| Global navigation satellite systems(GNSSs),represented by Bei Dou and GPS,can provide global navigation and positioning services and have become a critical space-time infrastructure.However,due to the influence of a variety of errors,the precision of standard navigation and positioning services provided by GNSS itself is only about 5 to 10 m,which is far from meeting the needs of high-precision positioning applications such as mapping and remote sensing,automatic driving,precision agriculture,and plate movement monitoring.To realize the dm-,cm-or even mm-level positioning accuracy,geodesists have put forward two precise positioning methods: differential positioning suitable for local-area users,and the other is precise point positioning(PPP)which is suitable for wide-area users.Although the PPP method can provide seamless and unified navigation and positioning services worldwide,due to the inherent limitations and vulnerability of GNSS itself,PPP has the problem of discontinuous positioning or even unavailability in the environment such as indoor,forest,tunnel,overpass,and urban canyon where signals are blocked.In addition,to achieve cm-level positioning accuracy,PPP usually requires tens of minutes of convergence time,which has always been a bottleneck restricting its development and application.In order to improve the performance of GNSS,a variety of satellite navigation augmentation systems are developed based on the primary navigation system.Nevertheless,traditional ground-based augmentation systems have limited coverage thus cannot provide global augmentation services.Besides,traditional satellite-based augmentation system based on the high-orbit satellite platforms mainly focuses on information augmentation,but the ability of signal augmentation is limited.In areas where the signal is obscured,it is still unable to provide continuously available navigation and positioning services and realize rapid PPP initialization.In recent years,domestic and foreign scholars have proposed to use the low earth orbit(LEO)constellation as a platform for navigation signal broadcast and augmentation information forwarding to augment the existing GNSS.With advantages such as rapid geometric changes,strong signal power,and global space-based monitoring coverage,the LEO constellation can complement the GNSS constellation and comprehensively improve the accuracy,integrity,availability,and anti-jamming capability of satellite navigation and positioning services,which has become an important development direction for the next-generation satellite navigation systems.Since most of the LEO-based navigation augmentation constellations are still in the early stage of system construction,the augmentation effect they can produce is not very clear,and how the system is optimized and implemented is at the stage of demonstration.Therefore,this thesis takes rapid and high-precision positioning as the starting point and conducts in-depth research on LEO constellation navigation-augmented GNSS rapid PPP methods and critical technologies.First,based on the simulated data,the performance of LEO augmentation under different constellation scales,orbital altitudes,space-time accuracy,noise levels,and positioning modes is demonstrated.On this basis,a genetic algorithm is adopted to design the hybrid LEO navigation augmentation constellations from the point of view that the constellation coverage is optimal and more conducive to the realization of rapid and precise positioning.In addition,under the premise of compatibility principle and within the scope authorized by the International Telecommunication Union(ITU),the frequency configuration and signal design of a LEO-based navigation augmentation system is analyzed from the point of view that the ambiguity parameters in high-precision positioning are easy to be resolved.Overall,this thesis aims to provide valuable references for constructing a LEO-based navigation augmentation system,and meanwhile overcoming the problems of long initialization time and low reliability in PPP.The main work and contributions of this thesis are as follows:1.Due to the lack of a large number of actual observations and information in the early stage of system construction,to demonstrate the expected performance of the future global LEO-based navigation augmentation system,an algorithm for simulating the ephemeris of the LEO navigation constellations and an algorithm for simulating the observations of the LEO navigation satellites by considering various observation errors and measurement noise are both given.The simulation system of ephemeris and observation data is independently developed and used to simulate the ground network observation data of various constellations,which lays a foundation for evaluating and verifying precise positioning performance.2.The performance of LEO navigation-augmented GNSS rapid PPP is comprehensively evaluated.Taking the traditional polar-orbit LEO constellation as an example,the effects of different constellation scales,orbital altitudes,space-time accuracy,noise levels,and positioning modes on navigation augmentation performance are discussed,respectively,and the systematic,comprehensive,and statistical conclusions have been obtained.Moreover,the critical technologies of LEO navigation-augmented GNSS fraction cycle bias(FCB)estimation and PPP ambiguity resolution(PPP-AR)under highly dynamic,short-arc,and small-coverage conditions have been successfully broken through.The results show that compared with ambiguity-float PPP solutions,the positioning accuracy in the east,north,and up components is significantly improved from 0.016,0.008,and 0.020 m to 0.003,0.002,and 0.011 m,i.e.,an accuracy improvement