X-ray Pulsar Signal Denoising Based On Signal And Noise Characteristic

X-ray Pulsar-based Navigation is a new kind of autonomous navigation technology,which has a huge potential for development of the spacecrafts in deep-space, near-earth,Geostationary orbit, and elliptic-orbit. X-ray pulsar signal is the basis of pulsar-basednavigation, which must objectively express the characteristic of pulsar. In fact, thesignal quality of X-ray pulsar is not ideal due to many reasons, such as the long distancebetween pulsars and earth, the complexity of cosmic background noise, the strongelectromagnetic interference, etc. These interferences not only decrease the quality ofsignal but also impact on the signal extraction and the estimation of navigationparameters. Therefore, extracting high-quality signal of X-ray pulsar is a very importantpre-propressing, and directly determines the efficiency and precision of position,velocity, attitude and timing.During the acquisition of X-ray pulsar signal, there are many kinds of artificialand non-artificial interference factors. The different mechanism of noises has differentstatistical characteristics and probability distribution. The fluctuation of period mayintroduce Gaussian noise, the stochastic of photon arrival may produce the Poissonnoise, the solar storm and X-ray storm may introduce impulsive noise. Obviously, it isdifficult to filter all kinds of noise using the same denoising method. Therefore, eachnoise properties mechanisms must be researched individually to obtain good denoisingresults. According to the characteristic of X-ray pulsar, pulse peak, pulse width, pulseprofile, the denoising method is trying to find the different characteristics betweenX-ray pulsar signal and noise, and distinguish the signal and noise to accomplish thedenoising. X-ray pulsar signal processing, as a new cross-field research field, has nospecified documents to introduce to filter the noise based on synthetic analysis of thecharacteristics of different kinds of noises.The importance of the X-ray pulsar signal processing is introduced on the basis ofthe navigation principle of X-ray pulsar. The characteristics of the noise sources inobserving system of X-ray pulsar are discussed. Then several denoising methods areproposed according to the mechanism and distribution of noise. The theories andmethods are verifed by the observing system RXTE. The main works can besummarized as follows.1Noise source and its character in observation process of X-ray pulsar signal. Theobservation principle and process are analyzed. Noises imported in X-ray pulsar acquisition, including Poisson noise caused by randomness of photon arrival anddetection, Gaussian noise caused by electronic readout and impulsive noise caused byelectrical interference and special astronomical phenomenon, are discussed. Themathematical model and statistical characteristics of the noise are established byanalyzing the source and feature of the noise. And the measured data from RXTE, thestructure, the method of signal extraction, are analyzed, which provides credible datasource for verification of the theory and method.2Considering Gaussian noise from the observation system of X-ray pulsar signal,the mechanism and feature of noise are analyzed. It is difficult to obtain high-qualitypulsar profile only by epoch folding. A Gaussian noise removal method based on dataoptimization and Daubechies wavelet is proposed. Considering the complexity of theobservation system for the X-ray pulsar signal, Firstly, the method selects out goodinterval from measured data, Secondly decomposes the signal by Daubechies wavelet,then filtering the Gaussian noise by threshold policy. The method, making the complexmatters easier, reduces computational complexity, but also Gaussian noise effectively.The processing platform of X-ray pulsar signal is in a resource-constrained device. Inorder to balance the complexity and performance, a denoising method based liftingscheme wavelet is proposed. The lifting scheme reduced the complex wavelettransformation to the basic matrix calculation. The SNR of pulsar profile based on thelifting scheme is improved by setting a thresholding value to removal the Gaussiannoise. The method is better for engineering implementation than the traditional one.3The Poisson noise introduced by the randomness of photon arrival and photondetection in observation system of X-ray pulsar signal is analyzed. Considering thePoisson property of X-ray pulsar signal, a Poisson noise suppression method based onVariance Stabilization Transform is proposed. The signal containing Poisson noise istransformed to Gaussian white noise by Haar-Fisz transformation. The noise is reducedby thresholding and inverse transform of Haar-Fisz. The indirect method filters thePoisson noise and extends the research space for X-ray pulsar signal. Besides，a methodbased on Bayesian multi-scale model is proposed by directly suppression the noiseinterference.4Considering impulsive noise characteristic in X-ray pulsar observation system, animpulsive noise suppression method based on wavelet packet transform is proposed.The detection algorithm named Linear Filtering could find out the peak number ofimpulsive noise by the impulsive noise characteristics, such as discontinuity, large frequency and irregulations. In wavelet packets domain, the impulsive noise overdifferent sections are filtered by thresholding policy. Especially, no noise signals doneed treatment. For the loss of time information in denoised processing, the denoisedsignal is interpolated to obtain high time resolution, which recovers a high-quanlityphoton pulsar profile.