GOSAT (Greenhouse gases Observing Satellite) / Ibuki
GOSAT (nickname Ibuki meaning “breath” or “puff”) 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) 9) 10)
• 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: Overview of organizations and function allocation in the GOSAT project (image credit: JAXA)
Figure 2: Artist's rendition of the deployed GOSAT spacecraft in orbit (image credit: JAXA)
The spacecraft bus is three-axis stabilized with a structure size of 2.0 m (length) x 1.8 m width) x 3.7 m height). The structure consists of the mission module in which the mission sensors (payload) are loaded and the bus module containing the bus components. The mission module and the bus module (CFRP cylinder) can be separated so that the assembly is performed easily. The mission module consists of the honeycomb panel reinforced by CFRP on the surface.
• The AOCS (Attitude & Orbit Control Subsystem) is based on a zero-momentum design, attitude is sensed by Earth sensors, star trackers, IRU (Inertial Reference Unit), and a GPS receiver. Actuation is provided by a RWA (Reaction Wheel Assembly) and by MTQ (Magnetic Torquers).
AOCS consists of the AOCE (Attitude and Orbit Control Electronics), the IRU (Inertial Reference Unit), the FSSA (Fine Sun Sensor Assembly), ESA (Earth Sensor Assembly), the GPSR (Global Positioning System Receiver), STT (STar Tracker), the RWA (Reaction Wheel Assembly), the VDE (Valve Drive Electronics), and the MTQE (Magnetic TorQuer Drive Electronics). Figure 3 presents a block diagram of the AOCS. 11)
Figure 3: Block diagram of the GOSAT AOCS (image credit: JAXA, MELCO)
• The EPS (Electrical Power Subsystem) uses a 50 V unregulated bus, the solar panels are of rigid padpole design with 3.8 kW of power (EOL), and 4 pairs of NiCd batteries with energy of 35 Ah for solar eclipse operations (note: the NiCd batteries have flight heritage).
• The PDL (Paddle Subsystem) consists of two paddle wings, deployment mechanisms, paddle drive mechanisms. The solar array paddles are folded and attached at the side panels of the satellite by the hold and deploy mechanisms during the launch phase. The two paddle wings are deployed by the ordnance controller. The length of the paddle wing is about 6 m from the attachment to the tip. One wing generates over 2.0 kW at the end of mission life (EOL) with the condition that the sunlight is normal to the paddle surface. The power needed to drive the bus subsystems is generated by one wing, and partial observation of the mission sensors is possible even if one paddle wing fails.
• The RCS (Reaction Control Subsystem) is a monopropellant hydrazine blowdown system. RCS consists of 2 tanks of 550 mm diameter, four 20 N thrusters ,eight 1 N thrusters, tubes, pressure sensors, filters and valves. If a thruster of the four 1 N thrusters fails, AOCE (Attitude and Orbit Control Electronics) switches the control thrusters to the other four 1 N thrusters automatically.
• MDHS (Mission Data Handling Subsystem). The data from mission sensors is multiplexed by MDHS, recorded in a memory, and send to DT subsystem. The memory size of MDHS is 48 GByte.
• DT (Direct Transmission Subsystem). The data from MDHS is modulated at the X-band modulator, and converted to RF signal. And it is amplified at the XSSPA and transmitted to the ground station.
• TTC Telemetry Tracking and Command Subsystem). TTC consists of TTC-RF and TTC-DH. It receives the command from the ground station, demodulates and distributes to each subsystem. It gathers telemetry data from each subsystem, edits, records and transmits to the ground station. It also has the autonomous function and increase the flexibility of the operation.
• The TCS (Thermal Control Subsystem) maintains the temperature of the satellite at moderate temperature range for the each component. Thermal control is performed passively using heat pipes, MLI and OSR, and performed actively using a heater controlled thermostat.
