Opportunities with VLBI transmitters on Galileo satellites

Abstract

Geodetic Very Long Baseline Interferometry (VLBI) is a space-geodetic technique based on the simultaneous observation of signals from extragalactic radio sources, known as quasars, with multiple radio telescopes distributed over the Earth. This technique is uniquely capable of determining the orientation of Earth in space and the positions of radio sources in an inertial frame. Consequently, VLBI plays a crucial role in deriving Earth Orientation Parameters and establishing the Celestial Reference Frame. It also contributes to the determination of the Terrestrial Reference Frame, which is a combined product of the several space-geodetic techniques. 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, allowing VLBI antennas to observe satellites alongside quasars. In this work, the feasibility and opportunities of VLBI observations to Galileo satellites are studied, including the estimation of station coordinates and satellite orbits. The investigations are based on Monte Carlo simulations, assuming that one or a few Galileo satellites are equipped with a dedicated VLBI transmitter. More precisely, the studies cover the estimation of VLBI station coordinates from VLBI observations to satellites, with the purpose of assessing frame ties. This study finds that at least two but better three Galileo satellites have to be equipped with a VLBI transmitter, with the best results if all satellites with a transmitter are placed in the same orbital plane. Station coordinates from 24-hour sessions can then be estimated with a precision at the centimetre level or better, emphasising the possibilities with this new observation type.Moreover, the sensitivity of VLBI observations to satellites in terms of the satellite position in the local orbital frame and the Earth rotation angle is examined by deriving and interpreting so-called Dilution of Precision (DOP) factors. The latter can be used in the scheduling process as criteria for selecting the most suitable scan for estimating the satellite position or the Earth rotation angle. Finally, the determination of the orbit of a Galileo satellite is investigated in terms of estimating kinematic positions of a satellite in the local orbital frame for short orbital arcs, as well as estimating the six orbital elements. In this context, the right ascension of the ascending node is of particular interest, as VLBI observations to satellites allow for its direct determination. The results indicate that the absolute orientation of the satellite orbit around the z-axis, expressed as right ascension of the ascending node, can be determined with VLBI observations to satellites with a precision of 40 microarcseconds, which corresponds to 0.6 cm arc length at the altitude of the Galileo orbit. Therefore, VLBI observations to satellites have the capacity to directly reference the satellite orbit with respect to the celestial reference frame.The findings in this work emphasise that mounting one or more VLBI transmitters on the next-generation of 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.