Close-Range Photogrammetry for Antenna Deformation Measurements

The knowledge of gravitational deformations at the receiving unit of VLBI antennas is one of the crucial components in achieving the GGOS accuracy goal of 1 mm in position on a global scale. Various geodetic methods for measuring the object geometry of the antenna’s receiving unit are known such as terrestrial laser scanning (TLS) or close-range photogrammetry to cite but a few. The advantage of a photogrammetric approach compared to surface measurements by a single TLS station is the complete coverage of the surface by varying camera positions. Thus, surface points potentially shaded by structural elements are also captured. Moreover, for signalized markers and under optimal conditions, close-range photogrammetry achieves an accuracy of up to 5 μm + 5μm per m. Due to the high accuracy requirements, a photogrammetric approach was chosen for the deformation mea- surements of the legacy 20 m Radio Telescope Wettzell (RTW) for the first time. To realize a stable an reliable block geometry configuration, a remote controlled unmanned aerial vehicle (UAV) was used as sensor platform. Signalized markers mounted at the receiving unit of the RTW were measured in ten elevation positions to investigate elevation dependent deformations. In each elevation position, image data were captured at least twice to obtain redundant data sets. Up to four flights were performed in selected elevation positions using different camera systems. This contribution presents the results of the photogrammetric analysis of this extensive data material