Research On Unsteady Thermal Analysis And Error Calibration Methods Of Star Sensor

Star sensor has the advatanges such as small volume,light mass and high precision,and so on.Thus,it is irreplaceable in spacecraft attitude determination and control system.In recent years,with the development of space technology,the denmand for the precision of star sensor is increasing.While during operating in orbit,space heat flux will cause star sensor optical system to be deformed,which affects the precision of star sensor directly.In this paper,unsteady thermal analysis,thermal deformation,and orbital error calibration of star sensor's optical system are studied.The main contents of the paper are as follows:Space heat flux calculation is the premise of spacecraft thermal analysis.The traditional method such as integration method and Monte Carlo method,used to calculate the space heat flux,are large computing and low computational efficiency.In the paper,a new method which based on Mapping Plane of Environment(MPE)and Reverse Monte Carlo(RMC)method is proposed to calculate the space heat flux,and the periodic variation of radiation intensity caused by revolution is considered.Initially,environment map is designed according to system structure model.Then,RMC method is used to dertermine random model of the position and direction of rays.Furthermore,backtrack these radiation heat rays,and determine the visibility between rays and receiving surface by MPE.Ultimately,statistical results of these radiation heat rays are obtained.The heat flux of different points on the system surface at different moment in the orbital period is simulated,and the correctness and validity of the proposed method are verfied.Regarding unsteady temperature distribution of star sensor caused by space heat flux,on the base of designing a star sensor optical system with large field of view and high precision,calculation results of space heat flux obtained in previous being are used as thermal boundary conditions,and node network model and thermal network balance equations are established for unsteady temperature response analysis.According to the characteristics of thermal network balance equations,a method based on Precise Exponential Integration is presented for solving nonlinear inhomogeneous equations,and unsteady temperature distribution results of star sensor optical system is obtained.Simulation results show that Precise Exponential Integration method is with high precision,not sensitive to step length,strong nonlinear adaptability,and better stability.Transient temperature distribution results show that: affected by space heat flux on-orbit,the temperature of star sensor optical system is not evenly distributed,periodic changing,and along the axial,radial and weeks up larger temperature difference;In the process of entering and leaving the shadow area,the temperature changes violently.And temperature difference is larger along the axial,radial and circumferential direction.Temperature changes violently in the process of in or out of the earth shadow.The influence of unsteady temperature distribution on the deformation of star sensor is analyzed.The temperature response is used as the boundary condition,and the thermo-structural variation law of the optical system is obtained by finite element method.Using the Zernike polynomial to fit the lens surface shape,the relationship between the structural deformation and the optical system imaging error is obtained.The error factors of the star sensor optical system are determined,and the influence of each error factor on the measurement accuracy of the star sensor is analyzed.For the optical system errors caused by thermal deformation,an optical system error model of star sensor is built based on the principle of star angular moment.In order to resolve the problem of multi-parameters,nonlinearity,parametric matrix orthogonal of error model,a two-step calibration method is proposed.The accuracy of this error model and the effectiveness of the two-step calibration method are verified by numerical simulation.Aiming at the orbital calibration of star sensor optical system errors,this paper presents an independent on-orbit calibration method for star sensor errors in the process of image point centroid extraction.The influence of optical system errors on the shape and energy distribution of star image spots is analyzed.A centroid extraction model of star image point with optical system errors is established by mapping relationships.In order to calibrate error parameters in the model,an improved kalman filter algorithm based on variable feedback parameters is proposed.Estimation accuracy and computational efficiency of the improved EKF are analyzed in simulation.Furthermore,the accuracy of the centroid extraction model,the validity of the improved EFK and the feasibility of star sensor optical system errors calibrated by centroid extraction model of image point,are verified.