|
|
I.INTRODUCTIONIn January of 2009, GOSAT which is the first satellite dedicated to observing greenhouse gases, that is Carbon Dioxide and Methane was launched. GOSAT stands for Greenhouse Gases Observing Satellite. GOSAT is loaded with two mission instruments which are called TANSO which stands for Thermal and Near Infrared Sensor for Carbon Observation altogether, and one of TANSO is Fourier Transform Spectrometer (FTS) which is the main instrument to observe the carobon dioxide and methane and another one is Cloud and Aerosol Imager (CAI) whose data are used to compensate the FTS data. GOSAT has been operated on orbit over five and half years and has accomplished the accuracy targets. Therefore it has provided a lot of useful scientific data sets to users and interesting articles for carbon source and sink evaluation have been produced and published. The data acquired globally are believed to be useful for the research about the global phenomena, and these results have been supporting to well understanding of carbon cycle. Currently, the importance of space-based carbon observation has been approved and desired the continuous observation in toward On the other hand, we learned through the GOSAT operation that the data accuracy is not enough for the political use such as the global warming countermeasures and what we should improve in the following mission to meet political requirements regarding the measurement instruments, processing algorithm and so on. Our experiences regarding observation performances as well as hardware design were reflected on the mission requirements on GOSAT-2. The requirements on GOSAT-2 observation are improvements of the observation performance of the FTS such as signal to noise ratio, increase of the number of the useful data and addition of the carbon monoxide observation band for a good understanding of CO2 and CH4 sources and sinks to elucidate the carbon cycle more precisely. The cloud and aerosol imager, CAI, data on GOSAT have been used to compensate the FTS data for the aerosol and to detect clouds. And the CAI-2 for GOSAT-2 will have the additional observation channels to reinforce the aerosol observation abilities and observes the air pollution. Based on the feasibility studies, the hardware system requirements were defined and the design was started. Table 1 shows the principal specification of GOSAT and GOSAT-2. GOSAT-2 is now under development and the specifications are tentative right now. Table 1.Specifications of GOSAT and GOSAT-2
II.THE FOURIER TRANSFORM SPECTROMETER(FTS) ON GOSAT:A.Outline of the FTS on GOSATThe FTS on GOSAT has three spectral bands in the SWIR, ShortWave InfraRed, region and one spectral band in the TIR, Thermal InfraRed, region, and measure the SWIR radiance reflected from the earth’s surface with two linear polarized light and TIR radiated by the ground and the atmosphere. The wavelength range of the three bands in SWIR, band 1,2 and 3, are 0.758 to 0.775μm, 1.56 to 1.72 μm and 1.92 to 2.08μm, respectively, and the one of the TIR, band 4, is 5.56 to 14.3 μm. The sampling interval of the interferogram is 0,2cm-1, and the spectral resolution, defined as the full width of half maximum (FWHM) of the instrument line shape(ILS), is lower than 0.6cm-1 for band 1 and lower than 0.27cm-1 for band two to four. The concentrations of CO2 and CH4, that is XCO2 and XCH4 are mainly retrieved from 1.6μm and 2.0μm band coupled with O2A band (0.76μm). Table 2 shows the major specifications of the TANSO-FTS. Table 2.the major specifications of TANSO-FTS
The measurement targets of the GOSAT is to observe CO2 and CH4 column density with relative accuracy of 1 % for CO2 and 2 % for CH4 at 1,000 km square spatial scale and in 3 months average. B.Details of the FTS on GOSATTANSO-FTS on GOSAT consists of three units, Optical Unit, Control Unit and Electrical Circuit Uni. Fig.1. shows a block diagram of TANSO-FTS. The optical Unit consists of a pointing mechanizm, a monitor camera, relay optics, a FTS mechanism, detectors and analog signal processors. The optical configuration including the band separation anad detector optics is illustrated in Fig. 2. Two-axes pointing mechanism rotating in the cross track and along track direction is used to point the line of sight to the target points, solar diffuser, black body and deep space as well as compensating the image motion due to the satellite orbiting. Two pointing mechanisms are installed and one of them is redundant one. Driving the pointing mechanism is performed according to the on-board table uploaded to the satellite. A small two-dimensional CMOS camera whose spacial resolution is 50m and FOV is 30km by 30 km which is about three times of the FOV of the FTS is installed in the FTS coaxially adjusted to the optical axis of the FTS to monitor the field of view of the FTS. The monitor camera images are acquired coincident with the FTS data acquisition. FTS mechanism is a double pendulum type interferometer with two cube-corner reflectors made from three gold-plated Zerodur plates on an invar structure, which creates a ±2.5cm optical path difference (OPD). The two cube-corner reflectors are mounted at the edges of a V-shape arms as indicated by the Fig. 3. These cube-corner reflectors are carefully aligned with each other to permanently maximize modulation efficiency with the minimum optical shear. The surfaces of the cube-corner reflectors are better than λ = 20 (RMS) at 633 nm. A maximum beam divergence smaller than 1 arc sec shows excellent orthogonality. The backsides of the cube-corner reflectors are gold coated to minimize thermal radiation coupling with the environment. The thickness of the beam splitter is selected to be larger than the maximum OPD to avoid