| Global Navigation Satellite System(GNSS)is the key infrastructure of providing Positioning,Navigation and Timing(PNT)services.Keeping GNSS as the central infrastructure,enriching PNT information sources,improving the reliability,security and robustness of PNT services,and promoting the construction of integrated-PNT system is the plan next coming up.GNSS is initially designed for ground users,the medium and high orbit satellites used in GNSS insure good satellite geometry for ground users.However,due to the signal transmission loss,the signals are weak,therefore,they can easily to be interfered.Also,signal interruption is easily to occur in sheltered areas such as urban canyon,indoor,forest and bridge,so it is challenging to provide reliable and continuous PNT service.In addition,the geometric configuration of the constellation composed of medium and high orbit satellites changes slowly,hence it takes a long convergence time to realize centimeter level positioning.Therefore,its application in ereas such as precision agriculture,automatic driving,intelligent surveying and mapping,intelligent logistics with high-precision positioning needs is limited.The first concern of this paper is how to improve the reliability of the current satellite navigation system and promote the real-time high-precision application.The traditional wide or local area augmentation systems improve the positioning accuracy by broadcasting correction and augmentation information.Its coverage is limited and a certain number of ground stations always need to be built.Considering the construction wave of large Low Earth Orbit(LEO)communication constellation,bringing LEO satellites into the navigation field,increasing the orbital diversity of navigation system,expanding the navigation application in challenging environment and improving realtime positioning performance are the current research hotspots.To accomplish this task,we need to design a reasonable LEO satellite constellation,and investigate the LEO satellite number,orbital altitudes and environments influence on LEO-augmented GNSS navigation performance.The second issue this paper addressed on is spacecraft positioning with GNSS sidelobe signals.Nowadays,orbit determination of spaceacraft mainly depends on the ground control system.This approach is expensive,the increasing fight missions burdens the ground control system and the spacecraft positioning accuracy is not very high.Extending GNSS PNT services for high orbit spacecraft will improve the orbit determination accuracy,incrase satellite autonomy and save costs.However,mostly spacecraft locate above the GNSS satellites,therefore,they can only receive sidelobe signals from the opposite side of the earth during most orbital periods,which makes the received signals are sparse and the satellite geometry is not very well.Nowadays,BDS-3 has finished its global construction,the number of global navigation satellites reach more than 100.Such rich navigation signals improve the signal geometry and the positioning performance.The third problem we concerned is,when the spacecraft approaches the moon,the GNSS signal attenuates seriously,and it is challenging to use GNSS siganls for lunar orbit and landers.Therefore,it is necessary to establish a reasonable lunar navigation constellation.To solve the listed three problems,this paper selects three Earth-Moon space navigation scenarios as ground,high orbit and lunar surface,and presents three contributions:LEO satellite augmentation,GNSS sidelobe signal positioning performance analysis and lunar navigation constellation design.The main work and contributions of this paper are listed as follows:1.The first work of LEO-augmentated GNSS navigation is LEO satellite constellation design.Firstly,this paper analyzes the air drag effects on LEO satellites,clarifies the reason that LEO satellites should be placed at altitudes between 900 km～1500 km.Further,in order to find the reasonable LEO constellation configuration with multi-objective,this paper fistly uses the non-dominated sorting genetic algorithm-Ⅲ(NSGA-III)to solve the problem of LEO satellite constellation design.LEO constellation is designed under two scenarios:navigation and augmentation.Navigation scenario refers to that LEO constellation provides PNT service as an independent navigation system.In this scenario,when the optimization objective is the global average GDOP and the number of satellites,the LEO constellation with altitudes of 900,1000,1100,1200,1300,1400 and 1500 km needs 264,240,210,210,200,190 and 180 satellites respectively.With these satellites,global average GDOP can be less than 3.When the optimization objective including global average GDOP,number of satellites and altitudes,LEO constellation orbital altitude of 1008.23 km and 252 satellites is recommended.In the augmentation scenario,the proposed configuration of polar LEO constellation with orbital altitudes between 900 km～1500 km is given when the global average number of visible LEO satellites is 4,5 and 6.Due to the uneven distribution visible satellites of the polar