Research On The Key Technologies Of The BDS/GPS Multi-frequency Network RTK Supporting Massive Concurrent Users

The Network real-time kinematic(RTK)technique can provide centimeter level real time positioning solutions and has been considered as the geo-spatial infrastructure.With increasing popularity,the Network RTK technique confronts a bottleneck that is how to support large scale network RTK systems so support high concurrency users.In the past,the high precision positioning services are professionals oriented and only support a few concurrent users.Currently,the precise positioning provides a spatial infrastructure to the artificial intelligence(AI)and countless location-awareness smart devices,such as the autonomous vehicle,unmanned aerial-vehicles(UAV),Robotics etc.Therefore,it is urgent to develop the approaches to support the large-scale network RTK systems.In this study,we investigated the key technologies of the network RTK support huge concurrent users from four aspects:(1)In this study,we proposed a self-organizing spatial clustering(SOSC)approach for network RTK virtual reference station users which automatically clustering online users to reduce the computation load at the server side.Experiment results indicate that the SOSC approach achieves comparable performance as the predefined grid approach under uniform distribution scenario and outperforms the grid approach under clustered distribution scenario in terms of time consumption,cluster number and mean distance to cluster center(MDTCC).The impact of applying SOSC on the user positioning is evaluated with real GNSS data and experiment results show that the impact of baseline length up to 10 km on positioning precision and reliability is limited and therefore safe for applying the SOSC approach.(2)We investigated ionospheric delay abnormal mitigation method with the spatial correlation,so that it improves the reliability of the regional ionosphere modeling.In this study,we compared the ionosphere interpolation methods with a provincial ground-based augmentation system(GBAS).Considering the reference station ambiguity may be fixed to incorrect integers,it is likely to produce irregular ionosphere estimates,which is affected by the incorrect ambiguity resolution.If the incorrect ionosphere delay is not properly handled,it may degrade the user's positioning performance.We proposed a new method called two dimensional Vondrak filter to mitigate the impact of incorrect ionosphere delay.This method exploits the spatial correlation between different reference stations and provide a smooth regional ionosphere model even in presence of ionosphere outliers.In this way,it can mitigation the impact of ionosphere outliers on the user positioning and improve the reliability of the regional ionosphere modeling.(3)We investigated the reference station multipath effect mitigation method with a multi-layer back propagation artificial neural network(BPNN).For the reference station,the multipath mode is relatively fixed,which is related to the signal incident angle.Based on this principle,we build a relationship between the multipath and the satellite incident angle with an 8-layer artificial neural network.This neural network can also be used to predict the multipath error for each satellite.The results indicates that the artificial neural network method can reduce the multipath effect by 50%,so that it can mitigate the multipath effect in real time and improve the ambiguity resolution reliability for the reference station networks.(4)We studied the positioning method of network RTK users based on BDS/GPS fusion,and verified the performance of the self-organizing spatial clustering method(SOSC)from user positioning accuracy prospect.This paper focuses on the unified spatial-temporal datum of BDS/GPS fusion positioning,and discusses the loosely coupled and tightly coupled models of fusion positioning and the design of extended Kalman filter for GPS Beidou fusion positioning.This paper also carried out theoretical research on the quality control in BDS/GPS fusion positioning,including cycle slip detection method based on triple-frequency carrier phase observation data,outlier detection and ambiguity resolution quality control,etc.In this paper,a cycle slip detection method using geometry-free and ionosphere-free(GFIF)combination and two MW combinations to determine triple frequency observation data is proposed,and a quality control method using generalized difference test(GDT)to improve ambiguity test.The user positioning performance of the SOSC algorithm is also verified with static and kinematic BDS/GPS data...