Research On Near Real-time Precise Orbit Determination For Luojia-1A LEO Satellite Using The BeiDou System

Luojia-1A is a scientific experimental satellite operated by Wuhan University,which is the first augmentation satellite that transmits LEO(Low Earth Orbiter)navigation signals in China.Luojia-1A can receive GPS/BDS dual frequency signals and transmit dual-frequency band ranging signals.For achieving the signal augmentation function,there is a high demand for the precision orbit and clock correction for Luojia-1A satellite.On the one hand,the centi-meter level precise orbit is also required for precise positioning applications.On the other hand,near real-time orbit product of Luojia-1A is of great significance for the generation of broadcast ephemeris.With the development of the Bei Dou navigation system,orbit determination for LEO satellites is still challenging.This thesis carries out research work on precise orbit determination for Luojia-1A satellite using the Bei Dou system for maintaining the high-precision reference datum of the Luojia-1A satellite.The research works included in this thesis are summarized as follow:(1)Determine centimeter level precision orbit of the Luojia-1A satellite using Bei Dou observations.This work includes the analysis of the characteristics of onboard BDS observations of the Luojia-1A satellite,which shows that the number of GEO satellite observations is more than 40% compared to the situation of collecting observations on Earth.Since the accuracy of the orbit products of the GEO satellites is not as good as that of the IGSO and MEO satellites,therefore,a weight reduction approach was adopted for the observations from GEO satellites.The precision of the Luojia-1A orbit is improved from 6.3 centimeters to 2.3 centimeters by adopting this weight reduction strategy.The precision improvement in the radial direction,alongtrack,and out-of-plane directions are 53.47%,47.29%,and 76.2%,respectively.Furthermore,the precision of the Luojia-1A orbit can achieve 2 centimeters by adjusting the interval of pseudo parameters.(2)Considering the satellite-specified and temporal variation of the Beidou clocks characteristics,an adaptive model for Bei Dou Clock prediction was investigated.Currently,clock prediction of the Bei Dou system is still challenging to meet the needs for near real-time precise LEO orbit determination.This research uses an adaptive model for the Bei Dou system based on the analysis of the characteristics of Beidou satellites clocks.Considering the temporal variation of Bei Dou clocks,an optimal model with cross-validation was addressed,the model that has minimum variation in the validation part is used for predicting the next 24 hours clock offsets.In this thesis,the adaptive model was compared with the ultra-rapid products released by GFZ and i GMAS.The comparison results indicate that the adaptive model can achieve32.1%～47.3% improvements in the prediction of 3 hours.(3)Precise near real-time orbit determination of the Luojia-1A satellite using the Bei Dou system.A rapid orbit determination of the Luojia-1A satellite is achieved by adopting the BDS rapid products released by Wuhan University.Comparing with the precise orbit products,the precision of the rapid orbit of the Luojia-1A satellite is 0.29 meters.By adopting the BDS ultra-rapid products and the weight reduction strategy of GEO observations,the near real-time orbit of the Luojia-1A satellite can achieve an accuracy of 1.1 meters.Furthermore,the near real-time orbit of the Luojia-1A satellite can be enhanced further to an accuracy of 0.6 meters by utilizing the predicted clock offsets which predicted by the adaptive model.