Thermal And Near infrared Sensor for carbon Observation (TANSO)
On board
the Greenhouse gases Observing SATellite (GOSAT)

Research Announcement

Appendix A

Outlines of GOSAT and TANSO Sensor

GOSAT (Greenhouse gases Observing SATellite) carries an observing instrument called
Thermal And Near infrared Sensor for carbon Observation (TANSO), which is composed of a
Fourier Transform Spectrometer (TANSO-FTS) and a Cloud and Aerosol Imager (TANSO-CAI)

A.1 Outline of GOSATGOSAT is scheduled to be launched into space by JAXA’s H-IIA rocket in early 2009
and will acquire data on the global distribution of carbon dioxide (CO2) and methane(CH4) (column abundance and altitude distribution) for at least five years.

Twoobservation sensors, TANSO-FTS and TANSO-CAI, are placed in the earth-facing (+Z)
plane of the satellite and operated by a three-axis attitude control system in such a waythat the sensors keep looking toward the geocentric direction. GOSAT secures power
supply needed for the operation of itself and the sensors on board with two sets of solar
paddle to be deployed facing towards the sun, records and playbacks the mission dataacquired by the sensors on the Mission Data Processor (MDP) subsystem, and transmits
the data down to the ground stations.
Figure A.1-1, Table A.1-1 and Table A.1-2 show an overview of GOSAT in orbit, itsmajor specifications and orbital parameters, respectively. Figure A.1-2 represents
nominal orbits of GOSAT.

Figure A.1-1 Overview of GOSAT in orbit

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Table A.1-1 Major Specifications of GOSAT
Specification items

Description

Size

Main body: H3.7m(X) × W1.8m(Y) × D2.0m(Z)(except for its extrusions)
Wing span: 13.7m

Mass

1,750 kg

Power

3.8 kw (EOL)Life time

5 years

Launch schedule

January 23, 2009

Table A.1-2 Orbital Parameters of GOSATParameters

Description

Orbit typeSun-synchronous, quasi-recurrent

Altitude against the earth

666 km at Equator

Inclination angle

98.06 deg

Orbits/day

14 + 2/3 revolutions/day

Orbits/recurrence

44 revolutions/3 days

Descending node time

13 hours ±15 minutes

Figure A.1-2 GOSAT’s nominal orbits

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A.2 Outline of GOSAT/TANSO-FTS
A.2.1 Outline of TANSO-FTS
Table A.2-1 below provides an outline of TANSO-FTS.
Table A.2-1 Outline of TANSO-FTS
Observation method

Earth-looking observation

Method ofSpectral measurement of atmospheric absorption by the Fouriertransform interferometermeasurement
Functions

1. Observes the atmosphere in visible, short wavelength infrared andthermal infrared bands looking toward the earth center.
2. Carries out observation over the land on lattice points.
3. Observes

the

same

footprint

duringone

interferogrammeasurement while the satellite is moving.
4. Multiple observations are carried out for a same footprint toimprove SNR.
5. Observes at a fixed angle (or fixed distance) interval in cross-track
direction during the lattice observations.
6. Returns to the same footprint after three days.
7. Observes sea area where sunglint is expected, using the two-axis(AT/CT) mechanism.
8. Conducts a combination of observations in lattice point, sunglint
and specific point modes.
9. Performs the following in-orbit calibrations:(1) Solar irradiance calibration in the visible and short wavelength
infrared bands and blackbody calibration in the thermal infraredband(2) Deep-space calibration in the visible-, short wavelength- andthermal-infrared bands(3) Annual lunar calibration on the sensitivity by pointing the satellite
and the two sensors to the moon
TANSO-FTS is composed of the following three units:(1) TANSO-FTS Optical Unit
(2) TANSO-FTS Control Unit
(3) TANSO-FTS Electrical Circuit Unit

Figure A.2-1 shows a block diagram of TANSO-FTS. Considering that the mission ofthis Project is to get useful results, redundancy is employed in the design of sensors
wherever possible. Figure A.2-2 shows an overview and the internal structure of the
optical unit of the sensor.

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Figure A.2-1 Block Diagram of TANSO-FTS

Figure A.2-2 Overview and Internal Layout of the Optical Unit of TANSO-FTS.

A.2.2 Specifications of TANSO-FTS(1) Fourier interferometer mechanism
Table A.2-2 summarizes the specifications of the Fourier interferometer mechanism andoutlines the scanning method.

