Precise Orbit And Clock Determination For BDS-3 Satellites Using Inter-satellite Link Observations

As an important part and development trend of modern Global Navigation Satellite System(GNSS),the inter-satellite link(ISL)is mainly used to realize the autonomous orbit determination of a navigation constellation or joint ground stations to improve the orbit determination accuracy.Due to political and geographical factors,it is currently difficult for China to evenly build ground monitoring stations on the global scale,which makes Beidou has the most urgent need for the ISL in four major global navigation satellite systems.Meanwhile,this also to promote China achieve an important breakthrough in terms of the ISL technology,and the ISL has been successfully applied to the Beidou-3navigation satellite system(BDS-3).On the one hand,it provides a very good opportunity for the research on the application of navigation satellite inter-satellite links,on the other hand,it also brings some challenges for the high-precision and high-performance BDS-3ISL data processing.Due to the sensitivity and confidentiality of the ISL,there is very little related information published abroad and rarely involves the key technologies.In China,scholars initially conducted a lot of research on the satellite orbit determination using simulation ISL data.With the successful launch of BDS-3 experimental satellites and BDS-3 satellites in recent years,related results based on real ISL data have been published successively.However,most of the current studies only give preliminary results of the ISL orbit and clock determination,and very little researchs were done on the refinement of the ISL data processing models.In addition,more BDS-3 satellites form more ISLs with good spatial geometry at present,thus the contribution of the ISL on BDS-3 satellite orbits and clocks need to be analyzed in detail.This paper focuses on BDS-3 precise orbit and clock determination based on intersatellite link observations.We have been solved several key issues,such as the corrections of BDS-3 ISL ranging deviations,BDS-3 satellite L-and Ka-band antenna phase center calibration,BDS-3 satellite yaw attitude model analysis based on the ISL observations.Thus,the geometric model of BDS-3 ISL data processing for orbit determination has been refined or established,and the software of ISL orbit determination and combined orbit determination using both satellite-ground and ISL observations have been developed.In addition,the proposed models,methods and data processing software have been verified with a large amount of real data.The main research work and contributions of this thesis are summarized as follows:(1)The basic situations of BDS-3 ISL topology and measurement scheme were introduced,and the observation model and main errors for BDS-3 ISL data processing in precise orbit and clock determination were analyzed.After converting bidirectional observations to the same epoch,the addition of dual one-way observations can form the clock-free observables and are used to satellite orbit determination,the subtraction of dual one-way observations can form the geometry-free observables and are used to satellite clock offset estimation.(2)We founded that there are link-dependent ranging biases in the BDS-3 ISL observations,and analyzed the characteristics of such ISL ranging biases,established the related correction model.Based on the 50-day ISL data from 20 BDS-3 satellites,the ranging biased of 171 two-way links and 342 one-way link observations were calculated,thus the corresponding correction models were obtained,and the results show that the largest ranging biases of the BDS-3 bidirectional links were about 18 cm,and the one-way links were up to 30 cm.Finally,the influence of these biases on orbit determination and clock offset estimation of BDS-3 satellites based on ISLs were analyzed.(3)Based on the 260-day L-band satellite-ground and Ka-band ISL observation data in 2019,different strategies were used to estimate the phase center offsets(PCO)of the BDS-3 satellite L navigation antennas and Ka phased array antennas.Compared with the L antenna PCO estimation using only the L-band satellite-ground data and the Ka antenna PCO estimation using only the Ka-band ISL data,the simultaneous estimation of L and Ka antenna PCOs by the joint satellite-ground and ISL data can achieve a higher accuracy.The results shows that the accuracy of L PCO estimation were 2.6 cm,4.7 cm,and 12.0 cm in the X,Y,and Z directions,and the accuracy of Ka PCO estimation were 2.5 cm,6.7 cm,and 4.1 cm in the X,Y,and Z directions,respectively.In addition,the L and Ka PCO estimates were also verified by corresponding results of orbit determination.(4)Based on the reverse kinematic precise point positioning(RKPPP)method,the epoch-by-epoch satellite Ka antenna PCO was calculated based on the ISL observations to estimate the yaw attitude of BDS-3 satellite,and the resulting estimated accuracy was 2.4 °.Based on the estimated actual yaw attitude,the yaw attitude control mechanism of BDS-3satellites provided by the two manufacturers was analyzed,and then a modified yaw attitude model for different manufacturers of BDS-3 satellites was proposed.For CAST satellites,when the ? angle is between ± 3° and the satellite orbit angle ? is between about[-6° ? 6°] or [174° ? 186°],the yaw maneuver is performed,and a modified Galileo model should be used during yaw maneuver;for SECM satellites,when ? angle is between ± 3°,the yaw maneuver is performed,and the BDS-3 model issued by CSNO should be used.The proposed model agrees with the estimated actual yaw attitude at the 3° level.(5)We developed the software for ISL orbit determination with only Ka observations and combined orbit determination with L+Ka observations,and validated the performance.The orbit results for 18 MEO and 1 IGSO satellites of BDS-3 show that,for the orbit determination with only ISLs,the 1D orbit overlap accuracy was 9.5 cm,and the SLR check accuracy(RMS)was 4 cm;for the orbit determination with global stations and ISLs,the 1D orbit overlap accuracy was improved from 4.0 cm to 3.2 cm and the SLR check accuracy was improved from 5.0 cm to 3.6 cm;for the orbit determination with inland stations and ISLs,the 1D orbit overlap accuracy was improved from 35.2 cm to 6.5 cm and the SLR check accuracy was improved from 10.9 cm to 3.9 cm.The orbit results for the first BDS-3 GEO satellite show that the use of ISLs can significantly improve its radial accuracy,from about 70 cm to 20 cm.(6)Based on the Ka-band ISL observations,the BDS-3 satellite clock estimation was performed,and the RMS of quadratic polynomial fitting residuals for estimated Ka-band clock offsets was 0.14 ns,which is better than that of L-band clock offsets from precision orbit determination.Based on the L and Ka band clock offsets,we compared the periodic characteristics,frequency stability,and the predicting performance of the BDS-3 onboard rubidium clocks and hydrogen clocks.The Ka clock offsets are not affected by orbit errors,which makes it a certain advantage in the characteristics analysis of satellite atomic clocks.The periodicity obtained by Ka clocks was significantly weakened compared to that of the L clocks,the frequency stability results of Ka clocks were mor stable,and the Ka clock offsets showed higher mid-and long-term prediction accuracy.