Study On Instrument BSDF Calibration Of The Spaceborne Environmental Trace Gas Monitoring Instrument(EMI-?)

Environmental trace gas Monitoring Instrument(EMI)is used for global trace gas monitoring.It was launched on the Gaofen-5 satellite on May 9,2018.The earth observation optical path(main optical path)is used to observe the earth's atmosphere to obtain the absorption spectrum of trace gases.The solar observation optical path(calibration optical path)is used to observe the sun through the diffuser to obtain the solar reference spectrum.EMI uses differential absorption spectroscopy technology for trace gas inversion.The calibration optical path is used for spectrum calibration and instrument radiation monitoring during the flight,and on-orbit absolute radiance calibration is not performed.EMI-? is a follow-up load of EMI.In order to improve the accuracy of on-orbit radiation calibration and expand the remote sensing data retrieval algorithm,it is necessary to obtain the bidirectional scattering distribution function(BSDF)that characterizes the relationship between the radiation characteristics of the main optical path and the calibration optical path,based on this parameter,the absolute radiance calibration of EMI-? in orbit based on the solar standard source can be realized.The BSDF parameter of the EMI-? is defined as the ratio of the irradiance response of the main optical path to the irradiance response of the calibrated optical path.This article first obtains the BSDF model according to the EMI-? calibration structure characteristics,and analyzes and discusses the influencing factors in the model,and calculates the theoretical value of the whole machine BSDF,which provides a reference for the analysis of the experimental test results.According to the requirements of BSDF calibration,the absolute irradiance calibration and absolute irradiance calibration of the main light path and the calibration light path are carried out respectively;this paper uses the integrating sphere method to calibrate the main light path,and the dark noise is deducted from the calibration data.Spatial response non-uniformity correction,and based on the linear response model of the load,obtain the absolute radiance calibration coefficient;use the direct divergence method to calibrate the irradiance of the calibration optical path,based on EMI-? on-orbit solar Calibration angle relationship,using a two-dimensional turntable to perform different angle calibration tests on the load,and obtain the absolute irradiance calibration coefficients corresponding to the PTFE diffuser reflector and the quartz diffuser reflector in the calibration optical path;During the process,the sources of uncertainty were also analyzed,and the absolute radiation calibration uncertainty corresponding to the four channels were all less than 5%,which met the requirements for the radiation calibration of the load.Using the obtained results of the absolute radiance and irradiance calibration coefficients,the BSDF parameters of the EMI-II machine are calculated.The parameters are a function jointly determined by the wavelength,the incident angle and the observation direction.In order to meet the on-orbit application requirements of EMI-II,the calculated BSDF parameters are interpolated in two dimensions based on the load on-orbit observation angle,with an azimuth angle interval of 5' and a pitch angle interval of 3'.At the end of the article,the on-orbit application scheme and verification method of BSDF parameters are proposed,and the attenuation characteristics of the diffuser are discussed.