TanSat ACGS On-orbit Wavelength Calibration Using the Solar Fraunhofer Lines
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摘要: 大气二氧化碳（CO2）探测仪（ACGS, Atmospheric Carbon dioxide Grating Spectrometer）搭载于中国全球二氧化碳观测科学试验卫星（TanSat），通过探测0.76 μm、1.61 μm、2.06 μm波段的反射太阳光谱，采用最优估计算法反演大气CO2浓度。满足高光谱分辨率和高精度CO2浓度反演需求，精确探测光谱波长的变化非常重要。本文以高分辨率太阳参考光谱的夫朗禾费吸收线作为参考基准，利用ACGS对太阳的观测光谱计算了ACGS三个谱段通道中心波长位置在一年内的变化情况。结果显示，三个谱段的波长变化在光谱分辨率10%以内，满足光谱定标精度需求。这种变化可能是由于仪器在轨状态变化引起，特别是在轨运行温度变化引起的。ACGS波长的微小变化需要在产品反演中进行修正。基于独立太阳夫琅禾费吸收线的在轨光谱定标方法不仅可以有效监测ACGS的光谱稳定性，还可以为L2产品的处理的提供参考信息。Abstract: Spectra measured using an atmospheric carbon dioxide grating spectrometer (ACGS) by the Chinese global carbon dioxide monitoring scientific experimental satellite (TanSat) in the bands of 0.76, 1.61, and 2.06 μm can be used for retrieving carbon dioxide (CO2) concentrations by fitting observations and simulations using an optimal estimation algorithm. Accurately detecting the change in the center wavelength is important because of its very high spectral resolution and accuracy requirement for product retrieval. Variations in the center wavelength for all the three bands of ACGS have been calculated at the locations of the Fraunhofer lines by comparing solar-viewing measurements and a high-resolution solar reference spectrum. Variations in magnitudes less than 10% of the spectral resolution for each band have been detected. Changes are probably caused by the vibration and instrument status difference between the ground and space, especially the temperature variation in the orbit. The scheme described herein can be used not only for monitoring spectral stability but also to gain spectral knowledge prior to the level-2 product processing. These minor temporal changes in the wavelength in the orbit should be corrected during the product retrieval.
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Key words:
- High-resolution /
- Carbon dioxide /
- Spectral calibration /
- Solar spectra
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图 1 实验室测定的（a）O2A带、（b）WCO2带和（c）SCO2带星下像元的中心区域的仪器线型（ILS）
Figure 1. Preflight ILS (Instrument Line Shape) functions at three adjacent pixels located in the central section of FPA (Focal Panel Arrays) for (a) O2A band, (b) WCO2 band and (c) SCO2 band
图 2 星下像元（a）O2A带、（b）WCO2带和（c）SCO2带波长及像元序号的对应关系
Figure 2. An example of wavelength as a function of pixel index in the focal plane at FOV 5 for (a) O2A band, (b) WCO2 band and (c) SCO2 band
图 3 （a）O2A带、（b）WCO2带和（b）SCO2带的Kurucz太阳光谱和选择的参考吸收线位置（红色×）μ
Figure 3. Kurucz solar spectra and locations of the Fraunhofer lines selected as a reference in (a) O2A band, (b) WCO2 band and (c) SCO2 band
图 4 O2A带2017年多普勒频移的时间序列
Figure 4. Time series of the Doppler shifts in the O2A band in 2017
图 5 WCO2带2017年多普勒频移的时间序列
Figure 5. Time series of the Doppler shifts in the WCO2 band in 2017
图 6 SCO2带2017年多普勒频移的时间序列
Figure 6. Time series of the Doppler shifts in the SCO2 band in 2017
图 8 O2A带9个空间像元波长偏移的时间序列
Figure 8. Time series of the wavelength shift for nine spatial FOVs in the O2 A-band
图 9 WCO2带9个空间像元波长偏移的时间序列
Figure 9. Time series of the wavelength shift for nine spatial FOVs in the WCO2 band
图 10 SCO2带9个空间像元波长偏移的时间序列
Figure 10. Time series of the wavelength shift for nine spatial FOVs in the SCO2 band
图 11 O2A带基准吸收线位置波长偏移量统计结果
Figure 11. Statistics of the wavelength shift at the locations of the selected Fraunhofer lines in the O2A band
图 12 WCO2带基准吸收线位置波长偏移量统计结果
Figure 12. Statistics of the wavelength shift at the locations of the selected Fraunhofer lines in the WCO2 band
图 13 SCO2带基准吸收线位置波长偏移量统计结果
Figure 13. Statistics of the wavelength shift at the locations of the selected Fraunhofer lines in the SCO2 band
表 1 TanSat ACGS主要光谱参数
Table 1. Spectral parameters of the TanSat ACGS instrument
探测波段 谱段范围
/nm光谱分辨率
/nm光谱
像元数光谱
采样率空间
像元数O2A 758～778 0.033～0.047 1242 ＞2 9 WCO2 1594～1624 0.12～0.14 500 ＞2 9 SCO2 2042～2082 0.16～0.18 500 ＞2 9 
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