Characterization of the Beam and the Aperture of the Onsala Twin Telescopes

Very Long Baseline Interferometry (VLBI) was originally developed to perform high angular resolution radio imaging, but it has also proven useful in geodesy. VLBI has played a key role in defining the International Celestial Reference Frame (ICRF), providing measurements of the Earth Orientation Parameters (EOP), and contributing to the International Terrestrial Reference Frame (ITRF) [1]. The legacy S/X VLBI systems have achieved positional accuracies of up to 5 mm.