FY-2水汽通道光谱响应测量大气污染评估和订正

Evaluation and Correction of Atmosphere Contamination on the Measurement of the Spectral Response of FY-2 Water Vapor Channels

由于无法实现真空测量,大气吸收作用会对实验室光谱响应(SRF)测量产生污染。为了提高光谱定标精度,通过敏感性试验定量评估了大气污染对水汽通道实验室光谱响应测量的影响,并以FY-2D为例进行了光谱响应污染订正,分析了光谱污染带来的通道辐射计算误差。不同测量环境的敏感性模拟结果表明:大气吸收导致光谱测量曲线产生显著波动,强吸收光谱处的响应减弱,致使依赖于SRF的通道辐射计算结果被高估。光谱污染带来的通道亮温偏差随着水汽含量的增加呈指数增长趋势,偏差大于0.5K且仅在等效水平路径小于1m相对湿度低于15%的干燥环境下小于1K。4m水平路径35%相对湿度的情况下,亮温偏差可大于4K。FY-2系列卫星水汽通道的光谱响应都存在不同程度的大气污染现象。利用水平大气透过率光谱,通过光谱比值的方法,对FY-2D的SRF进行订正。订正后SRF的异常波动被基本消除,曲线分布更加光滑合理。理论分析结果表明:大气污染导致FY-2D在典型大气条件下大气层顶的通道亮温模拟偏高2.2K,黑体辐射进而辐射定标高估7.6%。大气吸收对实验室光谱定标的影响非常显著,不仅对水汽通道对所有气体吸收通道都不能忽略。实验室光谱定标不能忽略大气吸收的影响,应该通过扣除大气透过率的影响的方式对测量光谱响应进行订正。

Spectral response is an important parameter of satellite instruments,and is the integrated performance of the reflective and transmission characteristics of each optical element and the spectral response characteristics of detectors.As do not have monochromatic light source devices for onboard spectral calibration,the spectral response in orbit is usually characterized by prelaunch evaluation in laboratory.Due to lack of vacuum environment,spectral response measurement in laboratory will be certainly contaminated due to atmospheric absorption between the calibration source and the instrument.In order to improve spectral calibration accuracy,influences of atmosphere contamination on the spectral response measurements of FY-2 water vapor channels were assessed based on a series of sensitivity tests using radiative transfer models（RTM）.Because of the atmospheric absorption,the spectral responses become unsmooth,and the structure shows good consistency with the transmittance distribution.The channel brightness temperatures（BT）simulated based on contaminated SRF will be overestimated,and the biases increase exponentially over water vapor column.The biases are greater than 0.5 K and less than 1 K only during extremely dry environment with humidity less than 15%and homogeneous path less than 1m.However,when the homogeneous path approaches 4m under a normal ambient condition where the humidity is around 35%,the simulation BT bias can reach 4 K.Atmospheric contamination on spectral response measurement is happen for all water channels of FY-2 series satellites.Spectral correction was implemented for FY-2D by removing horizontal transmittance influence using spectra ratio.The corrected spectral response curve becomes much smooth and the distribution is more physically reasonable.The contamination leads to a warm bias of 2.2 K for clear sky top of atmosphere BT simulation,and a 7.6%positive bias for the blackbody radiance as well as the radiometric calibration calculation.The influence of atmospheric contamination on spectral response measurement is serious,thus can＆#39;t be ignored not only for the water vapor channel but for all absorptive channels.The contamination to spectral measurements which can not be neglected should be corrected from dividing the original spectral response by atmospheric transmittance.