Study On Signal Characteristics Of X-ray Pulsar Based Navigation And Simulation Experiment System With Multi-physical Features

The X-ray pulsar navigation（XPNAV） is a novel astronomical autonomous navigation method, which can provide the measurement information of the position, velocity and timing for those vehicles travelling near Earth, into deep space or on interplanetary missions. Concentrating on the key technical problems unresolved in the XPNAV system, this dissertation investigates the X-ray pulsar signal characteristics, the high precision of pulse arrival time（TOA） estimation method, the spacecraft velocity estimation based on the pulsar Doppler signal and the XPNAV signal simulation technology. The main research work includes: 1. This dissertation studied the principle of the positioning and timing in the XPNAV system, and summarized the commonly used time and coordinate systems and the corresponding transformation. On this basis, we elaborated the transformation equation of the photon arrival time from the spacecraft to solar system barycenter（SSB）. 2. In view of the existing literature lacking systematic studies on the pulse profile stability, the statistical indicators such as the flow rate, pulse width, pulse radiation fraction, pulse intensity ratio, pulsar profile signal-to-noise ratio and geometric factor were introduced to survey the long-term profiles variation. The Cramer-Rao theory was also applied to analyze the effect of the pulse profile stability on navigation accuracy. The results show that the pulse profiles of the Crab pulsar have long-term stability and no systemic changes are discovered. The pulse profile intrinsic stability bring about the measurement error 34 ± 25 m for the XPNAV system. 3. For choosing the optimal X-ray energy level in the XPNAV, the 16 years observation data within the energy 2-60 keV, were utilized to analyze the characteristics of the pulse profiles respectively within 2-10 keV, 10-20 keV, 20-30 keV, 30-40 keV, 40-50 keV and 50-60 keV. Using geometric factor and profile SNR, the influence of the pulse profiles in different energy ranges on the XPNAV accuracy was surveyed. The results show that, for 30-60 keV energy range of the Crab pulsar, the improvement to the navigation precision is not obvious. 4. Aiming at the problem of low estimation precision for the pulse TOA, we deduced the epoch folding noise distribution from the CXB and X-ray photons from sources. A novel estimation method was proposed to improve the estimation precision, and the estimation performance of the proposed method was proved theoretically. The results show that the method has superior asymptotic properties, the estimation performance is close to the maximum likelihood method, but with much less calculation cost. 5. Aimed at the cosmic X-ray background（CXB） which directly effects the estimation precision of the pulse TOA, the characteristics of the flux and energy range of the CXB was studied. The mathematical model of the stationary CXB was established, and its effect on the pulse TOA deduced. The results indicate that the CXB in different space regions is stationary. For the energy range 2-10 keV and 2-60 keV, the flux of the CXB is 49 cnts/s and 215 cnts/s respectively. Under the condition of t_bst =50s and （?） =154cnts/s, the estimation errors of the range introduced by the CXB in the two energy range above are 2.97 km and 3.9 km. 6. In view of the spacecraft velocity estimation employing the X-ray pulsar signal Doppler Effect, the mathematical model of the velocity estimation for the near Earth orbit spacecraft was established under the framework of general relativity. An improved RANSAC x² statistical method was proposed to increase the observed frequency estimation accuracy in low SNR, and several experiments using the Crab pulsar observation data were used to validate the presented method. The results show that the proposed method significantly improves the estimation precision of the observation signal frequency by two orders of magnitude compared with the direct x² statistical method. For the velocity estimation of the RXTE satelite in the near-Earth orbit, the velocity estimation error is about ± 110 m/s using the Crab pulsar photon TOA data detected. 7. Contraposing the problems existing in the X-ray pulsar signal simulation system, such as the poor pulse profile accuracy, low rotation frequency accuracy and lack of physical characteristics, this dissertation presented a semi-physical navigation pulsar signal simulation based on the concept of visible light. The simulation system was designed and a series of experiments were carried out. The results show that the rotation stability ia improved to 10^-9 from the existing 10^-4, and the correlation coefficient between the observation and standard pulse profiles reaches 0.99.