GOSAT (Greenhouse gases Observing Satellite)

GOSAT is a JAXA mission within the GCOM (Global Change Observation Mission) program of Japan. The GOSAT mission goals call for the study of the transport mechanisms of greenhouse gases such as carbon dioxide (CO₂) and methane (CH₄).

The emphasis is on atmospheric monitoring to clarify the sources and sinks of CO₂ on a sub-continental scale. The overall mission objective is to contribute to environmental administration by estimating the Green House Gases (GHGs) source and sink on a sub-continental scale and to support the Kyoto protocol that was adsorbed at COP3/UNFCCC (3rd session of the conference in the framework of climate change) in 1997. The protocol calls for a reduction of greenhouse gases, in particular CO₂; it requires all parties to reduce their emissions by 5% below the level of the year 1990, for the period of 2008-2012. Specific GOSAT objectives are: ^{1) 2) 3) 4) 5) 6) 7) 8)}

• Observation of the CO₂ and CH₄ column density (CH₄ column density during orbital nighttime):

- at a spatial scale of 100-1000 km

- with relative accuracy of 1% for CO₂ (4ppmv, 3 month average) and 2% for CH₄

- during the Kyoto Protocol's first commitment period (2008 to 2012).

• Reduction of CO₂ annual flux estimation errors by half (0.54GtC/yr to 0.27GtC/yr) in identifying the greenhouse gas source and sink at subcontinental scale with the data obtained by GOSAT in conjunction with that from the ground-based instruments.

The mission priority is on:

- Short wave infrared observation

- CO₂ and CH₄ column density (during the orbital day time)

Secondary mission goals are:

- Thermal infrared observation

- CO₂ and CH₄ altitude profile

- CO₂ and CH₄ CH₄ column density (during orbital night time)

- Observation of other trace gases (O₃, etc.)

- Provision of other products (temperature profile, Earth radiation)

GOSAT is a joint project of JAXA (Japan Aerospace Exploration Agency) and NIES (National Institute of Environmental Studies) with instrument development/funding by Japan's Ministry of the Environment (MOE). In this arrangement, JAXA is responsible for the satellite and instrument development, launch and operation of the spacecraft (including data acquisition), while NIES is in charge of data analysis (algorithm development) and utilization.

Figure 1: Artist's rendition of the GOSAT spacecraft (image credit: JAXA)

Spacecraft:

The spacecraft bus is three-axis stabilized with a structure size of 2.4 m x 2.6 m x 3.7 m. The AOCS (Attitude & Orbit Control Subsystem) is based on a zero-momentum design, attitude is sensed by Earth sensors, star trackers and a GPS receiver. The EPS (Electrical Power Subsystem) uses a 50 V unregulated bus, the solar panels are of rigid padpole design with 4 kW of power (EOL), and 4 pairs of NiCd batteries with energy of 35 Ah for solar eclipse operations. The overall S/C mass is about 1750 kg with a payload mass of 391 kg. The overall design life is 5 years. The spacecraft is being manufactured by Mitsubishi Electric Corporation of Tokyo as the prime contractor of GOSAT.

Table 1: Some spacecraft parameters

Figure 2: Line drawing of the GOSAT spacecraft (image credit: JAXA)

Figure 3: Illustration of the GOSAT spacecraft (image credit: JAXA)

Launch: The launch of GOSAT is scheduled for August of 2008 on a JAXA launcher (H-IIA vehicle). The secondary payload on this flight is SDS-1 (Small Demonstration Satellite-1) of JAXA. The launch site is the Tanegashima Space Center, Japan.

Orbit: Sun-synchronous circular orbit, altitude = 666 km, inclination = 98º, revisit cycle of 3 days, LTAN (Local Time at Ascending Node) at 13:00 hours.

RF communications: The downlink is provided in X-band (8 GHz) with a data rate of 120 Mbit/s. The TT&C data link is not DRTS compatible (2 kbit/s uplink, 30 kbit/s downlink). Science data is received and level-0 processed at JAXA/EOC (Earth Observation Center) in Hatoyama, Japan.

Sensor complement: (TANSO-FTS; TANSO-CAI)

TANSO-FTS (Thermal And Near infrared Sensor for carbon Observation - Fourier Transform Spectrometer). TANSO-FTS features high optical throughput, fine spectral resolution, and a wide spectral coverage (from VIS to TIR in four bands). The reflective radiative energy is covered by the VIS and SWIR (Shortwave Infrared) ranges, while the emissive portion of radiation from Earth's surface and the atmosphere is covered by the MWIR (Midwave Infrared) and TIR (Thermal Infrared) ranges. These spectra include the absorption lines of greenhouse gases such as carbon dioxide (CO₂) and methane (CH₄). ^{9) 10) 11) 12) 13) 14) 15) 16) 17) 18)}

Figure 4: Schematic of the SWIR and MWIR/TIR radiative transfer in Earth's atmosphere (image credit: JAXA)

Figure 5 illustrates the spectral coverage and absorption lines of GOSAT observations. From these spectral data, CO₂, CH₄, and ozone (O₃), which are major GHG (Greenhouse Gases), are observed. The column density of CO₂ is mainly retrieved from the 1.6 µm region absorption lines, of which intensities are less temperature-dependent and not interfered by other molecules. The oxygen (O₂) A band absorption at 0.76 µm is being used to estimate the effective optical path length.

