Study On The Key Technologies Of Spectral Calibration For Airborne Hyperspectral Imager

As an image-spectrum merging technology,hyperspectral imager has been widely applied in many fields such as precision agriculture,ocean observation,urban planning,disaster monitoring and so on.The hyperspectral imager can not only obtain the geometric characteristics of the target,but also detect the target's spectral reflection characteristics quantitatively.Due to the imager's measurement accuracy of the target's spectral characteristics is highly dependent on the spectral precision of the instrument,it is necessary to perform high-precision spectral calibration on the hyperspectral imager.In this thesis,the crucial technology of spectral calibration of airborne hyperspectral imager is studied,and the problem of instrument's digital response distortion caused by water vapor in the laboratory spectral calibration process is solved.A spectral absorption method based on water vapor is proposed.In addition,this thesis proposes a new onboard spectral calibration algorithm based on atmospheric absorption characteristics with small-scale artificial Lambertian ground targets,which solves the impact of atmospheric underlying surface on the results of onboard spectral calibration.In this thesis,the theoretical basis and key technologies of spectral calibration are introduced in detail,and the validity and feasibility of the algorithms are proved by actual experimental data.The main content and innovation achievements of this thesis include the following aspects:1)This research proposes a laboratory spectral calibration method based on monochromator and the water vapor spectral absorption characteristics,which is based on the detailed analysis of the principle of hyperspectral imager spectral calibration process and the phenomenon of absorption of colored light happens during the process of laboratory spectral calibration.The laboratory water vapor spectral calibration method solves the distortion problems of digital response curve in the wavelength range of 1350 nm-1420 nm and 1820 nm-1940 nm under the condition of normal temperature and pressure.The experimental results showed that the water vapor spectral calibration algorithm enables the hyperspectral imager to overcome the distortion phenomenon of the digital response curve deviates from its intrinsic Gaussian shape caused by water vapor absorption in a non-vacuum environment by introducing the central wavelength offset,the spectral response half-width extension and the change in response efficiency.The method restores the intrinsic spectral response functions of the instrument located in the water vapor absorption wavelength range to the greatest extent,and the hyperspectral imager's laboratory spectral calibration accuracy is highly improved.2)This research corrects the system error of the monochromator's annotated spectral postion by establishing the difference function between the theoretical response curves and the actual response curves of the hyperspectral imager,and accomplishes the process of the high-precision laboratory spectral calibration of hyperspectral imager.The spectral calibration accuracy of the short-wave infrared module is improved from ±0.5 nm to ±0.125 nm.The thesis expatiates the principle and specific implementation steps of the laboratory spectral calibration method based on the monochromator and the water vapor spectral absorption characteristics.The calibration algorithm's effectiveness is analyzed using an airborne short-wave infrared hyperspectral imager and a single-wavelength semiconductor laser,and the spectral calibration accuracy of the method is evaluated quantitatively.The results suggest that the laboratory spectral accuracy of the short-wave infrared module of the hyperspectral imager can be calibrated better than 5% of the instrument's spectral resolution.This method not only presents some innovations in the accuracy of spectral calibration,but also reduces the cost of laboratory spectral calibration and simplifies the experimental operation steps.It is an innovation of the laboratory spectral calibration method of hyperspectral imager,and lays the foundation for hyperspectrometer's practical applications.3)This research presents an onboard spectral calibration method based on standard diffuse artificial Lambertian boards with a gradient of reflectivity,and the spectral response function's position shift caused by the difference between the laboratory and the actual flight environment of the hyperspectral imager is thoroughly investigated.According to the significant absorption of electromagnetic waves caused by atmospheric oxygen,carbon dioxide and water vapor,this method completes the onorbit spectral calibration of a hyperspectral camera by constructing a difference function of the simulated and actual equivalent pupil radiation curve.This method adoptes the non-uniformity correction technology,which corrects the smile spectrum bending phenomenon of hyperspectral images,making it possible to perform full-field spectral calibration based on the small-scale ground targets.4)The underlying surface of the atmosphere composed of complex types of features has a significant impact on the on-board spectral calibration results of airborne hyperspectral imagers.This research successfully eliminates the influence on the onboard spectral calibration results caused by the atmospheric underlying surface based on the artificial ground Lambertian targets with a gradience of reflectivity,the atmospheric correction of hyperspectral flying images and the spectral reflectance retrieve of ground objects.The influence of the calibration results is also analyzed in depth.The work of this research can reduce the uncertainty of onboard hyperspectral spectral calibration results and lay the foundation for large-scale applications of hyperspectral images.5)In this research,an onboard spectral calibration method based on ground-based artificial Lambertian targets with a gradience of reflectivity is employed to calculate the spectral offset of hundreds of thousands of frames of hyperspectral flight images.The 1 times of the spectral uncertainty ? of hyperspectral VNIR module is ±0.08 nm,2 times of ? is ±0.15 nm,and the 3 times of spectral uncertainty ? is ±0.23 nm.The 1 times of the spectral uncertainty ? of the short-wave infrared module is ± 0.13 nm,2 times of ? is ±0.25 nm and the 3 times of spectral uncertainty ? is ±0.38 nm.In this research,the high-precision onboard spectral calibration is no longer limited to the deployment area of the artificial ground targets,and the monitoring of the spectral state of the airborne hyperspectral imager during the entire flight process is achieved.The onboard spectral calibration accuracy is better than one-tenth the spectral resolution,which helps to improve the application ability of hyperspectral imager.