Measuring Focal Length Variations of VGOS Telescopes Using Unmanned Aerial Systems

Paper in proceeding, 2019

VLBI radio telescopes are large technical facilities whose structures are aected by several deformation patterns. In particular, temperature- and gravity-dependent deformations bias the estimated global telescope position and, therefore, if uncorrected, deteriorate the geodetic results that can be derived from the geodetic VLBI analysis. The rigidity of a telescope structure under varying acting forces is restricted by its structural properties. Large conventional radio telescopes are more affected by deformation effects than the new compact-designed VGOS antennas. The design document for the next generation VLBI system (today called VGOS) states <300 µm as requirement for the path length stability. A traceable metrological system that can be used to check this stability level must be at least three times better than the requirements. Close range photogrammetric methods fulfil these accuracy requirements but usually need a crane during the survey of a telescope. To avoid the latter, an unmanned aerial system was used for the first time to evaluate the possible deformation of the main reflector surface of the north-eastern of the Onsala twin telescopes (ONSA13NE). The focal length of the ring-focus paraboloid was derived in several elevation angles to study the gravitational deformation effects on the main reflector of this VGOS antenna.