of about 81.3,75.0 and 45.0%,respectively.Therefore,LEO navigationaugmented PPP-AR contributes significantly to positioning accuracy,especially for short-time mm-level positioning.In addition,by analyzing the coverage of polar-orbit constellations,it is found that the coverage of this type of constellation is uneven at different latitudes.The lower the latitude is,the fewer the number of LEO satellites can be observed.The number of visible LEO satellites is closely related to the augmentation performance of rapid PPP,especially in terms of convergence time.The results show that when the orbital altitude is fixed to 1100 km,and the polar-orbit LEO constellations with 72,144,216,288,and 576 satellites are used for navigation augmentation,an average number of 3.83,7.67,11.50,15.36,and 30.68 LEO satellites can be observed in the mid-latitude region(40°N – 60°N),respectively.The corresponding convergence time of ambiguity-float GPS PPP in static mode can be significantly shortened from 16.9 min to 4.3,2.6,1.9,1.6,and 1.1 min,which indicates that the addition of a small-scale LEO constellation in the early stage of system construction has a greater benefit on the improvement of PPP convergence speed.In contrast,the improvement efficiency gradually slows down in the later stage.3.To avoid the uneven global coverage of standard single polar or near-polar orbit LEO constellation and improve the ability of the system to augment the rapid,precise positioning,the optimal design schemes of hybrid LEO navigation augmentation constellations are proposed by using a genetic algorithm for the first time.The optimized hybrid constellation enables as many visible LEO satellites as possible and realizes globally even coverage when the total number of satellites is limited.The optimal constellation configuration can be obtained for different elevation mask angles and global coverage availability with different visible satellites when the type of constellation is fixed.The results show that when the total number of LEO satellites is not more than 100 for the target altitude of 1248.171 km,the distributions of the average number of visible satellites are pretty even at different latitudes.The mean values are5.49,5.44,and 5.47,with standard deviations of 0.44,0.18,and 0.28,for the optimized hybrid polar-orbit/Walker,orthogonal circular-orbit/Walker and Walker/Walker constellations,respectively.For the hybrid orthogonal circular-orbit/Walker constellation type,the necessary numbers of LEO satellites to realize globally even coverage with six visible satellites are 109,172,and 221 for elevation mask angles of 7°,15°,and 20°,respectively.For globally even coverage with four and five visible satellites with an elevation mask angle of 7°,the total required satellites are 90 and 93,respectively.All proposed hybrid constellations can provide100% global coverage availability with one to three visible satellites and meet the demand for globally seamless broadband Internet access services.4.The spectrum congestion of current multi-GNSS navigation signals in the L band leads to serious signal interference.It is challenging to apply for another two or more proper frequencies in this band for the future LEO-based navigation augmentation system.Under the premise of the compatibility principle and within the scope authorized by the ITU,the frequency configuration and signal design of a LEO-based navigation augmentation system are analyzed from the perspective of high-precision positioning AR.The appropriate signal frequencies from the L,S,and C bands are respectively selected to satisfy the specific integer ratios,enabling PPP-AR by directly fixing the L+S or L+C dual-band ionosphere-free(IF)ambiguities.Meanwhile,a high-efficiency modulation scheme with excellent spectrum roll-off characteristics,termed continuous phase modulation(CPM),is adopted to effectively avoid interference between proposed signals and the existing satellite navigation signals and other radio services at adjacent frequencies(such as radio astronomy and microwave landing systems).The propagation loss characteristics of LEO downlink signals in different frequency bands are also analyzed.The results show that at 5° elevation angle,the total losses are 180.2,181.8,195.6,and 196.5 dB for the proposed L-band,S-band,C-band,and GPS L1 signals,respectively.At 90° elevation angle,the corresponding losses are 168.8,169.0,175.2,and192.2 dB.Hence,in the case of identical satellite transmitted power,the received power of all proposed signals will be stronger than that of the GPS L1 signal,particularly for high elevation angles.Thanks to the frequency configuration of specific integer ratios,a new algorithm for dual-band undifferenced IF FCB estimation is proposed.Based on the simulated data,the quality of FCB products and PPP-AR performance are tested and analyzed.At reference stations,the quality of FCB products is evaluated according to the distribution of the remaining fractional parts after deducting the FCBs from the float ambiguities.The results show that after removing the FCBs,100.0,99.7,and 71.7% of the fractional parts are within ±0.15 cycles for GPS narrow-lane(NL),LEO L+S dual-band IF,and LEO L+C dual-band IF float ambiguities.At user stations,the static PPP results show that the convergence time of GPS PPP can be significantly reduced from 17.9 to 2.5 min with the augmentation of 5.44 LEO satellites on average.In terms of positioning accuracy,AR has significant advantages.Compared with ambiguity-float GPS+LEO PPP,GPS PPP-AR+LEO PPP-AR solutions in east,north,and up components significantly improve from 0.008,0.008,and 0.027 m to 0.002,0.003,and 0.011 m for 10-minute sessions,respectively. |
