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

Abstract

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.

Vienna VLBI and Satellite Software (VieVS) for Geodesy and Astrometry

“…VieVS is used for scheduling Austral sessions about once a month. It should also be mentioned here that VieVS has special scheduling options like the source-based strategy or the star-mode (McCallum et al 2017), and simulations have demonstrated that the performance of VieVS in terms of scheduling is similar to that of SKED (Sun et al 2014).…”

VieSched++: A New VLBI Scheduling Software for Geodesy and Astrometry

“…Besides this, there is another optional multiplicative factor of the score. It is possible to use a custom sequence of scans to different source groups as used by the STAR scheduling mode (McCallum et al 2017). When using this option, scores from scans to sources to the current target group are multiplied by a factor of 100 otherwise they are divided by a factor of 100.…”

The Australian mixed-mode observing program

“…2017 ; McCallum et al. 2017 ) from the AuScope array do have a positive impact. The intensive observing program with the AuScope VLBI network was interrupted, due to the upgrade to VGOS-compatible systems.…”

“…As emphasised in the latest realisation of the international celestial reference system (ICRS), ICRF3 (Charlot et al 2020), results for the southern hemisphere still clearly lag behind those of the northern sky, though regular as well as special observing campaigns (e.g. Plank et al 2017;McCallum et al 2017) from the AuScope array do have a positive impact. The intensive observing program with the AuScope VLBI network was interrupted, due to the upgrade to VGOS-compatible systems.…”