The Australian mixed-mode observing program.

Global geodetic VLBI is upgrading to its next-generation observing system, VGOS. This upgrade has turned out to be a process over multiple years, until VGOS reaches its full capabilities with the envisaged continuous observations. Until then, for the Australian stations, the upgrade means ceasing their legacy S/X observations, leaving a large gap in the global network as well as in the station time series. The Australian mixed-mode observing program is a series of sessions where the VGOS stations in Hobart and Katherine observe legacy S/X VLBI together with other stations in the region. This paper describes the technical details of these observations and their processing strategies and discusses their suitability for geodetic results by comparison with those of standard legacy S/X sessions. The presented mixed-mode sessions allow a continuation of the station time series, a benefit for the stations themselves as well as for future realisations of the terrestrial and celestial reference frames. A novel mode of observing is introduced and tested. The results are promising and it is suggested for acceptance into standard legacy S/X IVS observations, overcoming current gaps in the network due to VGOS upgrades and preventing a worsening of global results otherwise.

Probing a southern hemisphere VLBI Intensive baseline configuration for UT1 determination.

The deviation of Universal Time from atomic time, expressed as UT1-UTC, reflects the irregularities of the Earth rotation speed and is key to precise geodetic applications which depend on the transformation between celestial and terrestrial reference frames. A rapidly varying quantity such as UT1-UTC demands observation scenarios enabling fast delivery of good results. These criteria are currently met only by the Very Long Baseline Interferometry (VLBI) Intensive sessions. Due to stringent requirements of a fast UT1-UTC turnaround, the observations are limited to a few baselines and a duration of one hour. Hence, the estimation of UT1-UTC from Intensives is liable to constraints and prone to errors introduced by inaccurate a priori information. One aspect in this context is that the regularly operated Intensive VLBI sessions organised by the International VLBI Service for Geodesy and Astrometry solely use stations in the northern hemisphere. Any potential systematic errors due to this northern hemisphere dominated geometry are so far unknown. Besides the general need for stimulating global geodetic measurements with southern observatories, this served as a powerful motivation to launch the SI (Southern Intensive) program in 2020. The SI sessions are observed using three VLBI antennas in the southern hemisphere: Ht (South Africa), Hb (Tasmania) and Yg (Western Australia). On the basis of UT1-UTC results from 53 sessions observed throughout 2020 and 2021, we demonstrate the competitiveness of the SI with routinely operated Intensive sessions in terms of operations and UT1-UTC accuracy. The UT1-UTC values of the SI reach an average agreement of 32 µs in terms of weighted standard deviation when compared with the conventional Intensives results of five independent analysis centers and of 27 µs compared with the 14C04 series. The mean scatter of all solutions of the considered northern hemisphere Intensives with respect to C04 is at a comparable level of 29 µs. The quality of the results is only slightly degraded if just the baseline HtHb is evaluated. In combination with the e-transfer capabilities from Ht to Hb, this facilitates continuation of the SI by ensuring rapid service UT1-UTC provision.