利用AIRS高光谱数据开展HJ-1B热红外通道的多点交叉定标及验证

Multi-points cross calibration and validation of HJ-1B and AIRS thermal infrared channels

我国热红外传感器在轨辐射定标主要通过基于大面积均匀目标的实测数据开展的单点法场地定标以及与高精度中分辨率载荷的多点法交叉定标来实现.单点法场地定标耗时、费力,且无法计算定标系数的截距,与中分辨率载荷的多点法交叉定标受到入瞳辐亮度匹配算法的制约,两者都影响着我国在轨定标系数精度的提高.利用AIRS高光谱数据的光谱特征,针对国产HJ-1B卫星热红外通道进行多点交叉定标方法研究,通过选择临近成像时间、选择均匀区域消除时空差异,同时通过MODTRAN辐射传输模型开展逐像元计算,对不同像元间的观测几何因素进行归一化处理.通过基于单一均匀区域、不同时相的时间序列多点法和基于同一时相、不同均匀区域的均匀目标多点法交叉定标获得HJ-1B热红外通道定标系数,并以基于HJ-1B热红外通道查找表法获取的星上定标系数为基准,对多点法交叉定标所得定标系数进行验证.经过分析,通过剔除时间序列多点法中的异常点和调整均匀目标多点法中的匹配因子,所得定标系数模拟得到的等效离水亮温值与基于星上定标系数反演得到的离水亮温值偏差分别达到1.81和0.92 K.由结果可知,基于AIRS的HJ-1B热红外通道的均匀目标多点法交叉定标结果更精确可信.

On orbit radiometric calibration of thermal infrared sensors in China is mainly based on the single-point method with insitu measurements of large uniform targets and cross calibration with moderate resolution sensors which have high calibration accuracy. The single-point method is time-consuming and laborious, and the intercept can not be calculated. Accuracy of cross calibration with the moderate resolution sensors is restricted by the matching algorithm of entrance pupil radiances. Both of this two methods＇ insufficients have been affected the accuracy of on orbit radiometric calibration in China. According to the characteristics of AIRS hyperspectral data, multi-points cross calibration is performed between the AIRS and HJ-1B thermal infrared channles. The time and spatial varieties are eliminated through the similar imaging times and homogeneous regions. At the same time, radiative transfer calculation by MODTRAN is used for each pixel to normalize the observation geometry. Times series multi-points method with the single target, various times and homogeneous targets multi-points method with the single time, various targets are performed to obtain the calibration coefficients of HJ-1B thermal infrared channel. Onboard calibration coefficients calculated by the Look-up table method is used to validate the multi-points cross calibration results. After the elimination of abnormal points from the time series multi-points method and regulation of match factor with homogeneous targets multi-points method, the differences between the equivalent brightness temperature retrieved by the multi-points cross calibration and onboard calibration coefficients are 1.81 and 0.92 K, respectively. The results indicate that the homogeneous targets multi-points method for cross calibration of AIRS and HJ-1B thermal infrared channels has higher accurate.