Research On Acquisition And Tracking Of GPS Weak Signals Under Multipath Interference

In recent years, mature technology has made GPS receivers available. With higher positioningrequirement from the customers, application of GPS receivers under complex environment hasattracted enormous research interest. Weak-signal environment is one of the most common complexconditions, where multi-paths becomes the major interference. This work investigated the acquisitionand tracking of weak signal with multipath disturbance. It includes several parts, listed as follows:Firstly, the technical development of weak signal processing and anti multi-path interference wasintroduced. The mechanism of generating multi-path signal was also studied. L1C/A signal withadjustable carrier to noise ratio under multipath condition was obtained by coding. This providedreliable signal source for further research.Secondly, this work explored the influence of multipath interference on the acquisition andtracking of weak signal, respectively. The effects of multipath not only lead to a significant increase intracking error, but also cause the complete malfunction of making acquisition judgments. Signals withlow C/N ratio won’t be acquired successfully with conventional FFT phase acquisition algorithm, thetracking features would also be affected.Thirdly, a novel acquisition algor ithm for weak signals was examined. A modified method wasproposed by combining half-bits and full-bits algorithms. When the signal is too weak to use thehalf-bits algorithm, full-bits algorithm is adopted. However, the acquisition time is almost halved asacquisition results by half-bits algorithm is utilized. The newly-developed algorithm can guarantee aneffective and real-time acquisition.Fourthly, a novel acquisition judgment algorithm was developed. The presence of multipathinterference makes the original acquisition ineffective, so a novel algorithm was proposed to eliminatethe influence on the acquisition judgments brought by the multipath interference.Lastly, tracking algorithm for weak signal under multipath interference was researched.Adjusting factor was introduced into the Strobe related algorithms to modify the code loopdiscrimination, improving the multipath mitigation. The modified algorithm needs to providesatisfying carrier tracking features under weak signal environment, therefore, EKF algorithm wasintroduced to the carrier tracking algorithm to elevate S/N ratio and enhance the carrier trackingfeatures.