Research On The Key Technologies For GNSS Precise Positioning Augmented With LEO Constellation

The era of large constellation consisting of hundreds or even thousands of low earth orbit(LEO)satellites is coming.Many LEO satellites will also have the function of operating as navigation systems and autonomously broadcast the navigation ranging signals for positioning.In view of the advantages of stronger received signal and rapid change of spatial geometry,advantageous complementarities can be achieved between the LEO and existing GNSS constellations and fast precise positioning is expected to be realized.In this thesis,the research focuses on the key technologies for GNSS precise positioning augmented with LEO constellation.The key technologies include the constellation design,orbit simulation,observation simulation,orbit determination of LEO satellite,broadcast ephemeris design and performance evaluation of LEO constellation augmented GNSS positioning.The main works of this thesis are as follows:(1)Schemes of GNSS and several kinds of LEO constellations are designed.The ground tracks,sky plot,ground coverage and global distribution of PDOP value of all simulated constellations are comprehensively analyzed and the initial validation of LEO constellation augmented GNSS performance is carried out.Compared with GNSS constellation,LEO constellation has higher inclination which contributes to wider ground track coverage that even contains the polar area.Due to the faster motion speed,LEO satellite can streak much longer track than GNSS satellite during the same period.The extent of geometric change of 31 s for LEO satellite is equivalent to that of 20 min for GPS satellite.LEO satellite is closer to the ground and thus leads to smaller ground coverage of one satellite.To accomplish the global coverage of LEO constellation,over 100 satellites are required.The distribution of PDOP value shows that the large value appears in low-latitude area,the medium value appears in mid-latitude area,and the small value appears in high-latitude area,which is helpful for the improvement of navigation and positioning performance in high-latitude area.(2)Programs used for the simulation of precise ephemeris file,onboard LEO data and ground-based observations are developed.The formulas of zero-difference code and carrier phase observation simulations are derived in detail.The entire process of simulating observation is given as well.(3)Methods of kinematic precise orbit determination(POD)and autonomous orbit determination based on inter-satellite link data are studied.The orbit determination and accuracy evaluation of LEO constellation are conducted.The result shows that GREC kinematic POD can obtain higher accuracy of 1.3,1.0 and 0.9 cm in the radial,along-track and cross-track directions,respectively,better than that of 3.9,3.5 and 3.4 cm for GPS-only kinematic POD.For autonomous orbit determination with inter-satellite link data,the three-dimensional orbital errors of 60-and 192-satellite LEO constellations are 46 and 18 cm,respectively.(4)The broadcast ephemeris for LEO satellite is designed and the ephemeris fit interval is determined.When a 16-parameter non-singular broadcast ephemeris is adopted,to ensure the maximum user range error around 10 cm,the fit interval is suggested to be set to 20 min for the orbit altitude of 1000 km.A new broadcast ionospheric model named MNTCM-BC is developed and applied to LEO navigation message.The MNTCM-BC is comparable to the Klobuchar model in terms of complexity,but performs better in the characteristic descriptions of ionospheric temporal and spatial variations.The prediction accuracy is also better than the later with an improvement of about 30%.(5)The uniform mathematical model of fusion processing for high-,mid-and low-orbit satellites is proposed.The temporal and spatial datum are unified.The functional model and random model is established.The method of LEO constellation augmented GNSS precise point positioning(PPP)is exploited based on the current multi-system GNSS PPP theory.The factors such as the number of LEO satellites,the latitude of station,the orbit altitude,the orbit type and the sampling interval of observations are taken into consideration to fully analyze the impacts on the augmentation of PPP convergence speed.The results show that LEO constellation can dramatically accelerate GREC PPP convergence,and the more satellites,the shorter convergence time.In mid-latitude area,while introducing 60-,96-,192-and 288-satellite LEO constellation,the augmented GREC PPP convergence time can be shortened from 8.2 to 7.0,3.2,2.1 and 0.8 min,respectively.The convergence performance is more obvious in the scheme of LEO constellation augmented GPS-or BDS-only PPP,and the convergence time can be shortened by more than 90%from about 20 to 2 min and even within 1 min with 192-or 288-satellite constellation.Given the cost,the satellite number of 192 is suggested.Additionally,the performance of LEO constellation augmented GREC PPP is related to the latitude of stations,and the higher latitude,the better performance.The improvement is about 70%overall.In terms of the orbit altitude,the configuration of 1000 km is better than that of 600 km.The orbit type and sampling interval of observation have little influence on the augmentation performance.(6)The performances of LEO-only positioning,including single point positioning(SPP)and PPP,are tested.For the 192-satellite constellation,the visible numbers of LEO satellites are 5.7,6.4 and 19.1 at stations FAA1,NNOR and KIRU,respectively.These stations are located in low-,mid-and high-latitude areas,respectively.The SPP can achieve a positioning accuracy of meter degree.At station NNOR,the static PPP convergence time is 2.2 min for LEO-only and much shorter than 23.5 min for GPS-only.The final positioning accuracy indicates that millimeter and centimeter degrees can be obtained in the horizontal and vertical directions,respectively.