Abstract: Large VLBI telescopes undergo gravitational deformations which affect both geodetic and astronomic observations as well as the real reference point (RP) position (i.e. the reference point which is directly linked to and determined by the physics of the VLBI observations). As a consequence, the accuracy of eccentricity vectors determined with high precision terrestrial observations strictly depends on the possibility of univocally defining the geodetic instrument\u2019s RP to be surveyed and estimated: technique dependent effects (e.g. gravitational and thermal deformations for VLBI, phase centre variations for GPS, etc) bias RP positions and weaken and perturb the information contained in the eccentricity. The impact on combined geodetic products is remarkable; a proper definition of space geodetic instruments\u2019 RP must therefore account for possible biases that modify its theoretical position. Whether the problem must be directly addressed by each technique-specific Service is still an open issue. Indirect approaches based on high precision terrestrial observations have proved to be additional, accurate and independent tools for determining and monitoring the eccentricities at co-location sites. Nevertheless, a deeper and rigorous investigation on RP location\u2019s variations is at least as important and it is nowadays fundamental for each space geodetic instrument. To this respect, we are presenting the investigations on VLBI telescope\u2019s RP position that were carried out at Medicina and Noto (Italy) on the 32 m antennas: trilateration, triangulation and laser scanning observations were applied and combined to monitor the gravitational deformations which affect the telescope\u2019s structure and to derive an elevation dependent correction function for radio signal path
