From 2014 July until 2025 January, Gaia was scanning the sky at optical wavelength with high precision. With that the mission effectively extended its nominal life time of five years by far. This astrometric satellite mission of ESA (European Space Agency) has already set a remarkable record. The second data release (DR2) of 2018 April, for example, contains about 1.7 billion objects recorded with unmatched precision. The final results of this mission are expected to be made available towards the end of 2030 and will become a game changer for astrometry and observational astronomy. Besides many scientific achievements, the Gaia data products will allow for better technical applications, such as the more precise attitude control and the quicker convergence of the attitude determination of satellites with respect to the star background.

From the point of view of astrometry, the Gaia data products present intrinsically consistent solutions. Positions of astronomical objects, however, are known only relatively to each other very well. The absolute orientation of the data products remains unknown. The trajectory of the Gaia satellite is a Lissajous-orbit around the Sun-Earth Lagrange point 2. This Lagrange point is a gravitationally stable point within the multi-body gravitational field of Sun and Earth. It lies about 1.5 million kilometers away from Earth on the far side of Sun und travels evenly with Earth around Sun. Satellite operation at this place allows for the thermal conditions to be stable, while energy supply is given through the Lissajous-orbit. The orbit ensures that Gaia is not resting permanently at the umbra shadow of Earth. From this position, however, there is no way to establish a direct link to the Earth surface. One precise option to align Gaia data products with the Earth surface is by observing optical counterparts of extremely distant active galaxies, that are part of International Celestial Reference Frame (ICRF). By means of the space geodetic technique VLBI (Very Long Baseline Interferometry), the ICRF can be oriented with very high precision with respect to the Earth surface. Mounted on the Earth surface, the VLBI antennas observe the same extremely distant galaxies in radio wavelengths and provide the so-called Earth Orientation Parameters (EOP) with utmost absolute accuracy. Taking over the absolute orientation established by VLBI data analysis for the Gaia data products is thus a necessary step to enable the application of Gaia data products from ground or within near Earth space.

Our project AGORA supports the highly precise alignment of the Gaia data products through VLBI data analysis by delivering a state-of-the-art and consistent realization of ICRF based on VLBI data of IVS (International VLBI Service for Geodesy and Astrometry) during the entire duration of the satellite mission. To achieve this goal IVS and data obtained by partner organizations are utilized. One scientific objective of the project is the combination of the various types of VLBI observations into a consistent solution.

Many active galactic nuclei (AGN) and their optical counterparts, typically the core of the mother galaxy or one or several bright components of the jet, are included in the Gaia data products as well as in ICRF. Hence, these objects can be used as common points for the alignment in an optimal way. Optical counterparts of AGN, however, are extremely faint and hence, this approach works only for the faint fraction of the Gaia data products. For this reason, we plan to conduct observations of radio stars in project AGORA in order to align the bright fraction of Gaia data products as well. Radio stars can be relatively bright in optical wavelengths; in radio wavelengths, however, they are typically extremely faint. The observations of radio stars are thus carried out with a special VLBI observing technique called phase referencing.

Upon completion of project AGORA, transformation parameters, such as spin and spin rate, will be available for Gaia enabling the precise orientation of Gaia data products. Ground-based optical observations and other applications based on Gaia data products will be possible with absolute orientation and significantly improved precision.

Dhar, S., Heinkelmann, R., Beyerle, G., & Schuh, H. (2025). A VGOS CRF fully consistent with the S/X frames preliminary results. 27th European VLBI group for Geodesy and Astrometry (EVGA) Meeting 2025, (EVGA2025), Matera, Italy. Zenodo. https://doi.org/10.5281/zenodo.15236608

Lunz, S., Anderson, J.M., Xu, M., Heinkelmann, R., Titov, O., Lestrade, J.-F., Johnson, M.C., Shu, F., Chen, W., Melnikov, A., Mikhailov, A., McCallum, J., Lopez, Y., de Vicente Abad, P., Schuh, H. (2024). The impact of improved estimates of radio star astrometric models on the alignment of the Gaia bright reference frame to ICRF3. Astronomy & Astrophysics, 689, A134, https://doi.org/10.1051/0004-6361/202142081

Lunz, S., Anderson, J. M., Xu, M., Titov, O., Heinkelmann, R., Johnson, M.C., Schuh, H. (2023). Enhancing the alignment of the optically bright Gaia reference frame with respect to the International Celestial Reference System. Astronomy & Astrophysics, 676, A11, 23 pp.,  https://doi.org/10.1051/0004-6361/202040266