基于MODIS的GF-4/PMS遥感器交叉定标——以巴丹吉林沙漠为参考目标

Cross calibration of GF-4/PMS based on MODIS over Badain Jaran Desert

高分四号(GF-4)是中国首颗高分辨率静止轨道光学遥感卫星,搭载了一台可见/中波红外凝视相机,由于缺少星上定标系统导致无法提供高频次的星上辐射定标系数。为了更好的实现GF-4/PMS传感器辐射性能监测和数据定量化应用,本文选择了巴丹吉林沙漠南端均匀沙地,利用MODIS作为参考传感器对GF-4/PMS可见近红外波段进行交叉定标。为尽量减少地表不均匀性造成的交叉定标误差,使用较高分辨率的影像(Landsat 8/OLI)搜索巴丹吉林沙漠地区的均匀区域,确定为交叉定标试验区。为进一步减少遥感器成像角度和大气变化的影响,进行了MODIS成像角与GF-4成像角角度差约束(小于20°)、成像时刻约束(小于2 h)、云和成像质量问题,共选择2016年—2018年有效图像对13组,通过利用MODTRAN模拟计算二者光谱匹配因子进而实现交叉定标系数计算。结果表明:(1)本文计算的辐射定标系数在朗伯体假定和Ross-Li模型假定情况下具有较高一致性,忽略BRDF效应情况造成的计算误差在0.8%—4%,定标结果的不确定度在7.4%以内;(2)2016年至2018年GF-4/PMS遥感器辐射性能呈现缓慢下降特征,年衰减率小于1%。本文的方法可以有效地提高GF-4/PMS的辐射定标频率和定标精度,实现对GF-4/PMS传感器整个生命周期的辐射性能监测。

GF-4 is the first high-resolution geostationary orbit optical remote sensing satellite in China,and it is equipped with a staring camera that covers the visible-to-mid-infrared spectrum.Providing in-orbit radiometric calibration coefficients frequently is difficult due to the lack of on-board calibration devices.To effectively monitor the radiometric performance of GF-4/PMS and provide a solid basis for quantitative applications,the cross-calibration method was used in this study to derive the calibration coefficients over a selected calibration site located in the north of Badan Jaran Desert.The cross-calibration site was identified by searching the uniform area in a high-resolution image(i.e.,Landsat 8/OLI)to minimize the uniform impact on the cross-calibration results.To reduce the effects of the imaging angles of the sensor and atmospheric changes,the restraint was set to the imaging angle difference between MODIS and GF-4(less than 20°),the imaging time difference(less than 2 h),the cloud,and image quality.In total,13 image pairs were available from 2016 to 2018 and were used to calculate the cross-calibration coefficients after compensating for the spectral matching factors,which were simulated by MODTRAN.Results showed that(1)the cross-calibration coefficients calculated under the Lambertian assumption were highly consistent with those calculated under the Ross-Li BRDF assumption.The uncertainty was less than 7.4%,and the relative difference ranged between 0.8%and 4%when BRDF was neglected.(2)The radiometric performance of GF-4/PMS decreased slowly from 2016 to 2018,with 1%decrement per year.Thus,the proposed method can effectively improve the radiometric calibration frequency of GF-4/PMS with an acceptable accuracy and can be used for radiometric performance monitoring over the whole life cycle of the sensor.