基于敦煌场地定标的FY-3MERSI反射太阳波段在轨响应变化分析

On-Orbit Response Variation Analysis of FY-3 MERSI Reflective Solar Bands Based on Dunhuang Site Calibration

鉴于中分辨率光谱成像仪不能实现反射太阳波段的星上绝对辐射定标,提出了基于地表方向模型、矢量辐射传输模型6SV并联合MODTRAN吸收透过率校正的敦煌场替代定标新方法,4年的同步定标结果表明,除了水汽吸收中心波段之外,定标不确定度小于5%,而多数波段优于3%。以Aqua MODIS为辐射基准的大气顶辐射计算试验表明,正演与卫星观测间的平均偏差在波长<1μm的窗区波段小于3%,波长>1μm的小于5%(除了2.1μm波段);此外,经场地定标的MERSI表观反射率与MODIS具有很好的一致性。基于多年的场地定标结果发现:可采用二次多项式拟合定标系数的时间变化,进而实现逐天的定标更新;波长<0.6μm的波段衰变较大,波段8(0.41μm)入轨第一年的衰变率约为14%;在轨初期衰变最大,一年后趋缓,两年后部分波长>0.6μm的波段出现响应增加现象。

MERSI is the keystone payload of FengYun-3 and there have been two sensors operating on-orbit since 2008. The on- orbit response changes obviously at reflective solar bands （RSBs） and must be effectively monitored and corrected. However MERSI can not realize the RSBs onboard absolute radiometric calibration. This paper presents a new vicarious calibration （VC） method for RSBs based on in-situ BRDF model, and vector radiometric transfer model 6SV with gaseous absorption correction u- sing MOTRAN. The results of synchronous VC experiments in 4 years show that the calibration uncertainties are within 5 % ex- cept for band at the center of water vapor absorption, and 3% for most bands. Aqua MODIS was taken as the radiometric refer- ence to evaluate the accuracy of this VC method. By comparison of the simulated radiation at top of atmosphere （TOA） with MODIS measurement, it was revealed that the average relative differences are within 3 % for window bands with wavelengths less than 1 tim, and 5% for bands with wavelengths larger than 1 9m （except for band 7 at 2.1 9m）. Besides, the synchronous nadir observation cross analysis shows the excellent agreement between re-calibrated MERSI TOA apparent reflectance and MODIS measurements. Based on the multi-year site calibration results, it was found that the calibration coefficients could be fit- ted with two-order polynomials, thus the daily calibration updates could be realized and the response variation between two cali- bration experiments could be corrected timely; there are large response changes at bands with wavelengths less than 0.6 gm, the degradation rate of the first year at band 8 （0. 41μm） is about 14% the on-orbit response degradation is maximum at the begin- ning, the degradation rates slow down after one year in operation, and after two years the responses even increase at some band with wavelengths larger than 0. 6 μm.