GNSS Weak Signal Acquisition And Tracking

In order to promote the development of GNSS,the Federal Communications Commission(FCC)and the European Commission(EC)issued executive orders Enhanced 911(E911)and EI12 in 1996 and 2003 respectively,requiring the integration of navigation receivers into mobile phones to provide location and emergency call services for users.General GNSS receivers can't meet the new requirements of indoor GNSS positioning,which brings many new challenges to the design of receivers.In the indoor and dense urban complex environment,occlusion,multipath and interference are more serious.The power of GNSS signal will be seriously attenuated,which is 20-30 dBm lower than the normal signal.Therefore,it is difficult for receivers to acquire these weak signals.Even if they are acquired,the accuracy of tracking can't be guaranteed.Therefore,this paper focuses on the acquisition and tracking of weak signals.The main work is as follows:(1)The relationship between acquisition probability and threshold of coherent integration and incoherent integration is deduced.A full-bit coherent integration method is presented and optimized,which greatly reduces the computational complexity.The simulation results verify the correctness of the method.(2)Introducing the carrier frequency discriminators for weak signals,such as NSFM and FFT.The frequency refinement method of FFT frequency discriminator is improved,and Monte Carlo simulation is carried out to analyze their performances.FFT frequency discriminator is more susceptible to noise.When the carrier-to-noise ratio is greater than 12,FFT-Refine has higher accuracy than FFT-Coarse.With the increase of carrier-to-noise ratio,FFT-Refine has very high accuracy.And the length of data has little effect on accuracy,while the accuracy of FFT-Coarse is mainly limited by the length of data.NSFM-NonCoherent is insensitive to the change of carrier-to-noise ratio.When the carrier-to-noise ratio is higher than 15,the accuracy of NSFM-NonCoherent is higher than that of NSFM-Coherent,and NSFM-NonCoherent is not affected by data bit flipping.(3)A method of eliminating strong signal interference is proposed when strong signal interferes with weak signal.The main idea is to remove the strong signal from the original signal.This method only needs to obtain the carrier frequency and code phase of the strong signal.The influence of strong signal tracking error on the interference cancellation method is analyzed.The effectiveness of this method is verified by theoretical analysis and simulation experiments.