Pierguido Sarti

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 ...

Status of the Sardinia Radio Telescope project

We present the status of the Sardinia Radio Telescope (SRT) project, a new general purpose, fully steerable 64 m diameter parabolic radiotelescope capable to operate with high efficiency in the 0.3-116 GHz frequency range. The instrument is the result of a scientific and technical collaboration among three Structures of the Italian National Institute for Astrophysics (INAF): the Institute of Radio Astronomy of Bologna, the Cagliari Astronomy Observatory (in Sardinia,) and the Arcetri Astrophysical Observatory in Florence. Funding agencies are the Italian Ministry of Education and Scientific Research, the Sardinia Regional Government, and the Italian Space Agency (ASI,) that has recently rejoined the project. The telescope site is about 35 km North of Cagliari. The radio telescope has a shaped Gregorian optical configuration with a 7.9 m diameter secondary mirror and supplementary Beam-WaveGuide (BWG) mirrors. With four possible focal positions (primary, Gregorian, and two BWGs), SRT will be able to allocate up to 20 remotely controllable receivers. One of the most advanced technical features of the SRT is the active surface: the primary mirror will be composed by 1008 panels supported by electromechanical actuators digitally controlled to compensate for gravitational deformations. With the completion of the foundation on spring 2006 the SRT project entered its final construction phase. This paper reports on the latest advances on the SRT project.

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.

A Comparative Study of the Applied Methods for Estimating Deflection of the Vertical in Terrestrial Geodetic Measurements

This paper compares three different methods capable of estimating the deflection of the vertical (DoV): one is based on the joint use of high precision spirit leveling and Global Navigation Satellite Systems (GNSS), a second uses astro-geodetic measurements and the third gravimetric geoid models. The working data sets refer to the geodetic International Terrestrial Reference Frame (ITRF) co-location sites of Medicina (Northern, Italy) and Noto (Sicily), these latter being excellent test beds for our investigations. The measurements were planned and realized to estimate the DoV with a level of precision comparable to the angular accuracy achievable in high precision network measured by modern high-end total stations. The three methods are in excellent agreement, with an operational supremacy of the astro-geodetic method, being faster and more precise than the others. The method that combines leveling and GNSS has slightly larger standard deviations; although well within the 1 arcsec level, which was assumed as threshold. Finally, the geoid model based method, whose 2.5 arcsec standard deviations exceed this threshold, is also statistically consistent with the others and should be used to determine the DoV components where local ad hoc measurements are lacking.

Impact of network geometry, observation schemes and telescope structure deformations on local ties: simulations applied to Sardinia Radio Telescope

The 64-m Sardinia Radio Telescope (SRT) is currently under construction in Sardinia (Italy). To ensure future surveying and monitoring operations at an utmost level of accuracy, we aim at selecting the optimal design and the most cost-effective solution for the establishment of the local ground control network (LGCN). We simulate and test 45 data sets corresponding to 5 different network configurations. We investigate the influence of 2 LGCN geometries (14 or 8 ground markers) and 3 terrestrial observation schemes (based on redundant forward intersections or side shots) on the precision and accuracy of the conventional reference point (CRP) of SRT and the simulated tie vector with a global navigation satellite system (GNSS) station. In addition, thermal and gravitational deformations of the radio telescope structure are simulated as systematic errors introduced into the observations and their effects on the CRP estimates are quantified. The state-of-the-art of CRP surveying and computation, based on terrestrial indirect methods, is applied. We show how terrestrial indirect methods can estimate the position of the radio telescope CRP to the millimeter precision level. With our simulations, we prove that limiting the LGCN to a 8-point configuration ensures the same precision on the CRP obtained with a 14-point network. Furthermore, we demonstrate that in the absence of telescope deformations, side shots, despite the lower redundancy, preserve a precision similar to that of redundant forward intersections. We show that the deformations due to gravitational flexure and thermal expansion of the radio telescope cannot be neglected in the tie vector computation, since they may bias the CRP estimate by several millimeters degrading its accuracy but not impacting on its formal precision. We highlight the dependency of the correlation matrices of the solutions on the geometry of the network and the observation schemes. Similarly, varying the extent of telescope deformations, we show that the CRP estimate again depends on the combination of the network geometry and the observation schemes.

