FFT-Based Acquisition of GPS L2 Civilian CM and CL Signals

Mark L. Psiaki

Abstract:	Block processing Fast Fourier Transform (FFT) algorithms have been developed to do signal acquisition for the new CM and CL civilian codes that will appear on the L2 GPS frequency once the Block IIR-M satellites are launched. These algorithms have been developed to aid in the acquisition of weak GPS signals. FFT block processing can be used to significantly reduce the computational burden caused by the extended lengths of the new codes and by the long integration times needed to acquire weak signals. The target application for this technology is high-altitude spacecraft navigation. The new algorithms must work efficiently when the maximum allowable FFT batch size precludes the use of a single FFT for a full code period. Zero-padding and overlap-and- discard techniques are used to generate partial correlation accumulations at a range of pseudo-random number (PRN) code start times, and interpolation techniques are used to map these partial accumulations onto a desired grid of start times before summation into full accumulations. Frequency-domain techniques are used to reduce the number of FFT and inverse FFT (IFFT) operations that are needed if one performs the acquisition search over a range of frequencies. The new algorithms have been used to acquire simulated signals with carrier-to- noise ratios as low as 9 dB-Hz. They can reduce execution times in comparison to brute-force algorithms by factors ranging from 1,000 to over 30,000.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	457 - 473
Cite this article:	Psiaki, Mark L., "FFT-Based Acquisition of GPS L2 Civilian CM and CL Signals," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 457-473.
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