Monia Negusini

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.

GPS as a geodetic tool for geodynamics in northern Victoria Land, Antarctica

The VLNDEF (Victoria Land Network for DEFormation control) project started in 1999 with the aim of detecting crustal deformation in Northern Victoria Land (Antarctica) over an area that had never been surveyed by a dense GPS network before. After a brief summary of the Italian geodetic activities carried out since 1991, the paper presents the results obtained from the processing of data collected from 1999 to 2003. In particular, processing strategies were dealt with, in order to produce horizontal and vertical displacement maps through GPS observations. Absolute motions in a global reference frame have been investigated using a double approach, which allowed us to make considerable progress in detecting movements and standardizing the data analysis. The analyses provide absolute horizontal velocities ranging between 17 mm yr−1 and 8 mm yr−1, with greater motions in the northernmost area. The subtraction of the rigid plate motion provides relative displacements, which may contribute to the understanding of neotectonics and geology, whereas the pattern of the vertical crustal motions detected, with average values of +1.3 mm yr−1, is essential to detect the effect of Glacial Isostatic Adjustment (GIA) and other geophysical signals, and to redefine theory and numerical models used without any direct measurements.

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.

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.

Determination of tropospheric parameters within the new IVS Pilot Project

In April 2002 the International VLBI Service for Geodesy and Astrometry (IVS) set up the Pilot Project — Tropospheric Parameters, and the Institute of Geodesy and Geophysics (IGG), Vienna, was put in charge of coordinating the project. Seven IVS Analysis Centers have joined the project and regularly submitted their estimates of tropospheric parameters (wet and total zenith delays, horizontal gradients) for all IVS-R1 and IVS-R4 sessions since January 1st, 2002. The individual submissions are combined by a two-step procedure to obtain stable, robust and highly accurate tropospheric parameter time series with one hour resolution. The zenith delays derived by Very Long Baseline Interferometry (VLBI) are compared with those provided by the International GPS Service (IGS). At sites with co-located VLBI and GPS antennas, biases are found between the Global Positioning System (GPS) and VLBI derived zenith delays, although short-term variabilities generally show a good agreement. Possible reasons for these biases are discussed. Additionally, the time series submitted by the individual Analysis Centers are compared to the combined IGS time series for the CONT02 campaign of 15 days of continuous observations in the second half of October 2002.

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.

New Geodetic and Gravimetric Maps to Infer Geodynamics of Antarctica with Insights on Victoria Land

In order to make inferences on the geodynamics of Antarctica, geodetic and gravimetric maps derived from past and new observations can be used. This paper provides new insights into the geodynamics of Antarctica by integrating data at regional and continental scales. In particular, signatures of geodynamic activity at a regional extent have been investigated in Victoria Land (VL, Antarctica) by means of Global Navigation Satellite System (GNSS) permanent station observations, data from the VLNDEF (Victoria Land Network for Deformation control) discontinuous network, and gravity station measurements. At the continental scale, episodic GNSS observations on VLNDEF sites collected for 20 years, together with continuous data from the International GNSS Service (IGS) and Polar Earth Observing Network (POLENET) sites, were processed, and the Euler pole position assessed with the angular velocity of the Antarctic plate. Both the Bouguer and the free-air gravity anomaly maps were obtained by integrating the available open-access geophysics dataset, and a compilation of 180 gravity measurements collected in the VL within the Italian National Program for Antarctic Research (PNRA) activities. As a result, new evidence has been detected at regional and continental scale. The main absolute motion of VL is towards SE (Ve 9.9 ± 0.26 mm/yr, Vn −11.9 ± 0.27 mm/yr) with a pattern similar to the transforms of the Tasman and Balleny fracture zones produced as consequence of Southern Ocean spreading. Residual velocities of the GNSS stations located in VL confirm the active role of the two main tectonic lineaments of the region, the Rennick–Aviator and the Lillie–Tucker faults with right-lateral sense of shear. The resulting VL gravity anomalies show a NW region characterized by small sized Bouguer anomaly with high uplift rates associated and a SE region with low values of Bouguer anomaly and general subsidence phenomena. The East and West Antarctica are characterized by a different thickness of the Earth’s crust, and the relative velocities obtained by the observed GNSS data confirm that movements between the two regions are negligible. In East Antarctica, the roots of the main subglacial highlands, Gamburtsev Mts and Dronning Maud Land, are present. The Northern Victoria Land (NVL) is characterized by more scattered anomalies. These confirm the differences between the Glacial Isostatic Adjustment (GIA) modeled and observed uplift rates that could be related to deep-seated, regional scale structures. Remote Sens. 2018, 10, 1608; doi:10.3390/rs10101608 www.mdpi.com/journal/remotesensing Remote Sens. 2018, 10, 1608 2 of 27

Geodetic techniques applied to the study of the Etna volcano area (Italy)

Volcanic behaviour of Mt. Etna is due to the complex interaction between both the local and the regional stress field involving the eastern Sicily. Eruptions could trigger (be triggered?) during crust extension and/or compression, which are strictly linked with the dynamics of the lower mantle. In this study, very long baseline interferometry (VLBI) space geodesy technique has been used in order to study Etna volcano’s activity by means of the crustal deformations between Noto and Matera (located on the African and the Eurasian Plates, respectively). By analysing VLBI data, we obtained the behaviour of the baseline which crosses the Etnean area, from 1990 December to 2003 March, representing the time variations of the distance between the two geodetic stations; the linear trend of the baseline shows a general increasing, pointing out an extension of the crust between them. A detailed analysis of the Noto-Matera baseline allows the identification of three parts of the VLBI curve in the considered period. In the first part of the curve (from 20/12/90 to 09/02/94), VLBI data are rather poor and therefore no reliable consideration about correlation between crust movements and volcanic and seismicity activity has been made. In the second part of the curve (from 09/02/94 to 04/09/00), VLBI data are more frequent and show slightly fluctuations in the distance. Increasing in the extension and compression were observed in the central and in the final part of this period. In the third period (from 04/09/00 to 25/03/03), VLBI data are very sparse even if the time series was quite long; therefore, to fill gaps in the information, we analysed global positioning system (GPS) data. GPS technique performs continuous observations, and we were able to highlight both extensions and compressions in detail. Comparisons between the trend of Noto-Matera baseline length variations, volcanic activity and seismicity in the Etna area show the complexity of the development over time and space of the phenomenology determined by a deep cause which can be traced, in our opinion, to the interaction between the asthenospheric mantle, deep crust and surface crust. Therefore, we state that crustal distension and compression are determined by the lower pulsating mantle.

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.