| Navigation Positioning System(NPS) can be used to offer the Navigation Positioning Service for the conveyance of road, railway, the air and the sea. It can be used for the army and civilian, it has tratic function and business behalf. It consist of several secondary planet which go around the earth and blaze away wireless signal whitch has certain frequency. No matter where you are, on the land, in the sea or in the air, if only you have a receiver on hand, you can receive the signal eradiated by the secondary planet. The receiver choose at least four swatch to analyse, in this way, you can make sure where you are.The theory base of the GPS is the Spread Spectrum technologies. Compared to other communication technologies, Spread Spectrum technology has the advantages of low probability of intercepting, a high immunity to interference, hiding the useful information and multiple-access communication ability. In modern information times, it has been one of the high-technological communication means and is developing from military use to commercial environment. In this paper, the author mostly research on Direct Sequence Spread Spectrum (DSSS), the most ordinary one within spread spectrum techonologies.There are two nomal method of acquisition at present: order method(time domain, base on cosmically correlator) and parallel method(frequency domain, based on fast fourier thansform(FFT)). In chapter two of the paper, it researchs on the two method particularly and gives their advantages and disadvantages.In chapter three, it researchs on the p code acquisition deeply, Based on analyzing and contracting the ways referred by thesis and paper on GPS, this paper proposes a way of direct acquisition of the GPS P code. It is a way that uses FFT to achieve circular correlation by partition and zero padding and achieve frequency synchronisition. In this way, acquisition can be achieved very fast as it transfers two-dimensional search into parallel one-dimensional search. Also, it can be easily implemented in real GPS project for its low need in hardware resource. In the end, this paper gives the simulation result and the implemention result. GPS signal has the disadvantages of faintness and low sensitive. At the environment of war field, GPS receiver may face various interference. How to realize acquisition from the jam-signal is the research aspect of the GPS and this paper. In chpter four, it discusses the anti-jam arithmetic. When GPS receiver meet with the strong narrow band interference environment, it needs to ask the signal processing technique for help to enhance the anti-jam ability under the situation that needn't increase the signal bandwidth to ensure the acquisition module has enough plus.It discusses the FPGA realization project in chapter five and analyses every module in detail. In the basic-type receiver, the long code acquisition uses frequency domain method and choose the Altera Stratixâ…¡EP2S180 chips. In the anti-jam-type receiver, it uses time domain method and choose Xilinx xc4vlx100 chips. In the end, it debugs every module and the whole system. The test result shows the method advanced by the paper is right and feasible.The synchronisition ways proposed in this paper can realize acquisition function without the help of C/A code. It can be used in real GPS project, such as GPS receiver synchronisition, receiving base synchronisition. It also has fatal practical and economic significance to the research of our own NPS receiver. |
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