A Study Of Detection And Phase Measurement Technology For X-ray Pulsar Signals

Since proposal of pulsar navigation, about half a century elapses. But it does not attract much attention until the feasibility was confirmed ten years ago. Due to its special radiation characteristics and periodic stability, X-ray pulsar has been considered as an ideal navigation source, and shows its great application potential in spacecraft autonomous navigation. By now, space experiments have be carried out for autonomous navigation using X-ray pulsars. But this new technology is still on stage of theory exploration and feasibility validation。 Especially, the substantive research about pulsar navigation is infancy, the key issues limited the precision of X-ray Pulsar NAVigation (XPNAV), such as high precision observation for extremely weak signals, high efficiency and performance phase measurement and error correction of time transfer have not been solved well. This dissertation focus on the problems of signals detecting, integrated profile establishing and phase measurement, and the main content can be summarized as:(1) Several characteristic parameters which can be used in signal detection, such as periodic, profile and flux intensity, have been analyzed. It is feasible to detect or identify the pulsar signal using the periodic or the profile characteristics in frequency domain respectively. But for extremely weak signal which is complex to detect, these methods are not satisfactory. Studies have shown that, due to influence of harmonic and color noise, there are insufficient situations for the frequency domain method in X-ray pulsar signal detecting, and sometimes, the time domain method do better. In this paper, characteristics of integrated profile, flux intensity and periodic are used in X-ray pulsar detecting, and a time domain method based on Bayesian estimation is proposed. The experiments employing simulation data and RXTE data show that this method does better than the same kind of Gaussian distribution model based method, and the phase offset can be estimated simultaneously.(2) The basic approach to get navigation information of XPNAV is to measure the phase of integrated profile. For the photons series whose arrival time has been transferred to SSB, the traditional method is to fold the data into one period which should be solved firstly. This method depends on the precision of the period, and has not evaluation criterion. The correlation between period and integrated profile is analyzed in this paper. A minimum entropy method for direct profile folding without period is proposed and proved. Consequently, its ability of solving period is discussed. The efficiency is verified experimentally using simulated and RXTE data. The pulsar integrated profile commonly is considered as one dimensional distribution of the radiation region. The statistical analysis of great integrated profiles shows there are kernel components in these profiles, and then the Gaussian component decomposition method (GFSAP) is proposed to explain the radiation region of pulsar. We use the GFSAP to model pulsar integrated profiles. Since it approximates the profiles well, we apply this model to analyze the CRLB of integrated profile phase measurement to evaluate the effect of integrated profile components in phase measurement. The results show that the correlation among the components and width are two important factors relate to precision of phase measurement. In view of that, the frequency phase measurement method as Taylor FFT is essentially fitting of original function using addition of several harmonics. Keep which in mind, this paper propose the Gaussian fitting method based on minimum entropy criterion. The experiments show that the proposed method does better than Taylor FFT method in relatively low SNR condition.(3) The phase measurement using integrated profile requires high quality of profile, which lead to difficulty in application. This paper establishes a new probability model for pulsar signal to describe distribution of arrival photon frequency with respect to sampling time in observation duration. The analysis shows the good ability of this model in expressing relation between profile deformation and period error. Based on this model, this paper proposed a joint measurement method for phase and phase rate which takes Doppler as a parameter to formulate the directly folded profile. Then, the optimization method is used to resolve the two variables. Simulation experiments show that this method can estimate phase and phase rate without folding out undistorted profile, and its precision is better than Taylor FFT method.(4) From the performance of XPNAV, its position error is less than one tenth of the minimum ambiguity distance, so the ambiguity problem can be ignored when the system is working normally and continuously. But for the situations without initial position or the position record losing due to system fault, the ambiguity must be handled. The exhaustive search method is an effective method commonly used, whose shortage is high complexity. This paper proposed a kind of fast space search method which divides the ambiguity space to multiple subspaces. Then, the phase project increment is used to establish fast searching method based on full binary tree. Simulation experiments show that benefiting from the diminution of space scale and fast binary tree, the proposed method promoted the space search efficiency a lot.(5) By now, although the superiority of XPNAV is obvious comparing with other celestial navigation method, it is not a mature method. The traditional method which uses the star occultation of earth to determinate the spacecraft orbit and takes the angle between star and earth as information input is total different from XPNAV. Aiming at increasing navigation precision and autonomy, an information fusion method combined pulsar phase and star angel measurement is proposed. Based on Unscented Kalman Filter (UKF), real orbit parameters are used in experiment, and the results show than position and velocity estimation precision can be enhance efficiency. Further, the feasibility and possibility for the combination with other navigation method are discussed.(6) The simulation system on ground is very important for navigation system. It is especially significance for XPNAV due to impossibility to receive the X-ray pulsar signal in space. This paper preliminary designs a kind of signal simulation scheme, which focus on the establishing virtual reference frame, signal modeling and main hardware units.