GOSAT (Greenhouse Gases Observing Satellite) is a satellite to measure greenhouse gases from space with the Fourier
Transform Spectrometer (FTS). It measures spectra of Earth-reflected solar radiations with high spectral resolution of
about 0.2 cm-1, covering four spectral bands, including 0.76, 1.6, 2.0 and 14 micron wavelength regions. In first three
bands, we have two detectors for measuring two components of polarization. For the acquisition of one interferogram,
it takes 4 seconds or less, depending on the definition of measurement mode. Since the satellite moves in high speed, an
image motion compensation mirror (IMC) works to continuously stare the same position on the Earth surface during this
period of acquisition. For staring same position, the mirror is controlled by two-axis motors. Staring position could
slightly fluctuate around the right position, making the position of the instantaneous filed of view (IFOV) vibrate with an
amplitude of a few hundreds meters. Since the optical characteristics (such as the albedo) of the IFOV changes
place-by-place, the intensity of IFOV could also change due to this fluctuation. The intensity of IFOV could also change
due to the change of reflection angle, wind on the water surface, or other causes. During this period of the acquisition,
the optical path length and Doppler shift caused by the satellite moving could also change. In this paper, we examine the
effects of some of these kinds of disturbances to the signals of interferograms on resultant spectra and retrieval accuracies
of CO2, and discuss about the correction method to the interferogram and spectra.
For measuring weak radiations with high spectral resolution, the Fourier Transform Spectrometer (FTS) is a powerful
tool by virtue of its multiplex advantage. When we use this advantage for measuring Earth-reflected solar radiations from
a moving satellite in space, however, we encounter a difficulty that during the acquisition of the interferogram, the
optical characteristics of instantaneous filed of view (IFOV) could change. The change of IFOV radiance is caused by,
e.g., the fluctuation of line-of-sight of the spectrometer, the change of reflection angle, the glittering of the water surface,
or other causes. In this paper, we examine the effects of such pseudo signals contained in the interferograms on resultant
spectra and retrieval accuracies of CO2. Simulations showed that such pseudo signals could generate serious errors in
CO2 retrieval with the FTS of the GOSAT (Greenhouse Gases Observing Satellite). A method is shown to correct these
types of disturbances on the interferograms.
Together with these topics, we also discuss about other issues such as the ability of the measuring system of
GOSAT-FTS to resolve the vertical density profiles of gases or the method to avoid the uncertainty of the existing data
for high resolution spectra of solar radiance spectra.
A method has recently been developed to compress the number of channels of trace gas remote sounder preserving almost all the information content that the original data has1. In this method, the weighting function of the original channels is expanded with empirical orthogonal functions (EOFs), and a set of hypothetical radiances, whose weighting functions are the EOFs, are used for the analysis. It has been shown that the radiance data of 240 of original channels of CO2 spectrum at around 6207 cm-1 can be compressed to about 3 channels of hypothetical radiances with loosing negligible information content. This means that the information content of the vertical profile of trace gas, which the spectrum of the reflected solar radiation has, is not so much. In the present paper, the information content of the vertical profile is examined for typical two types of spectra of CO2 and CH4 absorption bands at around 1.6 and 2.1 μ regions. Another issue of this paper is the correlation between the "measurement error" of the hypothetical channels. Since the hypothetical radiance is generated by the linear combination of radiances of the original channels, it could have the correlation between the radiance errors in hypothetical channels. It is shown that the crrelation rapidly decreases with the decrease of the range of the data that generates the EOFs.
A method has recently been shown by Aoki to compress the number of channels of vertical atmospheric sounder preserving almost all the information content that the original data has. In this method the weighting function of the original channels is decomposed with empirical orthogonal functions (EOFs) and a system of hypothetical radiances, whose weighting functions are the EOFs, are constructed and used for the analysis. It has been shown that the radiance data of 1200 of original channels, which is obtained from the region 640-760 cm-1, can be compressed to 23 or less channels of hypothetical radiances with loosing negligible information content. In the present paper, the studies were performed on the a) the relation between the number of the EOFs that is required to reconstruct the weighting functions in sufficient accuracy and the spectral resolution, b) the dependence of the correlation between "measurement error" of hypothetical channels on the range of zenith angle of sample data of weighting function, from which the EOFs are generated, and c) the dependence of the information content on the spectral resolutions 0.1, 0.35 and 0.6 cm-1.
