The observations of geodetic Very Long Baseline Interferometry (VLBI) measurements refer to an invariant point of the radio telescope. This point is often called the reference point. For the interpretation of VLBI results it is necessary to know the position of this invariant point and its possible variation with time. Only the knowledge of the reference point makes it possible to combine VLBI results with results from other co-located techniques e.g. for the International Terrestrial Reference Frame (ITRF) (Ray and Altamimi, 2005) and in the framework of the Global Geodetic Observing System (GGOS). Within the GGOS an automated monitoring of the reference points and therefore of the local-tie vectors at co-location stations is discussed (Plag and Pearlman, 2008). In this paper a new mathematical model is described to allow such an automated monitoring of the reference point of an azimuth-elevation type radio telescope.