Research On The Key Technologies Of Multi-frequency And Multi-constellation GNSS Rapid Precise Positioning

As messaging services get smarter,there is increasingly urgent demand for high-precision and high-frequency spatio-temporal datum information.The emerging applications such as autonomous driving,smart city,all require both high-precision positions and critical timeliness to reach accurate positions.The precise point positioning(PPP)technique has been demonstrated as an effective tool in achieving high-precision positioning worldwide and has been widely used in scientific and civilian application.However,traditional PPP technique still suffers from the problem of long initialization time to achieve centimeter-level positioning accuracy,which also limits the wider applications of PPP in some time-critical applications.With the rapid development of BDS and Galileo as well as the modernization of GPS and GLONASS,the world of satellite navigation is undergoing dramatic changes.The multiconstellation fusion brings significant improvements in satellite visibility,spatial geometry,convergence,accuracy,continuity and reliability for precise positioning.Moreover,multifrequency observations can formulate different observation combinations to assist ambiguity solution(AR).Multi-constellation and multi-frequency signals open new prospects for fast AR of PPP.Currently,more and more reference stations are set around the world to provide the precise atmospheric information for users,which also brings new opportunity to enable the ambiguities fixed rapidly and further shorten the initialization time of PPP.Therefore,this thesis conducts research on the key technologies of multi-frequency and multi-constellation GNSS rapid precise positioning,focusing on multi-frequency and multiconstellation fusion processing,PPP fast ambiguity resolution,PPP-RTK technique,and finally establishes a set of rapid precise positioning service system based on multi-frequency and multi-GNSS observations to improve the accuracy,timeliness and reliability of precise positioning,especially for the positioning performance in complex environments.The main work and contributions of the thesis are as follows:(1)This thesis introduces the function models,stochastic models and parameter estimation methods for multi-frequency and multi-constellation precise point positioning.Starting from the basic GNSS observation equations,two general PPP models,i.e.,the ionosphere-free model,and the undifferenced and uncombined model,are introduced and compared.Then the ionosphere-free model and the undifferenced and uncombined model are extended for multi-frequency and multi-constellation PPP,respectively.The coupling relationships between the parameters such as the receiver clock offsets,the satellite clock offsets,the ambiguities and the code/phase signal biases are analyzed to determine the estimating parameters in each PPP model.In addition,the stochastic models of multifrequency and multi-constellation PPP are presented,and two commonly used parameter estimation methods are also introduced,including the sequential least square method and the Kalman filtering method.(2)A method for multi-frequency and multi-constellation code and phase bias estimation is proposed,by carefully considering the temporal and spatial characteristics of the code and phase biases for different GNSS constellations,different signal frequencies,as well as different observation types.The accuracy of the generated code bias products is better than0.1 ns,and the accuracy of the phase bias products is better than 0.1 cycle.Aiming at the problem of various and complex combinations of the signal biases caused by abundant GNSS signal resources,a method for absolute bias estimation is proposed to provide users with simple-formed and datum-unified multi-frequency and multi-constellation bias products.By introducing the absolute bias products as direct corrections to raw observations,undifferenced PPP ambiguity resolution can be achieved.(3)The temporal and spatial characteristics of BDS-3 code and phase biases are analyzed in detail for the new BDS-3 signals B1 C,B2a,B2 b,and B2a+b.It is found that both code and phase biases of BDS-3 B1 C signals are in good consistency for different signal components.The code and phase biases of different signals in BDS-3 B2 band also exhibit good agreement with each other.These unique features of BDS-3 new signals are of great significance for BDS-3 precise positioning and ambiguity resolution.Furthermore,a multi-frequency BDS PPP ambiguity resolution model is established,which takes full advantage of both the backward-compatible signals and the new BDS-3 signals.With this model,the five-frequency ambiguity resolution is achieved for BDS-2 and BDS-3 PPP.The time to first fix(TTFF)can be shortened to 25.5 minutes,and the positioning accuracy can be increased by 30-50 %.(4)This thesis establishes undifferenced ambiguity resolution models for multi-frequency and multi-constellation PPP in both ionosphere-free mode and undifferenced uncombined mode,respectively.The results show that the multi-frequency ambiguity resolution can significantly shorten the convergence time of PPP and improve the positioning accuracy,and the positioning performances are gradually improved as the number of frequencies increases.GPS+Galileo+BDS five-frequency PPP ambiguity resolution only takes 4.6 minutes to achieve centimeter-level positioning accuracy.Compared with the ionosphere-free model,the multi-frequency ambiguity resolution model using raw observations shows several advantages,such as simpler model forms,more extensible,and better positioning performances.In the context of the rapid development of GNSS and LEO constellations,this thesis proposes a method of LEO-augmented multi-frequency and multi-constellation PPP ambiguity resolution.With LEO-constellation augmentation,the positioning accuracy of millimeter-level in the horizontal components and centimeter-level in the height component can be achieved within 1minute.(5)This thesis proposes a method for multi-frequency and multi-constellation PPP-RTK ambiguity resolution based on raw observations,and establishes a multi-frequency and multiconstellation PPP-RTK service system to achieve rapid initialization and re-initialization.The contributions of multi-frequency and multi-constellation to PPP-RTK positioning are demonstrated in different urban dynamic scenes.In open and high-speed environments such as viaducts and expressways,the fixing rate of over 93% can be reached by multi-frequency and multi-constellation PPP-RTK,and the positioning accuracy of 2-3 cm can be achieved in the horizontal components.However,the positioning accuracy and stability of PPP-RTK are severely reduced in complex ground environments such as urban canyons.In this context,an inertial augmented rapid ambiguity resolution method is proposed to improve the continuity and reliability of PPP-RTK.In the case of GNSS outage of few seconds,INS observations can provide decimeter or even centimeter-level positioning results to fill in the position gap.Furthermore,a fast ambiguity recovery within 1-5 s could be achieved for PPP-RTK/INS after outages lasting up to 30 s while 8-18 s is required for PPP-RTK.