Figure 5: Spectral coverage of TANSO-FTS bands (image credit: JAXA)

The polarization of the scene flux is also acquired by measuring the P and S polarization simultaneously. The path radiance (P) is highly polarized while the surface reflected radiance (S) is less polarized as shown in Figure 6. In addition, as the instrument itself has the polarization sensitivity, the radiative transfer of SWIR is well defined by measuring and characterizing polarization.

Figure 6: SWIR range polarization schematic (image credit: JAXA)

The instrument was built by the ABB Bomem Remote Sensing Group of Quebec City, Canada, a Swiss-Swedish electrical engineering company under contract to NEC Toshiba Space Systems. The TANSO-FTS design employs a nadir-viewing instrument to monitor the greenhouse gases in the troposphere (the troposphere happens to be the main atmospheric layer in which the greenhouse effect is taking place) - and the nadir-viewing monitoring concept is considered the best scheme feasible to measure the radiative flux in the troposphere. The observation geometry is illustrated in Figure 7. The TANSO-FTS instrument has a mass of 250 kg, power consumption of 310 W, size: 1.2 m x 1.1 m x 0.7 m.

Figure 7: Illustration of the observation geometry (image credit: JAXA)

Table 2: Specification of TANSO-FTS (Greenhouse Gases Sensor)

The main TANSO-FTS elements are: scanning/pointing mechanism, relay optics, FTS, and detector arrays in the focal plane. A single FTS configuration was chosen with a beamsplitter capable of covering the required wide spectral range. The instrument employs a dual-pass flexible blade Michelson FTS (Fourier Transform Spectrometer) design as well as a diode laser sampling system to reduce the instrument size and mass. FTS is a double pendulum type interferometer with two corner cube reflectors. The maximum optical path difference of 2.5 cm provides an unapodized spectral resolution of 0.2 cm^-1 across a wide spectral range going from 0.75 - 15 µm with a ZnSe beam splitter and a fully redundant 1.31 µm DFB (Distributed Feedback) laser. A photoconductive (PC) HgCdTe sandwich detector (also referred to as MCT) in the MWIR/TIR ranges and a pulse-tube cooler provide high linearity and low-noise level performance. The TANSO interferometer accommodates an optical beam of more then 70 mm in diameter to provide the high throughput needed for Earth observation. The scan arm motion is induced by a voice coil actuator driven by a sophisticated control algorithm. The TANSO interferometer design uses well-proven technologies; it benefits from the space heritage of the ACE-FTS instrument operating onboard the Canadian SciSat-1 mission since February 2004.

The overall concept design/performance and operational scenarios of TANSO-FTS were verified with a BBM (Breadboard Model) instrument version, flown in an aircraft demonstration series (completion of test flights in May 2003).

The number of cross-track observation points is variable and can be selected in such a way as to satisfy the SNR and spatial resolution requirements. FTS employs a dichroic filter to be able to observe all spectral bands for all observation points.

Figure 8: Illustration of the TANSO interferometer (image credit: ABB, JAXA)

Figure 9: TANSO-FTS instrument (image credit: JAXA)

Figure 10: TANSO-FTS instrument components (image credit: JAXA)

Figure 11: TANSO-FTS optics and polarization (image credit: JAXA)

On the ground, the FTS interferograms are being transformed into spectra (which include the absorption spectra of GHGs) using FFT (Fast Fourier Transform) algorithms. The global GHG source-and-sink characteristics on a sub-continental scale are being retrieved from the global GHG distribution data with a chemical transfer model.

Figure 12: Overall data flow concept of the TANSO-FTS instrument (image credit: JAXA)

TANSO-CAI (Thermal And Near infrared Sensor for carbon Observation - Cloud and Aerosol Imager). TANSO-CAI is a radiometer in the spectral ranges of ultraviolet (UV), visible, and SWIR to correct cloud and aerosol interference. The imager has continuous spatial coverage, a wider field of view, and higher spatial resolution than the FTS in order to detect the aerosol spatial distribution and cloud coverage. Using the multispectral bands, the spectral characteristics of the aerosol scattering can be retrieved together with optical thickness. In addition, the UV-band range observations provide the aerosol data over land. With the FTS spectra, imager data, and the retrieval algorithm to remove cloud and aerosol contamination, the column density of the gases can be the column density of the gases can be retrieved with an accuracy of 1%.

Table 3: Specification of the TANSO-CAI instrument

Figure 13: Schematic of the TANSO-CAI instrument structure (image credit: JAXA)

TANSO instrument operations:

During the daytime period of the orbit both SWIR and MWIR/TIR of the FTS and the imager data are acquired; during the nighttime passage, only FTS MWIR/TIR data is acquired. At sunrise, the direct sunlight is introduced into the FTS through the spectralon diffuser plates for SWIR radiance calibration. Two diffusers with different exposure times are being used to correct the long-term diffuser degradation. In addition, the 1.55 µm diode laser light is introduced through the diffuser plate into the FTS to calibrate the instrument function onboard. The pointing mechanism views the deep space and inner blackbody periodically for the zero level and MWIR/TIR radiance calibration. ¹⁹⁾

Lunar calibration is achieved by rotating the spacecraft into the direction of the moon. This provides a stable calibration reference for both instruments. Lunar calibration is considered once per year.

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