Precise orbit determination for low Earth orbit satellites using GNSS:Observations,models,and methods

Spaceborne global navigation satellite system(GNSS)has significantly revolutionized the development of autonomous orbit determination techniques for low Earth orbit satellites for decades.Using a state-of-the-art combination of GNSS observations and satellite dynamics,the absolute orbit determination for a single satellite reached a precision of 1 cm.Relative orbit determination(i.e.,precise baseline determination)for the dual satellites reached a precision of 1 mm.This paper reviews the recent advancements in GNSS products,observation processing,satellite gravitational and non-gravitational force modeling,and precise orbit determination methods.These key aspects have increased the precision of the orbit determination to fulfill the requirements of various scientific objectives.Finally,recommendations are made to further investigate multi-GNSS combinations,satellite high-fidelity geometric models,geometric offset calibration,and comprehensive orbit determination strategies for satellite constellations.

Observable-specific phase biases of Wuhan multi-GNSS experiment analysis center’s rapid satellite products

Precise Point Positioning(PPP)with Ambiguity Resolution(AR)is an important high-precision positioning technique that is gaining popularity in geodetic and geophysical applications.The implementation of PPP-AR requires precise products such as orbits,clocks,code,and phase biases.As one of the analysis centers of the International Global Navigation Satellite System(GNSS)Service(IGS),the Wuhan University Multi-GNSS experiment(WUM)Analysis Center(AC)has provided multi-GNSS Observable-Specific Bias(OSB)products with the associated orbit and clock products.In this article,we first introduce the models and generation strategies of WUM rapid phase clock/bias products and orbit-related products(with a latency of less than 16 h).Then,we assess the performance of these products by comparing them with those of other ACs and by testing the PPP-AR positioning precision,using data from Day of the Year(DOY)047 to DOY 078 in 2022.It is found that the peak-to-peak value of phase OSBs is within 2 ns,and their fluctuations are caused by the clock day boundary discontinuities.The associated Global Positioning System(GPS)orbits have the best consistency with European Space Agency(ESA)products,and those of other systems rank in the medium place.GLObal NAvigation Satellite System(GLONASS)clocks show slightly inconsistency with other ACs’due to the antenna thrust power adopted,while the phase clocks of other GNSSs show no distortion compared with legacy clocks.With well-estimated phase products for Precise Orbit Determination(POD),the intrinsic precision is improved by 14%,17%,and 24%for GPS,Galileo navigation satellite system(Galileo),and BeiDou-3 Navigation Satellite System(BDS-3),respectively.The root mean square of PPP-AR using our products in static mode with respect to IGS weekly solutions can reach 0.16 cm,0.16 cm,and 0.44 cm in the east,north,and up directions,respectively.The multi-GNSS wide-lane ambiguity fixing rates are all above 90%,while the narrow-lane fixing rates above 80%.In conclusion,the phase OSB products at WUM have good precision and performance,which will benefit multi-GNSS PPP-AR and POD.

Consistency of MGEX Orbit and Clock Products

The analysis centers of the Multi-GNSS Pilot Project of the International GNSS Service provide orbit and clock products for the global navigation satellite systems(GNSSs)Global Positioning System(GPS),GLONASS,Galileo,and BeiDou,as well as for the Japanese regional Quasi-Zenith Satellite System(QZSS).Due to improved solar radiation pressure modeling and other more sophisticated models,the consistency of these products has improved in recent years.The current orbit consistency between different analysis centers is on the level of a few centimeters for GPS,around one decimeter for GLONASS and Galileo,a few decimeters for BeiDou-2,and several decimeters for QZSS.The clock consistency is about 2 cm for GPS,5 cm for GLONASS and Galileo,and 10 cm for BeiDou-2.In terms of carrier phase modeling error for precise point positioning,the various products exhibit consistencies of 2–3 cm for GPS,6–14 cm for GLONASS,3–10 cm for Galileo,and 10–17 cm for BeiDou-2.