Greenhouse gases Observing SATellite (GOSAT) is a Japanese satellite to monitor column density of greenhouse gases such as carbon dioxide (CO2) and methane (CH4) globally from space. GOSAT will be launched in 2008. The data measured by a GOSAT sensor and ground-based monitoring station data will be used into an atmospheric transport inverse model to identify source/sink amount of CO2 in a sub-continental scale. One of the main GOSAT sensors is a nadir-looking Fourier Transform Spectrometer (FTS), which covers Short Wavelength Infrared (SWIR) region to measure column density of CO2. National Institute for Environmental Studies (NIES) is promoting researches on CO2 and CH4 sensitivity analysis, error analysis, data retrieval algorithm study, ground-based/air-borne validation strategy, and a plan of inverse model study for the SWIR FTS. A Bread-board model (BBM) of the SWIR FTS was built and tested by ground-based and airborne measurements. Several sets of the CO2 and CH4 radiance spectra over rice fields were obtained by the test measurements, and it was confirmed that the airborne measurements with a vibration insulator are effective for onboard measurements. Moreover, several improvement items of BBM have become clear.
A system of satellite borne sensor is proposed for measuring the column amount of green house gases in the troposphere from observations of near infrared solar radiation in the sun glint region reflected from water surface of ocean and lakes. A high accuracy determination of the column amount of gases is achieved by measuring the difference of absorption line of greenhouse gas from that of oxygen with using tunable etalons of high resolving power. It is demonstrated by the ground- based measurements of the absorption line of carbon dioxide in the direct solar radiation that the column amount is obtainable with the standard deviation of about 0.4%. The satellite sensor system presented in this paper is compared with other methods from the viewpoint of retrieval error.
Some developments that are planned to be included into an information system for atmospheric applications are considered. A fast algorithm to retrieve temperature, pressure, and mixing ration altitude profiles of the atmosphere composition from high-resolution spectra that are recorded by the interferometer of a new generation on the satellite platform is discussed. The algorithm is distinguished by the analytical computation of the derivatives, entered the inverse calculations, and by the regularization technique. Validating this code, the MITI's IMG data, recorded on board NASDA's ADEOS satellite, is used in order to retrieve a realistic temperature profile of the low atmosphere up to altitudes of 20 km.
A conceptual design is given for the space borne sensor to measure the three dimensional distribution of trace gases in the troposphere. The sensor makes spectral measurements of trace gas absorption lines in the surface reflected solar radiation. Only a few absorption lines are selected by a narrow band pass filter and scanned by a tunable etalon with the resolution better than 0.1 cm-1. The sensitivity and vertical resolution of this sensor system to the variation the trace gas amount are examined with using radiative transfer model that includes the single scattering of solar radiation by air molecules and aerosols. Some examples of measurements are shown with a ground-based sensor for water vapor vertical profile.
New type of infrared instruments such as Tunable Etalon Remote Sounder for the Earth (TERSE) and High resolution Limb Infrared Absorption Spectrometer (HLAS) were proposed and studied for the future Japanese earth observation satellite program. This paper describes the results of feasibility study of TERSE and HLAS.
The spectral albedo of snow surface and the radiant flux density inside the snow are investigated by a multiple scattering model for the atmosphere-snow system. When the snow is composed of two layers with different grain sizes, albedos at the wavelengths shorter and longer than 1.4 micrometers are close to the albedos of the lower and upper layer, respectively. On the other hand, the spectral distribution of the radiant flux density inside the snow changes with the optical depth. When the optical depth of snow increases, the blue light at the wavelength from 0.45 to 0.50 micrometers becomes relatively stronger than other wavelengths. This effect is remarkable for the snow with large grain size.
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