Generalized Frequency-Domain Correlator for Software GPS Receiver: Preliminary Test Results and Analysis (PREPRINT)

Abstract : The generalized frequency-domain correlator (GFDC) is introduced in this paper within the framework of a software GPS receiver which implements a frequency-domain baseband signal processor. The use of a GFDC can offer several advantages. First, as a generalization of the FFT-implemented correlation with a block repetitive processing capability, it enables fast acquisition through simultaneous code delay and Doppler frequency search. Second, it produces the delay-Doppler map of (generalized) complex correlations from which the joint code delay and carrier phase and frequency error discriminator can be implemented for grid tracking with the joint code and carrier loops, thus blurring the line between acquisition and tracking. Third, the same operation is applicable, without any other changes except for the replica code, to both the BPSK codes such as GPS C/A-code and the BOC modulations such as M-code. Fourth, it can switch seamlessly among different types of correlation/matching operations with a wide peak base for acquisition and a narrow peak base for tracking. Fifth, the generalized correlation has a sharp peak with a reduced base width, thus making its timing estimation more accurate and less sensitive to multipath. In this paper, the generalized frequency-domain correlator (GFDC) is formulated and different implementation schemes for spectrum filtering are presented. The experimental data (a set GPS data recorded on a large aircraft) are used to demonstrate the ability of the GFDC to uncover closely-spaced multipath signals more easily than the conventional correlators. The practical issue of limited signal bandwidth and its effects on both the conventional correlators and the GFDC are analyzed via computer simulation.