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Spectral Parameters and Signal-to-noise Ratio Requirement for CO2 Hyper Spectral Remote Sensor
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摘要: 卫星短波红外CO2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CO2探测目标的可实现性。针对高光谱CO2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CO2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CO2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CO2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CO2浓度1%的变化所需要的信噪比是可以实现的。Abstract: With the stable increase of carbon dioxide (CO2) concentrations, space based measurement of CO2 concentration in lower atmosphere by reflected sunlight in near infrared band has become a hot research topic. Recently, 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 is going to be launched in 2015. Hyper spectral CO2 detector 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 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, 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.
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Key words:
- hyper spectral;
- CO2;
- remote sensing;
- TANSAT
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图 2 3种光谱分辨率和采样率的透过率光谱
Fig. 2 Transmittance spectra for three spectral resolutions and sampling ratios
图 3 两种探测器在两种光谱分辨率时CO2透过率光谱
Fig. 3 Transmittance spectra for two spectral resolutions of two detectors
图 5 两种探测器在不同光谱分辨率条件下的透过率相对误差
Fig. 5 Transmittance relative errors for two detectors under two spectral resolutions
表 1 LBLRTM模拟条件列表
Table 1 Simulation conditions of LBLRTM
模拟条件 参数 大气模型 美国标准大气 扫描函数 三角型函数 波数范围/cm-1 6237~6242 光谱分辨率/cm-1 0.0014, 0.07, 0.312, 0.5 FWHM内光谱采样数 1, 2, 4 
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表 2 探测器的采样间隔和在两种光谱分辨率下的采样率
Table 2 Sampling spacing and sampling ratio for two spectral resolutions of detectors
探测器 采样间隔/cm-1 采样率 FWHM为0.312 cm-1 FWHM为0.468 cm-1 A 0.234 1.33 2 B 0.117 2.67 4
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表 3 波数范围在6235~6245 cm-1的吸收通道中，两种探测器在不同光谱分辨率条件下的透过率平均误差
Table 3 Transmittance average errors for two detectors under two spectral resolutions, referred to those under the baseline in the absorption channels between 6235-6245 cm-1
探测器 FWHM为0.312 cm-1 FWHM为0.468 cm-1 A 2.41% 0.92% B 0.57% 0
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表 4 SCIATRAN模型模拟条件列表
Table 4 Simulation conditions of SCIATRAN
模拟条件 参数 背景CO2浓度 373.6×10-6 气溶胶条件 晴空无气溶胶 太阳天顶角 60° 地表反照率 0.15 狭缝函数类型 高斯型 光谱分辨率 0.08 nm 光谱范围 1594~1624 nm
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表 5 探测边界层CO2浓度变化的信噪比需求
Table 5 SNR requirements of detecting CO2 concentration variation in boundary layer
CO2浓度变化 f/% 信噪比 1×10-6 -0.0524 1900 2×10-6 -0.1048 950 3×10-6 -0.1571 640 4×10-6 -0.3252 300
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表 6 探测CO2柱浓度变化的信噪比需求
Table 6 SNR requirements of detecting CO2 concentration variation in the whole column
CO2柱浓度变化 f/% 信噪比 1×10-6 -0.1219 820 2×10-6 -0.2441 410 3×10-6 -0.3580 280 4×10-6 -0.4874 200
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