Noname manuscript No. (will be inserted by the editor) Determination of the local tie vector between the VLBI and 1 GNSS reference points at Onsala using GPS measurements 2 T. Ning · R. Haas · G. Elgered 3 4 the date of receipt and acceptance should be inserted later 5 Abstract Two gimbal-mounted GNSS antennas were installed on each side of the 6 radome-enclosed 20 m VLBI radio telescope at the Onsala Space Observatory. GPS 7 data with a 1 Hz sampling rate were recorded for five semi-kinematic and four kine- 8 matic observing campaigns. These GPS data were analysed together with data from 9 the IGS station ONSA with an in-house Matlab-based GPS software package, us- 10 ing the double-difference analysis strategy. The coordinates of the GNSS antennas 11 on the telescope were estimated for different observation angles of the telescope, at 12 specific epochs, and used to calculate the geodetic reference point of the telescope. 13 The local tie vector between the VLBI and the ONSA GNSS reference points in a 14 geocentric reference frame was hence obtained. The two different types of observing 15 campaigns gave consistent results of the estimated local tie vector and the axis off- 16 set of the telescope. The estimated local tie vector obtained from all nine campaigns 17 gave standard deviations of 1.5 mm, 1.0 mm, and 2.9 mm for the geocentric X, Y, 18 and Z components, respectively. The result of the estimated axis offset of the VLBI 19 telescope shows a difference of 0.3 mm, with a standard deviation of 1.9 mm, with 20 respect to a reference value obtained by two local surveys carried out in 2002 and 21 2008. Our results show that the presented method can be used as a complement to the 22 more accurate but more labour intensive classical geodetic surveys to continuously 23 monitor the local tie at co-location stations with an accuracy of a few millimetres. 24 Keywords VLBI radio telescope · geodetic reference point · axis offset · GNSS · 25 GPS · local tie vector 26 T. Ning · R. Haas · G. Elgered Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, SE-43992 Onsala. E-mail: tong.ning/rudiger.haas/gunnar.elgered@chalmers.se T. Ning GeoForschungsZentrum Potsdam, Telegrafenberg A17, Potsdam 14473, Germany E-mail: tongn@gfz-potsdam.de