D. Mayer

The AUSTRAL VLBI observing program

The AUSTRAL observing program was started in 2011, performing geodetic and astrometric very long baseline interferometry (VLBI) sessions using the new Australian AuScope VLBI antennas at Hobart, Katherine, and Yarragadee, with contribution from the Warkworth (New Zealand) 12 m and Hartebeesthoek (South Africa) 15 m antennas to make a southern hemisphere array of telescopes with similar design and capability. Designed in the style of the next-generation VLBI system, these small and fast antennas allow for a new way of observing, comprising higher data rates and more observations than the standard observing sessions coordinated by the International VLBI Service for Geodesy and Astrometry (IVS). In this contribution, the continuous development of the AUSTRAL sessions is described, leading to an improvement of the results in terms of baseline length repeatabilities by a factor of two since the start of this program. The focus is on the scheduling strategy and increased number of observations, aspects of automated operation, and data logistics, as well as results of the 151 AUSTRAL sessions performed so far. The high number of the AUSTRAL sessions makes them an important contributor to VLBI end-products, such as the terrestrial and celestial reference frames and Earth orientation parameters. We compare AUSTRAL results with other IVS sessions and discuss their suitability for the determination of baselines, station coordinates, source coordinates, and Earth orientation parameters.

Results from the regional AUSTRAL VLBI sessions for Southern Hemisphere reference frames

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.

Comparison of Site Velocities Derived from Collocated GPS, VLBI and SLR Techniques at The Hartebeesthoek Radio Astronomy Observatory (Comparison of Site Velocities)

Abstract Space geodetic techniques provide highly accurate methods for estimating bedrock stability at subcentimetre level. We utilize data derived from Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI) and Global Positioning Systems (GPS) techniques, collocated at the Hartebeesthoek Radio Astronomy Observatory, to characterise local plate motion and compare the solutions from the three techniques. Data from the GNSS station were processed using the GAMIT/GLOBK (version 10.4) software, data from the SLR station (MOBLAS-6)were processed using the Satellite Laser Ranging Data Analysis Software (SDAS) and the VLBI data sets were processed using the Vienna VLBI Software (VieVS) software. Results show that there is a good agreement between horizontal and vertical velocity components with a maximum deviation of 1.7 mm/yr, 0.7 mm/yr and 1.3 mm/yr between the North, East and Up velocity components respectively for the different techniques. At HartRAO there is no significant trend in the vertical component and all the techniques used are consistent with the a-priori velocities when compared with each other. This information is crucial in monitoring the local motion variations since geodetic instruments require a very stable base to minimise measurement errors. These findings demonstrate that station coordinate time-series derived with different techniques and analysis strategies provide comparable results.

Testing general relativity with geodetic VLBI: What a single, specially designed experiment can teach us

Context: We highlight the capabilities of 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 γ was estimated by global analyses of thousands of geodetic VLBI sessions. Here we estimate γ 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◦. 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. Results: We obtained γ with a greater precision (0.9 × 10−4) than has been 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 tests of general relativity.

Analysis Strategies for the Densification of the ICRF with VLBA Calibrator Survey Sources

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”.