As the Central Bureau for the Asia Pacific Reference Frame (APREF), Geoscience Australia were keen to transition to the most up-to-date realisation of a trusted global reference frame from the IGS, being IGS20. However, following adoption of ITRF2020/IGS20 there were apparent site-specific, centimetre-level coordinate inconsistencies between ITRF2020/IGS20 and ITRF2014/IGb14, concerningly presenting as an inconsistent height offset across the APREF network. The Asia-Pacific Reference Frame (APREF) is a network consisting of more than 1000 stations across the Asia-Pacific region, including ~700 Australian stations as well as global IGS core stations. For our routine analysis of the network, we process GPS-only double-difference observations in network mode and align them to the global reference frame of choice, using the Bernese software. We process daily solutions, and then stack them to generate weekly solutions. We then take these coordinates and apply the Australian Plate Motion Model to acquire GDA2020 coordinates, which is a plate-fixed national datum used widely across Australia, including to calculate the legally traceable coordinates we provide to station owners and operators. Taking the latest available products (satellite clock and orbit files, and antenna models) from the IGS, APREF (weekly) solutions are aligned to the IGS20 reference system (where IGS20 is the IGS realisation of ITRF2020), however, we found station-specific offsets of our APREF solutions between the ITRF2014/IGb14 and ITRF2020/IG2S0 due to updates in the ground antenna calibration values from igs14.atx to igs20.atx that reached up to 3 cm. Some of the antenna calibration values were updated post-release of ITRF2020 (updates between igsR3.atx and igs20.atx), which results in further inconsistencies at the coordinate level between the APREF solutions and ITRF2020 solutions. This presentation will discuss the challenges faced when implementing a new global frame of reference at the regional level and the impact on downstream users. We commend the IGS for their efforts in providing high quality, openly available products and services and would like to prompt conversation about the consideration of user requirements for the development of downstream products (such as regional reference frames). Abstract to be submitted to/presented at the American Geophysical Union (AGU) Fall Meeting 2023 (AGU23) - https://www.agu.org/fall-meeting

The Gaia optical astrometric mission has measured the precise positions of millions of objects in the sky, including extragalactic sources also observed by Very Long Baseline Interferometry (VLBI). In the recent Gaia EDR3 release, an effect of negative parallax with a magnitude of approximately −17 μas was reported, presumably due to technical reasons related to the relativistic delay model. A recent analysis of a 30-yr set of geodetic VLBI data (1993–2023) revealed a similar negative parallax with an amplitude of −15.8 ± 0.5 μas. Since both astrometric techniques, optical and radio, provide consistent estimates of this negative parallax, it is necessary to investigate the potential origin of this effect. We developed the extended group relativistic delay model to incorporate the additional parallactic effect for radio sources at distances less than 1 Mpc and found that the apparent annual signal might appear due the non-orthogonality of the fundamental axes, which are defined by the positions of the reference radio sources themselves. Unlike the conventional parallactic ellipse, the apparent annual effect in this case appears as a circular motion for all objects independently of their ecliptic latitude. The measured amplitude of this circular effect is within a range of 10–15 μas that is consistent with the ICRF3 stability of the fundamental axis. This annual circular effect could also arise if a Gödel-type cosmological metric were applied, suggesting that, in the future, this phenomenon could be used to indicate global cosmic rotation. <b>Citation:</b> Titov O, Osetrova A. Parallactic delay for geodetic VLBI and non-orthogonality of the fundamental axes. <i>Publications of the Astronomical Society of Australia</i>. 2024;41:e111. doi:10.1017/pasa.2024.111