Performance Modeling And Evaluation Of GNSS In Complex Scenes Of Town

Global Navigation Satellite System has an irreplaceable role in civilian and military areas. In civilian areas, GNSS is most widely used in urban areas. The heart area of every large city is full of high-rise buildings, GNSS signal is highly likely to be blocked by tall buildings, resulting in a decrease in the number of visible satellites. At the same time, the GNSS signal suffers from a large probability of surface reflection of various buildings, the occurrence of multipath is quite frequent and serious. Evaluating the performance of GNSS and studying the methods of improving the performance of GNSS in urban areas not only can promote the development and application of GNSS in urban areas, but also provide a reference for the construction of the Beidou System in china.We briefly describe the system composition of GNSS and the basic navigation principle. From three aspects of signal source, channel and receiver, the error sources of GNSS are analyzed in detail, which lays a theoretical foundation for analyzing the characteristics of GNSS in urban environment.Based on analyzing and summarizing classic GNSS performance and the parallel recursive model presented by Li Zuohu, a performance indicator are proposed for complex urban environment. The indicator is called "GNSS parallel recursive model in urban area". Based on the needs of GNSS users in civilian city,we analyzed the relationship between real-time, robustness, practicality and aviation navigation performance(Required Navigation Performance, RNP). We establish the evaluation index model of performance related to navigation signal and study the mechanism of the signal performance and the evaluation method. The GNSS signal of three typical navigation systems in the B1 frequency points is analyzed in detail, which is described from four aspects: accuracy, anti multipath performance, anti-jamming performance and compatibility performance.Based on the electromagnetic wave propagation theory, a real urban complex environment is used to establish the corresponding electromagnetic wave propagation models. A detailed analysis of the performance characteristics of this typical urban complex environment of GNSS is presented. We propose a new method of selecting the stars for the complex environment of the city. In this method, the satellite which can not be seen directly is removed with the help of 3D map, and then the star is selected by the way of angle and elevation, which can suppress the multipath effect in a certain extent. By establishing a typical model of the urban canyon, the simulation of several selection methods is carried out. The simulation results show that the DOP of this method is close to the result of the traditional method of angle and elevation and and the amount of calculation is about 25% less than the former.We study the algorithm, shadow matching, which can improve performance of GNSS in the urban canyons. With the ephemeris of GPS + GLONASS, an ideal model of shadow matching is established. The relationship between the height of the building, the width of the streets and the number of visible satellites is studied. The algorithm of shadow matching is implemented, and the effectiveness of the algorithm is verified. Because of the limitations of the one dimensional search method, a new method of helical two dimensional search is proposed in this paper. Based on the actual acquisition to the NEMA data and ephemeris data, combined with Baidu map, e urban 3D map, we tested the effect of shadow matching. The improvement of the accuracy of GNSS in urban areas by the way of shadow matching is verified. And we obtain a typical value which can describe whether it is a direct signal or not. Considering the role of shadow matching in the GNSS, the concept of fusion for shadow matching and the A-GPS is presented, and the reasons, the possibility and the shortage of the fusion are analyzed. Considering the reasons in several aspects such as computation, storage, and network transmission, the fusion design based on MS-Based is proposed. As the way of MS-Assisted is so popular, a software design of the shadow matching algorithm is proposed based on Android system.Finally, the full text of the study is summarized, and the direction of further research is pointed out.