Claudio Abbondanza

Laser Scanner and Terrestrial Surveying Applied to Gravitational Deformation Monitoring of Large VLBI Telescopes’ Primary Reflector

Laser scanning surveys were performed on the primary mirror of the very long baseline interferometry (VLBI) telescopes situated at Medicina and Noto observatories, with the specific purpose of investigating (1) gravity deformation patterns of the radio telescopes’ primary reflector and (2) the magnitude and relative variations of focal length as the antennas are steered in elevation. Both instruments have Azimuth-Elevation mounts and have 32 m parabolic mirrors which were surveyed in steps of 15° spanning the 90–15° elevation range. The scanning sessions were performed from two standpoints using a GS200 Trimble-Mensi; the sampling interval was set to 2 cm at a distance of 15 m. The complete surface of the main reflector at every elevation position was obtained by merging the two separate point clouds acquired from the two standpoints; each elevation is represented by at least 1.3 millions points. The merged clouds were compared for determining relative deformation patterns and magnitude. As the elevation ...

Local Ties and Co-Location Sites: Some Considerations After the Release of ITRF2008

Tie vectors (TVs) measured at co-location sites carry fundamental information for the computation of the International Terrestrial Reference Frame (ITRF). The combination of the different frames stemming from each space geodetic (SG) technique relies on the availability and accuracy of the relative positions between reference points of co-located SG instruments, i.e. TV. If, on the one hand, TVs accurate at 1mm level are sought to preserve the accuracy of the global frame and fulfill the requirements of the global geodetic observing system (GGOS), on the other hand, the assessment of TVs accuracy is not easy. Their accuracies are often questioned on the base of their agreement within the combination of SG solutions and the combination residuals. Though, the final discrepancies highlighted by the combination residuals do not depend uniquely on the accuracy of the TVs but are influenced by several factors of different origin. In this paper, we identify some of these factors and investigate their possible origin adopting different perspectives: local ties and terrestrial surveying, SG techniques and frames combination. Our purpose is to highlight some of the possible systematic errors in terrestrial and SG data analysis as well as to identify actions to be taken in the near future to mitigate the biases highlighted by the residuals of the combination. In contrast to what is commonly assumed, we show that the residuals are potentially influenced by a combination of biases affecting the TVs, their alignment and the SG solutions. Therefore, an objective evaluation of the error sources is necessary for each SG technique in order to improve their results as well as the combined SG products.

VLBI telescopes' gravitational deformations investigated with terrestrial surveying methods

Large VLBI telescopes undergo gravitational deformations which affect both geodetic and astronomic observations. In order to assess the extent and mag nitude of such deformations and to evaluate their effect on telescopes’ performances, terres trial urveying methods can be applied to monitor the telescopes’ structure at different pointing el evations. Finite Element Model analysis, laser scanner surveying, trilateration and triangula tion have been applied on the telescope in Medicina to estimate i) the deformations of the primary mirr or and to monitor ii) the position of the feed horn located at the primary focus and iii) the positi n of the vertex of the paraboloid. If detectable, these deformations modify the position of the p rimary focus and the signal path length and may therefore reduce the antenna gain and bias the phase o f the incoming signal. We are presenting the investigations performed on the Medicina VLBI t elescope, quantifying the magnitude of the deformations of the primary dish, the quadrupode and t he vertex and we are also presenting an elevation dependent model for signal path corrections.