A New Technology for GNSS Signal Fast Acquisition Within Three Seconds: Applicable to Current GNSS Receivers

H. Jun and C. Kee

Abstract:	Nowadays GNSS receivers are omnipresent navigation sensors even for persoonal positioning. In many cases, especially in E911, GNSS receivers are not working in ordinary times, but needed to find a position immediately as soon as push the button in emergency. Time to first fix (TTFF) is dominaltly dependent on the signal acquisition time. If receivers are not used in ordinary times or if receivers start without almanac or initial position information, the receivers should take cold start and search all candidate PRN numbers, Doppler shift and code phase shift. Currently there are two kinds of signal acquisition technologies. One is a sequential searching method used in common GNSS receivers. This method correlates received signal with locally generated signals in real-time, and changes Doppler offset and code phase shift of the local signal until the correlation value is over a threshold. The acquisition method spends 1ms for each cells, and the candidates are totally L*M*N cells, which are L-Doppler bins, M-code phase shift and N-PRN numbers. Therefore, this method needs long time (usually over 1 minute) to search all the candidate cells. The other is a DFT (Discrete Fourier Transform) method used in software GNSS receivers. This mehod stores sampled IF signals for code period (usually 1ms), and takes Fourier transform of these signals. Then the correlation values are acquired using DFT and inverse DFT process. This method reduces acquisition time dramatically (about 3-5 seccond) but needs high speed (about 5~7 MHz) storage apparatus, which can not be implemented in current receivers. Therefore, this method can be used only for future software receivers, which require different hardware architecture from the current one. This paper proposes a new technique for signal acquisition, which uses advantages of above two algorithms and can be implemented in the current receiver architecture with very fast acquisition time (below 5 seconds). This method stores the correlation measurements (1-ms accumulated I and Q) for only one PRN code changing Doppler shift and code phase shift in L*M cells. This process can be implemented in the current (H/W) receivers because it requires only 1ms-correlation measurements I and Q instead of all sampled signals. Then using this correlation data, the method can acquire Doppler and code phase shift of all the other candidate PRN signals. Therefore, this method doesn't need high speed sample and storage equipments like software receivers, but reduces signalreceiving time (1-2 sec) for acquisition as compared with a sequential searching method. This paper introduces a key idea for the algorithm, then shows a simulation and test results. This method is implemented in a Mitel-chipset based low-cost GNSS receiver made by Seoul National University, and 4~6 signals are acquired using this patent-pending method.
Published in:	Proceedings of the 2006 National Technical Meeting of The Institute of Navigation January 18 - 20, 2006 Hyatt Regency Hotel Monterey, CA
Pages:	895 - 907
Cite this article:	Jun, H., Kee, C., "A New Technology for GNSS Signal Fast Acquisition Within Three Seconds: Applicable to Current GNSS Receivers," Proceedings of the 2006 National Technical Meeting of The Institute of Navigation, Monterey, CA, January 2006, pp. 895-907.
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