Challenges for geodetic VLBI in the southern hemisphere

et al. 2016

J Geod

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 nextgeneration 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

Section: Processingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The AUSTRAL VLBI observing program

Plank

Lovell

et al. 2016

J Geod

“…In terms of nominal source positions, which are strongly correlated with the number of observations and sessions in which a source is observed, southern sources are much less precise than northern ones (e.g. Plank et al 2015, Figures 7 and 8).…”

Section: Introductionmentioning

confidence: 99%

Star Scheduling Mode—A New Observing Strategy for Monitoring Weak Southern Radio Sources with the AuScope VLBI Array

Publ. Astron. Soc. Aust.

Mayer

Bail³

et al. 2017

Self Cite

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.

“…These are realistic values for the current global VLBI results using one year of data (e.g. Plank et al 2015). We then run three different simulations: (1) source structureonly, where the observed minus computed (O − C) values entering the least-squares adjustment were only the delays due to source structure (O − C = τ ss ); (2) full simulations without source structure effects, accounting for the standard geodetic stochastic error sources (O − C = τ trop + τ clk + τ wn ); (3) full simulations (including source structure effects; O − C = τ trop + τ clk + τ wn + τ ss ).…”

Section: Simulation and Processingmentioning

confidence: 99%

“…These R-sessions are undertaken twice a week for 24 h, using a global antenna network of 7-11 stations that can change from one session to the next (e.g. Plank et al 2015). Because of their regularity and timeliness in correlation and analysis, these sessions constitute the majority of the IVS data and heavily contribute to its main products: the terrestrial reference frame, the Earth orientation parameters and the CRF.…”

Section: S I M U L At I O N M E T H O Dmentioning

confidence: 99%

On the estimation of a celestial reference frame in the presence of source structure

Plank

Shabala

Mon. Not. R. Astron. Soc.

et al. 2015

The spatial structure of sources making up the celestial reference frame (CRF) at radio frequencies is a systematic error source in very long baseline interferometry (VLBI) measurements. Using simulations, we investigate the effects of source structure on the CRF, determined by the actual observational programme of the International VLBI Service for Geodesy and Astrometry. This is done using the source structure simulator of the Vienna VLBI Software. Applying various mock two-component source models, systematic displacements of 10-80 μas in median source position offsets are found. These offsets are predominantly aligned with the direction of the jet. The simulations further show that slight changes in the source model can significantly change the estimated positions. We finally present a new parametrization of source positions in the analysis, namely along the jet direction and perpendicular to it, allowing us to significantly mitigate the effects of source structure on an estimated CRF.