Research On The Key Techniques Of Assisted GPS Positioning System

With the growing demand of Location Based Services, particularly for application of urban and indoor positioning, a high performance satellite positioning technology– Assisted GPS (A-GPS) has been a hot research focus in the navigation field. A-GPS positioning technology retains the characteristics of the conventional GPS technology with all-weather and all-day capacity, continuity and high precision. Moreover, it is not only compatible with various advantages of the wireless communications network, but also extends the application to the field of weak signal environment in which the conventional GPS receivers cannot work. Considering the A-GPS receiver can obtain the assistance data (including the approximate position, the ephemeris information, the satellite clock corrections, and etc.), this dissertation focuses on the key techniques of A-GPS positioning system for implementation and performance enhancement, including the uncertainty of pseudo-code phase and carrier frequency shift estimation method, acquisition algorithm for GPS receiver, A-GPS positioning algorithm in the weak singal environment.The uncertainty of pseudo-code phase and carrier frequency shift are extremely important for GPS signal acquisition in the weak signal environment. The traditional uncertainty estimate method is usually based on the valid almanac reckon, however, it can only provide rough range for the uncertainty of pseudo-code phase and carrier frequency shift due to the valid age of almanac data. In this dissertation, the effect of various factors on the pseudo-code phase and carrier frequency shift are analyzed. Considering that the valid ephemeris can be obtained by the A-GPS receiver from the assistance data, the uncertainty estimate methods based on the Taylor expansion for pseudo-code phase and carrier frequency shift are proposed. The simulation and experimental results show that the search space are 25 times less than these of uncompressing the search space when the approximate location error of receiver is 100km, and the uncertainty search space is sufficiently reduced.The acquisition algorithm is an important step for GPS receiver, and the goal is to choose the minimum mean acquisition time as without significant performance degradation. This dissertation investigates the acquisition model of square-law detector based on FFT frequency domain correlation, and the optimizing object function is proposed by the minimum mean acquisition time criterion. Given two cases, one specifying the false alarm probability and the other not, the parameters of acquisition system are optimized. The analytic formula of statistic performance for downsampling differential detection and standard differential detection under carrier frequency shift are derived for the first time. In order to minimize the mean acquisition time, the performance of detection are investigated. The numerical simulation results show that the standard differential detection and the square-law detection show better performance than the downsampling differential detection; and the standard differential detection shows better performance than the square-law detection at the low uncertainty of frequency.The pseudo-code phase can be only measured by the GPS receiver in the severely weak signal environment, so the full pseudo-range cannot be obtained. Considering that all or part of the satellite signal power is severely weak, the A-GPS positioning methods based on pseudo-code phase measurement and pseudo-code phase/pseudo-range combination are presented to avoid that the initiate value error of the receiver position is constrained within 150km to the Lambda positioning method. Comparing with the conventional GPS positioning method (obtain the pseudo-ranges for all satellite signals), it is shows that the positioning accuracy of the proposed methods is quite slightly worse than that of the conventional method.The research contributions in this dissertation can be directly applied and used for reference to Compass Navigation Satellite System in China.