Research On Key Technology Of Reverse RTK For Android Smartphones And Vehicle-based Experimental Validation

With the development of low-cost and low-power Global Navigation Satellite System(GNSS)chip technology and android operating system of smartphones,it is possible for intelligent devices such as smartphones to provide multi-system and multifrequency raw GNSS observation data,which provides the possibility of achieving high-precision positioning services for the public application.The method to achieve real-time,continuous and reliable high-precision positioning with smartphones has gradually become the hotspot.Due to the non-integer initial phase bias(IPB)on carrierphase observations,the double-difference ambiguity parameters cannot maintain the integer property,and because of the limitation of GNSS antenna manufacturing process and signal gain of smartphones,the satellite signal may lose lock frequently,which will increase the difficulty of cycle slip detection and repair.In addition,the power consumption and computing resources such as CPU and memory of smartphones are limited strictly,so it is difficult to allocate enough memory for high-precision positioning algorithms,which may lead to large calculation delays.To address the above problems,this paper proposes a velocity constraint-based time-differenced carrier phase(TDCP)cycle slip detection and repair method between epochs.Then a Reverse Real-Time Kinematic(R-RTK)system for smartphones based on the method is constructed.Finally,to verify the reliability and usability of the system,a large-scale,multi-scenarios dynamic on-board application test is performed.The main work and conclusions of this paper are as follows:(1)The quality of raw GNSS observations for Android smartphones is evaluated systematically,including the satellite tracking status,pseudorange and carrier-phase observation noise,carrier-to-noise ratio(C/N0)and doppler velocity accuracy.Results show that the carrier-phase observation noise of smartphones is about 2 cm,and the IPB parameters remain unchanged after starting the smartphones,so the ambiguity variation between epochs can maintain integer property without cycle slips.The satellite signal C/N0 of smartphone is about 10-20 d B-Hz lower than that of the receiver,and there is no obvious correlation between C/N0 and elevation angle of the satellite,so it is appropriate to use the C/N0 based weighed model.The doppler velocity accuracy is high and can generally be kept within 1dm/s.(2)To address the problem of frequent lock-lose for the satellite signal in complex environments,a single-frequency TDCP cycle slip detection and repair method based on doppler velocity constraints is proposed.Firstly,the combined observation is constructed by using the TDCP observations of satellite pairs with similar line-of-sight directions and the relative positions between epochs calculated by the doppler velocity,and the satellite pair without cycle slips through the combined residuals is determined.Subsequently,the least squares and partial ambiguity resolution methods based on optimal/suboptimal alternative group strategy are used for estimate and repair the ambiguity parameters.Finally,experiments are conducted to verify the reliability and efficiency of the method by using simulated data,results show that this method can accurately and efficiently detect multidimensional cycle slips in general scenarios(with a success rate of over 90%),with an average computational cost of less than 5ms,which can be applied in real-time high-precision positioning.(3)To address the problem of limited computing resources of smartphones,a reverse RTK system architecture for Android smartphones is proposed and constructed,and the proposed single-frequency cycle slip detection and repair method is deployed in it to reduce the computing pressure on the smartphones.Firstly,the function of three modules(Android APP,the cloud server and the CORS platform),the data interaction between modules and the data encoding format are designed and completed.Subsequently,a large-scale(Shenzhen,Wuhan and Beijing),multi-scenarios(urban open/blocking,highway open/blocking)dynamic on-board application test is performed to verify the reliability and usability of the system.The results show that Xiaomi MI8 can achieve sub-meter real-time dynamic positioning accuracy under the open sky environment,which can be improved up to about 70% compared with the positioning results provided by GNSS chips of smartphones.The single-frequency cycle slip detection and repair algorithm proposed in this paper can significantly improve the position accuracy by up to 30%?40% in obscured scenarios.The R-RTK system can operate stably and effectively in all scenarios,and the time delay is generally lower than 0.2s,which shows good positioning performance.