The next realization of the International Terrestrial Reference System, the ITRF2020, is planned to be released in 2021. Our joint VLBI Analysis Center VIE which runs between TU Wien and BEV is one of eleven IVS (International VLBI Service for Geodesy and Astrometry) analysis centres which provide VLBI input to the ITRF2020. The SINEX files submitted to the IVS Combination Center are produced with the Vienna VLBI and Satellite Software VieVS and contain unconstrained normal equation systems for station position, source coordinates and Earth orientation parameters. In this presentation, we document the included sessions and stations in our submission and introduce the Vienna terrestrial reference frame based on our contribution to the ITRF2020. In particular, we highlight special settings in the Vienna solution and assess the impact on the terrestrial reference frame.

The next International Terrestrial Reference Frame (ITRF), ITRF2020, will be released in early 2021 and preparations are entering the final phase. It will be realized by using the observations of the space geodetic techniques Very Long Baseline Interferometry (VLBI), Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). The Vienna VLBI group is planning to contribute to the ITRF2020. This poster will present the goals, the methodology and preparations for our contribution. In order to analyse the VLBI observation sessions, the Vienna VLBI and Satellite Software (VieVS) will be used. The poster will focus on the influence of applying the gravitational deformation and the atmospheric loading on the individual solutions and the ITRF. For this purpose, a selected list of more than 800 sessions of the last 40 years, which was released by the International VLBI Service for Geodesy and Astrometry (IVS) to verify the latest changes in the implementation, will be analysed and the results will be presented.

We are using various models and analysis strategies, such as galactic aberration, ray-tracing etc., to create different Vienna celestial reference frame (CRF) solutions. These solutions are then compared against the Gaia reference frame (Gaia-CRF2). This is done using a degree 2 vector spherical harmonics approach. The estimated parameters are used to investigate the impact of the various analysis methods on the differences between Gaia and the Very Long Baseline Interferometry (VLBI) CRF. We find that correcting for galactic aberration reduces the difference between the Gaia-CRF2 and the VLBI CRF significantly (30 μ as in D2 and 13 μ as in D3). Furthermore, we find that using a priori ray-traced tropospheric delays in addition with low absolute constraints on tropospheric gradients reduces the a_20^e parameter by 20 μ as. Using these analysis strategies we can explain almost all significant differences between the Gaia-CRF2 and the VLBI CRF. However, the vector spherical harmonic (VSH) parameter a_20^e is still highly significant and can not be explained by modeling and analysis choices from the VLBI technique.

Geodetic VLBI correlation poses a new challenge in the current VLBI activities at the research area Higher Geodesy at Technische Universität Wien (TU Wien). We are using the Distributed FX (DiFX) software correlator and the Haystack Observatory Postprocessing System (HOPS) on the Vienna Scientific Cluster 3 (VSC-3), which is a supercomputer located at TU Wien. We provide more technical details about the VSC-3 and information about activities related to correlation at TU Wien. Furthermore, we present tools to directly access the correlation and fringe-fitting output database with the Vienna VLBI and Satellite Software (VieVS) using the vgosDb format and we discuss postcorrelation processing aspects in VieVS based on currently correlated experiments.

We compare two Celestial Reference Frame (CRF) solutions made from Very Long Baseline Interferometry (VLBI) group delay observations in S/X band using vector spherical harmonics. In both solutions the same data set was used which consists of almost all observations since 1979 until the beginning of 2018. The same parameterization and models were used with the exception that in one of the solutions the effect of galactic aberration (GA) was corrected. The other solution serves as a reference. We show that the deformation of a CRF estimated with the whole set of VLBI observations can be described by a systematic dipole displacement with an amplitude of about 35 μas.

Six campaigns with a total of twenty-four Very Long Baseline Array Calibrator Survey (VCS) observing sessions were carried out with ten radio telescopes located on U.S. territory from 1994 to 2007. The aim of those astrometric sessions was to estimate source positions and to make snapshot images of compact radio sources. Coordinates of about two thirds of the sources in the ICRF2 catalogue are estimated from VCS sessions, most of them from two scans in one session only. Moreover, there are systematic errors due to the deficiencies of a continent-wide network for the estimation of Earth orientation parameters (EOP) and the linking between the celestial and terrestrial frame. We investigate the impact of EOP estimation on source positions for those sessions and we use polar motion estimates from the analysis of Global Navigation Satellite Systems (GNSS) observations to strengthen the solution. We find that there is a systematic effect up to 1 mas in the estimated source coordinates between a solution with fixed EOP coming from the GNSS techniques and a solution where the EOP are estimated in the Very Long Baseline Interferometry analysis. Furthermore, we discuss analysis strategies for these sessions including the proper use of datum or “transfer sources”.

