FPGA Implementation Of GPS Signal C/A Code Tracking

Global positioning system (GPS) is a satellite navigation system established to solve the navigating and positioning problems on land or sea and in the air. As a new generation of satellite navigation system, it not only has global, round-the-clock, continuous, high-precision navigating and positioning functions, but also possesses excellent interference immunity and confidentiality. Therefore, the development of GPS navigation technology has become ever more important in the advancement of navigation industry.In order to receive navigation messages and calculate their values, it is necessary to complete complex signal processing in the GPS receiver, while how to capture the satellite signals and track the C/A code is one of the most important issues. In this paper, the corresponding simulation results of GPS signals were provided based on the detailed GPS signal's structure analyses, FPGA features, and optimized algorithm.The contents of this paper include the following aspects:1. An in-depth study on the structure and theory of GPS signal is conducted, and the modulation mechanisms of GPS signal are explained in detail.2. To capture the GPS signal, a rapid acquisition method based on the FFT parallel code is employed. The incoming signals are firstly translated into frequency domain and correlated, and then the results are translated back to the time domain by using IFFT. Accordingly, a much more complex computation is avoided and the searching speed for GPS signals is much faster than sequential detecting technique.3. A non-coherent DLL is used to track the C/A code. The local carriers from carrier tracking loop turn the input signals into the baseband signal, and the baseband signals multiply the three different phase sequence of the local yards, then deal with cumulative results, at last, put the code phase and current carrier frequency into carrier tracking loop.4. Costas loop is used as the carrier tracking loop. It is similar to the C/A tracking loop in structure. Thus, only the simulation of carrier NCO has been given in this paper.In this paper, a novel approach by using FPGA to capture the GPS signals and track the C/A codes is employed. According to the characteristics of FPGA, various system optimizations could be realized by just changing the corresponding IP core or related VHDL program without changing any hardware. The advantages and flexibility of the novel approach are easily adapted to the different functions of the different GPS receivers.