GPS-based Precise Absolute and Relative Kinematic Orbit Determination of Swarm Satellites under Challenging Ionospheric Conditions

摘要

Global Positioning System (GPS) receivers have become standard equipment for the precise orbit and baseline determination of LEO satellites since the first installation of a dual-frequency GPS receiver on the TOPEX/Poseidon satellite in 1992. Kinematic orbit determination has attracted a lot of attention in recent years because it is free of models for correcting forces acting on the LEO satellite, which makes it possible to derive the gravity field from the kinematic orbits. The European Space Agency (ESA) Swarm mission launched on November 22, 2013, consists of three identical satellites in near-polar orbits, Swarm A and C flying almost side-by-side at an initial altitude of 460 km, and Swarm B flying in a higher orbit of about 530 km. Each satellite is equipped with a high precision 8-channel dual-frequency GPS receiver for precise orbit determination. Therefore, the Swarm mission offers excellent opportunities to provide temporal gravity field information derived from the kinematic orbits of the satellites for the gap between the Gravity Recovery and Climate Experiment (GRACE) and its follow-on mission (GRACE-FO). In order to contribute to these studies, a software based on Precise Point Positioning (PPP) was developed for kinematic orbit determination in this thesis. The kinematic orbits of Swarm satellites from January 2015 to December 2017 are computed and validated. The three-satellite formation opens up new opportunities to analyze the baseline determination (relative positioning). In this contribution, the kinematic baseline with fixed ambiguities is determined using the GPS double-difference method. Our investigations revealed that carrier phase observations with product baseline 03 can contain half-cycle ambiguities. The latter are repaired after reprocessing the raw GPS by the ESA in 2018, which increases the rate of ambiguity resolution significantly. Compared with the float solution, the quality of the baseline is significantly improved under strong ionospheric scintillations Carrier phase observations from onboard GPS receivers are strongly disturbed by ionospheric scintillations, which degrade the kinematic orbits over the geomagnetic equatorial and polar areas and, thus, the gravity field. The GPS carrier phase observations also suffer from different types of disturbances due to the different properties of ionospheric scintillations over the equatorial and polar areas. In this contribution, a new method is proposed to filter the high-frequency noise and repair the systematic errors in the phase observations, instead of eliminating or down-weighting the disturbed observations, in order to improve the quality of the kinematic orbits. The latter and the gravity field derived can be improved significantly. The systematic errors along the geomagnetic equator bands in the gravity field are also eliminated successfully.