| Over the years,the space-borne dual-frequency GPS technology has been got in-depth research and extensive applications in precisely determining the absolute and relative orbits of low Earth orbiters.The experimental results of precise orbit determination for many low Earth orbiters with different tasks in China and abroad based on dual-frequency GPS orbit demonstrate that,the absolute centimeter level orbit of only one satellite can be achieved through pure dynamic,reduced dynamic or kinematic approaches.Furthermore,the precision of baseline solution for satellite formation has already reached millimeter level.As the increasing requirements of satellite tasks and applications for the timeliness and accuracy of satellite orbits,this thesis has designed and realized the real-time and near real-time precise orbit determination system against onboard platform and ground environment,respectively.At present,in addition to the steadily developing GPS system,the BDS and other carefully designed global navigation satellite system and various types of augmentation systems are all under actively and orderly developing and growing.These systems can not only take on the all kinds of precise orbit determination tasks for low Earth orbiters independently,but also integrated and enhanced with each other for the purpose of providing more abundant,three-dimensional and effective observation information.On the basis of comprehensive investigation on corresponding current research situation at home and abroad,and thoroughly digesting and absorbing the already accumulating research findings,this thesis has made several attempts and exploration focusing on the research filed of absolute and relative orbit determination for low Earth orbiters and their formation using GPS and BDS.The leading research efforts and conclusions in this article can be summarized as follows:(1)Using the phase ambiguity fixing technology to enhance the performance of orbit determination for low Earth orbiters.In the precise reduced dynamic orbit determination of low Earth orbiters,using the GPS phase integer clock products to realize the satellite single differenced phase ambiguity integer constraints for the space-borne single receiver single antenna.Take the orbit determination of HY-2A satellite as an example,and estimate the cross-track biases in precise orbits based on float ambiguity solution using SLR normal point data,the results indicates that the cross-track biases fluctuated in the range of about-2?2 cm would always occurred in orbit determination,no matter whether or not calibrating the solar radiation pressure model when using float ambiguity solution.While the cross-track biases can be eliminated effectively when fixing the ambiguities to integer compared to the approach of antenna PCV correction.Moreover,take the orbit determination of HY-2A and JASON-3 as examples,analyze the performance of phase ambiguity fixing under dynamic constraints with different strengths.Finally,carefully analyze the improvements of applying integer ambiguity resolution on precise orbit determination from the aspects of ambiguity fixing successful rate,orbit differences between overlapping arcs and SLR data checking residuals.(2)Improve the performance of orbit determination for FY-3C when using BDS data only.At present,the large errors existed in the final orbit products of BDS satellites restrict the precision orbit determination of low Earth orbiters based on BDS only.By analyzing the manifestations of current BDS orbit errors,it can be seen that meter level biases always existed in GEO orbit products,and the magnitudes of daily biases are not exactly the same,so the precision of orbit determination would decreased when using the GEO observations directly.Furthermore,the decimeter level discontinuous jumps occurred easily in daily boundaries of IGSO and MEO orbit determination,so the daily jumps would decrease the precision of orbit determination when cross the daily boundaries.This article proposes the approach of simultaneously estimating GEO daily along-track and cross-track constant biases in FY-3C orbit determination,the experimental results demonstrate that the three-dimension orbit precision can be improved by 20%when compared to the orbits derived by excluding GEO.Against the daily boundary jumps in BDS orbit products,this article proposes the approach of high precision fitting daily BDS orbits in order to disconnect the disadvantageous effects of discontinuous orbits in adjacent days on FY-3C orbit determination.The results show that the three-dimension precision of FY-3C orbits can be improved by about 39%when compared to the orbits derived by interpolating BDS orbits directly.Eventually,use the GPS-derived orbits as reference,the precision of FY-3C orbits derived by using BDS only can achieve 3.9,2.4,1.5 and 4.9 cm in along-track,cross-track,radial and three-dimension,respectively.(3)Implement the relative orbit determination with millimeter-level precision for the GRACE twin satellite formation.Both the zero difference and station single difference combination of dual-frequency GPS phase observations can contribute to conducting the relative orbit determination for satellite formation.The zero difference observations have the advantages of more available amount,full corresponding parameter estimation,however,at the same time,the effects of GPS orbit and clock errors are introduced.While the station single difference observations have the advantages of mitigating the effects of common errors in the same GPS satellite,but at the same time,the parameters such as absolute satellite orbits cannot be estimated effectually.On the basis of the single satellite absolute orbit determination software,this article adds the relative orbit determination procedure using zero difference and station single difference combination of observations,and realizes the ability of integrating satellite inter K-Band ranging observation data so as to enhance the performances of relative orbit determination.Take the relative orbit determination of GRACE twin satellite formation in the same orbit as an example,systematically analyze the detail impacts of onboard antenna PCV,double difference ambiguity fixing,observation combination types,reduced dynamic or kinematic,whether integrate KBR or not on satellite inter baseline determination.The results indicate that the implemented relative orbit determination module has the capability of providing millimeter-level reduced dynamic and kinematic baseline solution for GRACE formation.(4)Design and realize the algorithms of onboard real-time and ground near real-time orbit determination.The problem that the navigation satellite precise orbit and clock products are difficult to obtain under the situation of real-time orbit determination for onboard platform and near real-time orbit determination in ground after receiving observation data.Besides,onboard orbit determination will also be restricted by certain constraints such as limited computing performance and memory space.Facing the condition that the broadcast ephemeris with large errors could be obtained only onboard and in ground,this article proposes the algorithm of calculating approximated positions and initial receiver clock errors using pseudo range data,later using adjacent epoch differenced phase data to perform the precise orbit determination.Design onboard reduced dynamic real-time orbit determination using square root information filter and phase epoch difference and estimating empirical accelerations,the results of simulated real-time orbit determination for GRACE twin satellites based on broadcast ephemeris shows that,the designed algorithm can provide long term steady consecutive decimeter-level orbit products.Design near real-time reduced dynamic orbit determination algorithm using batch least squares and phase epoch difference in ground.The simulated near real-time GF-3 orbits obtained using broadcast ephemeris demonstrates that,the proposed algorithm can produce orbit products with three-dimension precision in the range of about 7?22 cm and average about 14 cm. |
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