Direct P(Y)-Code Acquisition Algorithm forSoftware GPS Receivers

David M. Lin, James B.Y. Tsui, Dana Howell

Abstract:	The general approach for GPS P-code reception starts with C/A code acquisition which is then handed over for precise timing to the P-code tracking loop. However, if P-code tracking is interrupted in a jamming environment and C/A code acquisition is also denied, a long period of P-code signal is needed for correlation in order to increase the integration gain and achieve the required signal-to-jam ratio for re-aquisition. Several papers related to P-code direct acquisition for GPS hardware receivers have been published in previous ION conferences (1) . This paper will discuss an efficient coherent acquisition algorithm in the frequency domain. This algorithm was developed for digital signal processing in software GPS receivers to acquire long and weak P-code or C/A code sampled data. The algorithm breaks down a set of long data into M subsets and performs local correlation in the frequency domain. The local results are then coherently integrated by an M-point DFT (Discrete Fourier Transform) for global detection. M is the number of the Doppler bins which are derived from the total integration time and the total Doppler range to be covered. The calculation time can be significantly reduced by performing correlation in the frequency domain and by dealing with multiple smaller DFTs in block level instead of a single large DFT . Several sampled data sets which were generated by the hardware GPS signal simulator of Advanced Concept Exploitation Facility (ACEF) (2) at Sensors Directorate, Air Force Research Laboratory were used for technique development and the result demonstration. The reason for use of simulated data instead of measured real data was to avoid any security issues. The sample data are RF (Radio Frequency) modulated P-code plus C/A code, and the thermal noise which is 21 dB higher than signal was added in software.
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:	363 - 368
Cite this article:	Lin, David M., Tsui, James B.Y., Howell, Dana, "Direct P(Y)-Code Acquisition Algorithm forSoftware GPS Receivers," Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999), Nashville, TN, September 1999, pp. 363-368.
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