A Pulse Time Delay Estimation Method for XNAV: Non-Uniformly Epoch Folding

X-ray pulsar-based navigation (XNAV) is an ideal navigation mode for deep spacecraft. In order to overcome the time-varying frequency problem caused by the acceleration of spacecraft in the process of photons TOA data, this paper proposes an improved method called Non-uniform epoch folding (NUEF). Specifically, the paper analyzes the mechanism of the traditional EF method, and then divides the traditional method into two key stages: Density Analysis and Digital Filtering. Draws the conclusion that the key to limit the pulse time delay estimation accuracy of the traditional method is that the mathematical expectation of the photon density sequence obtained in the first stage of EF cannot maintain periodicity in a long observation time interval; Then the solution NUEF is given. NUEF method abandons the characteristic that bins uniformly distribute on time axis in traditional EF methods. This method uses the principle of equal phase interval to determine the position and length of each bin by introducing external velocity information, and theoretically overcomes the problem of time-varying frequency in the long observation interval; The simulation experiment verifies the effectiveness of NUEF method. Specifically, the pulse time delay estimation accuracy of NUEF method is improved compared to traditional EF method. And because of the elimination of the periodic search process, the computational complexity of the NUEF method has also decreased.