Research On GPS Multipath Effect Correction Technology In Urban Environment

GNSS positioning applications in urban environment receive coherent signals from extra frontal paths such as specular reflection,diffraction,and diffuse reflection,and are affected by multipath interference and non-line-of-sight signal(NLOS)reception,resulting in complex ranging errors.Most of the existing multipath suppression techniques discuss the error caused by NLOS reception and multipath interference separately to simplify the problem.In the actual urban environment,the multipath effect is the result of the coexistence and interaction of both multipath interference and NLOS reception.Therefore,comprehensive solutions should be considered.Some researchers defined "the effect of coherent signals from other active or passive excitation sources other than direct signal on direct signal interference" as generalized multipath,and studied various multipath effects in a more comprehensive and in-depth way.Based on the above analysis,this dissertation takes GPS system as the research object under the generalized multipath framework,studies the common and comprehensive effects of multipath in urban environment,and puts forward the corresponding correction model.The main research contents are as follows:1)The mechanism,error characteristics and related calculation methods of different types of multipath signals are studied.Qualitative and quantitative analysis of the error levels caused by different types of multipath signals is carried out.Finally,the temporal and spatial repeatability of multipath signals according to their geometric relationship is analyzed.2)Based on the common characteristics of various multipaths spatial repeatability,a carrier multipath mitigation method named T-MHM(trend surface analysis based multipath heterogeneous map)is proposed to deal with the coexistence of various multipaths in urban environment by the integration of trend surface analysis and MHM(multipath hemispherical map)method.Compared with the original MHM modeling method,T-MHM enhances the modeling and correction ability for both low-frequency and high-frequency multipath,and is effective for ranging errors caused by multipath interference and NLOS reception.In practical application,T-MHM can use larger lattice modeling to improve the computational efficiency and spatial coverage,and ensure the baseline solution accuracy,convergence speed and carrier phase residual correction rate.3)Furthermore,an advanced trend surface analysis based multipath hierarchical map(AT-MHM)is proposed to correct the pseudorange error in urban environment.Considering the anisotropy of the multipath within the grid for the pseudorange multipath characteristics,the linear trend term is used for the parameters with weak spatial heterogeneity to avoid overfitting.The multipath reduction performance of the proposed method on geodetic receivers and low-cost receiver platforms is evaluated.AT-MHM has improved the baseline accuracy of Trimble receiver and Ublox receiver by 45.7% and 72.9% respectively.4)Using measured data in typical environments to explain the coexistence and mutual influence of multiple multipath signals in urban environments: single NLOS signal,interference of multiple NLOS signals,and the transition between diffracted signals and direct signals;The direct-like nature of the diffraction signal is found,and the carrier error caused by it may exceed 2 weeks;on the basis of the above research,based on the lidar point cloud data,a NLOS error response considering the combined effect of multiple.