Research On Key Algorithms Refinement Of Multi-frequency Multi-GNSS High-precision Positioning And Time Transfer

High-precision positioning and time transfer are essential components of global navigation satellite system(GNSS)positioning,navigation,and timing(PNT).They are necessary for the operation of many critical infrastructure sectors.With the full completion of BeiDou Navigation Satellite System(BDS)and Galileo,the modernization of GPS and GLONASS,and the development of Quasi-Zenith Satellite System(QZSS)and Navigation with Indian Constellation(NavIC),the coexistence of multi-GNSS has taken shape.In multi-GNSS scenarios,algorithms refinement for highprecision GNSS positioning and time transfer has become the current research focus of GNSS.In this contribution,based on the theories of rank deficiency elimination and integer estimation,this thesis aims at four aspects:GNSS inter-system bias(ISB)estimation and application with the single-differenced(SD)model,short-term variation of GNSS receiver biases,ionosphere-weighted model,and undifferenced and uncombined(UDUC)GNSS time and frequency transfer with integer ambiguity resolution.The main contributions and innovations are as follows.1.The SD real-time kinematic(RTK)method of ISB estimation and application is proposed,and the joint estimation of overlapping frequency and non-overlapping frequency ISB is realizedThe following research is carried out considering that the traditional doubledifferenced(DD)model is challenging to achieve optimal fusion in multi-frequency multi-GNSS scenarios,First,based on the S-basis theory,the types of rank deficiency in the SD model are clarified,the full-rank SD functional model is derived,the estimable form of ISB under the SD model is given,and the joint estimation of overlapping frequency and non-overlapping frequency ISB is realized.Second,based on the SD model,the characteristics of ISB and its influencing factors are explored,and the processing strategies of different types of ISB are clarified.Third,the ISB estimated by the above model is used in high-precision RTK positioning,and the advantages of the SD model in ISB estimation and application are verified.2.The short-term variations of GNSS receiver biases and their influence law are systematically studied,and the correction models for GNSS receiver biases are established,which significantly improve positioning accuracyThe following research based on the SD model is carried out considering that the time-invariant assumption of GNSS receiver biases affects high-precision GNSS applications.First,the short-term variations of receiver differential code bias(DCB)and differential phase bias(DPB)are discovered.The relationship between short-term variations in GNSS receiver biases and receiver type and temperature is clarified.Second,it is found that the non-overlapping frequency ISB has an obvious correlation with the temperature,while the overlapping frequency ISB does not.Third,the functional correlations between the GNSS receiver biases and temperature are established.The gain of accounting for short-term variations of GNSS receiver biases to high-precision RTK positioning is verified.3.A stochastic model suitable for SD ionospheric pseudo-observations is constructed,which significantly improves the positioning performance of medium baselinesThe following research is carried out considering the unreasonable stochastic model of ionospheric pseudo-observations in the ionosphere-weighted model.First,a reasonable stochastic model of GNSS observations is constructed based on the SD ionosphere-fixed model and the least-squares variance component estimation(LSVCE).Second,based on a reasonable stochastic model of GNSS observations,the ionospheric delay is extracted using the SD ionosphere-float model,and the stochastic model of ionospheric pseudo-observations is constructed with LS-VCE again.Third,the stochastic model of the constructed GNSS observations and ionospheric pseudoobservations is applied to the SD RTK positioning with medium baselines,which significantly improves the success rate of integer ambiguity resolution and positioning accuracy.4.The UDUC GNSS time transfer method with integer ambiguity resolution is proposed,the impacts of integer ambiguity resolution and precise satellite clock products on time transfer are revealed,and picosecond time transfer is realizedThe following research is carried out considering the inflexibility of the ionosphere-free(IF)precise point positioning(PPP)in GNSS time transfer and the difficulty in realizing integer ambiguity resolution.First,starting from the UDUC observations,using the S-basis theory and external precise satellite orbit and clock products,a UDUC GNSS time transfer model with integer ambiguity resolution,namely the SCF(Satellite Clocks Fixed)model,is constructed.In addition,considering the influence of precise satellite clock products on time transfer,a UDUC GNSS time transfer model with integer ambiguity resolution and satellite clocks estimated,namely the SCE(Satellite Clocks Estimated)model,is constructed by using the S-basis theory and broadcast ephemeris.Third,based on the above model,the potential of GNSS time transfer is explored,and it is proved that GNSS has the ability to realize picosecond time transfer.