Inversion Of Characteristic Information And Motion Information Of Low Speed Non-resolved Targets In Near Space From Photometric Data

Near-space is the region of the atmosphere between an altitude of about 20 and 100 km.In this region,air is too thin to support flight of most aircraft and too thick for a satellite to sustain orbital operations.Vehicles created to fly in nearspace include stratospheric airships and high altitude balloons(HAB).Many aircrafts in the near-space region are for military purposes and thus have intelligence,surveillance,reconnaissance,and communication capabilities.Situational awareness of aircraft in near-space,which involves scouting,detecting,identifying,and tracking low-and high-dynamic vehicles in near-space,has been of importance for a long time,especially with the rapidly growing use of high-altitude balloons and airships.The exploratory research work is requested to develop methods to better identify the various low speed targets in near space and ultimately to keep such objects under the surveillance of opto-electronic equipment or radar.The radiation temperature of low speed targets in near space is approximately the ambient atmosphere temperature,which are too distant or too small(making them dim)to make themselves to clearly resolved imaging in the influence of atmospheric scatterance and optical system diffraction phenomenon.To address issues outlined above,this paper investigate a series of studies on this phenomenon.The main contents are as follow:Firstly,optical scattering characteristics of high altitude balloon in near space are researched based on the fundamental radiation theory in applied optics.Facet of balloon in near space is created according to the general idea of coordination transformation and surface mesh-creation.A bidirectional scatterance distribution function model is deduced by summing transparent object all scattering components,including specular,near-specular,diffuse and random diffuse components.Then a mathematical model of scattering characteristics of balloon in near space is built.An atmospheric modeling tool,MODTRAN,is used to model background radiance in the range of 0.36-1?m,3-5?m and 8-14?m and radiance of balloon in the range of 0.36-2.4?m.Secondly,ground-based detection ability of high altitude balloon in near space is analyzed based on the fundamental optic-electronic detection theory.Radiance model of detection system is built based on background radiation and radiation of high altitude balloon.The radiometric signal electrons in focal plane array produced by high attitude balloon in near space and background radiation are calculated precisely by considering the effects of atmosphere transmission,optical system imaging,detector and detector sampling.Then signal-to-noise ratio is deduced for high attitude balloon in near space.An atmospheric modeling tool,Modtran,is used to model radiance of self-emission,specular background reflection and diffuse background reflection.Radiation characteristic of balloon in complex atmospheric is analyzed.And the influence on optic-electronic equipment's SNR of specular reflectivity,diffuse reflectivity and integration time also is analyzed.To validate the previous analysis that the indirectly observable states for a HAB,such as position and velocity parameters,can be estimated from the non-resolved photometric and astrometric data,a ground-based opto-electronic experimental setup is built.Two sets of experiments are carried out under the same observation condition.A set of object images for the HAB were captured in clear weather by the ground-based opto-electronic detector.After zero field correction and flat field correction using the correction images,these non-resolved object images were processed using the PHOT program package in the IRAF software;the processed results are the wavelength-dependent non-resolved photometric data.The processing method is called aperture photometry.Thirdly,in order to obtain the shape and dimension information of low dynamic non-resolved object in near space,the optical detection experiments of high altitude balloon were carried out to investigate how to estimate object shape and dimension information using photometric data.In inversion course,two shape representation methods are used to describe object shape and three sets of regularization functions are used to restrict object shape.Based on the Fourier transform of photometric data and model data of the shape representation methods,the shape and dimension information of the non-resolved object are inversed combining with the point spread function of the optical system.The comparative results indicate that the prominent features presented in the object shape are similar,which shows that the features are extracted from the photometric data.Fourthly,an estimation method for indirectly observable parameters for a typical low dynamic vehicle(LDV)is presented.The estimation method utilizes apparent magnitude,azimuth angle,and elevation angle to estimate the position and velocity of a typical LDV,such as a HAB.In order to validate the accuracy of the estimated parameters gained from an unscented Kalman filter,two sets of experiments are carried out to obtain the non-resolved photometric and astrometric data.In the experiments,a HAB launch is planned;models of the HAB dynamics and kinematics and observation models are built,to use as time update and measurement update functions,respectively.Two sets of actual and estimated parameters are given to clearly indicate the parameter differences.Two sets of errors between the actual and estimated parameters are also given to show how the estimated position and velocity differ with respect to the observation time.The similar distribution curve results from the two scenarios,verify that non-resolved photometric and astrometric data can be used to estimate the indirectly observable state parameters(position and velocity)for a typical LDV.