channeling, and its surface has high quality (better than λ = 20 at 633 nm) and no AR coating to maintain high optical efficiency over a wide spectral range. Bare Zn–Se material has a spectrally flat index of refraction and transmittance higher than 65% over the wide spectral region from 0.76 to 15 μm. Modulation efficiency is a key parameter to characterize the FTS performance, especially at SWIR. In general, modulation at shorter wavelengths is lower and FTS application becomes more difficult. With careful manufacturing and screening of the optical components, the modulation efficiencies are 70±3% at 13,200 cm–1, 91±3% at 6200 cm–1, and 95±3% at 5000 cm–1 PFM). The swing arm is mounted on the beam splitter holder with a flexible blade. A fully redundant metrology sampling system using 1.31 μm distributed-feedback (DFB) lasers and two InGaAs detectors is used to sample the signal. The temperature of these diode lasers are controlled up to 0.001°C above or below 25.000°C using thermoelectric controller because the diode laser wavelength depend on the temperature strongly. The actual wavelength stability (Δλ/λ) is better than 10-7. All optical components making up the optical unit are mounted on the optical bench whose temperature is maintained within the range from 20 ° C to 26 ° C. This optical bench is mounted on the satellite structure with three kinematic mounts to isolate the thermal distortion. The scene flux passed the FTS mechanism is steered into the after optics(shown in Fig. 4), and is drawn in order of increasing wavelength. The each drawn scene flux is divided into two polarizations except Band 4. Layout of the components in the Optical Unit of the TANSO-FTS is shown in the Fig. 5. The after optics and detector unit including the pre-amp are mounted on the separate small bench from large one that other optics are mounted, and this small bench is mounted on the large one. The black body and the window to see the deep space for calibration are mounted on the side wall and the pointing mechanism is rotated about 90 degrees to see them. The solar diffuser are mounted on the top cover and the solar ray scattered on the diffuser is observed for solar irradiance calibration. C.THE ANOMALIES OF TANSO-FTS ON GOSAT ON ORBITAfter the launch, TANSO-FTS on GOSAT has encountered several anomalies. And we have investigated the root cause of these anomalies to reflect on the design of GOSAT-2.
Table 3.Anomalies happened on the GOSAT
Table 3 shows the summary ot the anomalies happened on the GOSAT. III.THE FOURIER TRANSFORM SPECTROMETER(FTS) ON GOSAT-2:A.Requirements for GOSAT-2 missionGOSAT has accomplished the accuracy targets, but on the other hand, we learned a lot of things on the instruments, processing algorithm and so on through the GOSAT operation, and we understood what we should improve in the following mission, and our experiences regarding observation performances as well as hardware design were reflected on the mission requirements on GOSAT-2. The principal requirement is to estimate the Carbon Dioxide flux with the accuracy of ±100% which means that it’s possible to distinguish between source and sink accurately. In addition to this requirement, it was required to evaluate the method to determine the anthropogenic emission. So the measurement accuracy required to meet these mission were studied and the target of the GOSAT-2 observation were defined as below.
Table 4.Observation targets of GOSAT-2
B.Measurement Requirement for TANSO-FTS-2 on GOSAT-2It’s necessary to improve the observation performances of the FTS such as signal to noise ratio, the number of the useful data and addition of the carbon monoxide observation band to meet the targets of the GOSAT-2 mission. Not only the performance of the FTS but also the one of the cloud and aerosol imager, CAI, data should be improved because the CAI data are used to compensate the FTS data for the aerosol and to detect clouds. And the CAI-2 for GOSAT-2 will observe the additional observation channels to reinforce the aerosol observation abilities and observes the air pollution. In this paper, CAI-2 is not mentioned. Based on the feasibility studies, the hardware system requirements were defined and the design was started. Table 5 shows the principal specifications of TANS-FTS-2 on GOSAT-2 compared with FTS on GOSAT. Table 5.Principal specifications of TANSO-FTS-2 on GOSAT-2 compared with FTS on GOSAT
C.Approaches to Achieving the targetsTo achieve the mission targets, it’s necessary to improve the concentration measurement precision of the CO2 and CH4. And the targets of the measurement precision is defined by the average in the area. So it’s effective to increase the number of the useful data and the signal to noise ratio of each data.
D.TANSO-FTS-2 Design Features
IV.CONCLISION:Fourier Transform Spectrometer on GOSAT is the first FTS which observe the SWIR region from space, and all of the observation targets had been satisfied. On the other hand, a lot of issues have been made clear such as the hard wear behavior as well as the data accuracy which is necessary for the political use. So new requirements for the greenhouse gases observation from space had been presented. And GOSAT-2 mission was started. GOSAT-2 adopts the same measurement method as GOSAT, that is Fourier Transform Spectrometer with cube-corner reflector. To improve the measurement accuracy, the signal to noise ratio will be improved and the number of the useful data will be incresed. To satisfy these improvements, the size of the cube-corner reflector, that is the effective aperture size will be expanded, and the intelligent pointing which is the function to avoid the clouds. We intend to launch GOSAT-2 in 2017 Japanes Fiscal Year. |
