我国西北部沙漠定标场网的地表反射率光谱特征模型

Surface Reflectance Spectral Characteristic Model of Desert Calibration Site Network in Northwest China

选取中国西北地区10个典型的用于辐射定标及仪器性能追踪的辐射定标场,在卫星过境时利用地面手持光谱仪和低空无人机(UAV)同步观测反射率光谱,系统比较了多个场地的反射率光谱差异,并开展了多个场地的光谱特征分析和参数建模研究。同一个场地一天内不同时间的光谱形状变化很小,光谱角小于5°,光谱幅度的变化主要受太阳天顶角和大气状况的影响,光谱幅度在不同天的同一时刻变化很小。不同场地的光谱形状和幅度差异较大。同一场地不同观测尺度下的光谱曲线基本吻合。通过分析发现,当波长小于1100 nm时,所有沙漠场的光谱曲线形状与三角反正切函数曲线相似。基于反正切函数进行地表反射率光谱建模,各场地实测光谱与模拟光谱的均方根误差均在0.6%以内,相关系数均在0.99以上,表明四参数光谱模型能够准确地描述沙漠场的反射率光谱。用模型计算的地表反射率替代场地实测的地表反射率进行FY-3D中分辨率光谱成像仪(MERSI)场地定标,可以发现,与基于实测光谱计算的结果相比,利用该模型得到的MERSI各个波段的相对偏差小于3%,表明构建的沙漠四参数反射率光谱模型可以很好地应用在MERSI的定标中。

Ten typical radiometric calibration sites are selected for radiometric calibration and instrument performance tracking in northwest China.Simultaneous observation of reflectivity spectrum is realized by ground hand-held spectrometer and low-altitude unmanned aerial vehicles(UAVs)during satellite transit.The reflectance spectral differences of multiple sites are compared systematically,and the spectral characteristic analysis and parameter modeling of multiple sites are carried out.The spectral shape of the same site varies little at different time in one day,and the spectral angle is less than 5°.The variation of spectral amplitude is mainly affected by solar zenith angle and atmospheric conditions,and the spectral amplitude varies little at the same time in different days.The spectral shapes and amplitudes of different sites are quite different.The spectral curves of the same site at different observation scales are basically the same.Through the analysis,it is found that when the wavelength is less than 1100 nm,the spectral curves of all desert sites are similar to the curves of triangular arctangent function.Based on the arctangent function,the surface reflectance spectral modeling is carried out.The root mean square error of the measured and simulated spectra of each site is within 0.6%,and the correlation coefficient is above 0.99.The results show that the four-parameter spectral model can accurately describe the reflectance spectra of desert sites.The surface reflectance calculated by the model is used to calibrate the FY-3D medium resolution spectral imager(MERSI)site instead of the measured surface reflectance,it is found that compared with the results calculated based on the measured spectra,the relative deviation of each band of MERSI obtained by the model is less than 3%,indicating that the desert four-parameter reflectance spectral model can be well applied to the calibration of MERSI.