Extended Kalman Filter Methods for Tracking Weak GPS Signals

Mark L. Psiaki and Hee Jung

Abstract:	A combined phase-locked loop/delay-locked loop has been developed for tracking weak GPS C/A signals. This work enables the use of the weak side-lobe signals that are available at geosynchronous altitudes. The tracking algorithm is an extended Kalman filter (EKF) that estimates code phase, carrier phase, Doppler shift, rate of change of Doppler shift, carrier amplitude and data bit sign. It forms a likelihood function that depends on the errors between accumulations and their predicted values. It recursively minimizes this likelihood function in order to track the signal. It deals with data bit uncertainty using a Bayesian analysis that determines a posteriori probabilities for each bit sign. A second filter is used to initialize the EKF. This batch filter starts with coarse carrier frequency and code phase estimates and refines them using maximum likelihood techniques while estimating the carrier phase and the first PRN code period of a navigation data bit. The resulting system can acquire and maintain lock on a signal as weak as 15 dB Hz if the receiver clock is an ovenized crystal oscillator and if the line-of-sight acceleration variations are as mild as those seen by a geostationary user vehicle.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	2539 - 2553
Cite this article:	Psiaki, Mark L., Jung, Hee, "Extended Kalman Filter Methods for Tracking Weak GPS Signals," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 2539-2553.
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