Extension and Optimization of the Local Geodetic Network at the Onsala Space Observatory

Since May 2017, the Onsala Space Observatory\ua0(OSO) has hosted the Onsala twin telescopes\ua0(OTT), two identical telescopes fulfilling the VGOSspecifications.\ua0The local geodetic ground network has\ua0to be extended to the area around the OTT to provide\ua0local tie vectors for combining different geodetic\ua0space techniques at the observatory. Furthermore,\ua0this network is essential for monitoring the temporal\ua0and spatial stability of the new radio telescopes. Both\ua0network configuration and measurement uncertainties\ua0of the terrestrial observations have a strong impact on\ua0the obtainable accuracy of the reference points. Network\ua0optimization procedures help to avoid misconfigurations\ua0and provide suitable network configurations.\ua0For OSO, an extended ground network and an optimal\ua0observation schedule are derived that fulfill the accuracy\ua0requirements for monitoring processes. The observation\ua0schedule, derived by a second order design\ua0optimization, focuses on a practical experience when\ua0using modern geodetic instruments

Measuring Focal Length Variations of VGOS Telescopes Using Unmanned Aerial Systems

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

Gravitational deformation of ring-focus antennas for VGOS: first investigations at the Onsala twin telescopes project

The receiving properties of radio telescopes used in geodetic and astrometric very long baseline interferometry (VLBI) depend on the surface quality and stability of the main reflector. Deformations of the main reflector as well as changes in the sub-reflector position affect the geometrical ray path length significantly. The deformation pattern and its impact on the VLBI results of conventional radio telescopes have been studied by several research groups using holography, laser tracker, close-range photogrammetry and laser scanner methods. Signal path variations (SPV) of up to 1cm were reported, which cause, when unaccounted for, systematic biases of the estimated vertical positions of the radio telescopes in the geodetic VLBI analysis and potentially even affect the estimated scale of derived global geodetic reference frames. As a result of the realization of the VLBI 2010 agenda, the geodetic VLBI network is currently extended by several new radio telescopes, which are of a more compact and stiffer design and are able to move faster than conventional radio telescopes. These new telescopes will form the backbone of the next generation geodetic VLBI system, often referred to as VGOS (VLBI Global Observing System). In this investigation, for the first time the deformation pattern of this new generation of radio telescopes for VGOS is studied. ONSA13NE, one of the Onsala twin telescopes at the Onsala Space Observatory, was observed in several elevation angles using close-range photogrammetry. In general, these methods require a crane for preparing the reflector as well as for the data collection. To reduce the observation time and the technical effort during the measurement process, an unmanned aircraft system (UAS) was used for the first time. Using this system, the measurement campaign per elevation angle took less than 30 min. The collected data were used to model the geometrical ray path and its variations. Depending on the distance from the optical axis, the ray path length varies in a range of about \ub1 1 mm. To combine the ray path variations, an illumination function was introduced as weighting function. The resulting total SPV is about − 0.5 mm. A simple elevation-dependent SPV model is presented that can easily be used and implemented in VLBI data analysis software packages to correct for gravitational deformation in VGOS radio telescopes. The uncertainty is almost 200 μm (2σ ) and is derived by Monte Carlo simulations applied to the entire analysis process

Zur Modellierung eines Ring-Focus-Paraboloids

radio telescope; ring-focus paraboloid

Erfassung der Hauptreflektordeformation eines Radioteleskops durch UAV-gest\ufctzte Nahbereichsphotogrammetrie

Deformationen am Hauptreflektor eines Radioteleskops verursachen systematische\ua0Abweichungen in den Messungen und f\ufchren zu einer Verf\ue4lschung der abgeleiteten\ua0globalen Position und somit zu einem verzerrten Netzma fstab im globalen geod\ue4tischen\ua0Bezugsrahmen. Um diese Systematiken zu minimieren, ist ein stabiler bzw. modellierbarer\ua0Strahlengang \ufcber den gesamten Arbeitsbereich des Teleskops notwendig. Zur messtechnischen\ua0Erfassung des Hauptreflektors konventioneller Radioteleskope werden gegenw\ue4rtig\ua0h\ue4ufig Laserscanner verwendet, die eine Einzelpunktgenauigkeit von wenigen Millimetern erreichen.\ua0F\ufcr diesen Beitrag wurde erstmals das elevationsabh\ue4ngige Verformungsverhalten eines modernen,\ua0kompakten VGOS-spezifizierten VLBI-Radioteleskops photogrammetrisch untersucht.\ua0Um den Hauptreflektor des Teleskops in mehreren Neigungen zu erfassen, erfolgte die photogrammetrische\ua0Datenerhebung durch ein Unmanned Aircraft Vehicle (UAV) als Sensorplattform\ua0f\ufcr eine leichtgewichtige Kompaktkamera. Im Rahmen der Untersuchungen wurde eine\ua0Einzelpunktgenauigkeit von ca. 50 μm in 21 unterschiedlichen Bildverb\ue4nden erreicht