Research Of The Some Key Technologies For Achieveing GNSS Augmentation With LEO Satellites

Currently,multi Global Navigational Satellite System(GNSS)with low earth orbit(LEO)constellation augmentation has become a research topic of much interest.More and more LEO satellites will provide navigation services.Benefiting their large geometric variations and stranger signals,the drawbacks of current GNSS can be complemented,therefore,precise positioning with less time can be achieved.Since LEO satellites enhance GNSS still in the stage of theoretical demonstration,some specific technical issues are short of researches.In this paper,several key technologies that may appear in the process of constructing GNSS augmented by LEO satellites are studied.The main content and conclusions are as follows:(1)The design of broadcast ephemeris is one of the key technologies to realize LEO enhance GNSS.Because the operating status of LEO satellites is more complicated than high and medium orbit satellites,the existing GNSS broadcast ephemeris model cannot be directly used to fit the precision orbits of LEO satellites.In addition,the emergence of small eccentricity,small orbit inclination,and polar orbit LEO satellites may be applied in the future designs of LEO satellites enhanced constellations.The orbital element type broadcast ephemeris used by navigation systems,such as Beidou,are influenced by singularity problems and it is difficult to successfully fit the corresponding parameters in these cases.In this paper,based on Kepler orbital elements,the first or second non-singular orbital elements,and the improved second non-singular orbital elements,respectively,broadcast ephemeris models suitable for LEO satellites are designed.And the fitting accuracy of these types of broadcast ephemeris designs is tested using simulated and real LEO satellites’ precise orbit data.It is found that the broadcast ephemeris model based on the improved second non-singular orbital elements can overcome the singularity problems and provide a valuable reference for the future LEO satellites broadcast ephemeris design.(2)Considered the better non-singularity characteristic of integration-type ephemeris model,in this paper,from another point of view,enhanced integration-type broadcast ephemeris models suitable for the motion status of LEO satellites are proposed.Similarly,their performance is verified by simulated and real LEO satellites’ precise orbit data.It is found that integration-type ephemeris models have an exceptional non-singularity characteristic,and they are suitable for small eccentricity orbits,small inclination,and polar orbits.Integration-type ephemeris models have better-extended ability.(3)Orbital elements and integration-type ephemeris models are compared.Experiment results show that integration-type ephemeris models have better non-singularity character.Although improved second non-singular orbital elements are used to remove the influence of singularity,after 7 to 8 times iteration can be convergence when ephemeris parameters are fitted.However,for the integration-type model,only 3 to 4 times are needed.Otherwise,with the increase of eccentricity,for orbital elements ephemeris models,their fit accuracy decreases a lot.But for integrationtype ephemeris models,their fit accuracy can keep still be stable.For the long-time description of satellites’ motion,orbital elements ephemeris models are more suitable.For integration-type ephemeris models,their fit accuracy decreases a lot,with the extent of fit interval.(4)For how to fit broadcast ephemeris problems,related deep studies have been carried out.In conventional broadcast ephemeris parameters fit algorithms,the analysis method is used to calculate the partial derivative of which the processes are laborious and prone to errors.Aiming at this problem,the method of calculating numerical derivative to calculate the partial derivative of the ephemeris parameters,which is convenient and straightforward.To verify the performance of numerical derivative,real Global Positioning System(GPS)satellites precise ephemeris released by the German research center for geosciences are used to fit ephemeris parameters.Experiments results show that using the method of numerical derivative,almost no effect on fit accuracy and workload in the calculation are released a lot.For the strategy of fit ephemeris parameters,in this paper,traditional least squares and QR factorization are compared and analyzed.Experiments results show that QR factorization can release the effect of ill-conditioned normal equations,decrease iteration times and improve the fit success rate.(5)The idea of precise position combined high,medium,and low earth orbit satellites has been studied by a lot of researchers.Related experiments results show that LEO satellites will play an important role in the future development of GNSS.However,in the process of establishing LEO satellites to enhance GNSS,the performance of constellations should be evaluated.Therefore,in this paper,the extended probability model is proposed which can evaluate the performance of one constellation on a global scale only using some basic parameters.The extended probability is verified by using real and simulated precise ephemeris.And the characteristics of Beidou-3 and GPS constellation on a global scale are analyzed.Based on the extended probability model,the role of another type of satellite,elliptical orbital satellites may play is studied.Experiments results show that if elliptical orbital satellites are introduced,constellation geometry distribution and satellites visibility in high latitude regions will be improved.What’s more,satellitebased augmentation services in high latitudes especially in polar regions can be provided by elliptical orbital satellites,which cannot be covered by the current GEO-based augmentation system.Last,the performance of the GNSS+constellation,in which high,medium,low elliptical orbital satellites are included,is evaluated by the extended probability model on a global scale.Compared with current GNSS constellations,the GNSS+ constellation performs much better.(6)One of the most critical problems in data preprocessing is cycle slip.Solving properly cycle slip is one of the important preconditions for achieving high precision positioning.Because the idea of low earth orbit satellites enhances GNSS,still under the stage of theoretical argument,some specific technical problems,such as cycle slip,are short of study.Currently,although some studies have pointed out that the pseudo-range noise and multipath effects have an obvious effect,all almost recent algorithms don’t consider how to reduce the effect of observation noise and multi-path.In this paper,we analyze the performance of different carrier smoothing of code pseudo-ranges methods.Then a carrier smoothing of code is used to aid the triple-frequency cycle slip detection and repair.Real high,medium earth orbit satellites observations,and simulated low earth orbit satellites observations are used to verify this cycle slip detection and repair method.Experiments results show that after the observations noise is reduced,the success rate of cycle slip detection and repair is improved obviously.