On-orbit Calibration Of Particulate Matter Polarimetry Suits

PM2.5 is an important component of aerosols,which has a major impact on the global climate and atmospheric environment.It is also one of main factors to cause the smog and haze weather and is one of key monitoring parametric targets of atmospheric pollution.Multi-angle multi-spectral polarimetric measurement technology is one of advanced means for retrieving aerosol comprehensive parameters in the field of passive remote sensing.Particulate Matter Polarimetry Suits(PMPS)is comprised of Directional Polarimetric Camera(DPC)and Particulate Observing Scanning Polarimeter(POSP),and could directly remote sensing PM2.5 concentration near the ground from space.PMPS is one of the most comprehensive aerosol polarimetry sensors at present.On-orbit calibration is an important means to ensure data accuracy and precision from spaceborne instruments,it is of great significance for the quantitative retrieval of aerosol comprehensive parameters.In this paper,on-orbit calibration of PMPS is researched,the research mainly includes two parts.One is on-board calibration of POSP,the other one is cross-calibration of DPC using high accuracy and precision data from POSP based on the same platform,the same observation geometry and the same four spectral bands.In detail,the measurement scheme of PMPS is studied,the technical characteristics of PMPS based on polarization cross-fire(PCF)are summarized,and the needs of on-orbit calibration is proposed.The measurement error model of POSP is established,the polarimetric accuracy influence of system errors is analyzed,the errors tolerance range is given,the laboratory calibration of POSP is builded,and on-board calibration methods of POSP is optimized.Non-polarization calibrator(NPC)and linear polarization calibrator(LPC)utilizing the Earth reflection provide two types light source with known polarization states.It is the optimal solution under the balance of calibration accuracy,calibration frequency and resource requirements.The influence of standard source error is analyzed,and the optimal azimuth of linear polarized light is 22.5 degree.NPC and LPC are designed in detail,and related tests are carried out.The Marius’s law is used to accurately determine polarization state of linearly polarized light,and the ideal unpolarized light is determined by iterating with the sub-satellite point data.A configuration of solar+solar diffuser+ratio radiometer is used to ensure onboard radiometric accuracy of POSP.All on-board calibrators are designed at the forefront of optical path to meet full aperture,full field of view,and end-to-end requirement.Due to super wide-angle field of view of DPC lens,on-board calibrator cannot be equipped.In order to ensure the measurement accuracy of whole frame data of DPC,cross-calibration between DPC and POSP based on field-of-view matching technology at the same platform is proposed.The key factors of spatial response matching are analyzed.The coefficients of relative transmittance,linear attenuation and absolute radiometric calibration coefficients of DPC are analyzed and calibrated.The calibration transfer is realized.In order to solve the key problems in cross calibration,an airborne system based on POSP and simultaneous polarization camera(SPC)is built.The airborne test based on PCF application are first carried out in China,and effective flight data of five trips is obtained under different altitudes and different weather conditions.The feasibility of cross-calibration field of view matching was verified.Polarimetric and radiometric accuracy will be ensused when PMPS is on orbit.And more high accuracy PM2.5 concentration near-ground will be monitored from space.It will provide more scientific data support to solve global climate problems and air pollution problems.Moreover,it could provide a valuable reference for on-orbit calibration of other polarimeters.