Study On Performance Evaluation And Radiometric Calibration Of The Earth Limb Annular Imager

As the most hot observation method nowadays, space-based limb remotesensing of atmosphere has effectively merits of high coverage similar to nadirviewing and high precision and vertical resolution similar to occultation, with anembodiment of multiplicity on task and continuous detection on time. The EarthLimb Annular Imager is a kind of remote sensor just like that. It theoretically givesreal time spectral scattering information in multi-directions of limb observing andnadir observing data in10degrees field too. This new detecting method is a firstinternational way which provides data for remote sensing of atmosphere based onmulti-directions and double field observing mode.An UV/visible Limb Annular Imager system with140～160degrees wide-fieldis developed in this paper based on the application demands of the earth limbatmospheric detecting on future platform with the support of863project. It includes6wavebands, each consisting of optics unit,electronics unit and mechanical shell,while12pyramids reflector,filter,lens and CCD detector units combining to formthe whole optics unit. According to the UV/visible spectral radiometriccharacteristics of earth limb atmosphere and the optics designing and emulating withZemax software, an optics unit of the Annular Imager which satisfied the demands isdesigned finally. With configuration characteristics of the system, the integrating of system is studied and the experimental prototype gives a perfect consistency with thetechnology demands.Over recent years, the research of space remote sensor in our country isvigorously developed, the testing and calibration of UV spectrometer,visible-infrared imager or imaging spectrometer is fully experienced. But UV imageror imaging spectrometer is just on the initial stage. Without any experiences onperformance evaluation and calibration, the huge140°～146°field test andcalibration is the biggest technical difficulty in this project.A series research about the designing,performance evaluation and radiometriccalibration is developed on that experimental prototype system as below:1. An UV spatial resolution testing device is designed and a feasible testing ofthe horizontal and vertical spatial resolution of the experimental prototype is made,while the results commendably meet the technical guide lines;2. The proper response range and conversion equation among differentintegrating times of the imager is confirmed by accurately measuring the linearitywith the help of a high stable,adjustable UV integrating sphere system;3. Signal-to-Noise calculative equation of remote sensor is analysised anddeduced. By combining with not only the atmospheric spectral radiance under aknown solar altitude angle and earth limb height but also the spectral transmissivequality of the experimental prototype's optics system, a theoretic SNR of each bandis estimated and a real testing is made, which gives a reference to the late project;4. Test and analysis is made on the polarization characteristics and its effectsof the prototype. Further research shows a major reason caused by the interferencefilter for that polarization effection, which gives new requirements for purchasingthe interference filter in later stages;5. An UV/visible high precision wavelength calibration and relative spectralresponsibility testing device is designed and effectively solved the limits about theband and energy for the traditional method. The precision of wavelength calibrationfor the prototype is±0.5nm; 6. The transfer method of spectral radiance of a large aperture UV/visibleintegrating sphere is studied. A field segmental spectral radiance calibration methodis put forward by analyzing the problems of the wide-field Annular Imager. Study ofthe spectral radiance calibration of the visible Annular Imager with both full andsegmental field is made using an aperture diameter of1.5meters visible integratingsphere. Then a segmental field method which is proved to be effective is used on thespectral radiance calibration of the ultraviolet Annular Imager;7. A special equipment for geometric calibration of the annular imager isdesigned, a corresponding relationship between effective area and the azimuth isgiven.Thought of the UV/visible Limb Annular Imager makes an international levelachieved in height and precision of space-based limb remote sensing in our country.It is a foundation stone in technology for the next formal step since the triumphantlydesigned of the prototype, which supplies a gap in domestic related domain.