Analyzing shape deformation and rigid body movement of structures using commonly misaligned terrestrial laser scanners: the radio telescope case

Terrestrial laser scanners (TLS) suffer from internal misalignments leading to systematic measurement errors. In most cases, these systematic errors surpass the magnitude of random errors. Hence, it is necessary to account for systematic errors within the deformation analysis in order to obtain unbiased results. Within this work, we present and compare several strategies for dealing with these TLS misalignments without the need of a previous calibration. These strategies are based on two-face measurements, an in-situ TLS calibration and a combination of both in a bundle adjustment. Furthermore, we analyze if changing measurement geometries, i.e., variations of the station of the instrument w.r.t. the object under investigation, improve the sensitivity of this bundle adjustment regarding the estimation of the calibration parameters. We investigate these strategies based on a specific example: The elevation-dependent deformation analysis of radio telescopes that are used for geodetic very long baseline interferometry (VLBI). Within one measurement campaign, the radio telescopes rotate around their elevation axes. For this rotation, we need to know if the telescopes' reference points are stable and if the radio telescopes' main reflectors deform. While the first possible deformation equals a rigid body movement, the second one equals a shape deformation. Our results demonstrate that the shape deformation as well as the rigid body movement are least affected by the TLS misalignments if measuring in two-faces, calibrating the scanner in-situ and varying the measurement geometry. Although we only draw our conclusions based on the empirical results of this specific example, they are transferable to other deformation analyses using terrestrial laser scanners.