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Table A.2-2 Outline of the specifications of the Fourier interferometer and its scanning method
Spectroscopy

Fourier interferometerNo. of ports

2 (single-pass)

Scanning method

Two corner cubes are attached to the ends of the V-shaped
swing arm. The arm swings to scan with the supported flexible
blades acting as the axis. (See Figure A.2-4.)

Data acquisition method

Scanning on both sides and data acquisition on one way

Beam splitter

ZnSe (without coating)

Sampling

Sampling by laser diodes

Signal processing

Interferogram data are transmitted down to the earth.

Figure A.2-3 illustrates the structure of the scanning mechanism. The scanning speed
stability required is 1％ or lower. The interferometer acquires interferogram on bothsides of the zero (optical) path difference (ZPD) location.
Max. path difference

ZPD

180.0

=±2.5cm

6300.0Interferometer turnaround6800.07300.07800.08300.08800.09300.0-20.0

Incoming path
Return path
Figure A.2-3 Scanning by the Fourier interferometer

Figure A.2-4 Layout of the double corner cubes, swing arm and flexible blades

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(2)Band configuration
TANSO-FTS has detectors for four bands, all of which have the same field of view(FOV). Table A.2-3 tabulates the wave number range, spectral resolution, and otherparameters for each band.
Table A.2-3 Wave number range and spectral resolution of the four bands used by FTS
Band

Band 1

Band 2

Band 3

Band 4

Band name

Visible

SWIR

SWIR

TIR

Polarimetric

Yes

Yes

Yes

No

5800 -

4800 -

observation
Wave number

12900 -1

Range

13200 cm

6400 cm-1

700 -15200cm1800 cm-1(Note 5)(Note 1)Out-of-band

TransmittanceTransmittanceTransmittanceTransmittancecharacteristics

of 0.1% or lessof 0.1% or lessof 0.1% or lessof 0.1% or less

(Note 2)in the ranges ofin the ranges ofin the ranges ofin the ranges of<12700cm-1<5000cm-1<4500cm-1<600cm-1>13400cm-1>6800cm-1>5500cm-1>3800cm-1

0.2 cm-1

0.2 cm-1

0.2 cm-1

0.2 cm-1

0.6 cm-1 or less

0.27

Spectral resolution(Note 3)
FWHM

of

the

instrument

cm-1

or

less

0.27

cm-1 or

0.27cm-1 or lesslessfunction (Note 4)
Targetsofthemeasurement(O2)

CO2, CH4, H2O

CO2, CH4,

CO2, CH4,→information

H2O,

H2O, etc.

on air pressure

information on& cirrus cloud

cirrus cloud

Note 1: 80% or above of the maximum efficiency can be achieved within the wave number
ranges of Bands 1, 2 and 3. As for Band 4, 60% or above of the maximum efficiency
can be achieved in the designated wave number range by converting the sensitivity ofthe detector into the quantum efficiency.

The efficiency here means the product of theefficiency of the optical unit, the quantum efficiency of the detector, and the response of
the amplifier.
Note 2: In the wave number range corresponding to the signal turn-around in the Fouriertransform, less than 0.01% is assumed.
Note 3: The spectral resolution is defined as (1 / (2 * Max. path difference)).

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Note 4: The full width at half maximum (FWHM) of the instrument function is defined as the
FWHM when radiation from a monochromatic light source is introduced in the full
FOV and the measured data are inverse-Fourier-transformed without apodization.
Note 5: In Band 2, the transmittance must be 0.1 or higher when the laser wavelength is
1.55μm (6460 cm-1), which is for measuring the instrument function in orbit.
(3) Methods of optics, band splitting and polarimetric observation
Table A.2-4 lists the summary of specifications for TANSO-FTS in terms of optics,
spectroscopy and polarization, whereas Figure A.2-5 provides an overview of the opticalsystem.
Table A.2-4 Summary of specifications for optics, band splitting and polarization
Effective aperture

φ68 mm (Bands 1-3), φ60 mm (Band 4)

F value

F=2 (F value for the optics-detector system to be installed
between the FTS sensor and the detector.)

Optical system

Reflective optical system. However, a refractive optical system
is used in the case of focusing on the detectors for Bands 1-3.

Aperture control

As for Bands 1-3, the corner cubes provide a control of theaperture.

Optics

While the aperture diameter is 68 mm, the movement of light
flux in association with FTS’s scanning remains within 1 mm
in the direction perpendicular to the optical path.
For Band 4, an aperture control is introduced inside the opticalsystem of the detector so as to suppress background light.
FOV control

The FOV is determined with the slit common to all bands, after
the light is modulated by the Fourier interferometer andconcentrated. (15.8 mrad: corresponds to 10.5 km when
projected on the earth’s surface.)

