Abstract
The International Terrestrial Reference Frame (ITRF) is derived by combining several space geodetic techniques. Basically, a meaningful combination of the geodesic space techniques is impossible without further geometrical information, i. e. local-ties. Local-tie vectors are defined between the geometrical reference points of space geodetic techniques at co-location stations. These local-ties are introduced during the inter-technique combination process, to overcome the weak physical connection between the space geodetic techniques. In particular, the determination of the reference point of radio telescopes or laser telescopes is a challenging task and requires indirect methods. Moreover, the Global Geodetic Observing System (GGOS) strives for an automated and continued reference point determination with sub-millimeter accuracy, because deviations in local-ties bias global results.
This investigation presents a modified approach for automated reference point determination. The new approach extends the prior work of Lösler but evades the synchronization between the terrestrial instrument and the telescope. Thus, synchronization errors are omitted and the technical effort is reduced. A proof of concept was carried out at Geodetic Observatory Wettzell in 2018. Using a high-precision, mobile laser-tracker, the reference point of the Satellite Observing System Wettzell (SOS-W) was derived. An extended version of the in-house developed software package HEIMDALL was employed for a mostly automated data collection. To evaluate the estimated reference point, the derived results are compared with the results of two approved models.
Zusammenfassung
Der International Terrestrial Reference Frame (ITRF) wird aus der Kombination verschiedener geodätischer Raumtechniken bestimmt. Aufgrund fehlender physischer Verbindungen zwischen den Messverfahren der Raumtechniken ist eine kombinierte Auswertung nur durch geometrische Zusatzinformationen sinnvoll möglich. Sogenannte local-tie Vektoren, die zwischen den geometrischen Referenzpunkten der Raumtechniken definiert sind, werden hierzu an Co-Location Stationen mit übergeordneter Genauigkeit bestimmt und in der globalen Lösung berücksichtigt. Insbesondere die Bestimmung der Referenzpunkte an Radio- und Laserteleskopen ist herausfordernd, da die Referenzpunkte nicht direkt taktil gemessen werden können. Weiterhin strebt das Global Geodetic Observing System (GGOS) eine automatisierte und kontinuierliche Bestimmung der Referenzpunkte mit Submillimetergenauigkeit an.
In dieser Arbeit wird ein modifiziertes Verfahren zur automatisierten Referenzpunktbestimmung vorgeschlagen, welches den bisherigen Ansatz von Lösler erweitert, hierbei aber das Problem der Synchronisation zwischen terrestrischen Instrument und Teleskop umgeht, sodass Synchronisationsfehler vermieden werden, und sich der technische Aufwand verringert. Am Geodätischen Observatorium Wettzell (GOW) wurde 2018 eine Messkampagne initiiert, um die Tauglichkeit des neuen Ansatzes zu evaluieren. Hierbei wurde mit einem hochpräzisen, mobilen Lasertracker der Referenzpunkt des Satellite Observing System Wettzell (SOS-W) bestimmt. Die Messung erfolgte in Anlehnung an GGOS weitgehend automatisiert mit dem selbst entwickelten System HEIMDALL. Die Ergebnisse des neuen Ansatzes werden mit den Lösungen von zwei weiteren, bewährten Modellen verglichen.
About the authors
Michael Lösler studied geodesy at University of Applied Sciences Neubrandenburg and graduated with a diploma in 2006. He developed a new mathematical model for automated reference point determination during his employment as scientific assistant of the DFG project: high-precision real-time reference point determination of VLBI radio telescopes for combining IVS and IGS reference frames at Karlsruhe Institut of Technology. He is a member of the IERS (International Earth Rotation and Reference System) Working Group on Site Survey and Co-location. Since 2014, he is employed at Frankfurt University of Applied Sciences where he keeps working on automated metrological methods for reference point determinations.
Cornelia Eschelbach studied geodesy at the Karlsruhe Institute of Technology between 1997 and 2002. Afterwards she was employed at the Geodetic Institute of the Karlsruhe Institute of Technology (chair for surveying and geodetic sensors) and received her Ph. D. on the subject determination of the refraction correction by modeling the momentum and heat flux in the roughness layer. In 2010 she was appointed on the professorship of surveying and applied geodesy at the Frankfurt University of Applied Sciences. Three years later she founded the Laboratory for Industrial Metrology that is focused on large volume metrology and parameter estimation.
Stefan Riepl studied physics at the University of Regensburg and acquired a Ph. D. at the Technical University of Munich with a thesis focusing especially on two color satellite laser ranging. After that he joined the Federal Agency for Cartography and Geodesy and in the following years was delegated to the Universidad de Concepcion, Chile for the setup and operation of the Satellite Laser Ranging module of the Transportable Integrated Geodetic Observatory (TIGO). Later he returned to the Federal Agency for Cartography and Geodesy to design and develop the Satellite Observing System Wettzell (SOS-W).
Acknowledgment
The authors thank Swetlana Mähler for technical support and providing the adapters of the cat eye reflectors, which enabled a free choice of positioning at the telescope structure.
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