VLBI observations to Galileo satellites

Space geodetic techniques are essential for the determination of global geodetic reference frames, which are the basis for every kind on positioning and navigation on Earth and in space. These reference frames are the realization of Earth-fixed and space-fixed coordinate systems, including the Earth orientation in space for the transformation between the two systems. One important component of Earth orientation is the non-uniform rotation about its axis in about 24 hours. The determination of the corresponding Earth rotation angle with microsecond-accuracy is of utmost importance for exact positioning at the millimetre-level with Global Navigation Satellite Systems, such as the United States Global Positioning System (GPS) or Galileo of the European Union. While GPS or Galileo rely on measurements between Earth orbiting satellites and antennas on the ground, Very Long Baseline Interferometry (VLBI) uses globally distributed radio telescopes and the observation of quasars, which are extragalactic radio sources billions of light years away. The primary observable in VLBI is the difference in arrival time of the signal from the quasars at the radio telescopes. VLBI is the only technique of the determination of the Earth rotation angle, as the satellite techniques suffer from unmodelled errors in the satellite orbits. The goal of project VLBI2Galileo is the transfer of the advantages of VLBI to Galileo with a new type of observation. The only geometric ties between the two space geodetic techniques stem from local measurements at co-location sites on the ground, as there is no link between the techniques in space. In this project, we investigate the benefit of VLBI transmitters on board Galileo satellites, i.e., we assume that Galileo satellites are emitting signals similar to quasars. These signals can then be observed with VLBI radio telescopes realizing ties between the techniques in space. In consequence, these observations will enable the direct observation of the Earth rotation angle by Galileo observations, which is currently not possible. Additionally, we investigate the application of specially designed time encoded signals for ranging between the Galileo satellites and the VLBI radio telescopes, which will particularly improve the radial component of the Galileo orbits. We are going to use the Vienna VLBI and Satellite Software (VieVS) for extensive simulations to optimize the schedules (at what time should which VLBI radio telescopes observe which satellites and quasars) for the best possible determination of Galileo orbits and references frames. Finally, we will be able to provide information about the accuracies, which can be achieved with these new observation types.

Very Long Baseline Interferometry (VLBI) is a measurement technique with multiple radio telescopes on Earth simultaneously receiving radio signals from space originating from extremely distant objects in the universe, mostly quasars. More specifically, VLBI is the only geodetic technique capable of determining the precise orientation of the Earth in space and realizing the celestial reference frame with the positions of quasars. VLBI also contributes to the determination of the terrestrial reference frame, a coordinate system for positions on Earth, which is essential for every kind of positioning and navigation, as well as the observation of tiny effects, such as sea level rise with a rate of about four millimeters per year. Conventional VLBI observes only natural radio sources located billions of light years away. This may change as there are plans to equip satellites with dedicated VLBI transmitters mimicking quasars, allowing VLBI antennas to observe satellites alongside quasars. This study investigates the potential of VLBI observations to satellites of the European Union satellite navigation system Galileo, assuming that these satellites are equipped with dedicated VLBI transmitters. Since Galileo satellites currently do not carry such transmitters and therefore cannot be observed with VLBI, the analyses are based on simulations under the premise that one or more Galileo satellites are equipped with VLBI transmitters. More precisely, the studies cover the estimation of VLBI station coordinates from VLBI observations to satellites, with the purpose of assessing the tie between two different reference frames - one derived from observations to satellites and one derived from observations to quasars. This study finds that at least two, but preferably three, Galileo satellites must be equipped with a VLBI transmitter, with the best results if all satellites with a transmitter are placed in the same orbital plane. Moreover, the determination of the orbit of a Galileo satellite from VLBI is investigated in terms of estimating positions of a satellite in the local orbital frame for short orbital arcs, as well as estimating the six orbital elements, describing the satellite orbit. In this context, the absolute orientation of the satellite orbit with respect to the celestial reference frame is of particular interest, as VLBI observations to satellites allow for its direct determination. The findings in this work emphasize that mounting one or more VLBI transmitters on next-generation Galileo satellites will clearly enhance the opportunities with space geodesy. Also, the investigations and experiences from this thesis are very valuable for the upcoming Genesis mission of the European Space Agency, which will carry a VLBI transmitter.