Research On Precise Orbit Determination Of Low,middle And High Satellite Network Based On Regional Ground Stations

In recent years,with the gradual upgrade and deployment of GNSS navigation systems such as GPS,GLONASS,BDS,and GALILEO on the medium and high Earth orbits(MEO/HEO),the development of multi-purpose low Earth orbit(LEO)constellation has gradually become a research focus in various countries.China has also proposed to build a space-based information system integrating the positioning,navigation,timing,remote sensing and communications(PNTRC)services.Obviously,a LEO constellation with integrated navigation,remote sensing and communication functions will be a primary goal in the near future.The navigation augmentation to the existing GNSS is a core service capability of such a LEO constellation.Being used as a position reference of GNSS navigation and LEO augmentation system,the high-precision orbits of satellites are fundamental for the navigation performance enhancement,and it would require a globally well-distributed network of a large number of ground tracking stations(GTS).However,for China to deploy and maintain a global GTS network could be difficult due to unreasonable international concerns and possible adverse interference.Hence,it is necessary to consider other feasible options.One consideration is to achieve the high-precison orbit determination and time synchronization of all satellites in the HEO/MEO/LEO network(HML-Net)based only on a regional GTS network and a LEO augmentation system.Therefore,this dissertation focuses on the centimeter-level precision orbit determination(POD)of all satellites in the HML-Net only with a small number of GTSs.The POD solution scheme for the entire HML-Net is designed having two steps.First,the POD of all GNSS satellites and some LEO satellites is pursued in an integrated orbit determination(IOD)manner by multi-level data fusion of GNSS observations from a China-region GTS network and those IOD-participating LEO satellites,Inter-Satellite-Links(ISL)ranging measurements between GNSS satellites,Ka-Band Ranging(KBR)measurements between LEO satellites,and Satellite Laser Ranging(SLR)observations,etc..Then,based on the GNSS orbit and clock products generated in the first step,each of the remaining LEO satellites is processed individually in the POD process to obtain their highly precise orbits.Three POD techniques should be applied in this solution scheme,including the GNSS/LEO IOD using only GNSS data,the IOD based on multi-level data fusion,and the POD of LEO satellites with space-borne GNSS measurements in which IOD ephemris of GNSS satellites is used.In view of the difficulties existing in these three POD techniques under the condition that only regional GTSs are available,this dissertation has carried out in-depth and extensive research and analyses.The main research findings are summarized as follows:1? Key factors limiting the accuracy of GNSS/LEO IOD only with regional GTSs are raised and analyzed in detail.Accordingly,a series of processing refinement methods are proposed to deal with these factors.First,a step-by-step weighting method is proposed to account for the difference in the geometric strengthes of the geosynchronous(GEO),inclined geosynchronous(IGSO)and medium orbiters(MEO)satellites provided by their GNSS observations.Then,the approach of constraining the to-be-determined orbits using a known position sequence is applied to weaken the POD solution instability and improve the relatively low accuracy of GNSS satellites that are most likely caused by short tracking arcs and low volume of measurements from a regional GST network.Third,recognizing two features,which are the difference in the GNSS observation quality of LEO satellites and the formation configuration by some LEO satellites,the approximate formula of Helmert variance estimation is used to determine proper weights for the observation data of LEO satellites to account for the data quality difference,and the inter-satellite baselines are formed to carry out double-difference ambiguity resolution,respectively.Finally,the sequential implementation of these four methods constitutes a comprehensive GNSS/LEO POD/IOD refinement scheme.2? The regional IOD experiments,named BDS-2+GPS+8LEO and GPS+12LEO,based on the real data are carried out to verify the effectiveness of the above processing refinement methods.Meanwhile,LEO's enhancements on the IOD performance of GNSS satellites,the influences of GTS distribution and number of LEO satellites on the IOD are also analyzed in detail based on these IOD experiment results.The results show that,the four refinement methods can improve the accuracy of regional IOD gradually and their full implementation can improve the orbit accuracy of GNSS satellites by at least 25%.More importantly,based on the comprehensive refinement scheme,even if only a small number(5?8)of GTSs are available,as long as LEO satellites participating in the IOD are sufficient,the average orbit accuracy of the IGSO/MEO satellites can eventually reach 3?8cm level.3? The characteristics of multi-level data,i.e.the GTS/LEO/ISL/KBR/SLR data,are analyzed,an approach for multi-level data fusion is presented,and the optimized processing strategies using ISL and SLR data are explained specifically.Based on the real data,the optimized processing strategies using ISL and SLR data are experimentally verified.The results have demonstrated that,compared with only using the orbit-derived ISL measurements,the use of both the orbit-derived and clock-derived ISL measurements can improve the IOD orbit accuracy of BDS-3 satellites by 20%?30%,where the clock rate parameter for each BDS-3 satellite is estimated.With regard to the SLR data,estimating the positions of SLR stations,the Earth rotation parameters and satellite orbits simultaneously is beneficial to the improvement of the BDS-3 orbit accuracy.4? Based on the optimized ISL/SLR processing strategies,the IOD experiments,labelled BDS-3+ISL+SLR and GPS+12LEO+KBR+SLR,are performed with the real data.The results show that,in cases there are 7,11 and 13 regional GTSs,the average orbit accuracy of overlapping arcs of the BDS-3 satellites reaches 5.2cm,9.2cm and 7.4cm,respectively,and the SLR validation accuracy are 4.3cm,4.9cm and 4.6cm,respectively.In addition,with 5 and 7 regional GTSs in the IOD,the average orbit accuracy of GPS satellites reaches 6.5 and 4.9cm,respectively.5? The error characteristics of the resulting ephemeris generated by regional IOD are analyzed in-depth.The line of sight(LOS)error of the ephemeris varies slowly and smoothly,but some small random changes occur in the LOS error,so it is proposed to model the LOS error with a first-order Gauss-Markov process,and the LOS error is estimated in the LEO POD filtering process,in order to reduce its negative effects on final orbits of the LEO satellites.Test results have demonstrated that,the LOS error can be absorbed effectively and has less impact on the accuracy of final orbits after its estimation.The orbit accuracy can be improved by 40%,and the final average orbit accuracy could reach ?10cm level.6? On the basis of all these processing refinement methods,a complete POD experiment of the entire satellite network is performed by using GPS/12LEO/KBR/SLR real data.The results show that,with 5 and 7 regional GTSs,the average orbit accuracy of the GPS satellites in the IOD reaches 6.7cm and 5.0cm,respectively,and the average accuracy of2?7cm is achieved for 11 LEO satellites participating in the IOD.As for the remaining LEO satellites,a slightly worse accuracy of 12.3cm and 9.9cm,respectively,is realized.The experimental results have demonstrated that,using the POD solution scheme proposed in this dissertation,the orbits of all satellites in the entire network can be determined with centimeter-level accuracy,even if only a small number of regional GTSs are used.