On Pulse Phase Estimation and Tracking of Variable Celestial X-ray Sources

A. Robert Golshan and Suneel I. Sheikh

Abstract:	In this paper, the arrival times of X-ray photons from variable celestial sources, measured by an X-ray detector onboard a spacecraft, are modeled as a non-homogeneous Poisson random process with a periodic or quasi-periodic rate function. The problem of pulse phase estimation is addressed for the case in which the observed signal frequency is assumed to be constant and known over the observation interval. A maximum-likelihood phase estimator (MLE) is derived, and its performance is analyzed in a Monte-Carlo simulation. The Cramer-Rao theoretical performance bound is also derived and evaluated for this parameter estimation problem and then compared against the MLE results. Next, the problem of pulse phase estimation is considered for the case in which dynamical motion of the spacecraft causes the observed signal frequency to vary considerably over the observation interval. The constant frequency assumption is not valid in this case, and the observed signal exhibits doppler shifts that may cause the pulse arrival times (or phase epochs) at the detector to drift gradually and in an unpredictable manner over the observation interval. For this scenario, a pulse phase tracking algorithm is proposed that consists of an MLE followed by a digital phase-locked loop (DPLL). Photon arrival times are processed through the proposed algorithm, and the DPLL tracked phase and frequency are plotted to demonstrate its dynamical tracking capability.
Published in:	Proceedings of the 63rd Annual Meeting of The Institute of Navigation (2007) April 23 - 25, 2007 Royal Sonesta Hotel Cambridge, MA
Pages:	413 - 422
Cite this article:	Golshan, A. Robert, Sheikh, Suneel I., "On Pulse Phase Estimation and Tracking of Variable Celestial X-ray Sources," Proceedings of the 63rd Annual Meeting of The Institute of Navigation (2007), Cambridge, MA, April 2007, pp. 413-422.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In