Determination of Precise ITRF Positions for Ground-station Satellite Tracking Antennae

Stefan Leinen and Matthias Becker

Abstract:	On numerous ground-stations satellite tracking antennae are operated for the purpose of station-to-satellite communications, satellite tracking for precise orbit determination, Very Long Basline Interferometry (VLBI), etc. Especially the last mentionend applications require precise positions of the antennae in a global coordinate reference system. The International Terrestrial Reference Frame (ITRF) of the International Earth Rotation Service (IERS) is today's most accurate realization of such a global reference system. In this paper a procedure for the determination of the satellite tracking antenna position in the ITRF is outlined. The position is defined in a pure geometric sense as the intersection of the azimuth and elevation rotation axes of the antenna and is named antenna reference point (ARP). The procedure was elaborated in the course of the works to update the ESA Tracking Site Directory (ESA TSD) in the years 2002-2005 which serves as database for the ARP positions of the antennae used by ESA. The paper describes the general methodology of works including terrestrial measurements, data evaluation models and quality assessment for a selected example from ESA TSD. The ARP determination consists of three principal steps: At first ITRF coordinates have to be determined for a GPS marker on-site by means of GPS measurements with high absolute accuracy. This can be easily achieved from static GPS observations and data evaluation within the International GPS Service (IGS). Secondly a local geodetic network around the antenna will be observed by means of GPS as well as by terrestrial measurements (levelling and tacheometry). For this step a sophisticated 3D data integration model considering geoidal heights with respect to the reference ellipsoid (GRS 80 = Geodetic Reference System 1980) is used. Finally in the third step the measurements of the antenna itself and the calculation of the ARP are carried out. For this a new method was developed named Direct Method since it is based only on measurements directly to the rotatable elevation housing of the antenna in various azimuth and elevation positions without need for any further information. The 3D position and parametrization of both rotation axes of the antenna are obtained from calculation of best fitting circles in 3D and computation of the axes' intersection. Results are shown for the case of a new 35m Deep Space Antenna at Cebreros (Spain). GPS, total station, and leveling observations were carried out in order to fully satisfy the requirements of the integrated 3D data model. The suitability and applicability of the proposed procedure, especially the newly developed Direct Method, is demonstrated. For its validation the ARP was also determined by means of a second method, Antenna Traverse Method, requiring informations and measurements from the antenna manufacturer. The big advantage of the Direct Method is its applicability to any antenna with azimuth and elevation rotation mechanism directly from geodetic measurements without requiring extra information. It is shown that the antenna reference point can be determined with an accuracy of approx. 5 mm by the Direct Method which is independently confirmed by the Antenna Traverse Method.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	1058 - 1068
Cite this article:	Leinen, Stefan, Becker, Matthias, "Determination of Precise ITRF Positions for Ground-station Satellite Tracking Antennae," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 1058-1068.
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