Research On GNSS Signal Acquisition Techniques In Weak Signal Environment

The explosive growth of applications and requirements on Location Based Service(LBS)has made Global Navigation Satellite System(GNSS)positioning technology an essential part of smart devices.This puts higher requirements on the positioning of GNSS signals in harsh environments such as indoors and urban canyons.When the receiver works in the weak signal environment,the GNSS signal arriving at the receiver is very weak,therefore,how to improve the sensitivity,and ensure the positioning accuracy and reliability in the weak signal environment has become research focus and hotspot.In the high-sensitivity GNSS receivers,signal acquisition is one of the most critical and core technologies in the baseband signal processing procedure.In order to improve the capture sensitivity,GNSS receivers usually need long coherent integration period and many non-coherent integration times,which means the increase of acquisition time and the complexity.Furthermore,the new BOC modulated GNSS signal with the GNSS construction boom has also brought new challenges to the traditional acquisition technology.Therefore,this dissertation focuses on the pseudo-code and frequency acquisition algorithms for the GNSS signal in the weak signal environment.The main work and innovative achievements of this thesis are outlined as follows:(1)The characteristics of commonly used FMDBZP(Fast Modified Double Block Zero Padding)algorithm are analyzed in detail,then,aiming to solve the large computational shortcomings of FMDBZP algorithm,a combined using FMDBZP with differential correlation and Discrete Cosine Transform(DCT)filtering method is proposed.By employing the energy concentration characteristics of the DCT transform and the differential correlation to eliminate the Doppler frequency offset,the proposed method will simplify the structure of the receiver and improve the capture sensitivity.In order to expand the frequency search range,two weak GNSS signal acquisition algorithm based on frequency domain search method are proposed.These algorithms take advantage of the equivalent characteristics of the frequency domain cyclic shift and the time domain frequency compensation to expand the Doppler search range.Simulation analysis shows that the two algorithms can effectively improve the sensitivity of GNSS signal detection under large frequency offset(2)In order to overcome the problem of detection sensitivity reduction caused by Doppler shift,two Doppler frequency estimation algorithms are proposed based on the analysis of the influence of frequency offset.The two algorithms use the result of Fast Fourier Transformation(FFT)to constrain and estimate the Doppler frequency offset,and get a high-precision frequency offset estimation result,then,the high-precision frequency estimation is used to perform secondary acquisition of GNSS signals.Theoretical analysis and simulation results show that the two modified algorithms can effectively improve the accuracy of Doppler estimation and improve the detection probability.(3)Aiming to solove the ambiguity problem of BOC(Binary Offset Carrier)modulation signal widely used in the new generation GNSS system,based on the analysis of BOC signal correlation function,a general unambiguous BOC signal acquisition method is proposed.The algorithm decomposes the subcarrier waveform according to the modulation type,then,constructs a single peak correlation result according to the synthesis method of the correlation function.The new acquisition algorithm can completely eliminate the side peaks and achieve unambiguous capture;furthermore,the algorithm can be applied to the capture of the QMBOC signal in the BDS-3 B1 C.(4)After analyzing the long-range ranging code and the secondary carrier modulation characteristics of BDS-3 signal,a fast frequency-domain filtering acquisition algorithm for Beidou B3 I signal is proposed.The algorithm performs frequency domain symbol correlation processing on the partial matched filtering(PMF)-FFT output of each code phase to eliminate the influence of symbol hopping,so as to achieve fast capture of Beidou signal under data bit transition.In order to solve the problem that the traditional acquisition algorithm consume long time and large computational amount of long ranging code,a method of determining compressed sensing signal acquisition using Wash-Hardmann transform is proposed,and the capture performance of the algorithm is analyzed.Simulation results show that the proposed method can effectively capture the B1 C signal and save the computation burden.