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 MELCO (Mitsubishi Electric Corporation), Kamakura Works, Japan as the prime contractor of GOSAT. 12) 13) 14)
Table 1: Some spacecraft parameters
Figure 4: Illustration of the deployed GOSAT spacecraft (image credit: JAXA) 15)
Figure 5: Illustration of the GOSAT spacecraft in launch configuration (image credit: JAXA)
Spacecraft environment survey equipment:
Use of CAMs (Monitor Cameras): A total of 8 CAMs are being accommodated at strategic locations on GOSAT with the objective to monitor the spacecraft exterior in orbit. The CAMs are capable of capturing clear images during the eclipse using LED (Light-Emitting Diode) light sources. The images acquired by the CAMs are being used to grasp the satellite status accurately (e.g. deployment status of the solar array paddles, contamination during the rocket fairing separation event); they also play important roles by responding to anomalies promptly. 16)
Figure 6: Illustration of a CAM device (image credit: JAXA)
TEDA (Technical Data Acquisition Equipment). TEDA is onboard space environment measurement system with the objective to monitor the orbital radiation environment. TEDA consists of four LPT1-4 (Light Particle Telescope) assemblies and one HIT (Heavy Ion Telescope) device. LPT discriminates electrons, protons, and alpha particles and analyzes their quantitative energy, while HIT characterizes the fluxes and energy distributions of heavy ions having masses from that of helium (He) to iron (Fe). 17)
Figure 7: Flight models the the TEDA devices (image credit: JAXA)
JAXA experienced unfortunate failures of the solar array paddle and the power system on the Earth observing satellites ADEOS and ADEOS-II in 1997 and 2003, respectively. As GOSAT has been the 'first satellite' that JAXA initiated its development following these setbacks, the policy of 'achieving high reliability' was designated as the 'first priority' for the design and development phases of GOSAT.
To achieve high reliability, the GOSAT project adopted the following policies for its development:
- Maximum utilization of flight-proven components
- Elimination of single-point failure probabilities with the provision of functional redundancy
- Thorough tests of the mission duty cycles.
Figure 8: Allocation of the TEDA devices on GOSAT (image credit: JAXA)
Launch: The launch of GOSAT took place on January 23, 2009 on a JAXA launcher (H-IIA vehicle). The launch site is the Yoshinobu Launch Complex at the Tanegashima Space Center, Kagoshima, Japan (launch provider: Mitsubishi Heavy Industries, Ltd.). The seven secondary payloads on this flight are: 18) 19)
- SDS-1 (Small Demonstration Satellite-1) of JAXA (~100 kg)
- SOHLA-1 (Space Oriented Higashiosaka Leading Association-1), Japan (50 kg)
- SpriteSat (Tohoku University), Japan (microsatellite of ~50 kg)
- PRISM (Picosatellite for Remote?sensing and Innovative Space Missions) of ISSL of the University of Tokyo, 5 kg
- Kagakaki (SORUNSat-1), Japan, 20 kg
- KKS-1 (Kouku Kosen Satellite-1) of Tokyo Metropolitan College of Industrial Engineering), nanosatellite of 3 kg
- STARS-1 (Space Tethered autonomous Robotic Satellite-1) of Kagawa University, Japan, ~ 10 kg.
Figure 9: Schematic view of the secondary payloads (image credit: JAXA)
Orbit: Sun-synchronous circular orbit, altitude = 666 km, inclination = 98º, revisit cycle of 3 days, LTAN (Local Time at Ascending Node) at 13:00 ± 0.15 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. Another acquisition station is Svalbard (Spitzbergen, Norway). 20)
• Summer 2013: The GOSAT spacecraft and its payload are operating nominally (four years on orbit as of January 23, 2013). TANSO-FTS, the main instrument of GOSAT, has been continuously measuring CO2 and CH4 distributions globally every three days. The data of the two payloads, TANSO-FTS and TANSO-CAI, are processed by JAXA to Level 1 products. Their higher level products are processed by NIES (National Institute for Environmental Studies) and distributed to researchers and general users through GUIG (GOSAT User Interface Gateway). 21) 22)
• December 2012: The GOSAT data of global CO2 fluxes on a monthly and regional basis for the one-year period between June 2009 and May 2010 is now being distributed publicly. These flux values were estimated from ground-based CO2 monitoring data and improved GOSAT-based CO2 concentration data. 23)
• Fall 2012: The GOSAT spacecraft and its payload are operating nominally.
