Research And Application Of High-precision Automated Vicarious Calibration Method Based On Dunhuang Test Site

Considering the need of radiation calibration such as high precision,high frequency and traceability,it has become a trend to use standardized observation sites such as RadCalNet to obtain data and use unified processing methods to realize automated observation and automated calibration.The establishment of these sites can continuously provide surface reflectance data,atmospheric parameters and other data,and improve the evaluation frequency of radiation measurement accuracy of remote sensors.At the same time,the automated observation of the same instrument and the unified and standardized processing can reduce the uncertainty caused by different factors.At present,there are some shortcomings in the automated calibration of the site,such as space under-sampling caused by "point instead of plane" measurement,spectral under-sampling caused by channel instruments,and insufficient description of BRDF characteristics of the site to meet the application requirements of satellite remote sensors at different observation angles.Moreover,the differences between the empirical models adopted by each station,such as aerosol model and atmospheric model,and the actual situation directly affect the calibration accuracy,especially in the water vapor absorption band.In addition,based on the practical basis of AVCS,a self-developed automated calibration system deployed in Dunhuang test site,the main problems facing AV CS at present are analyzed,including lack of mature traceability,inversion algorithm of observation parameters,automated calibration method of the whole system,reliability evaluation and application of long time series.To solve the above problems,based on the observation data of the automated vicarious calibration system deployed in Dunhuang test site in the past 5 years,this paper carried out the research and application of the automated calibration method,and completed the following contents:(1)Research was carried out on PSR solar photometer direct channel calibration,water vapor observation channel calibration and atmospheric parameter inversion algorithms.CE318 with accurate calibration coefficient was used for algorithm verification and comparison.The results show that the aerosol optical depth and atmospheric water vapor content obtained by PSR after field calibration are in good agreement with those obtained by commercial instrument CE318,and the maximum deviations are 0.015 and 0.16g/cm2,respectively,which meet the requirements of practical calibration applications.(2)The influence of different aerosol models on radiometric calibration is analyzed,and the results show that improper selection of aerosol models in radiometric calibration calculation will have a great impact on the calibration results.The optical properties of the aerosol in Dunhuang test site were calculated by using CE318 direct and sky scanning signals.The results showed that the aerosol particles in Dunhuang test site showed a bimodal distribution dominated by coarse modal particles,which was in good agreement with the global dust aerosol and the desert-type aerosol in 6SV model.The measurement of aerosol model parameters can further improve the accuracy of automated calibration.At the same time,the basic atmospheric profile data is modified by compiling 6S V code and applied to automated calibration.The results show that the Dunhuang field atmospheric model is closer to the mid-latitude summer atmospheric model.In the absence of measured particle spectral distribution function and other data,some ideas and references are provided for radiometric calibration based on Dunhuang test site.(3)A high-precision calculation method of surface reflectance based on channel automated observation data is proposed,which solves the problems of undersampling of ATR spectrum,underrepresentation of "point instead of plane" and systematic calculation of surface reflectance in direction,and realizes real-time acquisition of surface reflectance in hyperspectral direction.When applied to automated calibration,the average relative deviation between the TOA reflectance predicted by AVCS and the results measured by Terra MODSI is less than 3.2%except for the water vapor observation band,and most bands are less than 2%.Compared with the application results of RadCalNet site,this method shows higher accuracy and stability.(4)The whole process automated calibration method based on AVCS is researched and the software integration is carried out to realize the automated observation and automated calibration of AVCS.Using the observation data of Aqua/Terra MODIS,SNPP VIIRS,Landsat7 ETM+,Landsat8 OLI and other highprecision remote sensors with different resolutions and observation angles,the longterm time series calibration evaluation and validation of AVCS were conducted.The results showed that the difference between the TOA reflectance predicted by AVCS and all the comparison satellite observations was small.The mean relative deviation was less than 3.5%(4.9%for ETM+661.4nm band and 6.9%for SWIR2 band).The high accuracy,stability and reliability of AVCS long-term automated calibration results are verified.(5)AVCS was used for long-term continuous calibration of domestic FY-3D MERSI-Ⅱ and GF1 WFV.The results show that all bands of FY-3D MERSI-Ⅱ have varying degrees of change during 2019-2022,among which the short-wave change amplitude is greater than 29%.The radiation properties of GF-1 WFV changed little in recent years,and the annual change rate was less than 0.31%.The domestic satellites after calibration show good consistency with the observation results of Aqua MODI S and Landsat8 OLI.(6)A cross calibration method based on AVCS is proposed,and the AVCS observation data is used as a bridge to realize the cross calibration of two remote sensors.Compared with the traditional cross calibration method,this method breaks the limitation of synchronous and near-synchronous observation,and can match the approximate observation conditions in a wider time range,thus increasing the calibration frequency.Using this method,the calibration of twin sensors,such as Aqua/Terra MODIS and Sentinel-2A/Sentinel-2B MSI,which basically have no synchronous crossing points,is successfully realized.This method reduces the contribution of AVCS measurement uncertainty to calibration,especially in the water vapor observation band,and effectively improves the calibration accuracy of water vapor observation band.Paying close attention to the trend of high-precision and high-frequency automated calibration,the research and application of the high-precision automated calibration method based on the Dunhuang test site carried out in this paper can meet the application requirements of long-term continuous on-orbit calibration and performance monitoring with different resolutions and different observation angles.At present,long time series high-frequency and high-precision automated calibration has been realized.It will directly support the operational calibration and quantification applications of domestic satellite remote sensors.