光栅色散型成像光谱仪的在轨光谱定标方法研究

On-Orbit Spectral Calibration Method of Grating Dispersive Imaging Spectrometer

精确的光谱定标是定量化反演地物信息的前提与基础。光栅色散型可见近红外成像光谱仪(VNS)主要用于海洋水色遥感和海岸带监测,采用推扫式成像方式,工作波段范围覆盖400~1 040nm,空间维视场像元总数为1 024,共设置256个光谱通道,光谱采样步长为2.5nm。针对仪器入轨后可能发生的光谱通道中心波长漂移或通道宽度展宽问题,基于光谱特征曲线匹配思想,提出了利用太阳大气廓线和星上定标器镨钕特征光谱进行在轨光谱定标的新方法。开展了在轨光谱真实性检验与定标的地面模拟实验,采用最小差值与相关系数联合算法对数据进行了处理。以大气氧气吸收763nm波段为例,介绍了在轨光谱定标的步骤。给出了太阳夫郎和费517nm、Pr-Nd玻璃685nm和氧气吸收763nm三个典型波段对应VNS的光谱通道的定标结果:三个通道穿轨视场Smile效应幅度相近,约为0.6nm;中心波长漂移方向和大小各异,分别为0.707,-0.369和0.293nm;对穿轨方向各像元的测量值进了二次曲线拟合,763nm通道标准偏差小于另外两个通道,三个通道的光谱定位精度较高优于0.176nm。为成像光谱仪开发出一种适用的在轨光谱定标算法。

Precise spectral calibration is the premise and base for quantitative radiance inversion of Earth scenes .The grating dispersive visible near-infrared imaging spectrometer （VNS） is used for ocean color remote sensing and coastal zones monitoring .A push-broom method is applied by this instrument .It is operated in the solar-reflected spectrum with wavelength range from 400 to 1040 nm .256 spectral channels with a nominal 2.5 nm interval and 1024 cross-track pixels ,corresponding to spectral and spatial dimensions ,are arranged on the focal plane .Spectral parameters including the center wavelength and bandwidth of the hyperspectral instrument may vary after launching ,due to external environmental changes or self-performance degenerations . For the sake of coefficients update ,an on-orbit spectral calibration method is presented in this contribution .The algorithm is based on a spectrum-matching technique using atmospheric absorption features ,solar Fraunhofer lines and Pr-Nd characteristic spectra of the on-board calibrator .Last squares and correlation coefficients are applied to process the data collected in the on-or-bit spectral calibration simulation experiments .The procedure is introduced by taking an example of the oxygen absorption 763 nm band .Fraunhofer lines 517 nm ,Pr-Nd glass characteristic spectra 685 nm and oxygen absorption 763 nm are selected as three typical bands ,corresponding to three channels of the visible near-infrared imaging spectrometer （VNS） .Their spectral recalibration results are reported as follows .Cross-track smile effect amplitudes are similar ,about 0.6 nm while different center wavelength shifts ,0.707 ,-0.369 and 0.293 nm respectively .The standard deviation of the channel of 763 nm is smaller than the other two ,deriving from second order polynomial fits to measurements across-track ,and spectral position precisions of the three channels are better than 0.176 nm .A practical on-orbit spectral calibration algorithm is proposed for the imaging spec-trometer .