FFT Acquisition of Periodic, Aperiodic, Puncture, and Overlaid Code Sequences in GPS
Chun Yang · 2001
This paper presents a study of FFT-implemented circular correlation and its application to fast direct acquisition of GPS codes. This includes the periodic C/A-codes, practically non-periodic P(Y)-codes, never-repeating cryptographic M-code, and puncture acquisition (PA) codes which have been proposed to aid direct M acquisition as well as overlaid codes which are created by surface-reflected GPS signals extended beyond one code chip. FFT operates on blocks of incoming and replica code samples, thus providing simultaneous search over the entire block of code phases. It is straightforward to work with periodic codes for circular correlation. However, it is not obvious for puncture codes and long codes in particular. One major concern is how to ensure that the incoming and replica code samples contained within the working block could be correlated. In addition, it is quite possible that the data bit sign may reverse in the middle of an integration interval. Furthermore, how to efficiently make use of complex FFT when the data length is not a power of two or highly composite is critical for practical implementation. These design and computation issues are properly formulated in this paper and pertinent acquisition schemes are suggested.