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Currently there are two kinds of signal acquisition technologies. One is a sequential searching method used in common GNSS receivers. It spends 1 ms for each cells, and the candidates are totally L*M*N cells, which are T-Doppler bins, M-code phase shift and N-PRN numbers. Therefore, it 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. It stores sampled IF signals for code period (usually 1 ms), and takes Fourier transform of these signals. Then the correlation values are acquired using DFT and inverse DFT process. It reduces acquisition time dramatically (about 3-5 seconds) but needs high speed (about 5~7 MHz) storage apparatus and high-end processor, which can not be implemented in current receivers. Therefore, it can be used only for future software receivers, which require different hardware architecture from the current one. Our new technique takes advantages of above two algorithms and can be implemented in the current receiver architecture with very fast acquisition time (below 5 seconds). It is based on newly found orthogonal property of cross-correlation sequence. Two different cross-correlation sequences made by different PRN combination have no correlation each other. But only when two cross-correlation sequences are made by same PRN combination with same delay, they have correlation. It means that the cross-correlation sequence can be used as a raw received signal, and acquisition information of all the satellites in view can be found from it. The method stores the correlation measurements (l-ms accumulated I and Q) for only one PRN code by shifting code phase. After that we can do the sequential search on that measurement using software approach. This process can be implemented in the current (H/W) receivers because it uses correlator output instead of sampled signal. Now, we can search all the PRN, code shift and doppler offset information with only one strip of cross-correlation sequence. And it need just 0.1 seconds' signal-receiving time in 12 channel low cost GNSS receiver. Simulation and experimental test results will be presented. |