Band splitting

After FOV being narrowed through the slit and the light

Band splitting

collimated, the light is splitted through the dichroic filter one
after another starting with Band 1.
Removal of

A narrow band pass filter is set up at the optical system of theout-of-band lightdetector for each of Bands 1-3.

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Polarization

For Bands 1-3, the polarization beam splitter is installed in the

Polarization

optical system of the detector for simultaneous observation of
two polarizations.

光路切替機構Figure A.2-5 Overview of the optical system(4) Band-to-band/sensor-to-sensor registration
Band-to-band/sensor-to-sensor registrations are determined as follows:(1) Band-to-band registration within FTS: 0.05 pixels or less(2) Registration between TANSO-FTS and TANSO-CAI: 0.5 pixels of TANSO-CAI or
less(3) A monitoring camera (for checking the FOV of FTS) is attached at the light entrance
section of FTS for the sake of determining registration with TANSO-CAI.

A.2.3 Observation mechanism of TANSO-FTS(1) Principles of TANSO-FTS observation
The sun can be regarded as a blackbody light source which has an absolute temperatureof slightly less than 6,000 K.

It emits light mainly in the range covering from
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ultraviolet to visible and short wavelength infrared bands. Most of visible and shortwavelength infrared light emitted by the sun reaches the earth’s surface while some isabsorbed and scattered by clouds and aerosols on the way. An earth-observing satellite
detects light reflected by the ground and returned to the satellite through the atmosphere.
The measured light provides information on the concentrations of CO2, CH4, and water
vapor (H2O), whose absorption bands are in the visible and short wavelength infrared
range.
The ground surface and clouds radiate thermal infrared waves with intensities
commensurate with their temperatures and their own wavelength characteristics.

There

are numerous absorption bands of major and minor atmospheric constituents in thesebands. Each atmospheric constituent has its own absorption bands within the visible
and short wavelength infrared bands or thermal-infrared band. Though the strength ofabsorption varies with spectra of each constituent, the use of databases established inlaboratories will help identify the relationship among the wave number within theabsorption bands, absorption strength, and constituent concentration.

The basicprinciple of molecular spectroscopy goes as follows: spectroscopic observation is
performed by detecting the absorption spectra of the atmospheric molecules of interest,
the detected light is converted into electric signals (photoelectric transfer), radiance perspectrum is calculated, and the amount of atmospheric molecules is derived from theradiance level.

A space-borne observing sensor points its scanning mirror at the target to be observed,
introduces the observed light into the system, converts it into electric signals at thedetector assigned to each bandwidth through the diffraction grating, interferometer oranother types of spectrometer, and transmits electric signals onto the ground. The data
received on the ground will be analyzed to extract necessary information, which willthen be transformed into spectral data, based on which the volume of each atmosphericconstituent is calculated.
(2) Principles of the Fourier interferometer
The Fourier interferometer first splits the light with its beam splitter (BS) into two withdifferent optical path lengths, which interfere with each other. It acquires the spectra of
the light source by inverse-Fourier-transforming the interferograms obtained whilechanging continuously the path length difference.
This method is characterized by the following two factors:(1) High gain of the lightA-9

(2) Acquisition of spectra over a wide wavelength range at a high spectral resolution
The Fourier interferometer has come into practical use owing to the development ofadvanced computers, which has made inverse Fourier transform faster, andhigher-accuracy mechanical scanning by means of laser range finders. Figure A.2-6
shows a diagram describing the principles of the Michelson interferometer.

Typically,

M1 and M2 are a fixed mirror and a moving mirror, respectively.

In case of

TANSO-FTS, however, M1 and M2 are installed on a single swinging arm so that thetwo mirrors move by the same distance with opposite phases. As a result, the path
difference is doubled, making it possible to scan faster. Thus, the interferometer canachieve both a high spectral resolution and a high spatial resolution without sacrificing
one or the other.

M1: Fixed mirror

Light source

M2: Moving mirror
When the moving distance is d,
the path difference is 2d.

Detector

Figure A.2-6 Principles of Michelson Interferometer
Further, space-borne interferometers are designed with attention paid to the vibration
environment while being launched on rockets and the thermal environment in outerspace.
The Fourier interferometer acquires signals called interferogram as shown in Figure
A.2-7 below.

