A Complex Channel Structure for Generic GNSS Signal Tracking

T. Lueck, J. Winkel, M. Bodenbach

Abstract:	The modernization of existing and the implementation of new global and local navigation satellite systems introduce many new signals at different RF frequencies and modulations. Although the general structure of all these signals appears very different, it is still possible to define a unified approach for signal acquisition and tracking. The present paper discusses a new structure for a hardware-based tracking channel to accommodate all various modulations currently available for GNSS signals. While the current GPS C/A code is simply a BPSK modulation, the up-coming GPS Block IIR-M and IIF, respectively will introduce a time multiplexed binary offset code (TMBOC) modulated signal. On the other hand Galileo makes use of a composite BOC (CBOC) and AltBOC modulated signal. In addition, with GPS L2C a time division multiplex (TDM) between the L2C medium length and L2C long length spreading code is used, which generates the need to mask some portions of the received signal out of the correlation process, which otherwise would increase the noise level. This paper will summarize the different modulation techniques currently used or planned for satellite navigation signals. Based on this, their similarities and differences are analyzed with respect to the correlation process and the subsequent signal tracking. The objective of the paper is to define a generic and efficient channel structure that is capable of tracking all known signal structures. Due to the large number of different modulation techniques of the current and future GNSS signals, several design variations of correlator channels are introduced and discussed. They can handle all of these different modulations, and switching between different signal structures is simply a matter of configuration. A trade-off in terms of implementation efficiency is performed. Among the more interesting features introduced in those designs are the ability to track different sub-channels of the same carrier in one single channel. Simultaneous tracking of Galileo data and pilot channel is possible, as well as tracking pilot and data channel of GPS L2C in one single channel. Due to the complex structure, the same channel can also be used to correlate against the complex AltBOC modulated, wide band Galileo E5 signal. While the Galileo E1 and E6 spreading codes are based on optimized memory codes, especially the long code of the new GPS L2 civil signal with a length of 1.5 seconds or 767250 chips is far too big to be reasonably implemented as memory code. Therefore the complex channel also implements an interface to generated codes. This interface does not only allow to track generated codes like E5 (which can however also be handled as memory code) or L2CL, but also to inject encrypted codes like Galileo public regulated service (PRS), encrypted commercial service (CS) or GPS P or W code for (semi-) code-less P-code tracking. Based on IFEN’s professional receiver series NavX-RPS 1000, which was already deployed for Galileo payload and in orbit testing and also acts as the ground reference receiver for the German Galileo Test Environment (GATE), the complex channel is implemented in FPGAs which are used for signal correlating. Tests based on simulated signals as well as on signal in space are presented.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	252 - 258
Cite this article:	Lueck, T., Winkel, J., Bodenbach, M., "A Complex Channel Structure for Generic GNSS Signal Tracking," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 252-258.
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