Pointing-dependent Variation of a Radio Telescope Reference Point – First Investigation at the Onsala Space Observatory

Journal article, 2025

The International Terrestrial Reference Frame (ITRF) is a highly accurate realisation of the International Terrestrial Reference System (ITRS), an Earth-fixed global geodetic reference system. It forms the backbone for almost all scientific investigations of the dynamic system Earth and is realised through the combination of several space-geodetic techniques. The positions of geometric reference points of these space-geodetic techniques, such as radio telescopes used for very long baseline interferometry (VLBI), are the physical realisation of the frame on Earth. The precision and the reliability of such a reference frame depend on the spatio-temporal stability of the involved reference points defining the datum. In this contribution, the reference point stability of a modern VGOS radio telescope at the Onsala Space Observatory is investigated. The aim is to quantify the magnitude of changes, especially when the radio telescope rotates about the azimuth and elevation axes, and to identify a suitable location for permanent monitoring instrumentation to monitor the spatio-temporal variation of the reference point position. Based on measurements with a highly precise laser tracker, horizontal and vertical variations were detected. The horizontal variations of approximately ±250 μm indicate a slightly asymmetrical arrangement of the telescope components. The vertical variations of about ±15 μm are quite small and indicate minor deviations in the construction work of the radio telescope.