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Rüdiger Haas
Type of contribution: poster

A GPS-based local-tie vector at the Onsala Space Observatory

Tong Ning, Rüdiger Haas, Gunnar Elgered Chalmers University of Technology, Department of Earth and Space Sciences, Onsala Space Observatory, SE-439 92 Onsala (Sweden)

So-called local-tie vectors at geodetic co-location stations are important pieces of information for the international terrestrial reference frame (ITRF). The Onsala Space Observatory is a co-location station contributing to the ITRF and operates equipment for geodetic Very Long Baseline Interferometry (VLBI) and Global Navigation Satellite System (GNSS) observations since several decades. The local-tie vector between the reference points of the instruments used at Onsala for VLBI and GNSS was previously observed several times by classical geodetic measurement techniques. In the summer of 2013 two gimbal-mounted GNSS-antennas were installed on the radio telescope, one on each side of the telescope dish, and several 24 hour long measurement campaigns were performed. Five semi-kinematic and four kinematic campaigns were conducted during July to September 2013. For the semi-kinematic campaigns, the telescope was pointed to different azimuth and elevation directions, spending up to 30 minutes at one position. The kinematic campaigns were conducted during normal geodetic VLBI-sessions where the telescope is continuously in motion and either tracking radio sources (slow motion) or slewing to new targets (fast motion). The GPS-data recorded with 1 Hz sampling during these campaigns were analyzed together with data from the ONSA site with a in-house developed software applying the double-difference analysis strategy with ambiguity-resolution. Phase-centre variation corrections were calculated for the rotated GNSS-antennas and applied in the data analysis. The resulting positions of the two GNSS-antennas on the telescope were used to determine its reference point and axis offset. Doing so, the so-called local-tie vector between the reference point of the telescope used for geodetic VLBI and the ONSA GNSS-monument was determined directly in the geocentric GPS-based reference frame. The results from the nine campaigns show standard deviations of 2.1 mm, 0.8 mm and 2.6 mm for the X, Y- , and Z-axis respectively. The disagreement with respect to the coordinate differences calculated from the ITRF2008 coordinates of the reference points at epoch 2013 are 0.8 mm, 0.2 mm and 8.5 mm for the X-, Y- and Z-components. The result for the axis offset has a standard deviation of 2.9 mm and differs by 0.5 mm from the results derived from classical geodetic measurements in 2002 and 2008.