Research On In-orbit Spectral Calibration And Polarization Correction Technology Of Visible/near-infrared Pushbroom Imaging Spectrometer

As important means of marine resources survey and environment monitoring,the satellite ocean remote sensing technology has been widely used in water primary productivity assessment, fishery resources, marine pollution detection and prevention, etc. With the continuous growth of space remote sensing technology, higher radiometric calibration precision requirements for the sensors have been put forward."Wide-band Imager" is designed for ocean color remote sensing and belongs to a low-earth orbit space platform, which will be launched into in this year. The visible and near-infrared module is the major subsystems of the mission. This dissertation focuses on improving the onboard radiometric measurement accuracy of the push-broom dispersive grating visible and near-infrared imaging spectrometer. Two factors have been intensively accounted for in radiometric calibration are spectral characteristic changes and polarization response of the instrument. And this research describes the development of both in-orbit spectral calibration method and polarization correction method. The main contents include:1) The visible and near-infrared imaging spectrometer system design, optical alignment method and on-board calibrator are analysed based on its specification requirements. The performance parameters and radiomatric calibration results of the instrument are derived through testing experiments before launching.2) The impact of spectral shift on the radiometric accuracy of the visible and near-infrared imaging spectrometer is assessed by the simulation model. Results show that significant influences arise near the wave range of Solar Fraunhofer lines, oxygen and water vapor absorption features. Under typical observing conditions, when the bandwidth of 10 nm, 5% radiometric measurement error is introduced by 1nm central wavelength drift, when the drift 2.5nm, measurement errors> 10%.3) The ‘channel synthesis technology’ is developed through theoretical analysis of spectral response function relationships between the full-frame mode and the sub-channel mode. Sub-channel spectral response function can be synthesized by the full-frame spectral response function, which is the base of in-orbit spectral calibration for sub-channel mode.4) An in-orbit spectral calibration method is proposed based on a spectrum matching technique of solar Fraunhofer lines, atmospheric absorption lines and spectrum features of the praseodymium neodymium glass diffuser illuminated by halogen tungsten lamp belonging to the on-board calibrator. In-orbit spectral calibration simulation experiments were carried out using field diffuse reflectance standards and the internal light source of on-board calibrator, respectively. Correlation coefficient combined with the least square optimization algorithm is applied to process the experimental data. The results show that the algorithm is sensitive to changes of the spectral characteristics of the imaging spectrometer, and the standard deviation of the center wavelength shift is less than 0.176 nm. Comparing with the ‘direct contrast method’, the accuracy is improved by an order of magnitude.5) A polarization response equation is derived based on the Stoke vectors and Mueller matrix, then a comprehensive analysis is performed on polarization influences in the calibration of sensors. On this basis a polarization correction algorithm is proposed for the imaging spectrometer. Radiometric measurement corrections are obtained by the polarization response coefficient. Taking the 797 th pixel of cross-track in CH10 as an example to validate the algorithm, the imaging spectrometer is illuminated by the partially polarized light with a known Stokes vector. Results show that the algorithm is suitable for polarization response correction. Under the condition that the incident light is completely polarized with a known azimuth angle, depending on polarization sensitivities of different spectral channels and field of views, the uncertainty of top-of-atmosphere radiation measurement can be reduced by 1%-6%, and the residual polarization sensitivity is less than 0.4%. The correction algorithm can be used to improve the radiometric measurement accuracy of water-leaving radiance in ocean remote sensing.6) An instrument polarization response test system is established. Polarization response amplitudes and phase angles of the visible and near-infrared imaging spectrometer prototype was derived, which vary with spectral channels and the field of view. 980 nm band polarization sensitivity average of 4.69%, while 413 nm band with an average of 1.88%. A polarization response parameter lookup table of 14 spectral channels × 1024 pixel fields of view is provided as a reference for the in orbit polarization correction of the instrument.