Exploiting the Orthogonality of L2C Code Delays for a Fast Acquisition

A.R.A. Moghaddam, R. Watson, G. Lachapelle, J. Nielsen

Abstract:	The GPS modernization effort is now providing the first new signals for general civilian use since the initial launch of GPS. The newly launched L2C (civilian) signal is a Time Division Multiplexed (TDM) signal with a unique structure designed to alleviate some inherent limitations in the structure of the legacy C/A code on L1. The two TDM codes forming the overall L2C signal structure—namely the CM (Moderate Length) and CL (Long Length) codes—are combined to form the full L2C code. The dataless property of the CL code is potentially valuable for situations where receiver sensitivity is critical. However, the CL code is very long (in comparison with the C/A code or the CM part of L2C), making direct acquisition of the code phase challenging. In addition, the hardware or software resources required to conduct a direct CL code search are costly or prohibitively complex for many implementations. Normally, therefore, a CM code acquisition is conducted prior to CL acquisition. A fast scheme for acquisition of the CL code is desirable for situations in which acquisition of CM is difficult or impossible. In this paper, one approach for fast acquisition of long PRN codes is discussed and analyzed with respect to performance on the L2C code. The acquisition approach is based on the orthogonality of shifted versions of a long PRN code such as CL, exploiting the excellent cross correlation properties of shifted versions of such a code in the process. New local codes—referred to as Hyper Codes herein—are created that contain more information per chip than the original local code. By using this class of local codes in the receiver, fast acquisition of the code offset in the CL segment of the L2C is viable in an acquisition based on the standard FFT approach without CM assistance. In this method, there is a tradeoff between speed and reliability of acquisition; the amount of degradation in reliability is analyzed and discussed with respect to complexity improvements.
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	1233 - 1241
Cite this article:	Moghaddam, A.R.A., Watson, R., Lachapelle, G., Nielsen, J., "Exploiting the Orthogonality of L2C Code Delays for a Fast Acquisition," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 1233-1241.
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