Figure 10: Monthly Global Map (Aug. 2012) of the CO2 column-averaged volume mixing ratios in 2.5º x 2.5º mesh (image credit: NIES) 24)
- Vicarious calibration field campaign with in-situ measurements and aircraft under-flight of GOSAT collaborated with NASA ACOS (Atmospheric CO2 Observations from Space) research.
- Intercomparison attempt with other TIR sensors of IASI and AIRS
- Annual degradation monitoring at uniform desert sites.
• June 2012: OCO/GOSAT collaboration.
Figure 11: GOSAT mission schedule (image credit: JAXA) 28)
Figure 12: The CAI instrument image was acquired on Nov. 4, 2011 when Ibuki flew over Thailand and Cambodia (image credit: NIES)
Legend to Figure 12: The water is shown in blue, terrestrial vegetation in red, and clouds in white. The white line is the contour line of mean sea level.
• October 2011: Using observational data from GOSAT and ground-based data, the estimation of monthly regional CO2 sources and sinks (net fluxes) and their uncertainty was carried out. It was demonstrated that the CO2 concentration data retrieved from GOSAT soundings can reduce the uncertainty of fluxes estimated from ground-based data alone. 30)
• Using GOSAT data, a research team from JPL, Germany, and Japan has shown that it is possible to pick up this fluorescent glow from space over the entire planet, and thereby infer details about the health and activity of vegetation on the ground. Terrestrial GPP (Gross Primary Production) constitutes the largest flux component in the global carbon budget, however significant uncertainties remain in GPP estimates and its seasonality. 31)
The fluorescence signal can be measured from space using high resolution spectra covering Fraunhofer lines (narrow absorption features in the solar spectrum) in the 660–800 nm range. By measuring the fractional depth of these lines, Fs can be accurately estimated, independent of scattering and albedo effects. For the retrieval of steady?state solar induced chlorophyll fluorescence, the project used radiance spectra measured in the red spectral range between 756–759 nm and also 770.5–774.5 nm, recorded by the TANSO instrument. The solar? induced fluorescence signal Fs was retrieved using an iterative least squares fitting technique. A unique and critical step in the data processing is the correction of an observed zero?level offset in acquired GOSAT O2 A-band spectra. Without correction, the offset strongly biases Fs because its impact on Fraunhofer line depth is indistinguishable from fluorescence.
After correction, the annual average of Fs clearly reveals the contrast between highly active vegetation and barren or snow?covered surfaces (Figure 13 a). Fluorescence maxima appear over tropical evergreen forests as well as the eastern United States followed by Asia and central Europe. Overall, the global map of chlorophyll fluorescence also captures many small?scale features such as enhanced signal in southeastern Australia or the comparatively low values of the Iberian Peninsula. The temporal evolution of fluorescence is of particular interest because the seasonal variation of atmospheric carbon dioxide is dominated by the seasonality of GPP and respiration. The research team observed a pronounced seasonal cycle in the northern hemisphere as well as seasonal shifts in the location of maximum fluorescence in the tropics (Figure 13 b). The southern hemisphere, conversely, exhibits a far smaller seasonal variability.
Figure 13: (a) Annual average (June 2009 through May 2010) of retrieved chlorophyll?a fluorescence at 755 nm on a 2° x 2° grid. Only grid?boxes with more than 15 soundings constituting the average are displayed. (b) Latitudinal monthly averages of chlorophyll fluorescence from June 2009 through end of August 2010 (image credit: JPL, JAXA, Ref. 31)
• The GOSAT mission is operating nominally in the summer of 2011. The overall functions and performances are good and well within design objectives. The radiometric response has been carefully monitored and calibrated. In addition, proper corrections have been applied for the level 1 processing. 32)
Figure 14: Overview of the GOSAT data process flow (image credit: JAXA)
On-orbit operation: Grid observation is a nominal operation mode for TANSO-FTS. In parallel, sun glint over the ocean and target observations such as validation points, mega cities, power plants, are inserted between nominal grid observations. Every day, an observation plan with a series of time and pointing angles is uploaded from the ground.
