A Vision Aiding Method To Detect And Mitigate GPS Multipath In The Urban Environment

With the Global Positioning System?GPS?being applied more and more widely,users now put forward higher and higher requirements about the positioning accuracy.But the measuring errors seriously restrict the positioning accuracy.In the process of positioning algorithm,the systematic errors?including satellite clock bias,atmosphere refraction error,receiver clock bias?can be eliminated or reduced by some ways,but the accidental errors?usually it means multipath error?are closely related to the local properties,so they are difficult to correct through general models,then the multipath error becomes the main source of the high-precision GPS measuring errors.Especially in the dense urban area,where the multipath effect which includes non-line-of-sight?NLOS?and multipath interference is more seriously.In view of the present research on the shortage?high cost of hardware,limited performance?of multipath error handling,this paper proposes a kind of GPS multipath detection technology based on visual aids to the multipath signals detection in the urban environment,to improve the positioning accuracy in the urban area.This paper first introduces and studies the GPS pseudorange positioning principle and the key factors affecting the positioning accuracy,focusing on the multipath error and ionospheric error.And proposes a method to detect the multipath interference based on the Carrier to Noise Ratio?C/N₀?of satellite signals.The traditional pseudorange differential positioning uses the Carrier-Smoothed-Code?CSC?method to suppress the multipath effect and receive noises,but the performance is limited by the Code-Carrier divergence?CCD?effect,so this paper compares and analyzes the current CCD detection methods,meanwhile,the sigma-inflation method is used to compensate the non-perfect distribution of the constructed test statistics during the design of CCD detection thresholds,the results of CCD detection thresholds test reflect the Constant False Alarm Rate?CFAR?performance in the fault-free condition.Second,this paper does detailed research on three-dimensional?3D?reconstruction based on the binocular vision,including the calibration of binocular stereo vision system,stereo matching and 3D reconstruction process.In the calibration of binocular stereo vision system,using Zhang Zhengyou's camera calibration method to compute camera intrinsic parameters.Calibrating template corner's image coordinates are obtained by handwork and Harris corner point detection algorithm.In the stereo matching process,deeply studying the principle and implementation process of stereo matching algorithm,then using Scale Invariant Feature Transform?SIFT?stereo algorithm to obtain coordinates of the matching point in two image coordinate systems.According to the basic principle of stereo vision,this paper introduces the basic method to obtain the spatial point location in the camera coordinate system based on the matching point and camera intrinsic and extrinsic parameters.On this basis the reconstructed of the building is done,then according the relative position of receiver,building and the visible satellites,detecting the NLOS reception GPS signals,and in the position calculation process using appropriate methods to deal with them,such as remove them or low their weight,at last,based on the C/N₀ value design the weighted model for multipath interference satellite signals,to achieve the purpose of improve the positioning accuracy.Finally,in order to validate feasibility of the proposed multipath detection and suppress method,the Single Point Positioning?SPP?test has been conducted.The results of the traditional SPP and the proposed SPP in which the multipath signal is suppressed show,when remove the NLOS satellite signal,the positioning accuracy is improved by 59.02%;when remove the NLOS satellite signal and weight the satellite signal of the rest based on the C/N₀value,the positioning accuracy is improved by 71.94%.This illustrates the proposed NLOS multipath detection method and multipath interference detection method based on the C/N₀are feasible and effective.And the proposed NLOS multipath detection method can provide technical support for GPS multipath detection in the urban environment.