Inversion And Accuracy Verification Of Spectral Calibration Parameters For Spaceborne Infrared Hyperspectral Instrument

In order to meet the requirements of high-precision detection and application,the spectral resolution of the spaceborne infrared hyperspectral instrument is required to be less than1cm^-1,the radiometric bias can be expected reach less than 0.5K,and the spectral frequency accuracy is required to reach 10.00ppm.Since the spectral accuracy directly affects the radiometric calibration,the infrared interferometer need to perform spectral calibration and accurate evaluation of the spectral accuracy before the data application,which has become one of the most critical techniques for the calibration of infrared interferometer.Fengyun-3D?FY-3D?was successfully launched on November 15,2017.The High-spectral Resolution Infrared Atmospheric Sounder?HIRAS?onboard the FY-3D satellite is a new infrared interferometer which was manufactured completely by China.The accuracy and quality of hyperspectral data is the basis of quantitative aplication,which is closely related to the fabrication technology of detector and data preprocessing techniques.For the multi-detector infrared interferometer,the detector pixel that deviates from the main optical axis receives the radiance coming from different direction within a solid angle,and has the off-axis effect.Spectral calibration is to calibrate the frequency of instrument observation spectrum to the correct position.The correction of the off-axis effect is directly dependent on the sampling laser wavelength,the size of detector pixel,and the position parameters?these parameters are called instrument spectral calibration parameters?.The instrument parameters were obtained from the ground tests before launch,but exist measurement errors,which need to be adjusted again.In this study,based on the spectral calibration parameters of the FY3D/HIRAS ground test instrument and a more accurate forward model LBLRTM,off-axis spectra were simulated.The sensitivity of off-axis spectrum to the laser wavelength error and the off-axis distance error of the detector was studied respectively.Based on the physical relationship between the spectrum and the off-axis distance of the detector,the spectral calibration is optimized by fine-tuning the position parameters of the detector,aiming to improving the accuracy of spectral calibration.In the spectral calibration accuracy evalution,the spectral accuracy is calculated based on the cross-correlation method.Since the corss-correlation method depends on the selection of spectral regions,this study further analyzes the sensitivity of spectral region selection to spectral deviation calculation,and proposes the most accurate speatral accuracy evaluation method.Excellent spectral regions improve the accuracy and efficiency of spectral accuracy calculation.In addition,the use of cross-correlation method requires the observation spectrum of clear field of view.This study also developed HIRAS clear scene detection scheme,which can efficiently acquire a large number of clear-sky observation spectra and support subsequent applications.The main conclusions are summarized as follows:?1?The sensitivity of the off-axis spectrum to the error of the laser wavelength and the error of the off-axis distance of the probe are studied.The results show that the change of spectral frequency shift are consistent with the change of laser wavelength.When laser wavelength changes by 1.00ppm,can cause spectral shift about 1ppm.The change of off-axis distance of the detector and the change of spectral frequency position can be regarded as the direct proportion under the condition that the off-axis distance changes by less than 3%.For example,a 1%off-axis distance change can cause a frequency offset of about 5.00ppm.Additionally,the off-axis effect is also depends on the frequency.The high wavenumber have the more serious off-axis effect.?2?Based on the in-orbit data,the retrieval test of the position parameters of the detector is carried out.The results show that the off-axis detector positional parameter has a maximum adjustment of about 1.8%compared with the ground test.The spectral correction can be performed based on the new spectral calibration parameters.The spectral calibration accuracy is better than the instrument design specification.The spectral calibration bias of the long-wave band is better than 1.00ppm,the standard deviation is 1.40ppm,the spectral calibration accuracy of the mid-wave band is about 1ppm,the standard deviation is 1.20ppm,and the spectral calibration accuracy of the short-wave band is better than 3.50ppm,the standard deviation is 5.60ppm.The retrieval of the detector position parameter is applied to the spectral calibration of other independent clear-sky observation spectra,and the spectral accuracy can reach the accuracy consistent with the test results,the overall spectral accuracy is better than 2.00ppm,and the standard deviation is less than 3.00ppm.?3?The selection of spectral region must be considered when calculating spectral accuracy by cross correlation method.The sensitivity of spectral region selection ia analyzed,and the results show that the spectral region cannot be arbitrarily chosen,because if the spectral region contains the atmospheric window,the number of absorption lines is correspondingly reduced,and the absolute error introduced can reach 3.05 and 3.35 ppm respectively in the long wave and short wave bands,respectively.Infrared band water vapor absorption line is dense,however,the spatial and temporal distribution of water vapor is not uniform,its strong absorption makes the radiation energy lower,so the strong absorption position of water vapor needs to be avoided in the middle wave band.Studies have shown that in the long-wave bands:select the region where water vapor and stable carbon dioxide absorption overlap,the spectral region starting wavenumber is from 716 cm^-1 to 719 cm^-1,for example as the region[716,766]cm^-1.In the mid-wave bands:Select the region containing the strong absorption band of methane and nitrous oxide.The spectral region starting wavenumber is from 1267 cm^-1 to 1273 cm^-1,for example,the region[1270,1320]cm^-1.In the short-wave bands:avoid The strong absorption band of carbon dioxide and the atmospheric window,and have the spectral regions[2159,2209]cm^-1.Based on the choice of spectral region,the average HIRAS spectral calibration accuracy is less than 2.00ppm in all three bands.The standard deviation is less than 2.00ppm in long-wave and mid-wave bands,and about 4.00ppm in short-wave bands.?4?The cross-correlation method for calculating the spectral accuracy should be under the clear scene.This study also developed a clear scene detection scheme for infrared hyperspectral instruments,which can efficiently and accurately obtain large-area of clear-sky samples and support subsequent applications.At present,HIRAS spectral accuracy monitoring,radiometric accuracy assessment monitoring,HIRAS data assimilation application in the Grapes model,atmospheric composition inversion and other scientific research and operational work are all using the clear scene detection results developed in this research.