Comparison of availability and reliability among different combined-GNSS/RNSS precise point positioning

With emergence of the BeiDou Navigation Satellite System(BDS), the Galileo Satellite Navigation System(Galileo), the Quasi-Zenith Satellite System(QZSS)and the restoration of the Global Navigation Satellite System(GLONASS), the single Global Positioning System(GPS) has been gradually expanded into multiple global and regional navigation satellite systems(multi-GNSS/RNSS). In view of differences in these 5 systems, a consolidated multi-GNSS/RNSS precise point positioning(PPP) observation model is deduced in this contribution. In addition, the performance evaluation of PPP for multi-GNSS/RNSS is conducted using a large number of the multi-GNSS experiment(MGEX) station datasets. Experimental results show that multi-GNSS/RNSS can guarantee plenty of visible satellites effectively. Compared with single-system GPS, PDOP, HDOP, and VDOP values of the multi-GNSS/RNSS are improved by 46.8%, 46.5% and 46.3%, respectively. As for convergence time, the static and kinematic PPP of multi-GNSS/RNSS are superior to that of the single-system GPS, whose reliability, availability, and stability drop sharply with the increasing elevation cutoff. At satellite elevation cutoff of 40 °, the single-system GPS fails to carry out continuous positioning because of the insufficient visible satellites, while the multi-GNSS/RNSS PPP can still get positioning solutions with relatively high accuracy, especially in the horizontal direction.

Modeling and Prediction of Inter-System Bias for GPS/BDS-2/BDS-3 Combined Precision Point Positioning

The combination of Precision Point Positioning(PPP)with Multi-Global Navigation Satellite System(MultiGNSS),called MGPPP,can improve the positioning precision and shorten the convergence time more effectively than the combination of PPP with only the BeiDou Navigation Satellite System(BDS).However,the Inter-System Bias(ISB)measurement of Multi-GNSS,including the time system offset,the coordinate system difference,and the inter-system hardware delay bias,must be considered for Multi-GNSS data fusion processing.The detected ISB can be well modeled and predicted by using a quadratic model(QM),an autoregressive integrated moving average model(ARIMA),as well as the sliding window strategy(SW).In this study,the experimental results indicate that there is no apparent difference in the ISB between BDS-2 and BDS-3 observations if B1I/B3I signals are used.However,an obvious difference in ISB can be found between BDS-2 and BDS-3 observations if B1I/B3I and B1C/B2a signals are used.Meanwhile,the precision of the Predicted ISB(PISB)on the next day of all stations is about 0.1−0.6 ns.Besides,to effectively utilize the PISB,a new strategy for predicting the PISB for MGPPP is proposed.In the proposed strategy,the PISB is used by adding two virtual observation equations,and an adaptive factor is adopted to balance the contribution of the Observed ISB(OISB)and the PISB to the final estimations of ISB.To validate the effectiveness of the proposed method,some experimental schemes are designed and tested under different satellite availability conditions.The results indicate that in open sky environment,the selective utilization of the PISB achieves almost the same positioning precision of MGPPP as the direct utilization of the PISB,but the convergence time of MGPPP is reduced by 7.1%at most in the north(N),east(E),and up(U)components.In the blocked sky environment,the selective utilization of the PISB contributes to more significant improvement of the positioning precision and convergence time than that in the open sky environment.Compared with the direct utilization of the PISB,the selective utilization of the PISB improves the positioning precision and convergence time by 6.7%and 12.7%at most in the N,E,and U components,respectively.

ZHBDCORS Multi-GNSS数据质量分析

在北斗导航卫星系统(BeiDou navigation satellite system,BDS)迈进全球服务之际,对珠海市北斗连续运行基准站(Zhuhai BeiDou continuously operating reference station,ZHBDCORS)系统2014年以来积累的全球导航卫星系统(global navigation satellite system,GNSS)连续观测数据进行质量检核,分析其质量变化原因,评估系统的BDS支持度。结果表明,ZHBDCORS站点接收机及解算软件均不可跟踪BDS-3信号,且未完全跟踪BDS-3卫星播发的BDS-2信号;部分基准站受观测环境影响,数据完整性差,多路径误差明显,应考虑迁站升级;同等观测条件下,BDS呈现更强的抗遮挡能力。