constellation,the hybrid LEO constellation configuration with the same average visible satellites is introduced.Finally,aiming at optimizing the configuration of GNSS+LEO constellation,a hybrid LEO constellation configuration with minimum number of LEO satellites is given.In this paper,the parallel computing method is used in the optimization design process.For double objective optimization problem,the parallel computing method is 27.3 times faster than serial computing method.2.The second work of LEO-augmentated GNSS navigation is LEO Precision Point Positioning(PPP)and the rapid convergence of LEO-augmentated GNSS PPP ambiguity.For LEO PPP,the rich number of LEO visible satellites can shorten the convergence time to less than 1 minute.Besides,the polar orbit constellation is used to analyze LEO-augmented GNSS PPP performance under different orbital heights,different number of satellites and different occlusion environments.The results show that 96 Leo constellations at an altitude of 900 km can shorten the convergence time of GPS PPP by 84%and the convergence time of GPS+BDS combined PPP by 82.6%.96,180,240 and 360 LEO satellite constellations can reduce the convergence time of GPS PPP from 27.5 min to 4.4,3.4,2.7 and 1.1 min respectively.When 180 LEO satellites are used,the convergence time of GPS PPP can be shortened to 18.4,4.3,1.8 and 1.4 min at 500 km,700 km,1100 km and 1300 km.Setting the cut-off altitude angles of 10°,20 °,30° and 40°,the positioning performance of LEO satellites combined with GNSS PPP will not be affected at 10°,20° and 30°,but the GNSS PPP cannot be enhanced at the cut-off altitude angle of 40°.3.For GNSS sidelobe signal navigation performance analysis,this paper selects five types of spacecraft:GEosynchronous Orbit(GEO),Gosynchronous Transfer Orbit(GTO),Highly Elliptical Orbit(HEO),Earth-Moon transfer orbit and halo orbit After simulating GNSS antenna radiation model and spaceborne antenna gain,GNSS spaceborne observations are simulated.The Doppler shift,signal carrier noise ratio(C/N0),number of visible satellites,Geometric Dilute of Precision(GDOP)and Single Point Positioning(SPP)performance are analyzed,and the importance of sidelobe signal in spacecraft positioning is emphasized.The results show that the Doppler frequency shift of the five types of space vehicles is generally greater than that of static ground users,and sometimes greater than that of LEO satellites.C/N0 of the main lobe signal is about 15 dB higher than that of the side lobe signal.GTO and HEO spacecraft receive highest siganls near perigee,which is around 67 dB-Hz.92.7%,86.5%,86.5%,78.4%and 68.9%of the received signals are sidelobe signals for GEO,GTO,HEO,Earth-Moon transfer orbit and halo orbit.In terms of the number of visible satellites,the BeiDou satellite navigation System(BDS)visible satellites of five types of spacecraft is 25.2,25.5,22.8,13.1 and 1.1 respectively.Under the GREC(BDS+Global Positioning System+GLObal Navigation Satellite System+Galileo)combination,the number of visible satellites of five types of spacecraft is 85.2,87.3,18.9,8.4 and 4.7 respectively.All types of spacecraft except halo can achieve 100%positioning availibility.In terms of GDOP value,the average GDOP of GEO,GTO and HEO under GREC combination is 2.0,1.1 and 2.3 respectively,and the GDOP value of Earth-Moon transfer orbit is about 103.The SPP 3D RMS of GEO,GTO and HEO spacecraft is 11.3,5.9 and 18.5 m respectively.The positioning result of the Earth-Moon transfer orbit is at km level.4.For the lunar navigation constellation design,firstly,this paper clarifies the numerical method of generating Lagrange point orbit family,and selects halo orbit for lunar navigation constellation design.In order to select a reasonable orbit for lunar navigation constellation,the lunar surface coverage of halo orbits with different amplitudes and positions is analyzed.The analysis results show that the Near Rectilinear Halo Orbit(NRHO)is of good coverage for the north and south poles,and the small amplitude orbit near L1/L2 point can cover the front/back of the moon.According to this,this paper selects two NRHOs and one small amplitude orbit near L1 and L2 points respectively,and a total of six orbits can realize one-fold coverage of 96.46%for the lunar surface.Further,six additional orbits are selected,and two satellites are distributed in each orbit.The initial phase of the two satellites differs by half an orbit period,resulting a constellation of 24 satellites.This constellation ensures four visible satellites for the entire lunar surface.Also,the proportion of lunar average PDOP value less than or equal to 5 is more than 96.96%,and the proportion less than or equal to 10 is more than 99.26%,which is a rather good satellite constellation geometry.Furthermore,two NRHOs and two small amplitude halo orbits are selected to analyze the range of nadir angles respect to lunar surface points.It is suggested that the NRHO beam angle should be 40° and the small amplitude halo orbit beam angle should be 10°. |
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