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8 November 2014 Spectral parameters and signal-to-noise ratio requirement for TANSAT hyper spectral remote sensor of atmospheric CO2
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With the stable increase of carbon dioxide (CO2) concentrations, space based measurements of CO2 concentration in lower atmosphere by reflected sunlight in near infrared band has become a hot research topic at present. Recently, the instruments sensitive to total CO2 column data in near-surface have become available through the SCIAMACHY instrument on ENVISAT and TANSO-FTS on GOSAT. The developing hyper spectral CO2 detector in China carried by TANSAT will be launched in late 2015. Hyper spectral CO2 detector is designed to provide global measurements of CO2 in lower troposphere. It employs high resolution spectra of reflected sunlight taken simultaneously in near-infrared CO2 (1.61μm and 2.06μm) and O2 (0.76μm) bands. Associating climate change with the observation requirements of carbon sources and sinks, the feasibility of making CO2 column concentration measurements with high-resolution and high-precision is studied by high resolution atmosphere radiation transfer model. The effects of key specifications of the hyper spectral CO2 detector such as spectral resolution, sampling ratio and sign-to-noise ratio (SNR) on CO2 detection are analyzed combining the scientific requirements of CO2 measurements of China. The typical characteristics of hyper spectral CO2 detector on TANSAT are grating spectrometer and array-based detector. To achieve the column averaged atmospheric CO2 dry air mole fraction (XCO2) precision requirements of 1×10-6-4×10-6, hyper spectral CO2 detector should provide high resolution at first to resolve CO2 absorption lines from continuous spectra of reflected sunlight. Compared to a variety of simulated spectral resolutions, the spectral resolution of hyper spectral CO2 detector on TANSAT can resolve CO2 spectral features and maintain the moderate radiance sensitivity. Since small size array detector-based instruments may suffer from undersampling of the spectra, the influences of spectral undersampling to CO2 absorption spectra are studied, the results indicate that sampling ratio should exceed 2 pixels/FWHM to ensure the accuracy of CO2 spectrum. Signal-to-noise ratio is one of the most important parameters of hyper spectral CO2 detectors to ensure the reliability of CO2 signal. SNR requirements of CO2 detector to different detection precisions are explored based on the radiance sensitivity factors. The results show that it is difficult to achieve the SNR to detect 1×10-6-4×10-6 CO2 concentration change in the boundary layer by solar shortwave infrared passive remote sensing, limited by the instrument development at present. However, the instrument SNR to detect 1% change in the CO2 column concentration is attainable. The results of this study are not only conductive to universal applications and guides on developing grating spectrometer, but also helpful to have a better understanding of the complexity of CO2 retrieval.
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Qian Wang, Zhong-Dong Yang, and Yan-Meng Bi "Spectral parameters and signal-to-noise ratio requirement for TANSAT hyper spectral remote sensor of atmospheric CO2", Proc. SPIE 9259, Remote Sensing of the Atmosphere, Clouds, and Precipitation V, 92591T (8 November 2014);

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