VLBI radio telescopes are large technical\ua0facilities whose structures are aected by several\ua0deformation patterns. In particular, temperature- and\ua0gravity-dependent deformations bias the estimated\ua0global telescope position and, therefore, if uncorrected,\ua0deteriorate the geodetic results that can be derived from\ua0the geodetic VLBI analysis. The rigidity of a telescope\ua0structure under varying acting forces is restricted by\ua0its structural properties. Large conventional radio\ua0telescopes are more affected by deformation effects\ua0than the new compact-designed VGOS antennas. The\ua0design document for the next generation VLBI system\ua0(today called VGOS) states <300 \ub5m as requirement\ua0for the path length stability. A traceable metrological\ua0system that can be used to check this stability level\ua0must be at least three times better than the requirements.\ua0Close range photogrammetric methods fulfil\ua0these accuracy requirements but usually need a crane\ua0during the survey of a telescope. To avoid the latter,\ua0an unmanned aerial system was used for the first\ua0time to evaluate the possible deformation of the main\ua0reflector surface of the north-eastern of the Onsala\ua0twin telescopes (ONSA13NE). The focal length of the\ua0ring-focus paraboloid was derived in several elevation\ua0angles to study the gravitational deformation effects\ua0on the main reflector of this VGOS antenna
