The Study Of Onboard Radiometric Calibration For FY-3C/MERSI Teb

The onboard radiometric calibration for satellite instrument is a most efficient method to improve the measurement accuracy.MODIS(Moderate Resolution Imaging Spectroradiometer)uses a variable temperature 270K to 315K blackbody for its TEB(Thermal Emission Band)onboard nonlinear calibration,but the Chinese MERSI(Medium Resolution Spectral Imager)on the FY(Fengyun)-3C satellite uses a constant-temperature?284K blackbody for its TEB onboard calibration.So the NSMC(National Satellite Meteorological Center)has not offered official calibration coefficients of MERSI TEB because MERSI's radiometric calibration is still being tested.This work focuses on selecting the optimized calibration algorithm for onboard radiometric calibration of MERSI TEB to realize and promote the application of FY-3C and other satellites'thermal inf'rared data.In this study,the onboard radiometric calibration of FY-3C/MERSI-TEB is carried out by using the onboard blackbody at a constant temperature and the deep space as reference.The calibration coefficients a0,a1 and a2 are determined by regression analysis between the estimated radiance L and the readout counts dn of the two references for three cases:linear calibration,semi-quadratic calibration and full-quadratic calibration.The onboard calibration can be achieved quickly by using the processing software of FY-3C/MERSI TEB,which is developed with IDL(Interface Description Language).The characteristics of the three algorithms' coefficients are analyzed,respectively.The zeroth and first-order coefficients obtained by linear calibration showed obvious differences between each CCD detector and each mirror side,and remain stable between each individual scan for certain detector and mirror side.The estimated coefficients obtained by semi-quadratic calibration were highly similar to linear calibration's coefficients,but they were still tending to underestimate the difference between each CCD detector.The estimated coefficients obtained by quadratic calibration were not only tending to underestimate the difference between each CCD detector and each mirror side,but also bringing substantial random changes between individual scan.We can conclude that the estimated coefficients a0 and a1 obtained by linear calibration can fully reflect the basic characteristics of remote sensorThe three algorithms' validity can be verified by comparing the difference between inferred blackbody radiance LiBB and standard blackbody radiance LiBB,and by comparing the difference between inferred blackbody brightness temperature TiBB and equivalent blackbody brightness temperature TeBB.Their relative errors between LiBB and LsBB are all smaller than 0.01%,and the temperature differences between TiBB and TeBB,are all smaller than 0.25K,which indicate that LiBB vs LsBB and TiBB vs TeBB both coincide very well.Also,the LiBB vs LsBB and TiBB vs TeBB differences for all three algorithms show obvious variations between individual detectors and mirror sides,but are relatively stable from scan to scan,which show very similar variation trends to the coefficients of linear calibrationThus,the MERSI's currently used constant-temperature blackbody has its intrinsic shortcoming and cannot accurately determine the second-order coefficients a2 which is related to the instrument cavity changes.At the present,the linear calibration can be regard as the optimized algorithm for FY-3C/MERSI TEB onboard radiometric calibration.In the future,to better characterize the on-orbit response of CCD detectors,the feasible solution is to develop variable-temperature blackbody,similar to that of MODIS.