Observations of radio sources near the Sun

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:1806.1129

Energy targeting and minimum energy distillation column sequences

The determination of distillation column configurations that consume the least total energy is studied. The novel contributions of the proposed design methodology for finding global minimum energy column sequences presented in this article include: (1) the definition of a total stripping line distance function for any sequence, (2) a robust energy targeting strategy that provides a continuously differentiable description of column sequences, (3) the flexibility to use any phase equilibrium model, (4) the ability to find column sequences that contain non-pinched, minimum energy columns within a sequence, and (5) the ability to include heat integration. The proposed energy targeting approach, which is used in conjunction with the two-level design methodology of Amale & Lucia (2008b), is shown to be a reliable and effective tool for finding minimum energy distillation column sequences. A number of example problems are presented to show the efficacy of the proposed design methodology. © 2009 Elsevier Ltd. All rights reserved

Observing APOD with the AuScope VLBI Array

The possibility to observe satellites with the geodetic Very Long Baseline Interferometry (VLBI) technique is vividly discussed in the geodetic community, particularly with regard to future co-location satellite missions. The Chinese APOD-A nano satellite can be considered as a first prototype—suitable for practical observation tests—combining the techniques Satellite Laser Ranging (SLR), Global Navigation Satellite Systems (GNSS) and VLBI on a single platform in a Low Earth Orbit (LEO). Unfortunately, it has hardly been observed by VLBI, so major studies towards actual frame ties could not be performed. The main reason for the lack of observations was that VLBI observations of satellites are non-standard, and suitable observing strategies were not in place for this mission. This work now presents the first serious attempt to observe the satellite with a VLBI network over multiple passes. We introduce a series of experiments with the AuScope geodetic VLBI array which were carried out in November 2016, and describe all steps integrated in the established process chain: the experiment design and observation planning, the antenna tracking and control scheme, correlation and derivation of baseline-delays, and the data analysis yielding delay residuals on the level of 10 ns. The developed procedure chain can now serve as reference for future experiments, hopefully enabling the global VLBI network to be prepared for the next co-location satellite mission

Characteristics and results of two years of a VLBI southern hemisphere intensive observing program

The parameter dUT1 (UT1-UTC, difference of universal time to atomic time) is essential for the transformation between celestial and terrestrial reference systems, inherent in precise navigation and positioning applications. Geodetic Very Long Baseline Interferometry (VLBI) is the only technique to directly observe dUT1. Real-time or near-real-time navigation tasks are dependent on rapid access to Earth orientation estimates or predictions. On a rapid turnaround basis, dUT1 is provided via so-called intensive sessions, which are routinely observed daily for one hour on one or sometimes more baselines. All currently operational intensive sessions are observed using northern hemisphere stations only. In a joint initiative of TU Wien, the University of Tasmania, the Hartebeesthoek Radio Astronomy Observatory, and later on also ETH Zurich, we set up the southern hemisphere intensive observing program (SI). The SI sessions are observed with three VLBI telescopes all located south of the equator: HART15M (South Africa), HOBART12 (Tasmania), and YARRA12M (Western Australia). Observations including HOBART12 are observed in mixed-mode configuration, using the VGOS receiver in Hobart and the legacy systems at the two other stations. By January 2022, we have successfully observed, correlated, and analyzed more than 50 SI sessions from the years 2020 and 2021. The resulting dUT1 values from the southern intensives are compared with dUT1 from the EOP 14 C04 series and with the results of other "northern intensives". The residuals with respect to C04 of the SI are on the same level as those of the INT1 and INT3 sessions and also match the level of agreement between all the various southern and northern intensives series

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.ISSN:1343-8832ISSN:1880-598

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.ISSN:0949-7714ISSN:1432-139