Research On Precise Orbit Determination Of BeiDou Navigation Satellite System

As a global navigation satellite system,BeiDou Navigation Satellite System(BDS)is independently established and operated by China.Since the system formally provided Asia-Pacific regional service on December 27 th,2012,it has begun to play an important role in the field of transportation,marine fisheries,hydrological monitoring,weather forecasting,and so on.As the precision of navigation satellite orbits is an important performance index of system service capabilities,providing highly precise orbits of BeiDou satellites is of important practical significance to improve the competitiveness and the application of the system.In addition,BDS precise orbits are also the core products of international GNSS Monitoring and Assessment Service(iGMAS)which is being actively constructed by China.But as a newcomer of the navigation satellite systems,research on precise orbit determination of BDS is still in its infancy.Compared with such mature system as GPS,the theory is not perfect,and the orbit precision is needed to improve.In this background,referred to the experience of the existing systems,the methods of BDS orbit determination should be developed by considering the own features of BDS and absorbing the guide of the latest development of precise orbit determination of navigation satellites.It is of great importance to improve the development and improvement of BDS.Therefore,research is carried out in this paper tightly around the core issue of precise orbit determination of BDS.At first,the states of navigation satellite systems and their orbit determination methods are summed up.And the basic theory of BDS orbit determination is introduced.In the following,the methods of BDS precise orbit determination are studied on in detail,such as single-system orbit determination,multi-system orbit determination,post-processing for precise orbit determination,BDS orbit determination with LEO,and real-time orbit determination.The main work and innovation points are summarized as follows:(1)Considering the high correlation between the tangential element of GEO and double-difference ambiguity,the classic double-difference dynamic method is improved,and the method which is to determinate BDS precise orbits by joint use of double-difference phase and carrier-smoothed code is proposed.The applicability of the new method is validated by real date analysis.The comparison of the zero-difference dynamic method and the new double-difference dynamic method is conducted.The results show that the new double-difference dynamic method can improve the precision of GEO orbits,and the zero-difference dynamic method can improve the precision of MEO orbits,and they can get comparably precise orbits of IGSO.These results are dominated by different calculating modes of the two methods,different movement characteristics of three types of satellites,as well as the local structure of observation stations.(2)Factors impacted on the double-difference ambiguity resolution of BDS in joint use of phase and code data are analyzed theoretically.The basic principle of double-difference ambiguity fixing of BDS by using QIF method is deduced.And the applicability of the method is analyzed by real data.The results show that a certain effect of double-difference ambiguity fixing can be achieved with the method of QIF.But the success rate of ambiguity fixing is not very high on the whole due to limited observation condition.After ambiguity fixing,it has limited effect on the improvement of BDS orbit precision.In particular,it requires further analysis and study on the method of double-difference ambiguity fixing about two GEO satellites.(3)The frequently-used method of zero-difference multi-GNSS joint orbit determination of BeiDou satellites and its data processing flow are given.And it is compared with the zero-difference dynamic method of the single system by real data analysis.The results show that the orbit precision of both methods is roughly equal under the condition of the current observation geometry of BDS.But if there are not stable atomic clocks on the ground,the precision of clock determination in the multi-GNSS joint orbit determination is better because of introducing more stable GPS time as the reference.(4)The frequently-used method of multi-GNSS joint orbit determination is improved,and a method of zero-difference multi-GNSS joint precise orbit determination of BeiDou satellites based on ambiguity fixing is present.The analysis of real data shows that the mean RMS of BDS orbits in the three-dimensional direction is improved 21.8%.The tangential orbit precision of three types of BDS satellites is improved to the greatest degree,which of GEO is most obviously improved.(5)The principle and formulas of the parameter pre-elimination and back substitution and the parameter conversion are derived.The principle of normal equation stacking is given.From orbit multi-subnet combination and orbit multi-period combination,post-processing for BDS precise orbit determination is firstly studied on in detail.The basic principle and data processing steps of orbit multi-subnet combination are present.The primary processing steps are derived based on parameter conversion.The analysis shows that the method can play a very important role in promoting network structure,improving the precision of the subnet solutions,and increasing calculation efficiency.The method of orbit multi-period combination is studied deeply.The processing method of osculating elements and dynamic parameters is given.And the combination principle of pseudo-stochastic pulse is focused on.The analysis shows that when pseudo-stochastic pulse is considered at the single day boundary,the precision of multi-period combination orbit of BDS is improved obviously.The precision of BDS orbits can get better,when the length of arcs is extended in a certain range by the method in the paper.(6)The four methods of geometric approach,dynamic approach,dynamic smooth approach and reduced-dynamic approach,which are usually used in LEO space-borne GNSS precise orbit determination,are introduced.The real-data analysis shows that the reduced-dynamic approach is the best method.The method,the precision of which can reach the level of cm,can compensate the deficiency of dynamic model by introducing empirical parameters.(7)A rapid parameter estimation method of pseudo-stochastic pulse is proposed.The method,which is based on the characteristic of pseudo-stochastic pulse and the theory of parameter pre-elimination and back substitution,can avoid resolving the reversion of normal matrix with large dimension,and help improving the efficiency of BDS orbit determination with LEOs.The analysis shows that the parameter estimation of the method is the same with least squares method,but has obvious advantage of calculation efficiency.(8)The method of BDS orbit determination with LEOs is proposed.Base on weighted least squares estimation,the principles of fixing and limiting LEO orbits in the method are proved coincident.And a priori constraint of LEO orbits should be given carefully.The data analysis shows that the orbit precision of navigation satellites can be improved obviously after processing GNSS observation of LEO together when the stations are only in the regional area or sparse in the whole world.As for BDS orbit determination with LEOs,the orbit precision of GEO is improved significantly,and the tangential precision of GEO can reach the similar level with that of MEO and IGSO.The tangential precision of MEO is also improved mostly in the three directions.And the orbit precision of IGSO is improved a little.With LEOs,the precision of three types of satellites can reach the considerable level.(9)A method of real-time GEO orbit determination is proposed based on adaptively robust filtering.The method can detect outliers by using dynamic information based on the principle of “3s ” during no maneuver period,and reduce the effect of outliers during maneuver period.And then the effect of outliers on the real-time GEO orbit determination can be processed well based on guaranteed efficiency.And meanwhile,the processing of real-time GEO orbit determination during maneuver period and during no maneuver period is combined smoothly by using adaptively filtering based on the new constructed time-interval adaptive factor.On the condition of some station layout and observation precision,the precision of real-time orbit determination can reach several decimeters during no maneuver by the method.Even though during maneuver,the distortion won't appear and the precision can also be better than five meters.