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The current realization of the International Celestial Reference Frame at 8.4 GHz, the ICRF3-SX, is computed from Very Long Baseline Interferometry measurements starting in 1979 until March 2018. The concentration of the majority of VLBI telescopes in the Northern Hemisphere reflects itself in the unequal distribution of observations to radio sources over declination which causes the ICRF3-SX to be weaker in the south. One of the current VLBI observing programs active in the Southern Hemisphere is the Australian mixed-mode program (AUM) which started to be organized in July 2018. In this presentation, we show the benefits of the AUM for the celestial reference frame and also discuss its current limitations.

In principle, we can use geodetic Very Long Baseline Interferometry (VLBI) measurements to calculate all types of Earth orientation parameters (EOP), namely celestial pole offsets, polar motion, and UT1-UTC. However, the sensitivity toward the single EOP and the resulting accuracy strongly depends on the network extension, the session duration, and the recording rate. We can expect high-quality estimates from sessions with a well-distributed observation network designed for EOP determination, such as the R1 and R4 sessions. On the other hand, it takes quite a long time from the actual observation to the delivery of the analysis results. The 24-h sessions observed within the Australian mixed-mode program (AUA/AUM) do not provide a globally extended network of stations. Still, they involve the future potential to deliver results with a short latency. Under this aspect, we investigate the possibilities to determine different sets of or single EOP from the AUA and AUM sessions observed since 2020. The networks usually comprise the Australian stations (Ho, Hb, Ke, Yg) and the telescopes in South Africa (Ht) and New Zealand (Ww). Due to the spatial limitation of that constellation, the estimation of the complete set of EOP is not always favorable. Therefore, different analysis schemes are tested, including treating the 24-h sessions similarly to Intensive sessions. A subset of the telescopes (Hb, Yg, Ht) have been or are involved in observing the so-called Southern Intensive sessions since 2020. In addition to the results of the 24-h sessions, we present the UT1-UTC estimates derived from the latest Southern Intensives (2022, 2023).

VLBI satellite tracking is a popular topic in geodesy at the moment. Attributed with the potential to solve the pending problem of inter-technique frame ties, the prospect of success provides the impetus of ongoing research in that area. Very Long Baseline Interferometry (VLBI) is a well-probed space geodetic technique used to determine the Celestial Reference Frame (CRF), the Terrestrial Reference Frame (TRF) and the Earth Orientation Parameters (EOP) in between. Alternatively, VLBI is generally used in spacecraft tracking. Technology in that area has been rapidly advancing in the last years. An overview of present realizations of VLBI spacecraft tracking, including data processing on the example of the Japanese lunar mission SELENE is part of this work. Today’s most precise and reliable realizations of the TRF rely on the measurements of several space geodetic techniques and the corresponding inter-technique ties. For future improvement, and also for a rigorous determination of the whole system of CRF-EOP-TRF, alternative methods for connecting the various space geodetic techniques, establishing precise frame ties, are urgently needed. A promising solution is the use of VLBI satellite observations, either in combination with a so-called space tie realized by a dedicated satellite or by directly observing satellites of the Global Navigation Satellite Systems (GNSS) with VLBI. The study of successful realizations of VLBI tracking and identifying proper applications for practical geodetic value summarizes this work. With the goal to process real data, the Vienna VLBI Software VieVS is extended for the possibilities to schedule, simulate and analyze VLBI satellite observations. Some details on the corresponding delay modeling are provided. The technique of VLBI satellite tracking is introduced, discussing some practical issues as well as the latest developments. This thesis contains a detailed simulation study of VLBI observations to satellites, identifying adequate observing strategies for the precise determination of antenna coordinates on Earth in the satellite’s system. For satellites at heights between 1000 and 20000 km, adequate observation strategies are found that allow the determination of the station coordinates at the level of a few millimeters. Therefore, the approach of weekly solutions in chosen, meaning that one satellite is observed by either a regional or a global antenna network during seven consecutive days. For the investigated satellites at 2000 and 6000 km, a feasible observing interval of 1 minute is found. Assuming turbulent tropospheric conditions, in regional networks of 6-7 stations with baseline lengths between 2000 and 3000 km, weekly 3D position rms between 3 and 14 mm can be expected, depending on the orbit, respectively the height, of the observed satellite, as well as on the changing geometry between the observing baselines and the target satellite. For global networks, a considerable high number of observing telescopes is needed, about 16-32, with expected accuracies at the same level as in regional networks for a higher satellite at 6000 km height and about a factor of two worse for a very low satellite at 2000 km. In the case of VLBI observations to a satellite of the GNSS, alternative observing strategies are needed. In this thesis the combination with a classical VLBI session observing extragalactic radio sources or the observation of a satellite constellation are introduced. The careful assessment of the presented results reveals valuable application of such observations in the area of frame ties, strongly supporting immediate realization and ongoing research on the topic of VLBI satellite observations.

As a combined product of four space geodetic techniques, the International Terrestrial Reference Frame (ITRF) substantially relies on the ties between the various techniques. The tie vectors, however, usually determined via classical surveying at co-location sites, differ from space geodesy solutions by up to a few centimeters. Observing satellites with the Very Long Baseline Interferometry (VLBI) technique is a topic of increasing interest for the establishment of space ties. VLBI observations to the upcoming GENESIS mission dedicated to realize a space tie would directly link the techniques and strengthens the connection in the combination to the ITRF. We present a study of practical considerations for VLBI satellite tracking. With an expected launch date of 2027, we focus our investigations on the efforts needed in the VLBI community to successfully support the GENESIS satellite mission. Current VLBI data formats and telescope hardware are not designed for satellite tracking. Therefore, this work examines the practicality of current VLBI standards for the use of satellite VLBI. We investigate aspects of the process chain from different satellite tracking approaches with the Australian VLBI network to correlation of the signals. Furthermore, we analyze visibilities of the GENESIS satellite mission assuming a realistic VLBI network., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Geodetic Very Long Baseline Interferometry (VLBI) data are capable of measuring the light deflection caused by the gravitational field of the Sun and large planets with high accuracy. The parameter $\gamma$ of the parametrized Post-Newtonian (PPN) formalism estimated using observations of reference radio sources near the Sun should be equal to unity in the general relativity. We have run several VLBI experiments tracking reference radio sources from 1 to 3 degrees from the Sun. The best formal accuracy of the parameter $\gamma$ achieved in the single-session mode is less than 0.01 percent, or better than the formal accuracy obtained with a global solution included all available observations at arbitrary elongation from the Sun. We are planning more experiments starting from 2020 using better observing conditions near the minimum of the Solar activity cycle., Comment: Proceeding of the EVGA 2019 Meeting. arXiv admin note: substantial text overlap with arXiv:1806.11299

Context. We highlight the capabilities of the geodetic VLBI technique to test General relativity in the classical astrometric style, i.e., measuring the deflection of light in the vicinity of the Sun. Aims. In previous studies, the parameter was estimated by global analyses of thousands of geodetic VLBI sessions. Here we estimate from a single session where the Sun has approached two strong reference radio sources 0229+131 and 0235+164 at an elongation angle of 1-3 degrees. Methods. The AUA020 VLBI session of 1 May 2017 was designed to obtain more than 1000 group delays from the two radio sources. The Solar corona effect was effectively calibrated with the dual-frequency observations even at small elongation from the Sun. Results. We obtained with a precision better than what is obtained through global analyses of thousands of standard geodetic sessions over decades. Current results demonstrate that the modern VLBI technology is capable of establishing new limits on observational test of General Relativity., Comment: Accepted for publication in Astronomy and Astrophysics