敦煌辐射校正场对地辐射计现场定标研究及应用

Research and Application of the Calibration of the Ground Radiometer in Dunhuang Test

针对实验室定标周期长、长途运输影响定标精度等问题,开展了通道式辐射计ATR的现场定标方法研究。在SRBC现场定标中引入超光谱辐照度仪自动观测数据来计算天空漫射照度,利用SRBC法计算了定标系数和地表反射率,将其与人工利用ASD型光谱仪测量的地表反射率进行对比,并对地表反射率计算的不确定度进行分析。研究结果表明,SRBC法定标系数计算的ATR反射率与ASD型光谱仪测量的相对偏差优于1.4%,反射率计算不确定度优于2.78%~4.35%,SRBC法在实际应用中具有较高的精度和系统优势。将SRBC定标系数应用于卫星自动化替代定标,近3年的自动化替代定标结果与AQUA/MODIS星上定标系数具有很好的一致性,各通道单次定标相对偏差基本在5%以内,平均百分偏差优于3.58%,能很好监测跟踪卫星载荷的运行情况,验证了SRBC法的有效性和适用性。

When the satellite is in orbit for a long time,the remote sensor will be affected by factors such as the space environment and the aging of components,which will cause the phenomenon of radiation response decay.To meet the quantitative requirements of satellite data and the monitoring of remote sensor performance change,continuous and precise calibration of remote sensors is required.At present,the vicarious calibration based on the reflectance-based method is widely used.A large area of uniform and stable targets are used as the radiometric calibration site.When the satellite transit,the surface reflectance and atmospheric parameters of the site are synchronously measured manually,and the apparent reflectance is calculated in combination with the radiative transfer model,to obtain the calibration coefficient of the satellite sensors.It takes much time,manpower,material and financial resources to complete an effective calibration test.At the same time,due to the limitations of weather conditions,the calibration frequency is basically maintained at the level of once a year.With the increase in the number and types of optical satellites,the radiometric calibration based on the traditional vicarious calibration method can not meet the needs of remote sensing quantification in time.In recent years,a method of automated vicarious calibration based on high frequency,high precision,low cost and other considerations has been adopted internationally.This method uses unattended and automated observation equipment to measure the surface reflectance and atmospheric parameters and combines with the radiation transfer model to realize the high-time calibration of remote sensors.The automated vicarious calibration observation network such as the Radiometric Calibration Network and the Automated Vicarious Calibration System has been deployed in Railroad Valley Playa in the United States,La Crau in France,Baotou in China,Dunhuang in China,etc.for satellite calibration and verification.Surface reflectance is an important parameter for automated vicarious calibration.Considering the long-term field observation and operation stability,the channel-type radiometer is usually used.The automated test-site radiometer is the core equipment of the automatic vicarious calibration system in the Dunhuang radiometric calibration test site,which provides surface reflectance parameters.In order to ensure the high-precision measurement of reflectance in the long-term automated vicarious calibration process,the high-precision calibration of the automated test-site radiometer is generally completed in the laboratory.The calibration process needs to be completed by returning to the laboratory.The long calibration cycle will cause the loss of observation data,and long-distance transportation vibration will also affect the calibration accuracy.The damage to the automated test-site radiometer observation area caused by manual mobilization,installation and disassembly will seriously affect its consistency with the satellite transit observation area.In addition,light sources standard lamp and sun are used for instrument calibration and surface reflectance calculation,respectively in the satellite automated vicarious calibration.There are differences in spectral distribution between the two.To solve these problems,the on-site calibration method of the channel-type automated test-site radiometer was studied.The automatic observation data of the hyperspectral irradiance meter was introduced into the solar-radiation-based calibration method to calculate the sky diffuse illuminance.The acquisition methods and calculation processes of light source,aerosol optical thickness,diffuse to total ratio and other parameters used in the calibration of the automated test-site radiometer and satellite automated vicarious calibration were consistent.The solar-radiation-based calibration method is used to calculate the calibration coefficient and surface reflectance,and the calculated surface reflectance is compared with the manually measured surface reflectance using ASD spectrometer.Then the uncertainty of surface reflectance calculation is analyzed.The results show that the relative deviation between the reflectance of the automated test-site radiometer calculated by the solar-radiation-based calibration method and the reflectance of artificial ASD measurement is better than 1.4%,and the uncertainty of reflectance calculation is better than 2.78%~4.35%.Solar-radiation-based calibration method has high accuracy and system advantages in practical application.The calibration coefficient of the automated test-site radiometer calculated by the solar-radiation-based calibration method is applied to the satellite’s automatic vicarious calibration.The automatic vicarious calibration results in recent three years are in good agreement with the AQUA/MODIS on-board calibration coefficient.The relative deviation of single calibration of each channel is basically within 5%,and the average percentage deviation is better than 3.58%.It can monitor and track the operation of satellite load,and verify the effectiveness and applicability of the SRBC method.