Can We Really Promise a mm-Accuracy for the Local Ties on a Geo-VLBI Antenna
For the next-generation geodetic VLBI network a 1 mm positioning accuracy is anticipated. The accuracy should be site-independent consistent, reliably controlled, and traceable over long time periods. There are a number of remaining limitations. These include random and systematic components of the delay observable itself, various antenna-related errors, and especially a proper handling of local ties at multi-technique sites.
At the Metsähovi Fundamental Station operated by the Finnish Geodetic Institute there are a CGPS and Glonass receivers, both in IGS network, a SLR (currently under renovation), a DORIS beacon, a superconducting gravimeter, and a 14.5 m radio telescope owned by Metsähovi Radio Observatory of the Helsinki University of Technology. Between five and eight geodetic VLBI sessions are conducted annually.
We tested a method to simultaneously measure the tie of the VLBI antenna to the GPS network by tracking the movement of two GPS antennas attached to the radio telescope during geodetic VLBI sessions. We used kinematic trajectory solutions of the two GPS antennas to calculate the orientation and the reference point of the VLBI antenna.
In this paper we describe the data acquisition, calculation model, some error sources and test results of four campaigns. The position of the reference point is time, temperature, antenna elevation and azimuth dependent. We propose that in the future, the position should be tracked permanently during geo-VLBI campaigns with attached GPS antennas.
KeywordsRadio Telescope Laser Tracker Superconducting Gravimeter Global Reference Frame Elevation Axis
We would like to thank T. Lindfors, A. Mujunen, M. Tornikoski, J. Kallunki, E. Oinaskallio and H. Rönnberg of the Metsähovi Radio Observatory of the Aalto University for their invaluable help.
Elevations and azimuths of GPS satellites were computed using “wheresat” of GPStk. (http://www.gpstk.org/bin/view/Documentation/WebHome). The Metsähovi model was programmed with Octave language (http://www.octave.org).
This work was partly supported by the Academy of Finland project 134952.
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