| Low-Earth-Orbit(LEO)satellite is an important platform for earth observation and space environment scientific researches.The determination of its high-precision orbit is of great significance for ocean monitoring,earth gravity field inversion,earth electromagnetic field detection,topographic mapping,atmospheric and space environment detection and other scientific researches.In recent years,the Global Navigation Satellite System(GNSS)has become a research hotspot in determining the orbit of LEO satellites,which can provide geometric orbits independent of the dynamic models,and has been an important method for precise LEO orbit determination.However,because of the fast speed of LEO satellites and the sophisticated space environment,it is difficult for the GNSS receivers to track satellite signals.The collected GNSS data are characterized by short and large number observation arcs,which shows poor continuity and brings two vital problems in precise LEO orbit determination.First,the ambiguities are difficult to fix to integers.Second,the satellite orbit is easily affected by the geometric distribution of GNSS satellites,resulting in poor stability.Therefore,this paper proposes a clock constraint method based on the characteristics of on-board atomic clocks of LEO satellites,studies an ambiguity splicing method between adjacent arcs based on phase fractional deviation(UPD)method,and discusses the influence of non-differential ambiguity fixing on kinematic precise LEO orbit determination.These methods effectively enhance the continuity of data and improve the precision and stability of LEO orbits.The main research work and conclusions are as follows:(1)introduced the theories and methods of GNSS precise kinematic orbit determination of LEO satellites systematically,including the time and coordinate system,precise point positioning(PPP)kinematic orbit determination methods,PPP ambiguity resolution,error sources,random models,as well as the methods of parameter estimation,etc.,which laid a foundation for subsequent PPP kinematic orbit determination researches;(2)a clock constraint method based on the stability of on-board atomic clocks was proposed,which effectively improved the precision of LEO orbit determination and reduced the gross error caused by the poor geometric distribution of GNSS satellites.The method is verified and analyzed with the data of GRACE satellite for one month.The results show that the new method can significantly improve the accuracy and stability of PPP float solution,fixed solution and real-time float solution.The radial accuracy can be improved by 20%-40%,and the along and cross accuracy can be improved by 5%-20%.For the epochs with only 3-4 GNSS satellites for positioning,the radial,along and cross average accuracy is better than 3cm seperately,among which the radial accuracy can be improved by about 40%-60%,and the along and cross accuracy by about 6%-40%.(3)an ambiguity splicing method based on UPD method was presented,which effectively solved the difficulties of short observation arcs and poor data continuity.The experimental analysis with GRACE satellite data shows that the wide-lane and narrowlane ambiguities can be well spliced,and more than 70% of the narrow-lane ambiguities can be spliced successfully.The orbit accuracy can be significantly improved by ambiguity splicing method,in which the accuracy of radial and along direction can be improved by about 10%-40%,and the cross direction can be improved by more than50%.(4)the Carrier-range method was extended to determine the LEO satellite orbits,and the results were compared and analyzed with the results of ambiguity fixed solution and ambiguity splicing method.The results show that Carrier-range,ambiguity fixed solution and ambiguity splicing results are similar in positioning accuracy with the difference less than 6mm in the radial,along and cross directions. |
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