VLBI-GPS Eccentricity Vectors at Medicina's Observatory via GPS Surveys: Reproducibility, Reliability and Quality Assessment of the Results

Local ties are key elements for the computation of combined space geodetic products: the different ITRF realizations determined by merging single technique geodetic reference frames rely on the availability of accurate eccentricities (or intra-site vectors) linking the instrumental reference points (RPs) at co-located observatories. If the RPs cannot be physically materialized and/or directly measured, an indirect approach can be used for their estimation: it is entirely based on a geometrical conditioning applied to observed points’ positions of the space geodetic instruments. Intra-site vectors are typically measured by combining terrestrial observations of angles, distances and height differences along with ancillary GPS measurements for assuring a correct alignment of the eccentricity into a global frame (terrestrial local ties). An alternative indirect approach can be used entirely relying on GPS measurements: it may efficiently be applied to VLBI-GPS eccentricities as well as at all co-location sites where ITRF tracking points can be surveyed with GPS technique (GPS-based local ties). In principle such an approach offers advantages with respect to that terrestrial, since it is (i) faster, (ii) semi-automatic and (iii) it provides an immediate eccentricity vector alignment into a global frame. This paper presents the results of the computation of the VLBI-GPS eccentricity at Medicina based on the GPS procedure, with the aim of assessing its quality and repeatability. To this respect, two GPS measurement sessions were performed in 2002 and 2006: the positive results obtained with the first campaign led us to repeat the survey in 2006 in order to better investigate the potential of GPS-based ties. 2002 and 2006 GPS-based ties were compared with terrestrial local ties surveyed at Medicina’s observatory in 2001, 2002, 2003. Each of the intra-site vectors was estimated by applying different configurations of the geometrical conditioning with the aim of investigating the effects on the RPs estimations in the terrestrial and in the GPS case. The results demonstrate the efficiency and the precision of the GPS approach for eccentricity vector computation which can be used as an alternative to terrestrial based ties.

VLNDEF Project for Geodetic Infrastructure Definition of Northern Victoria Land, Antarctica

Scientific investigations in Antarctica are, for many different reasons, a challenging and fascinating task. Measurements, observations and field operations must be carefully planned well in advance and the capacity of successfully meeting the goals of a scientific project is often related to the capacity of forecasting and anticipating the many different potential mishaps. In order to do that, experience and logistic support are crucial. On the scientific side, the team must be aware of its tasks and be prepared to carry out observations in a hostile environment: both technology and human resources have to be suitably selected, prepared, tested and trained. On the logistic side, nations, institutions and any other organisation involved in the expeditions must ensure the proper amount of competence and practical support.

GPS and Radiosonde Derived Precipitable Water Vapour Content and its Relationship with 5 Years of Long-Wave Radiation Measurements at “Mario Zucchelli” Station, Terra Nova Bay, Antarctica

The Italian “Mario Zucchelli” Station (-74° 41′ 55′′.6997 N, 164° 06′ 10′′. 5887 E), situated at Terra Nova Bay, Northern Victoria Land, is equipped with a permanent Global Positioning System receiver (TNB1), continuously observing since 1998. “Mario Zucchelli” is an Antarctic experimental facility where a large number of scientific observations are carried out, either permanently or seasonally. In particular, an observatory devoted to atmospheric physics is located at Campo Icaro, 2.5 km from the base: it is a Clean Air Facility where several atmospheric measurements are carried out. Since 2000, long-wave radiation measurements are performed routinely using a Kipp&Zonen CNR-1 net radiometer.

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.