Evolution and obtained expertise in reference point determination at the GIK

The International Terrestrial Reference System (ITRS) is realized by geodetic space techniques, which are linked by local-ties at the observation stations. Therefore, reliability and high accuracy are the main requirements for the determination of the different reference points in the corresponding local reference frame. At the Geodetic Institute of the University of Karlsruhe several scientific studies have been carried out to determine the reference point of radio telescopes at different observation stations and with different calculation models. Starting in 2002 the IVS reference point at Onsala Space Observatory was determined by 3D-circle fits with error propagation using full covariance information, which is custom procedure nowadays. In 2008 a completely new mathematical model was established able to fulfill future requirements as for example minimizing downtime of the telescope. Geodetic measurements with a tacheometer respectively a lasertracker at the Fundamentalstation Wettzell and the Onsala Space Observatory yielded a good verification of the model. This contribution shows the evolution and obtained expertise in reference point determination at the Geodetic Institute of the University of Karlsruhe by comparing the results of different campaigns at the Onsala Space Observatory

Refraktionskorrekturbestimmung durch Modellierung des Impuls- und Wärmeflusses in der Rauhigkeitsschicht

In dieser Arbeit wird ein neues Modell zur Berechnung der Refraktionskorrektur bei Zenitwinkelmessungen vorgestellt, das auf visurnahen Turbulenzmessungen basiert. Der hierzu nötige integrale vertikale Temperaturgradient entlang der Messtrecke wird aus Turbulenzparametern ermittelt. Vergleiche dieser berechneten Temperaturgradienten mit gemessenen Temperaturgradienten zeigten sehr gute Übereinstimmungen, sodass eine Reduktion des Refraktionseinflusses um mehr als 75 % möglich ist

Terrestrial monitoring of a radio telescope reference point using comprehensive uncertainty budgeting

During the 15-day-long global very long baseline interferometry campaign CONT14, a terrestrial monitoring campaign was carried out at the Onsala Space Observatory. The goal of these efforts was to monitor the reference point of the Onsala 20 m radio telescope during normal telescope operations. Parts of the local site network as well as a number of reflectors that were mounted on the 20 m radio telescope were observed in an automated and continual way using the in-house-developed software package HEIMDALL. The analysis of the observed data was performed using a new concept for a coordinate-based network adjustment to allow the full adjustment process in a true Cartesian global reference frame. The Akaike Information Criterion was used to select the preferable functional model for the network adjustment. The comprehensive stochastic model of this network adjustment process considers over 25 parameters, and, to describe the persistence of the observations performed during the monitoring with a very high measurement frequency, includes also time-dependent covariances. In total 15 individual solutions for the radio telescope reference point were derived, based on monitoring observations during the normal operation of the radio telescope. Since the radio telescope was moving continually, the influence of timing errors was studied and considered in the adjustment process. Finally, recursive filter techniques were introduced to combine the 15 individual solutions. Accuracies at the sub-millimeter level could be achieved for the radio telescope reference point. Thus, the presented monitoring concept fulfills the requirement proposed by the global geodetic observing system

Permanent monitoring of the reference point at the 20m radio telescope Wettzell

To achieve the aims of the VLBI2010 agenda and of the Global Geodetic Observing System (GGOS) an automated monitoring of the reference points of different geodetic space techniques, such as Very Long Baseline Interferometry (VLBI), is desirable. The resulting permanent monitoring of the local-tie vectors at co-location stations are essential to obtain the sub-millimeter level in the combinations. For this reason a monitoring system was installed by the Geodetic institute of the University of Karlsruhe (GIK) to observe the 20m radio telescope for VLBI at the Geodetic Observatory Wettzell from May to August. A specially developed software from the Geodetic Institute of the university collected data from automated total station measurements, meteorological sensors, and sensors in the telescope monument (e.g., Invar cable data). A real-time visualization directly offered a live view of the measurements during the regular observation operations. Additional scintillometer measurements allowed refraction corrections during the post-processing. This project is one of the first feasibility studies aimed at determining significant deformations of the VLBI antenna due to, for instance, changes in temperature

Permanent Monitoring of the Reference Point of the 20m Radio Telescope Wettzell

To achieve the goals of the VLBI2010 project and the Global Geodetic Observing System (GGOS), an automated monitoring of the reference points of the various geodetic space techniques, including Very Long Baseline Interferometry (VLBI), is desirable. The resulting permanent monitoring of the local-tie vectors at co-location stations is essential to obtain the sub-millimeter level in the combinations. For this reason a monitoring system was installed at the Geodetic Observatory Wettzell by the Geodetic Institute of the University of Karlsruhe (GIK) to observe the 20m VLBI radio telescope from May to August 2009. A specially developed software from GIK collected data from automated total station measurements, meteorological sensors, and sensors in the telescope monument (e.g., Invar cable data). A real-time visualization directly offered a live view of the measurements during the regular observation operations. Additional scintillometer measurements allowed refraction corrections during the post-processing. This project is one of the first feasibility studies aimed at determining significant deformations of the VLBI antenna due to, for instance, changes in temperature

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

Permanentüberwachung des 20m VLBI-Radioteleskops an der Fundamentalstation in Wettzell = Permanent monitoring of the 20 m VLBI-radio telescope at a major station in Wettzell

Permanente und automatische Überwachungsmessungen der Verbindungsvektoren zwischen den Referenzpunkten verschiedener Raumverfahren wie beispielsweise Very Long Baseline Interferometry (VLBI) oder dem Globalen Navigationssatellitensystem (GNSS) sind maßgeblichen Forderungen im Rahmen des Global Geodetic Observing System (GGOS) und der Agenda VLBI2010, um die angestrebte Submillimetergenauigkeit zu erreichen. Am Geodätischen Observatorium in Wettzell wurde aus diesem Grund ein Monitoringsystem eingerichtet, welches das 20m VLBI-Radioteleskop überwacht. Im Rahmen dieser Machbarkeitsstudie sollte geprüft werden, ob sich signifikante Deformationen, hervorgerufen beispielsweise durch Temperaturänderungen, feststellen lassen und welche Größenordnungen sie besitzen. In the agenda VLBI2010 and the framework of the Global Geodetic Observing System (GGOS), an automated monitoring of the reference points of different geodetic space techniques like Very Long Baseline Interferometry (VLBI) or Global Navigation Satellite System (GNSS) and therefore of the local-tie vectors at co-location stations are desirable to obtain the submillimeter level. For this reason a monitoring system was installed to observe the 20m VLBI radio telescope at the geodetic observatory Wettzell. This project is one of the first feasibility studies aimed at determining significant deformations caused by the VLBI antenna due to e.g. changes in temperature

Zur Modellierung eines Ring-Focus-Paraboloids

radio telescope; ring-focus paraboloid