Fast Direct P(Y)-Code Acquisition Using XFAST

Chun Yang, Juan Vasquez, James Chaffee

Abstract:	Global Positioning System (GPS) precision code (or its encrypted version), P(Y)-code, was not originally designed for direct acquisition per se but rather through the hand-over from its corresponding acquired coarse acquisition code or C/A code. However, direct acquisition of the P(Y)-code becomes desirable, not only because it offers code-ranging of higher accuracy and access to a second frequency, but also because it has a higher tolerance to jamming and spoofing than the C/A code. In an extreme case where the C/A code is totally jammed, spoofed, or disabled, direct acquisition of P(Y) code is the only viable option available. Conventional P(Y)-code acquisition relies on the hand-over from an acquired C/A code. Newer designs toward direct acquisition without first acquiring C/A code utilize “hard” parallelism for code search. By “hard”, we mean massive physical correlators in parallel, to contrast with “soft” parallelism in which fast algorithms run in a computer over the same data segment many times for different search parameters. In addition to P(Y)-code in GPS applications, very long code sequences are widely used in spread-spectrum communication and measurement. To succeed in fast direct acquisition of these very long code sequences, a novel approach, named as the Extended Replica Folding Acquisition Search Technique (XFAST), is presented in this paper. After examining the property of cross correlation among code subsequences, this paper provides a technical description of the XFAST approach and its main concepts including extended replica, code folding, circular correlation, and ambiguity resolution. Preliminary simulation results are also presented, together with discussions of design parameters.
Published in:	Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999) September 14 - 17, 1999 Nashville, TN
Pages:	317 - 324
Cite this article:	Yang, Chun, Vasquez, Juan, Chaffee, James, "Fast Direct P(Y)-Code Acquisition Using XFAST," Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999), Nashville, TN, September 1999, pp. 317-324.
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