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Radiance
signalofDistance traveled by the movable mirror
Figure A.2-7 Interferogram data
When this interferogram is inverse-Fourier-transformed, the spectra absorbed in the air
are obtained, as shown in Figure A.2-8. The spectra depicted here are those of thesunlight absorbed by H2O, CO2, and CH4 in the atmosphere as observed by the GOSAT
ground test model in November 2005.

CH4 absorption

H2O absorption

Intensity
CO2 absorption

Wavelength [µm]Figure A.2-8 Atmospheric scattering spectra observed by the GOSAT groundtest model at the top of Mt. Tsukuba in November 2005
Since the satellite can acquire observation data fairly frequently, the amounts ofatmospheric constituents can be derived from the above spectral data and plotted into a
global distribution diagram, as shown in Figure A.2-9.
example of global distribution data provided by satellites.

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There has been no such

Figure A.2-9 Global distribution of carbon dioxide (CO2)
The global distribution of net CO2 flux, as shown in Figure A.2-10, can also be derived
from the satellite data referred to above, and by using atmospheric inverse transportmodels.

←SourcegC/m2/day

Sink→

Figure A.2-10 Global distribution of net CO2 flux
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Furthermore, a global distribution of CO2 can also be obtained, as shown in Figure
A.2-11 in three-dimensional (3D) image, using the above net CO2 flux distribution andatmospheric transport models.
January

August← Low density

High density →Figure A.2-11 3D global distribution of CO2 obtained based on
the net CO2 flux distribution using atmospheric transport models(3) Observation by GOSAT
In case of GOSAT, Figure A.2-12, GOSAT observes visible and SWIR radiation of thesun reflected by the earth’s atmosphere and by the earth surface and thermal radiation in
TIR band from the earth surface and the atmosphere.

【TIR】
Figure A.2-12 Observation by GOSAT
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TANSO-FTS observes the light reflected by the earth in the range covering from visible
0.76 μm to thermal infrared 4.3 μm. (See Figure A.2-13.) The vertical distribution of
air temperature is measured in TIR bands, whereas the air pressure can be obtained fromthe absorption by oxygen molecules (O2) and the air column abundance is derived byreferring to the absorption by O2, which has much higher concentration remaining
constant than CO2.

GOSAT Spectral Coverage

250

Band 1

Band 220

100

Spectral Radiance（W/str/m2/micron）

200

10

50
0
0.758

O2
0.763

0.768

CO2

0
1.57

0.773

CH4
1.62

1.67

150
Band 3
10

100

CO2

5
0
1.98

50

2.03

Band 4

2.08

CO2
O3

CH4

0
0

3

6

9

12

15

Wavelength (micron)Figure A.2-13 Observation band of TANSO-FTS and air absorption bands

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A.3 Overview of GOSAT/TANSO-CAI
A.3.1 Overview of TANSO-CAI
Table A.3-1 below explains the purposes of the TANSO-CAI sensor.

Purpose 1

Table A.3-1 Purposes of TANSO-CAI
Assessment of theDetermines whether data should be discarded or not in

effectiveness ofcase there is a thick cloud. It is desirable that the cloud

FTS footprint area

thickness can be assessed in addition to the cloudPurpose 2

coverage.
Detection of clouds

Provides information necessary for correcting the errors

and aerosols in FTS

caused by clouds and aerosols in FTS footprint area.

footprint area and

Evaluates the characteristics of cloud and aerosol (e.g.

correction of

optical thickness, type). Thus, it is desirable that theresulting errors

spectral characteristics of aerosol can be derived.

TANSO-CAI is expected to derive the types and optical thickness of aerosol of each type.
To this end, the bands selected for CAI observation must be those where there is noabsorption by atmospheric dominant constituents, and, consequently where the best
signal-to-noise ratio (SNR) is attained, and where the spectral characteristics of the optical
thickness of aerosols can be observed. In addition, scattering by aerosols is significantly
polarized, in general; thus polarimetric observation is preferred.
However, as
TANSO-CAI observes at a fixed angle very close to the nadir, the degree of polarization isquite low, which makes it difficult to estimate the aerosol amount. Therefore, morestress is put on securing enough number of bands than polarimetric observation inchoosing the bands.
Given these considerations, the four bands shown in
Table A.3-2 were selected for TANSO-CAI.

