Leo Precise Orbit Determination And Integrated Precise Orbit Determination Of Leo And Multi-GNSS Satellites

With the increasing demanding of global communication and scientific research,the large low Earth orbit(LEO)constellation has become a new research hotpot.Many LEO constellation projects has proposed their design of navigation augment function.In order to achieve LEO-augmented positioning service,the precise knowledge of LEO satellites orbit is a prerequisite.Meanwhile,the large LEO constellation can be expected to reduce the dependence of GNSS satellites orbit on the ground network and signicantly improve the orbit accuracy of GNSS satellites.This thesis mainly focus on the LEO precise orbit determination(POD)and integrated POD of large LEO constellation and multi-GNSS satellites.We firstly diccussed several issues about LEO POD,including the influence of update of force model on POD,performance of GPS and BDS dual-system POD and LEO POD during the geomatnetic storm.Then,the integrated orbit modeling of large LEO constellation and GNSS satellites was investigated in detail.The study mainly focus on the design of LEO constellation,LEO and GNSS orbit simulation,GNSS observations simulation,the design of POD scheme and performance assessment of integrated POD.The main works of this thesis are as follows:(1)The impact of the update of atmosphere drag model and earth radiation pressure is evaluated.In therms of drag model,the perfomance of DTM94,DTM2013 and NRLMSISE00 semi-empirical atmosphere density models in LEO POD is investigated.The result shows that the update of atmosphere density model can slightly improve the orbit accuracy of LEO satellites.Compared with the DTM2013 model scheme,the NRLMSISE00 model scheme can achieve a better orbit quality for LEO satellites.The CERES model is employed to model the earth radiation pressure for LEO satellites.With the earth radiation pressure model,the orbit of LEO satellite can obtain accuracy improvement of a few millimeters or even centimeters.The SLR validation results show that after considering earth radiation pressure,the SLR biases of GRACE-A,SwarmA,Swarm-B,Jason-2 and Jason-3 orbit are reduced by 77.3%,59.0%,47.2%,46.8% and 47.4% respectively,while the STD values are reduced by 29.5%,34.0%,28.8%,26.7% and 29.0% respectively.(2)The performance of GPS and BDS dual-system POD is investigated.The results show that limited by the poor quality of BDS orbit product and few onboard BDS observations,the BDS-only POD for FY-3C and FY-3D satellite can only achieve decimeter-level precision.FY-3D satellite presents a better performance in overlap comparison than FY-3C satellite and its averaged RMS of overlap differences is 77.7%,24.1% and 71.5% smaller than that of FY-3C in the along-track,cross-track and radial components.The significant internal consistency improvement of FY-3D BDS-only solution can be attributed to the large increase of onboard BDS measurements for FY-3D.Only using onboard GPS observations,FY-3C and FY-3D orbit can achieve the precision of centimeter-level.However,due to the poor quality of BDS products(particularly GEO satellites),the average RMS of overlap differences for GPS+BDS(GC)dual system solution are obviously larger than that of the GPS-only solution.Nevertheless,with the BDS GEO satellites excluded,GC(w/o GEO)solution achieves a smaller orbit overlap difference than GPS-only solution.Compared with GPS-only solution,the GC(w/o GEO)solution of FY-3C and FY-3D satellite can be improved by 7.1% and 7.7% respectively.This demonstrates that the LEO orbit can benefit from the GPS+BDS dual-system combination when the high-accuracy BDS products are available.(3)The performance of precise orbit determination of Swarm constellation during the geomagnetic storm event on 07-08 September 2017 is analyzed.The result shows that during the geomagnetic storm,the drastic fluctuation of the atmosphere density increase the difficulty of drag force modeling for LEO POD.The quality of Swarm orbit was largely degraded during the storm when using the same quiet-time POD strategy of estimating drag parameters per 6h.The magnitude of orbit degradation reached centimeter-level for Swarm-B and decimeter-level for the lower flying pair(Swarm-A and Swarm-C).This negtive impact of atmosphere density can be largely suppressed by estimating a more frequent drag parameter coefficient(with every 30 min instead of every 6 h).With the refined POD scheme,Swarm orbit can achieve a similar precision of few centimeters during the storm as that in the quiet time.The storm time influence of higher-order ionospheric effects is also discussed.The GPS observations errors of higher-order ionospheric effects indeed get an increase during the geomagnetic storm and the second-order effect is the main contributor,even exceeding 1 cm.However,no evidenced orbit accuracy improvement is visible after correcting these ionospheric effects.(4)The integrated precise orbit determination of FY-3C,FY-3D,GPS and BDS satellites is performed.The three integrated POD schemes are designed,including FY-3C+GPS+BDS,FY-3D+GPS+BDS and FY-3C+FY-3D+GPS+BDS,and their POD performance is assessed.The overlap comparison results show that FY-3D satellite presents a stronger enhancement for GPS and BDS orbits than FY-3C satellite.Compared to the FY-3C solution,the orbit precision of GPS,BDS GEO,IGSO and MEO satellites for the FY-3D solution is respectively improved by 3.3%,51.6%,2.1% and 5.0%.Obviously,GEO satellite achieves largest precision improvement and the precision improvement in the along-track component dominates GEO orbits improvement.As expect,the integrated processing of FY-3D and FY-3C onboard data can further improve both BDS and GPS orbit precision.With respect to the FY-3D solution,the orbit overlap differences of FY-3C+FY-3D solution are on average reduced by 9.7%,4.1%,20.5% and 18.9% respectively for GPS,BDS GEO,IGSO,MEO satellites.(5)The integrated orbit model of large LEO constellation and multi-GNSS satellites is proposed.The large LEO constellation-augmented multi-GNSS precise orbit determination(POD)is performed.The potential influence factors of the integrated POD,including LEO satellites number,altitude,and orbit type as well as ground stations number and distribution,are analyzed and discussed in detail.The multi-GNSS POD results show that compared with ground-based POD,remarkable accuracy improvement of over 70% can be observed for all GNSS satellites when the integrated POD is performed.Particularly,BDS GEO satellites can obtain a centimeterlevel orbit and achieve the largest orbit accuracy improvement,which can reach up to about 98%.Meanwhile,the influence of LEO satellites number,orbital types and altitude on the integrated POD is assessed.Compared with the 60-LEO and 66-LEO schemes,the 96-LEO scheme presents an orbit accuracy improvement of about 1 cm for GEO satellites and 1 mm for other satellites because of the introduction of more LEO satellites.However,the increase of LEO satellites leads to a sharp rise in the computational time.In terms of the orbital types,the sun-synchronous-orbiting constellation exhibits a slightly better performance than the polar-orbiting constellation in augmentation capability.As for the LEO altitude,the LEO constellation orbiting at the altitude of 1400 km is found to having a stronger enhancement for the whole GNSS constellation than that of 600 km and 1000 km.Furthermore,the GNSS POD presents a stronger dependence on the distribution of ground stations than the stations number when a large number LEO satellites are introduced.This is because the GNSS satellite orbits are by a large extent determined by the onboard observations from large LEO constellation in the processing of integrated POD.With 60 synchronous-orbiting satellites,the integrated POD using 8 regional stations can obtain centimeter-level GNSS orbits,while the integrated POD using 8 global stations can achieve a similar orbit accuracy of millimeter level like that using a denser global network of 65 and 22 stations.