The improvement of the local ties between different observation methods (GPS, VLBI, etc.) improves the quality of the ITRF considerably. The IVS reference point of a VLBI radio telescope is defined as the intersection between the azimuth- and elevation-axis or, if they do not intersect, the intersection of the right-angle projection from the elevation-axis onto the azimuth-axis. In the past, these axes have been estimated by fitting 3D circles, e.g. (Eschelbach et al., 2003) or (Dawson et al., 2006). The data acquisition for the determination of the circles requires that the telescope has to be moved into clearly defined positions; therefore, the basic station process (data gathering for the intrinsic telescope task) is disturbed. In this paper we present an alternative mathematical model, which computes the reference point without circle fitting. This algorithm does not need observations from predefined telescope positions and therefore the station's downtime can be reduced. The parameter estimation of this non-linear problem is implemented in two steps. At first we are using the Levenberg-Marquardt-Algorithm for a pre-evaluation to find stable approximate values (Madsen et al., 2004), which we use for the main least-square-model in a second step.