Study Of Attitude Determination For Spacecraft Based On X-ray Pulsars

X-ray Pulsar-based Navigation(XPNAV)has been studying widely for several decades,which can provide time,position,and attitude for the near-Earth,deep-space and interplanetary spacecrafts.In view of the high autonomy and reliability,strong anti-interference and low-cost of XPNAV,it may be the mainly employed navigation method in the near future for deep-space exploration missions.The dissertation focuses on the problem of attitude determination relying on pulsars.The author’s major contributions are outlined as follows:1.Aiming at autonomously determining the attitude of spacecraft,a method only using X-ray pulsars is proposed.The method establishes a cylindrical collimator measurement model firstly.The X-ray scanner employs two cylindrical collimators and one X-ray detector to observe the radiation photon energy of pulsars.And then the light curve is obtained according to the characteristics of response function and the mathematical model.To fully represent the randomness of pulsar radiation signals,the concept of information entropy is introduced.The principle of minimum error entropy is employed to fit attitude parameters and work out aspect solution of pulsar in the spacecraft coordinate system.According to the known position of pulsars in the celestial coordinate system,the spacecraft attitude with respect to the celestial coordinates can be calculated.At last,combined with the observation data from NASA’s HEASARC database,numerical experiments are given to verify the effectiveness of the method in this paper.The simulation results indicate that the precision of attitude determination only using X-ray pulsars can meet the requirements of spacecraft attitude determination mission.With the given conditions,the accuracy of parameters fitting by the algorithm of minimum error entropy is superior to least square method.2.An attitude determination method for a deep-space mission spacecraft is derived and its performance analyzed.The attitude determination system is composed of Inertial Measurement Units(IMU),an X-ray Pulsar scanner,onboard computers and electronic devices.IMU data is processed to maintain real-time knowledge of spacecraft attitude relative to an inertial reference frame.X-ray pulsar scanner data is processed using Unscented Kalman Filter(UKF)to estimate and correct the attitude determination errors and the gyro drift compensation errors.The results of two X-ray pulsars availability analysis for deep-space attitude determination mission are presented.Linear covariance analysis techniques are used to evaluate nominal attitude determination performance,the effects of sensor measurement accuracy variations,and the effects of gyros misalignment errors.3.The filter bank of "UKF+EKF" is designed to improve the reliability of combination attitude determination system.UKF is conducted as a main filter,while EKF is the backup one.A semi-physical simulation scheme is devised to verify the integrated attitude determination algorithm based on the data from NASA HEASARC.Results of a nonlinear simulation analysis of attitude determination performance are also shown.Compared to only using pulsars or IMU,the attitude determination accuracy of integrated algorithm is improved by 20%-35%,and the performance of processing nonlinear state functions for UKF is superior to EKF.It can satisfy the system requirements of high precision and stabilization.4.One of the important characteristics of pulsar radiation is polarization.It is considered not only as a probe for recognizing the structure of magnetic field,but also as a lighthouse for estimating attitude of spacecraft via the orientation information between pulsar and detector.Although the polarization of pulsar has been studied for decades,application to attitude determination of spacecraft has hardly been researched and reported until recently.This paper puts emphasis on analysis of the feasibility for applying polarization information to attitude estimation.To this aim,stability factor(SFR)and observation fluctuation factor(OFR)are introduced to analyze stability of pulsar’s polarized position angle.On the basis of EPN data,sevaral simulated instances are conducted to demonstrate that the accuracy requirement of attitude determination can be met via polarization measurement.Pulsar’s SFR is evaluated by means of simulated polarization data,and OFR is utilized to analyze the relationship of fluctuation extent and observation time.The results are shown that polarized measurement of candidate pulsars PSR B0470-28 and PSR B2319+60 can reach the specification for attitude determination.5.The accurate attitude determination for spacecraft has always been a significant issue in deep-space mission.This paper describes an approach which improves the performance of attitude estimation using X-ray pulsar for spin-axis stabilized spacecraft.The imperfect alignment of sensor’s view axis with spin axis of spacecraft introduces modulation errors to the light curve of photons emitted from the specified pulsar,which affects the outcome of the attitude estimation system.In order to reduce the effects of modulation to a minimum,the demodulation to observed pulsar’s light curve is derived in detail.The second order model and the parameter fit method are employed to estimate the spacecraft attitude by X-ray pulsar’s energy measurement.Numerical experiments are shown that precise attitude estimation can be obtained with this method.