Very Long Baseline Interferometry (VLBI) is the space-geodetic technique directly connecting the Terrestrial Reference Frame realized by positions of Earth-based stations with the Celestial Reference Frame (CRF) defined by a set of extragalactic radio sources (so-called quasars) well distributed throughout the sky. The signals from the radio sources which are in a distance of several billion light years have to travel through the intergalactic, interstellar, and interplanetary space before arriving at the Earth. On their way they are deflected by the gravitational fields of huge masses, e.g., by the central black hole of our Galaxy, the Sun or massive planets such as Jupiter or Saturn. Together with any un-modelled motion of the SSB relative to the galactic centre, these effects cause an apparent change in the quasar positions. In the conventional VLBI model used for recent ICRF realizations, the theoretical group delay, i.e., the time difference between the arrivals of the plane radio wave at the two Earth-based stations, is computed in the Solar System Barycentric (SSB) frame and the positions of quasars are assumed to be constant. However, with the ambitious goal of the ICRF3 and the long VLBI observation history of more than three decades, it is now crucial to reconsider these assumptions and to assess those subtle effects on the source positions.

In the project Galactic VLBI, the main focus is on the investigations of the apparent motion of extragalactic radio sources and estimation of astronomical parameters from VLBI. We will analyse the VLBI sessions since 1984 with the Vienna VLBI Software (VieVS) and estimate the parameters of interest directly from the measurements in a global solution. These parameters are, e.g., the SSB acceleration vector towards the galactic centre, the mass of the galactic centre, dipole and quadrupole parameters described by spherical harmonics in source positions, or the post-Newtonian parameter gamma in the model of the gravitational deflection of radio waves. We will support the estimation with sophisticated simulations of those astronomical effects on all observed quantities, like Earth orientation parameters or station coordinates. Furthermore, dedicated observation schedules allowing an investigation of the gravitational delay due to the black hole in the galactic centre or due to planet occultations will be created. The findings of the project Galactic VLBI will help to refine the theoretical VLBI model and conceptions about our Galaxy, and to improve constraints on cosmologic models.