Precise GNSS And LEO Orbit Determination With Zero-Difference Ambiguity Resolution

The rapid development of multi-frequency and multi-system integrated Global Navigation Satellite Systems(GNSS)has provided a strong motion to get high-precision and high-reliability GNSS satellite orbit and clock products to expand GNSS precise applications.Meanwhile,the construction of low-earth-orbit(LEO)satellites and constellations for scientific research and communication tasks is also in full swing.The orbit accuracy of LEO satellites has to be improved.Therefore,the research on satellite orbit determination technology needs continuous refinement.On the one hand,more precise and proper dynamic model needs to be studied for each kind satellite.On the other hand,it is also important to strengthen the GNSS observation model by calibrating the errors and phase ambiguity in GNSS observations.Presently,most GNSS precise orbit and clock products from the International GNSS Service(GNSS)are based on double-difference(DD)ambiguity resolution(AR)strategy,while the LEO orbit products are mainly float solutions.This thesis focuses on improving the Multi-GNSS and LEO satellite precise orbit determination(POD)by using zero-difference(ZD)AR.Firstly,the theory and method of Multi-GNSS Uncalibrate Phase Bias(UPD)estimation and the ZD integer ambiguities determination are studied.Next,the mathematical model of Multi-GNSS POD and precise clock estimation(PCE)using carrier-range,which is the integer-ambiguity-recoverd phase observations,is discussed.Then the Multi-GNSS POD and PCE with ZD AR is performed and evaluated.Finally,based on the integer clock products,the LEO satellite ZD AR is performed for the reduced-dynamic(RD)POD and kinematic POD,as well as the real-time kinematic POD.The main work of this thesis are as follows:(1)The model of Multi-GNSS UPD estimation and integer ZD ambiguity determination was established to generate the Multi-GNSS Carrier-range observations.By using Multi-GNSS data in DOY(Day of year)060-210,2019 from a globally distributed MGEX(IGS Multi-GNSS)network,the POD and PCE with the legacy solution are firstly performed.Next,the wide-lane(WL)and narrow-lane(NL)UPDs are estimated with the obtained float ambiguities meanwhile the integer ambiguities are resolved.Finally,the obtained integer ambiguities are calibrated for corresponding phase observations and the so-called carrier-range is got.To investigate the performance of ZD AR,the stability and accuracy of WL and NL UPD are analyzed.The WL UPDs of GPS,Galileo and BDS-3 satellites are quite stable,with the standard deviations(STDs)of 0.029,0.025 and 0.022 cycles,respectively,while the stability of the wide lane UPD of BDS-2 satellites is slightly worse,with the STD of about 0.050 cycles,which may caused by the inferior range observation quality.For the NL UPD results,the STDs of GPS and Galileo are 0.017 and 0.018 cycles,respectively,and that of BDS is 0.053 cycles.This may be because the POD results of the BDS satellite are slightly worse,resulting in a slightly lower accuracy of the IF combination ambiguity.In addition,95% of GPS and Galileo's NL ambiguity residuals are distributed within 0.15 cycles,and the percentage of BDS has also reached 82%.(2)Multi-GNSS POD and PCE models based on Carrier-range observations were established,and the orbit accuracy was comprehensively evaluated.Firstly,the orbital day boundary discontinuous error is analyzed.The results of GPS and Galileo with ZD AR is about 18% less than those of DD AR solution,the improvement in cross-track is most evident,which reached 25%.For the BDS-2 IGSO satellite,the results of ZD AR solution is distinctly smaller than those of DD AR solution in along-track,cross-track and radial components,with improvements of around 30%.The BDS-2and BDS-3 MEO satellites showed similar improvements in along-track and cross-track,which amount to 50% and 30%,respectively.Next,the orbit differences with the MGEX precise orbit products from CODE and WUM are assessed.The orbit accuracy of GPS and Galileo satellites with ZD AR solution is improved by 7% in along-track in comparison with DD AR solution.The BDS-2 IGSO satellites are observed a 20-30% improvement in cross-track,while the BDS-2 and BDS-3 MEO satellites presented a 10-20% improvement in along-track and cross-track.Moreover,the SLR residuals results showed that the ZD AR could reduce the residuals STD by0.21,0.27 and 0.14 cm for BDS-2,BDS-3 and Galileo satellites,respectively.(3)The rapid estimation of Multi-GNSS 30s-interval integer clock using carrier-range observations is implemented,and the obtained integer clocks are assessed with both static and kinematic precision point positioning(PPP).Firstly,the stability of the integer clock was evaluated by the STD results of the differences with the WUM product.It was found that the stability was comparable with the legacy clocks.Then,the reduction of compute time in PCE using carrier-range observations are discussed.When using 134 MGEX stations to preformed the PCE for G,GC,GCE and GCER systems using raw phase observations and carrier-range,respectively,the compute time could be reduced by 50%.With the obtained Multi-GNSS integer clock,the static and kinematic PPP are calculated.For the static PPP,the positioning convergence time using G,GE,GC and GREC observations could be reduced by 14%,12%,18%,and 19% with ZD AR.For the kinematic PPP,the corresponding percentages are 13%,3%,29% and 20%,respectively.Besides,the convergence times of static and kinematic fixed PPP AR using the GREC four-system observations are3.95 and 5.76 minutes,respectively.(4)The LEO RD POD with ZD AR was performed with integer clock products,and the orbit accuracy was comprehensively analyzed.Swarm-A/B/C and GRACE-FO1/2 data of DOY065-245 in 2019 is employed in POD and AR.The NL ambiguity fixing rates of the Swarm satellites are about 97%,while those of GRACE-FO satellites are 95%.Besides,more than 97% of the Swarm NL ambiguity residuals are within 0.15 cycle,while the percentages of GRACE-FO satellites are92%.For the 4-hour orbit overlap RMS,the ZD AR solution showed obvious improvement in cross-track,with an 30% improvement for Swarm satellites,and an60% improvement for GRACE-FO satellites.In addition,the 3D RMS improvement is20-40%.For the orbit difference with precise science orbits(PSOs),the 3D RMS of GRACE-FO satellites are improved by 44%.The satellite laser ranging(SLR)residuals were also evaluated,and the STD of Swarm satellites with ZD AR was 10%smaller than that of float solution,while the improvement of GRACE-FO satellites are 26%.Moreover,the GRACE-FO orbits were also assess using KBR observations and an residuals reduction of 32% could be observed with ZD AR solution.These results strongly demonstrated that the ZD AR could significantly improve the orbit accuracy and precision for RD POD.(5)The post-processed and real-time kinematic POD with ZD AR was performed with integer clock products,and the orbit accuracy was comprehensively analyzed.Swarm-A/B/C and GRACE-FO1/2 data of DOY065-245 in 2019 is employed in POD and AR.The ambiguity fixing rates and residual distributions results are approximately identical.For the 4-hour orbit overlap RMS,most of the results of ZD AR solution were 0,but a small part of the error was extremely large,which might be caused by the inconsistently fixed ambiguity between two POD arcs.As a result,the ZD AR solution presented a inferior average value but a superior median value.For the orbit difference with PSOs,the ZD AR solution showed an improvement of about 25% in along-track and cross-track.For the SLR residual results,the residuals STD of Swarm and GRACE-FO satellites were both about 20%smaller than those of float solution.In addition,the real-time kinematic POD for Swarm-A was performed using observations in August 2018.The average time to first fix(TTFF)was 25 minutes,and the ambiguity fixing rate reached 90%.Compared with the float solution,the real-time orbit accuracy could improved by 20% with ZD AR.