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Table A.3-2 Observation bandwidth of TANSO-CAI and selection criteria
Criteria
Band 1 (0.380 μm)

No O3 absorption in the ultraviolet range wherereflectance is low on the ground

Band 2 (0.674 μm)

No interference between the rise of reflectance on thevegetation and absorption by O2 B and H2O bands.

Band 3 (0.870 μm)

No interference with absorption by H2OBand 4 (1.60 μm)

The maximum wavelength width is achieved while
avoiding the absorption by H2O, provided that theabsorption by CO2 and CH4 can be corrected.

At thesame time, the band should be free of any impact ofpossible effect on the detector cut-off due to temperaturefluctuation.
TANSO-CAI is composed of the following two units.
(1) TANSO-CAI Optical Unit
(2) TANSO-CAI Electrical Circuit Unit
Figure A.3-1 is a block diagram of TANSO-CAI.

Figure A.3-1 Block diagram of TANSO-CAI
An overview of TANSO-CAI is given in Figure A.3-2 below.

Figure A.3-2 Overview of TANSO-CAI

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A.3.2 Specifications of TANSO-CAI(1) Bandwidth and performance
Table A.3-3 below summarizes the specifications of TANSO-CAI.
Table A.3-3 Specifications of TANSO-CAI
Band 1

Band 2

Band 3

Band 40.380 ± 0.005

0.674 ± 0.005

0.870 ± 0.005

1.60 ± 0.01

<0.02

<0.02

<0.02

<0.10

Out-of-band

1％ or lower in the

1％ or lower in the

N.A.

1％ or lower incharacteristics (i)ranges of <0.36 andranges of <0.658

the(μm)(Note 2)>0.4

and >0.692<1.0 and >1.69

Out-of-band

N/A

1％ or lower in the

1％ or lower inrange of >0.696

the ranges ofCenterwavelength(μｍ)(Note 1)
Wavelength width(μｍ)(Note 1)characteristics (ii)(μm)(Note 2)<0.840ranges

of

N/A

and

>0.890Out-of-band

0.15％ or lower incharacteristics (iii)the range of >0.45

N/A

N/A

N/A

N/A

N/A

N/A

(μm)(Note 3)
Out-of-band

0.03％ or lower incharacteristics (iv)the range of >0.7(μm)(Note 3)
Polarization

None

Note 1: The center wavelength and wavelength width are specified based on the first andsecond moments, taking into consideration the spectral characteristics of the
optical system, filters and detectors for an overall efficiency.
Note 2: The sensitivities in the range specified in ‘out-of band (ⅰ)’ should be lower than
1% of that at the central wave length, and the sensitivity in the range specified in‘out-of-band (ⅱ)’ should be lower than 0.1% of that at the central wave length inorder to avoid the H2O absorption band in the above described bands.
Note 3: The sensitivity in the range specified in ‘out-of-band (iii)’ must be 0.15% or lower
and in the range specified in ‘out-of-band (ⅳ)’ 0.03% or lower, of that at thecorresponding center wavelength.
(2) Instantaneous FOV and the look angle in the cross-track direction
The instantaneous FOV is set at 500 m (in Bands 1, 2, 3) and 1.5 km (in Band 4) at thenadir point. The look angle in the cross-track direction is set at ±35 degrees, with which

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the sensor can observe the entire globe during daylight hours in three days except band 4.
Figure A.3-3 below illustrates the geometries of the instantaneous FOV and the normal
FOV.

【Band 4】

【Bands 1, 2, 3】

Figure A.3-3 Instantaneous and normal FOV of TANSO-CAI

A.3.3 Observation mechanism of TANSO-CAIThe column abundances of CO2 and CH4 are derived from the amount absorbed in the
optical path between the sun, the ground surface and the satellite.

The path length isobtained through the amount of absorption for the 0.76 μｍ band by O2 whose density is
known, with TANSO-FTS.

As cloud or aerosol on the path affect the effective path

length, correction is needed if there is any.

In order to improve the accuracy ofcorrecting errors caused by clouds and aerosols, electronic scanning imagers withmultiple bands covering ultraviolet, visible and short wavelength infrared bands are
installed, which also make it possible to observe cloud coverage and aerosols over theland and ocean areas. The FOV in the cross-track direction is used for ascertaining the
spatial distribution of aerosols over a wide range (1,000 km).
Registration between TANSO-FTS and TANSO-CAI data should be performed after thelaunch using the FOV check camera installed in the TANSO-FTS sensor. Figure A.3-4
shows a sample of CAI observation data.

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Figure A.3-4 A CAI observation image (equivalent to Band 2)
generated based on MODIS data

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