hertz)/(Velocity of light) = 107/(wavelength in nanometres)

= 104/(wavelength in micrometres)

Occasionally, one will see the cm-1 referred to as the wavenumber.

This is a description, NOT A UNIT!


Two-Beam Interference

Recall the expression a A x 2 2 2co s( co s( { / } )

The intensity is therefore I a A 2 2 24 co s (

Replacing A2 by Iin, the incident intensity of each wavetrain, we get

I I Iin in

4 1 22co s ( [ co s( ]

It is evident that I will alternate between 4Iin and 0 as changes through an amount 1 /

These two extreme conditions are termed constructive anddestructive interference, respectively (although any intermediate condition is equally possible).


Source

Input Frequency

Detector

M oving ArmVelocity V cm /sec

Condenser

Collim ator

PRINCIPLE OF THE MICHELSON INTERFEROMETER

cm -1Beam

splitte

r

Output Frequency = 2.V. Hz


K

I

J

d

Geometric Equivalent of a M ichelson Interferometer

M irror 1 Im age of M irror 2Beam 2

Beam 1 The geometric path differencebetween beams 1 & 2 is IJ + JK

where

IJ+JK = d/cos( +d.cos(2./cos( = 2.d.cos()

This expression can also be described by a number of wavelengths: n = 2.d.cos().If the interferometer is embedded in a medium of refractive index , this becomes n = 2..d.cos().

As stated earlier, we prefer to work in a frequency unit = 1/ so, finally, we get

n = 2...d.cos()

which is the fundamental equation for most types of interferometer.


Spectral Resolution (1)

Spectral Resolution is a measure of the ability of the spectrometer to discriminate between adjacent spectral features.

As we shall see, the spectral resolution of a Fourier Transform Spectrometer (henceforth FTS) is inversely proportional to the maximum path difference employed.

It must, however, be recognized that improved spectral resolution comes at the price of reduced signal-to-noise ratio, so the maximum path difference employed must be carefully “tuned” to the specific problem to be addressed. In short – enough but not too much!


Normalized Shapes of Doppler & Lorentz Lines

Normalized Frequency, ( )/

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

No

rmal

ized

Ab

sorp

tio

n C

oef

fici

ent

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Doppler

Lorentz


The 1976 US Standard Atmosphere Temperature Profile.Zones important to TES are indicated

Temperature, kelvins

210 215 220 225 230 235 240 245 250 255 260 265 270 275 280 285 290

Alt

itu

de,

km

0

5

10

15

20

25

30

35

40

45

50

Troposphere

Tropopause

Stratosphere


Weak Line Full-Width-at-Half-Maximum, cm-1

0.0001 0.001 0.01 0.1 1

Alt

itu

de,

km

0

5

10

15

20

25

30

35

40

45

50

Range for typicalLorentz lines

Range for typical Doppler

lines @ 1000 cm-1

TE

S L

imb

Vie

win

g

TE

S N

adir

Vie

win

g

Variation of Typical Linewidths with Altitude


Spectral Resolution (2)Now understanding what is desirable for spectral resolution, how does this apply to an FTS? Remembering an earlier equation for the monochromatic case:

I I Iin in

4 1 22co s ( [ co s( ]

we can readily change it (and some notation) for the broad-band case:

I x F d F x d( ) ( ) ( ) co s( )

0 02

The first integral is simply the area under the spectrum (i.e., is an additive constant that can be ignored). The second integral will be recognized as the cosine Fourier Transform of F ( )

Now, Fourier Transforms are defined over infinite ranges in both domains. We overcome the problem in the frequency domain through the use of optical filters. That is, we forceoutside some range In the path difference (x) domain, it is a different story.

F ( ) 0

Because we can only change path difference (scan) over some finite range Xmax (in either sense, or both), this is mathematically equivalent to multiplying the “infinite” interferogram by a boxcar function that = 1 over the scanned range and is zero elsewhere.

Now, a multiplication in interferogram space is identical to a convolution in spectrum space. The Fourier Transform of a boxcar is the well-known “sinc” (= sin[x]/x) function which therefore serves to define the spectral resolution of an ideal FTS.


The sinc (= sin(x)/x) function

Frequency offset in cm-1 * Maximum path difference in cm

-5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Am

plit

ud

e

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0


Reality


View of the TES engineering model interferometer retroreflector


2A3 1A1 1A2


TES Instrumental Line Shape (ILS)CO2 Laser 0001 - 0200 P(20) line (nominally @ 1046.8543 cm-1)

Model prediction

Frequency, cm-1

1046.6 1046.7 1046.8 1046.9 1047.0 1047.1 1047.2

Rel

ativ

e am

plit

ud

e

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0181 cm-1


+ Z

+ X

+ Y

A u ra S p a cecra ft C o o rd in a te S y stem

T E S

L im b V iew -6 4 .2 o

S p a ce V iew -7 2 o

~ 3 5 k mG eo id S u r fa ce

N a d ir

+ 4 5 In -tra cko

-45 In -trac ko

-45 C ross -tr ack

o

+ 4 5 C ross -tr ack

o

T E S

F ligh t P a th ~ 3 5 0 k m

1 6 k m a im p o in t

TES Observation Geometry


TES Operating Modes

Global Survey: 16 orbits of nadir & limb observations repeated every other day. This is the source of Standard Products.

Stare: Point at a specific latitude & longitude for up to ~4 minutes.

Transect: Point at a set of contiguous latitudes & longitudes to cover ~850 km.

Step-&-Stare: Point at nadir for 4 seconds (5.2 seconds with necessary reset). Spacecraft has moved ~40 km. Point at nadir again. Repeat indefinitely.

Limb Drag: Point at the trailing limb (16 second scans). Repeat indefinitely.

These last 4 modes constitute Special Products that are obtained only when no Global Survey is scheduled.


TES Step & Stare, Stare and Transect Modes

x x

STEP & STARE: A set of nadir footprints spaced about 35 km apart. Can be indefinite.

STARE: Point at a specific latitude & longitude for up to 210 seconds.

TRANSECT: A set of exactly contiguous latitudes & longitudes in a line up to 885 km long.


•Fourier transform spectrometer Fourier transform spectrometer •Wavelength response: 3.3 to 15.4 micronWavelength response: 3.3 to 15.4 micron•One scan every 4 or 16 sec. (0.1 cmOne scan every 4 or 16 sec. (0.1 cm-1-1 or 0.025 cm or 0.025 cm-1-1 res.) res.)•Four optically-conjugated 1x16 pixel detector arraysFour optically-conjugated 1x16 pixel detector arrays•Spatial resolution of 5 x 8 km at nadir & 2.3 km at limbSpatial resolution of 5 x 8 km at nadir & 2.3 km at limb•Passively cooledPassively cooled•2-axis gimbaled pointing mirror2-axis gimbaled pointing mirror

TES specifications


TES

Nadir View

Limb View

TES on the Aura Spacecraft


AURA Launch; 2004 July 15, 03:02 PDT


Level 1A: Produces geolocated interferograms.

TES Algorithm Overview

Level 1B: Produces radiometrically and frequency calibrated spectra with NESR.

Level 2: Produces VMR and temperature profiles.

Level 3: Produces global maps.


CONCLUSIONS

TES is fulfilling its promise to provide the first-ever global overview of the key constituents of tropospheric chemistry

and their inter-regional transport

For more information:

http://tes.jpl.nasa.gov

2005 March 15Oxford Spring School 1 The EOS AURA Tropospheric Emission Spectrometer (TES) 1:...

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