Research On The Acquisition Of C/A Code In Weak Signals Enviromnet Of GPS Receiver

With the development of Global Navigation Satellite System, high-sensitivity GPS gets general concern and weak signal GPS receiver becomes the focus of research. Common GPS receiver loses the ability to work under weak signal. For example, to provide indoor positioning capability in wireless devices, or orbit determination of Geostationary and high Earth orbit satellites require the receiver to work with serious signal attenuation. This paper researches the key technique of GPS receiver for such applications, i.e. acquisition of C/A (coarse/acquisition) code in high-sensitivity receiver. The software receiver acquisition algorithms are developed. They also can be modified to fit any other GPS signal or other navigation systems, for example Galileo or CNSS.At first, this paper introduces the structure of GPS signal, and GPS signal's transmission lost. Then the generation reasons for GPS weak signal have been analyzed. Meanwhile, corresponding power has been presented and carrier wave's Doppler Frequency Shift has been descried. Math Model has been established to explain the acquisition technology of GPS signal.Second, the basic theory of GPS signal acquisition has been described and on the basic of traditional time domain acquisition, GPS signal frequency domain acquisition has been researched. The speed of correlation and time for acquisition can be improved, which realized weak signal process. Steps and features of frequency domain acquisition have been simulated and analyzed. Acquisition technology in the condition of weak signal is one of the focuses of this paper. To improve sensitivity and process gain, accumulation is necessary. Therefore, the decision variable of coherent, non-coherent and differential accumulation has been achieved and the features have been compared. Features of two classical methods, half bit and full bit algorithms have been analyzed and performance of non-coherent and differential accumulation has been simulated.Last but not least, to satisfy the demand of software receiver, this paper proposed a configurable project for acquisition of weak signal. This project included average down sampling technique which is used for improving the usage of hardware resource, and the compensation of code Doppler frequency shift for reducing sampling errors. Meanwhile, full bit algorithms and differential accumulation have been improved. One has decreased the FFT operation amount with the same sensitivity and the other has increased the sensitivity while the FFT operation amount remains the same. For circular correlation with optimal data bits, a method for decreasing the process amount has been proposed.