The Medicina and Noto VLBI antennas are Az-El telescopes that experience gravitational deformations as they move in elevation. The ideal parabolic shape of the primary mirrors is therefore perturbed and the dishes are deformed according to the elevation pointing position of the antenna. Receivers at different frequencies, in particular the S/X geodetic receivers, are placed on the quadrupode, at the primary focus position; they also experience a displacement due to gravitational forces as the elevation changes. A third effect induced by gravity is the sag which might be possibly experienced by the dish as the elevation changes. The determination of the contribution and magnitude of all the different effects are of primary importance. The realization of an elevation dependent gravitational deformation model that can be implemented in the VLBI data analysis is our target; it would allow to quantify and correct any bias of gravitational origin which affects the observations. In order to face this complex task, terrestrial laser scanning and terrestrial observations have been applied to the antenna of Medicina and Noto. The VLBI dishes’ movements in elevation prevent full visibility of the inner part of the parabola from the ground: ad hoc supports were therefore installed nearby the antenna secondary focus allowing a complete laser coverage of the inner dish surface at different elevations. The raw data acquired with the laser scanner intrinsically define clouds of points expressed with respect to an instrumental reference system; in order to connect the observed points to an external reference system, it is necessary to relatively align the different clouds using tie points and moreover ad hoc terrestrial surveys are required to frame the laser survey in to the external reference system. The surveys and their results will be presented, along with the data analysis procedure and the most recently estimated deformations.

Medicina and Noto VLBI Radiotelescopes: gravitational deformations evaluated with terrestrial laser scanning

MONTAGUTI, SIMONETTA;VITTUARI, LUCA;
2007

Abstract

The Medicina and Noto VLBI antennas are Az-El telescopes that experience gravitational deformations as they move in elevation. The ideal parabolic shape of the primary mirrors is therefore perturbed and the dishes are deformed according to the elevation pointing position of the antenna. Receivers at different frequencies, in particular the S/X geodetic receivers, are placed on the quadrupode, at the primary focus position; they also experience a displacement due to gravitational forces as the elevation changes. A third effect induced by gravity is the sag which might be possibly experienced by the dish as the elevation changes. The determination of the contribution and magnitude of all the different effects are of primary importance. The realization of an elevation dependent gravitational deformation model that can be implemented in the VLBI data analysis is our target; it would allow to quantify and correct any bias of gravitational origin which affects the observations. In order to face this complex task, terrestrial laser scanning and terrestrial observations have been applied to the antenna of Medicina and Noto. The VLBI dishes’ movements in elevation prevent full visibility of the inner part of the parabola from the ground: ad hoc supports were therefore installed nearby the antenna secondary focus allowing a complete laser coverage of the inner dish surface at different elevations. The raw data acquired with the laser scanner intrinsically define clouds of points expressed with respect to an instrumental reference system; in order to connect the observed points to an external reference system, it is necessary to relatively align the different clouds using tie points and moreover ad hoc terrestrial surveys are required to frame the laser survey in to the external reference system. The surveys and their results will be presented, along with the data analysis procedure and the most recently estimated deformations.
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Montaguti S.;Vittuari L.; Sarti P.;Negusini M.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11585/56539
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