The operation cycle lasts 12 days with 3 patterns together with grid observations:
- Pattern A, no sun glint and no target observations except for limited validation sites
- Pattern B, sun glint and target observations requested by research announcement users
- Pattern C, sun glint and limited validation sites.
Each pattern continues for 3 days and the order is patterns A, B, C, and B. Once a month, the back side of the solar diffuser plate, the analog circuit electric, and the ILSF (Instrument Line Shape Function) calibrations are being performed.
Figure 15: GOSAT long-term operation schedule and data distribution (image credit: JAXA, Ref. 32)
• The GOSAT/Ibuki mission is operational in 2011 (two years on orbit as of January 23, 2011).
Acquired data are provided to research and public users with applying the Level 1 and 2 processing. The algorithm for Level 1 and 2 are also being updated during this phase, in order to improve the data quality for precise measurement of CO2 and CH4. - After the launch, the on-orbit characterization of performance, calibration, and health monitoring of TANSO has been continuously conducted for the updating of the Level-1 and -2 processing algorithms. - During the over one-year operational period, a few undesirable anomalies were noticed on the measurement data which are: 33) 34)
1) Pointing instability during acquisition:
2) Shift of ZPD (Zero Path Difference) position: The turnaround time is a loss of observation time which has to be minimized. During turnarounds, both FTS and pointing mechanisms are activated at the same time. Micro vibrations caused by the pointing mirror motion affect the laser fringe count. Sometimes the controller misses laser fringes at the turnaround position and ZPD shifts gradually (Ref. 32).
3) Degradation of sampling laser intensity
4) Offset of pointing positions.
Unfortunately, some of these anomalies affect the data qualities. To minimize the degradation of the data quality, counter operation techniques are applied as well as the additional quality assessments (quality flags are set).
• In Feb. 2010, JAXA completed the initial validation of the concentration of carbon dioxide (CO2) and methane (CH4) based on the analysis results of observation data in the clear region taken by GOSAT/Ibuki. Accordingly, JAXA started to provide the above results (Level 2 products of CO2 and CH4 column densities) as well as information on cloud covering to the general public on February 18, 2010. 35)
• Oct. 30, 2009: Recently, an initial calibration of Level 1 data products, radiance spectrum and images observed by IBUKI, has been completed and the project will begin to release them to general users.
Note: The greenhouse gases are regularly observed with 286 ground observation stations from 59 countries (Figure 16) and the data is distributed worldwide through World Data Center for Greenhouse Gases (WDCGG) which is operated by JMA (Japan Meteorological Agency) and WMO (World Meteorological Organization). 37)
Figure 16: Distribution of WDCGG observation points (image credit: JAXA)
• In late May 2009, the GOSAT mission is operating in an initial calibration and validation mode. The initial calibration of the mounted sensors, as well as tuning of the computer processing system, is underway at JAXA and at NIES (National Institute for Environmental Studies). 38)
• The TEDA instrument on GOSAT has been sending its observation data since its turn on 31 January 2009. The initial observed data of TEDA shows reasonable agreement with model predictions. GOSAT TEDA is expected to reveal the temporal and spatial structure of space radiation environment in detail in its planned five years mission (Ref.17).
• In April/May 2009, GOSAT/Ibuki is in an initial calibration and validation mode. A preliminary analysis of clear-sky carbon dioxide and methane column averaged dry air mole fraction over land was performed for the period April 20-28, 2009. These carbon dioxide and methane data show a hemispheric gradient, with largest values in the Northern Hemisphere, broadly in agreement with existing ground-based measurements. 39) 40)
• “First light” light of the sensors TANSO-FTS and TANSO-CAI occurred on Feb. 7, 2009 during the course of an initial functional checkup when the instruments were activated and observed some regions over Japan. 41) 42)
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). 43) 44) 45) 46) 47) 48) 49) 50) 51) 52) 53) 54)
Figure 17: Schematic of the SWIR and MWIR/TIR radiative transfer in Earth's atmosphere (image credit: JAXA)
Figure 18 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 18: 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 19. In addition, as the instrument itself has the polarization sensitivity, the radiative transfer of SWIR is well defined by measuring and characterizing polarization.
