| With the rapid development of Low Earth Orbit(LEO)satellite technology,more than 15 LEO constellation plans have been announced internationally,involving tens of thousands of LEO satellites.The emergence of large LEO constellations has provided new opportunities for LEO-enhanced GNSS,which has gradually become the frontier and hot spot of domestic and foreign research.Due to the advantages of low orbital height and fast motion,LEO satellites can be used as dynamic satellite-based monitoring stations by carrying onboard GNSS receivers.LEO onboard observations help to enhance the precision orbit determination of BDS satellites,especially under the distribution of regional ground stations.By transmitting navigation signals to the ground,LEO satellites help improve the geometry strength of GNSS satellites and shorten the convergence time of real-time Precision Point Positioning(PPP).In the research and practice of LEO-enhanced GNSS,the following problems were found:first,the increase of LEO satellites will lead to a sharp increase in onboard observation and calculations when LEO satellites participate in the integrated orbit determination of medium-and high-orbit navigation satellites.Second,the orbit accuracy of LEO satellites will be poor due to the influence of the ground station receiver clock,when the LEO ground observations are used for orbit determination.Third,in the LEO satellite-enhanced GNSS positioning service,the LEO satellite has the problem of inconsistent hardware delay,which makes the LEO onboard receiver clock cannot be directly used by users.In view of the above problems,this paper carried out research on four aspects:LEO onboard observations enhanced GPS/BDS precise orbit determination,integrated orbit determination based on LEO ground observations,integrated clock estimation based on LEO onboard and ground observations,the experiment and verification of LEO satellites enhanced GPS/BDS PPP.Based on the Position And Navigation Data Analyst(PANDA)software platform,the data simulation module and navigation enhancement service module of the LEO constellation are independently developed.The main work and achievements of this paper are as follows:(1)Aiming at the problem of onboard observation redundancy in the integrated orbit determination of LEO and GPS/BDS satellites,a selection method of LEO satellites based on LGDOP value for measuring satellite distribution is proposed.By calculating the LEO satellites combination corresponding to the minimum LGDOP value,this method selects a few LEO satellites distributed evenly around the world to participate in GPS/BDS integrated orbit determination.The proposed method reduces the number of participating LEO satellites while achieving the same orbit accuracy improvement effect of GPS/BDS satellites.Based on the actual onboard observations of GRACE-FO and Feng Yun-3C/D(FY-3C/D)satellites,this paper verifies the enhancement effect of LEO satellites on GPS/BDS satellites’ orbit determination accuracy.The results show that the participation of GRACE-FO and FY-3C/D satellites can enhance the geometric strength of the regional ground stations network,and the orbit determination accuracy of GPS and BDS GEO,IGSO,and MEO satellites can be improved by 28.2%,48.0% 43.7%,and 92.0%,respectively.The results based on the simulated observations show that when the number of LEO satellites selected to participate in the integrated orbit determination reaches about 20-30 according to the proposed selection method of LEO satellites,the orbit determination accuracy of regional ground stations distribution can be improved by 80%,and that of global ground stations distribution can be improved by nearly 40%.When the number of LEO satellites is further increased,it is difficult to significantly improve the orbit determination accuracy of GPS/BDS satellites,but will lead to a significant increase in the calculation.Therefore,about 20-30 LEO satellites selected by the proposed method participate in the integrated orbit determination of GPS/BDS satellites,which can reduce the computation as much as possible while improving GPS/BDS orbit accuracy.This method can achieve the effect of balancing orbit determination accuracy and computational efficiency.(2)Aiming at the problem of poor accuracy when LEO ground observations are used for orbit determination,an orbit determination method for LEO satellites with fixed ground receiver clock information is proposed.This method can realize orbit determination of LEO satellites only using LEO ground observations when the onboard observations are missing,and can also realize integrated orbit determination of LEO satellites with LEO onboard and ground observations,which can improve the orbit accuracy of LEO satellites.The results based on the simulated observations show that when LEO ground observations from global 136 and 40 stations and regional 15 stations participate in the integrated orbit determination of LEO satellites,the float solution accuracy of reduced dynamic orbit determination based on onboard observations can be improved by 17.7%,7.6%,and 1.5%,respectively,and the fixed solution accuracy can be improved by 30.8%,16.5%,and 5.5%.With the simulated real-time IKPOD mode,the participation of LEO ground observations can shorten the average convergence time of satellite orbit determination to 9.38 minutes,which is23.97% higher than the traditional KPOD model.Since onboard observations of 30 LEO satellites selected by the LEO satellite selection method are involved in GPS/BDS integrated orbit determination,the orbit accuracy of GPS/BDS satellites is very high,and it is difficult to further improve with a small amount of LEO ground observations.(3)Aiming at the LEO satellite onboard receiver clocks cannot be used as the LEO satellite clocks by users,a clock estimation method of LEO satellites by integrating the LEO ground and onboard observations considering LEO satellite hardware delay bias is proposed.The LEO onboard receiver clocks and LEO satellite clocks will absorb the different code delays during receiving and transmitting signals respectively,so the determination of the LEO satellite clocks must rely on ground tracking stations.After accurately estimating the LEO satellite hardware delay bias,the proposed clock estimation method by integrating the LEO ground and onboard observations makes full use of the good continuity of LEO onboard receiver clocks,which can improve the estimated accuracy of LEO satellite clocks.The results based on simulated observations show that the average RMS and STD values of LEO satellite clocks using the integrated PCE(IPCE)method can reach 0.27 ns and 0.15 ns,respectively.The improvements in the average RMS and STD are 42.6% and 60.5%,respectively,compared with the traditional PCE method in which only LEO ground observations are involved.Therefore,the participation of LEO satellite onboard observations can shorten the convergence time and improve the continuity and precision of LEO satellite clocks.(4)Based on the estimated GPS/BDS and LEO orbit and clock products and GPS/BDS UPD products,LEO-enhanced GPS/BDS PPP with ambiguity resolution is implemented.In addition,the improvement effect of positioning performance with jointly estimated GPS/BDS and LEO products is verified.The results based on the actual GPS/BDS ground observations and simulated LEO ground observations show that the average convergence time of GPS/BDS PPP fixed solution is shortened from14.5 min to 12.8 min with the LEO enhanced GPS/BDS products compared with the traditional products estimated by ground stations.The positioning accuracy of East,North,and Up directions are improved by 11.7%,6.7%,and 5.8%,respectively,and the float solution also has the same performance improvement.Compared with the traditional LEO orbit product estimated by onboard observations and LEO clock product estimated by ground observations,the LEO orbit and clock products estimated by onboard and ground observations have more significant enhancement effects on GPS/BDS PPP float and fixed solutions.The average convergence time of LEO/GPS/BDS PPP fixed solutions can be reduced from 12.8 min to 4.9 min with jointly estimated LEO orbit and clock products compared with GPS/BDS PPP.And the positioning accuracy of East,North,and Up directions can also be improved by 32.7%,25.3%,and 28.4%,respectively. |
