Fast Direct P(Y)-Code Acquisition Using XFAST

摘要

Global Positioning System (GPS) precision code (or itsrnencrypted version), P(Y)-code, was not originallyrndesigned for direct acquisition per se but rather throughrnthe hand-over from its corresponding acquired coarsernacquisition code or C/A code. However, direct acquisitionrnof the P(Y)-code becomes desirable, not only because itrnoffers code-ranging of higher accuracy and access to a second frequency, but also because it has a higherrntolerance to jamming and spoofing than the C/A code. Inrnan extreme case where the C/A code is totally jammed,rnspoofed, or disabled, direct acquisition of P(Y) code is thernonly viable option available.rnConventional P(Y)-code acquisition relies on the hand-overrnfrom an acquired C/A code. Newer designs towardrndirect acquisition without first acquiring C/A code utilizern“hard” parallelism for code search. By “hard”, we meanrnmassive physical correlators in parallel, to contrast withrn“soft” parallelism in which fast algorithms run in arncomputer over the same data segment many times forrndifferent search parameters.rnIn addition to P(Y)-code in GPS applications, very longrncode sequences are widely used in spread-spectrumrncommunication and measurement. To succeed in fastrndirect acquisition of these very long code sequences, arnnovel approach, named as the Extended Replica FoldingrnAcquisition Search Technique (XFAST), is presented inrnthis paper. After examining the property of crossrncorrelation among code subsequences, this paper providesrna technical description of the XFAST approach and itsrnmain concepts including extended replica, code folding,rncircular correlation, and ambiguity resolution. Preliminaryrnsimulation results are also presented, together withrndiscussions of design parameters.