CO_2高光谱遥感仪器的光谱参数和信噪比需求

Spectral Parameters and Signal-to-noise Ratio Requirement for CO_2 Hyper Spectral Remote Sensor

卫星短波红外CO_2遥感获得大气低层CO2浓度信息,已成为目前国际热点研究领域。结合气候变化及碳源、汇观测需求,利用高精度大气辐射传输模式研究了高光谱分辨率、高精度CO_2探测目标的可实现性。针对高光谱CO_2探测器光栅分光、阵列探测器特点,分析了光谱分辨率、光谱采样率等关键技术指标对CO_2探测的可能影响;基于辐射敏感度因子分析了不同探测精度要求下的信噪比需求。结果表明:高光谱CO_2探测器首先应具有足够高的光谱分辨率,以便从太阳反射连续谱段中分辨出CO2吸收线;为保证CO_2光谱的准确性,光谱仪所用探测器面元应该保证光谱采样率大于2;尽管探测边界层内CO2浓度1%变化所要求的信噪比难以达到,但探测整层大气CO_2浓度1%的变化所需要的信噪比是可以实现的。

With the stable increase of carbon dioxide（CO2） concentrations,space based measurement of CO2concentration in lower atmosphere by reflected sunlight in near infrared band has become a hot research topic.Recently,instruments sensitive to total C02 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 is going to be launched in 2015.Hyper spectral CO2detector is designed to provide global measurements of CO2 in lower troposphere,employing high resolution spectra of reflected sunlight taken simultaneously in near-infrared CO2（1.61 μm and 2.06 μm） and O2（0.76 μm） bands.Associated with climate change and 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.In consideration of the application requirements,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.Typical characteristics of hyper spectral CO2 detector on TANSAT are grating spectrometer and arraybased 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,influences of spectral undersampling to CO2 absorption spectra are studied,indicating that sampling ratio should exceed 2 pixels/FWHM to ensure the accuracy of CO2 spectrum.SNR is one of the most important parameters of hyper spectral CO2 detectors to ensure the reliability.SNR requirements of CO2 detector to different detection precisions are explored based on the radiance sensitivity factors.Results show that it is difficult to achieve 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 condition and level at present.However,the instrument SNR to detect 1%change in the CO2 column concentration is attainable.These results are not only conductive to universal applications and guides on developing grating spectrometer,but also helpful to better understand the complexity of CO2 retrieval.