(1) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, United States (bsoja@jpl.nasa.gov), (2) Geoscience Australia, Canberra, Australia, (3) Technische Universität Wien, Vienna, Austria, (4) University of Tasmania, Hobart, Australia, (5) Hartebeesthoek Radio Astronomy Observatory, Krugersdorp, South Africa, (6) Shanghai Astronomical Observatory, Shanghai, China, (7) Institute of Applied Astronomy, Saint Petersburg, Russia, (8) National Geographic Information Institute, Space Geodetic Observatory, Sejong, South Korea, (9) SYRTE, Observatoire de Paris, Paris, France

The Vienna VLBI and Satellite Software (VieVS) is state-of-the-art Very Long Baseline Interferometry (VLBI) analysis software for geodesy and astrometry. VieVS has been developed at Technische Universität Wien (TU Wien) since 2008, where it is used for research purposes and for teaching space geodetic techniques. In the past decade, it has been successfully applied on Very Long Baseline Interferometry (VLBI) observations for the determination of celestial and terrestrial reference frames as well as for the estimation of celestial pole offsets, universal Time (UT1-UTC), and polar motion based on least-squares adjustment. Furthermore, VieVS is equipped with tools for scheduling and simulating VLBI observations to extragalactic radio sources as well as to satellites and spacecraft, features which proved to be very useful for a variety of applications. VieVS is now available as version 3.0 and we do provide the software to all interested persons and institutions. A wiki with more information about VieVS is available at http://vievswiki.geo.tuwien.ac.at/.

The International Celestial Reference Frame suffers from significantly less observations in the southern hemisphere compared to the northern one. One reason for this is the historically low number of very long baseline interferometry radio telescopes in the south. The AuScope very long baseline interferometry array with three new telescopes on the Australian continent and an identical antenna in New Zealand were built to address this issue. While the overall number of observations in the south has greatly improved since then, a closer look reveals that this improvement is only true for strong radio sources (source flux densities >0.6 Jy). The new array of small very long baseline interferometry antennas has a relatively low baseline sensitivity so that only strong sources can be observed within a short integration time. A new observing strategy, the star scheduling mode, was developed to enable efficient observations of weak sources during geodetic sessions, through the addition of a single more sensitive antenna to the network. This scheduling mode was implemented in the Vienna very long baseline interferometry Software and applied in four 24-h sessions in 2016. These observations provide updated positions and source flux densities for 42 weak southern radio sources and significantly reduce the formal uncertainties for these sources. The star scheduling mode now allows the AuScope very long baseline interferometry array to undertake greater responsibility in monitoring sources in the southern sky, without significantly weakening the session for geodetic purposes.

The African VLBI Network will be a pan-African network of radio telescopes comprised of converted redundant satellite Earth-station antennas and new purpose-built radio telescopes. The first of these antennas, in Ghana, is currently being converted to a radio telescope and current funding is estimated to permit the conversion of two more antennas in Africa. These antennas will initially be equipped with a 5-GHz and 6.7-GHz receiver and the next receiver likely to be fitted is a 1.4–1.7-GHz receiver. While it would be advantageous for the AVN antennas to be able to participate also in geodetic and astrometric VLBI observations, there is no funding currently for this. In this paper we re-visit the scientific justifications for the AVN in an attempt to optimize the AVN for each science case, both astronomical and geodetic.

The current realization of the ICRS, the ICRF2, was published in 2009. New stations were implemented, and the amount of data from the southern hemisphere increased dramatically. The demands on the accuracy of the celestial reference frame are higher than ever, with the GAIA mission providing a catalog in the visible spectrum with comparable accuracy. These advances in VLBI and new demands on accuracy entail the necessity of a new version of the celestial reference frame which will be called ICRF3. We will report on the progress and plans of the Vienna group to estimate such a reference frame. Differences in our solution (especially in declination) compared to other groups are discussed. Furthermore, we examine issues which arise during the estimation process such as a declination bias (a maximal offset of about 150 μas in declination).

With the transition to VGOS, co-located radio telescopes will be common at many sites. This can be as a sibling telescope when a VGOS antenna is built next to a legacy one, or as the concept of a twin telescope with two identical VGOS antennas. The co-location of two antennas offers new possibilities in both operation and analysis. The immediate question for observing with sibling/twin telescopes is the applied observing strategy and its realization in the scheduling software. In this contribution we report about our efforts implementing new scheduling modes for sibling and twin telescopes in the Vienna VLBI Software. For the example of the sibling telescope in Hobart, several types of sessions will be discussed: an improved tag-along mode for the 26-m antenna (Ho), a proper implementation of the twin-mode using the antenna with the shorter slewing time, and an astrometric support mode enabling the observation of weak sources with the AuScope array.

The AUSTRAL observing program is an initiative led by the Australian AuScope VLBI antennas in collaboration with radio telescopes in Warkworth, New Zealand, and Hartebeesthoek, South Africa. In 2014 the number of AUSTRAL sessions increased tremendously. Comparing recent results to the standard products achieved in global VLBI sessions regularly undertaken by the International VLBI Service for Geodesy and Astrometry (IVS), better accuracies in terms of baseline length repeatabilities are found for these regional AUSTRAL sessions. The network of (almost) identical small and fast telescopes as well as the technical equipment at all stations allows for new observing modes and improved operations, as such serving as a testbed for the future VLBI Global Observing System (VGOS). Special AUST-Astro sessions are used for dedicated astrometry of sparsely observed radio sources in the southern sky, as well as for detecting new radio sources for geodesy. In 2015, the AUSTRAL program will be further increased and final steps are now being undertaken for full VGOS compatibility of the three AuScope VLBI antennas. We present the latest results of the AUSTRAL sessions and give an overview of the multiple areas of research they support.