Earth-based VLBI in the Galactic Frame

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.

The project "Earth-based VLBI in the Galactic Frame" covered research for a better estimation of extragalactic radio source positions. Very Long Baseline Interferometry (VLBI) is a space-geodetic technique that directly connects the terrestrial reference frame realized by positions of Earth-based antennas with the celestial reference frame defined by a set of radio sources. In 2018, the third all-sky realization of an extragalactic frame at radio frequencies, designated as ICRF3, was published by the ICRF3 working group established by the International Astronomical Union. The importance of a precise celestial reference frame is in providing a stable grid to navigate space missions, to monitor Earth orientation in space with extreme accuracy, or to study the plate tectonics on the Earth, and thus it allows a better understanding of the natural hazards from, e.g., earthquakes. This project supported the realization of the ICRF3 in its both novel features: 1) modeling of the Galactic acceleration in the VLBI data analysis, and 2) determination of source positions in several radio bands. In this project, a new method to detect the Galactic acceleration from VLBI measurements was developed. This method is based on fitting the Earth scale factor corrections estimated for each individual radio source within a global VLBI adjustment. These findings carried out within a working group on Galactic Aberration of the International VLBI Service for Geodesy and Astrometry served as a basis for our recommendations to the ICRF3 working group. ICRF3 includes for the first time position of extragalactic sources at several radio frequencies. In addition to the classic 8.4 GHz, positions of several hundreds of sources are reported at 24 GHz and 32 GHz. The reason for including more radio frequencies is the different impact of the source structure on the apparent source position, or the fact that for applications lacking dual-frequency observations for ionosphere calibrations, moving to higher frequencies quickly reduces charged particle effects. In this project, analyses of VLBI measurements at the newly established 24 GHz frequency were carried out providing a basis for the inclusion of the data in the arising celestial frame. Furthermore, in this project, research of astronomical VLBI data at 15 GHz from a so-called MOJAVE program was carried out. It has been shown that this astronomical VLBI program with its differences to a geodetic one is feasible for studying source structure contribution to the observed path delay relevantly for geodetic applications. The findings of this research will contribute to a further refinement of the next celestial reference frames.