Figure 19: 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 20. 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 20: 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 cryocooler 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 21: Schematic view of the TANSO interferometer (image credit: ABB, JAXA)
Figure 22: Photo of the TANSO interferometer (image credit: ABB, JAXA)
Figure 23: TANSO-FTS instrument (image credit: JAXA)
Figure 24: TANSO-FTS instrument components (image credit: JAXA)
Figure 25: 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 26: 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
TANSO-CAI consists of two units, CAI-OPT (Optical unit) and CAI-EL (Electronics unit). CAI-OPT is a radiometer with collecting optics, linear array detectors, preamps and analog to digital converters. CAI-EL is almost the same as FTS-EL.
Figure 27: 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 TANSO-FTS and the TANSO-CAI 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. 55)
The TANSO operation on orbit during daytime is illustrated in Figure 28. The FTS normally observes by separate pointings in the cross-track direction with 800 km swath by the pointing mirror. The CAI 3 bands cover the FTS observation swath with 1000 km for cloud detection within the FTS field of view, while only band 4 is slightly narrow. The FTS observations are combined with normal observation of 5 points cross-track with acquisition of on-orbit calibration data, sunglint observation over the ocean, and target observations for calibration and validation sites and tracking of large cities and vicinities. Figure 28 shows the observation pattern during the daytime on 5 June 2010. The red shows the normal grid observation. The continuous green over the ocean shows the sunglint observation to look at strong shining ocean areas. The target observations are operated over China and the U.S. east coast colored in green. 56)
Figure 28: TANSO operation with FTS and CAI. The FTS observations consist of grid observations globally, sunglint observations over the ocean, and target observations over calibration, validation points and large cities (image credit: JAXA)
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.
The TANSO-CAI instrument features less on-board calibration than the FTS interferometer. The sensitivity and stability against time and temperature variations were characterized in pre-flight tests. On-board calibration uses a blackbody and nightside observations. The lunar observation will be operated once a year with particular pixels at full moon. 57)
Figure 29: Schematic view TANSO on-orbit observations (image credit: JAXA)
Opportunities for coordinated CO2 observations from GOSAT and OCO
GOSAT of JAXA and OCO (Orbiting Carbon Observatory) of NASA are the first two satellites designed to make global measurements of atmospheric carbon dioxide (CO2) with the precision and sampling needed identify and monitor surface sources and sinks of this important greenhouse gas. Because the operational phases of the OCO and GOSAT missions overlap in time, there are numerous opportunities for comparing and combining the data from these two satellites to improve our understanding of the natural processes and human activities that control the atmospheric CO2 and it variability over time. 58)
Comparisons of GOSAT and OCO measurement approaches: GOSAT retrieves XCO2 from the same CO2 and O2 absorption bands used by OCO, but uses a high resolution Fourier transform spectrometer (TANSO-FTS) rather than a grating spectrometer to make its measurements. An independent Cloud and Aerosol Imager (TANSO-CAI) is used to identify cloudy scenes. The grating and FTS techniques both offer unique advantages for this application. For example, TANSO-FTS provides greater spectral coverage and slightly higher spectral resolution, while the OCO instrument provides greater spatial resolution and slightly higher signal-to-noise ratios in each sounding. Comparisons of XCO2 retrievals from these two measurement techniques could help to identify and correct subtle measurement biases that might otherwise be missed.
Combining the OCO and GOSAT datasets would benefit the carbon cycle science community by increasing the spatial coverage and decreasing the interval between observations by either satellite, alone. To combine these datasets without introducing biases, the OCO and GOSAT measurements must be validated by a common measurement standard. Fortunately, the OCO and GOSAT mission plans are quite synergistic, providing numerous opportunities for cross validation.
Table 4: Overview of some mission parameters of GOSAT and OCO
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.