The Impacts Of Attitude, Solar Radiation And Function Model On Precise Orbit Determination For Gnss Satellites

The development of satellite navigation systems has been booming since April27,1995, when the US Global Positioning System (GPS) was fully operated. The former Soviet Union and its follow-Russia, the European Union, China, Japan and India have developed or have been developing their own global or regional satellite navigation systems, named GLONASS, Galileo, BeiDou, QZSS, and IRNSS respectively, in order to meet the demands of state security and economic interests. The development of Multi Global Navigation Satellite System (Multi-GNSS) could not only provide stable and accurate positioning, navigation, and timing service, but also meet the demands of high-end users from Geodesy and other disciplines. However, the most essential products for the high-end GNSS applications is the high accurate orbit and clock products in the unified time and space reference system.The factors which have influences on the Precise Orbit Determination (POD) for GNSS satellites include two categories:the geometric corrections and dynamical forces. Thanks to the improvement of force models and error models of GNSS observables, the1D accuracy of orbits can be achieved better than2.5cm and3.0cm for the GPS and GLONASS satellites, respectively. However, there still are some challenges and issues to be investigated for POD of BeiDou, Galileo-IOV and other new coming satellites, which include:(1)POD with observables in three or even more frequencies;(2)POD for different types of GNSS satellites, such as GEO, IGSO, and MEO;(3)POD for GNSS satellites in none yaw-steering mode;(4)POD for Galileo-IOV satellites with centimeter level accuracy.This dissertation focuses on the impacts of attitude, solar radiation pressure (SRP), and function model on POD for GNSS satellites. The main goal of the dissertation is to improve the orbit accuracy for BeiDou IGSO and MEO satellites during the yaw attitude maneuver periods. Generally, the contributions of this dissertation include:(1) The eclipsing crossing or noon and midnight yaw maneuvers of GPS BLOCK II/IIA, GPS BLOCK IIR, GPS BLOCK IIF, GLONASS-M, and QZSS Michibiki satellites and their impacts on POD have been investigated. Based on the revised kinematic precise point positioning approach, the yaw attitude of BeiDou IGSO and MEO satellites are also analyzed. The results demonstrate that BeiDou IGSO and MEO satellites use two kinds of yaw attitude modes, which are yaw-steering and yaw-fixed. Once the orbit angle equals90°as well as the angle of Sun above the orbit plane are not greater than4°, the yaw attitude mode switches to yaw-fixed mode from yaw-steering mode. In other hand, the yaw attitude mode will switch to yaw-steering mode when the orbit angle equals90°as well as the angle of Sun above the orbit plane are not less than4°.(2) The algorithms for estimation of nadir angle dependent as well as azimuth and nadir angle dependent PCVs are presented. In addition, the impacts of PCOs on the satellite orbit, clock, troposphere, site coordinates, and frame scale are also presented. The PCOs and PCVs of BeiDou IGSO, MEO, and Galileo-IOV satel-lites are estimated and compared with the corresponding Europe Space Agency’s estimated values. The standard deviation of PCVs differences of BeiDou IGSO and MEO satellites are less than1mm in all nadir angles. However, the differences of PCOs are a bit larger maybe due to the different tracking stations, software pack-ages and frame scales used. Specifically, the X component differences for IGSO and MEO satellites are37.4mm and26.0mm, respectively, whereas they reach to-50cm and12.1cm for Z component. The impacts of the estimated PCVs and PCOs on satellite orbits, troposphere, and coordinates are assessed. The results demon-strate that the overlap orbit differences reduce significantly for BeiDou MEO and Galileo-IOV satellites, and the accuracy of sites coordinates in up direction is also improved. However, the estimated PCOs have a bit influence on IGSO orbits and troposphere.(3) The impact of attitude modes (yaw-steering and yaw-fixed) and SRP models on BeiDou GEO satellites POD has been analysed. Although the attitude modes has a bit impact on overlap orbit differences, the SLR validation results are improved with the actual yaw-fixed attitude. In addition, the orbit derived with CODE SRP model and one additional constant acceleration in along-track direction show best orbit quality indicated by overlap differences. However, the SLR validation indicates that the accuracy is the best for the orbit determined with adjustable Box-wing model. The above results indicate the orbit accuracy could be improved further with better SRP model.(4) The reason for accuracy degradation of BeiDou IGSO and MEO satellites during yaw attitude switch periods has been analysed, and the strategy for POD improve-ment has also been proposed. Based on the analytical SRP and thermal radiation pressure model, the force variations are investigated and confirmed that they are mainly distributed in the along-tack direction. Hence, one additional constant ac-celeration in along-track direction is added to CODE SPR model to improve the POD accuracy. The results demonstrate that the accuracy for POD in the yaw-fixed regime reach almost the same level of that in yaw-steering regime for both IGSO and MEO satellites. Furthermore, if the additional acceleration is estimat-ed as the piece-wise constant parameter, the overlap orbit differences are reduced significantly for IGSO during yaw attitude switch periods(5) The orbit and clock products in the united space and time reference system are determined for GPS, GLONASS, BeiDou, and Galileo using the Ionosphere-Free (IF) observables by the so-called ’one-step’ and ’two-step’ POD strategy. Compared with IGS final orbit and clock products, the results show that the determined orbits and clocks achieve the same accuracy as GFZ and CODE ones. This is in the top level of all IGS Analysis Centers (AC) in terms of quality. Meanwhile, the accuracy of Galileo-IOV satellites are almost same with other ACs, whereas ours BeiDou products are better than others.(6) The orbits of all Galileo-IOV satellites have a bias of about-4-5cm, and a clear dependence of the SLR residuals and RMS of clock residuals on the elevation of the Sun above the orbital plane are shown. In addition, SLR residuals show a high correlation with the clock residuals and orbit angle. The reason for these systematic errors is most probably related to SRP mismodeling issues and indicates the current SRP model should be improved. And the constrained clock or combination of SLR and GNSS observations for POD could be a new approach for refining the SPR model. (7) Two kinds of function models for POD with raw observables directly are presented and compared. The orbit and clock products are determined with raw and IF obser-vations, respectively. Although the results demonstrate the quality of orbits, clocks, earth rotation parameters computed with raw observations are improved slightly than that derived with IF observations, data processing time increase dramatically. Hence, it is not suitable for POD with raw observations directly.