Signal Analysis And Processing Based On X-ray Pulsars Navigation

Autonomous navigation in deep space is a difficult problem. At present the spacecraft navigation method mainly are celestial navigation, satellite navigation, inertial navigation and earth-based navigation. Navigation for Spacecraft close to earth can be implemented through earth-based navigation or satellite navigation system, such as GPS, GLONNASS and COMPASS. For those in deep space, earth based navigation system is the only option, whose performance is low because of the distance from earth and the restraint of earth size. X-ray pulsars are potential for navigation in deep space as an alternative. Based on simulation of signal, estimation of pulse frequency and phase, and influence of the general relativity, the following work is completed:The basic theory of X ray pulsar navigation is presented, including the characteristics of pulsars, the relativistic references of the space-time, the principle of positioning and the navigation database. The concept, classification, distribution and radiation properties of pulsars are presented, as well as the characteristics of pulsars which is suitable for navigation. The relativistic references of the spacetime is investigated, including spacetime interval, definition and implementation of reference system, and definition and conversion of time scales. The establishment of pulsar navigation database should contains: selection of navigation pulsars, development of phase model and the creation of pulse profile.A numerical simulation algorithm for the reconstruction of photon TOAs is derivated from distributive function. Based on the fact that the arrival of photons is a nonhomogeneous poisson random process, the modle of the signal of X-ray pulsars is built. The signal of pulsar PSR B0531+21 was simulated, and chi-square goodness-of-fit-test is ultilized to demonstrate that the simulated TOAs follow the poisson distribution. Epoch folding is implemented on the reconstructed signal, and observed pluse profiles were obtained. It shows that observed pulse profiles approaches to the standard template profile as observation time increses, which demonstrates that the simulation algorithm is feasible, and the signal of X-ray pulsars can be reconstructed effectively.Frequency of the pulsars signal received by the detector is shifted by Doppler effect, and the actual frequency can be obtained by estimators. the best approch to search signal frequency is sample variance method so far. it is showed that the somple variance is convex discretely, and an optimal search method is proposed according to the analyse of monotonicity of sample variance against frequency. For the case of unconstant frequency, an approach aided by IMU is proposed to transform unconstant frequency to constant frequency. Simulation demonstration is implemented. Relationship between time bins and search precise is analysed by simulation, and emprical formula is obtained, which is able to improve the search precise and velocity.Estimation of pulse phase is crucial for X-ray pulsar navigation. Three phase estimators(the least square method, the correlation method and the maximum likelihood estimation)are prensented and analyzed. A modified maximum likelihood estimation is proposed. Variance of each estimator is formulated, and comparision among the estimators is implemented in precision and computational complexity. The results show that these algorithms are asymptotically unbiased; the least square method and the cross-correlation method is not the minimum variance unbiased estimator; Maximum likelihood estimator is the minimum variance unbiased estimator, but it is time-consuming; The modified maximum likelihood estimator is excellent in computational complexity by taking epoch folding, while the estimation result is as accuracy as maximum likelihood estimator.Algorithem of positioning is proposed in the theory of general relativity for X-ray pulsar navigation. The procedure of positioning is presented. The influence of timetransfer from proper time to coordinate time on frequency estimation,and the influence of Parallax and Shapiro effect on phase difference is investigated. It indicates that frequency estimation can be implemented directely when the spacecraft is sufficiently far away from the solar.X-ray pulsar navigation(XNAV) is an important an potential deep space navigation technology. The simulation, analysis and processing of X-ray pulars' signal are investigated in this thesis, which is aimed at improving the precision through sigal processing. And it prepares basic work for the research of XNAV in the theory of general relativity.