31 research outputs found
TERRESTRIAL SURVEYING APPLIED TO LARGE VLBI TELESCOPES AND ECCENTRICITY VECTORS MONITORING
No full textAbstract: Large VLBI telescopes undergo gravitational deformations which affect both geodetic and astronomic observations as well as the real reference point (RP) position (i.e. the reference point which is directly linked to and determined by the physics of the VLBI observations). As a consequence, the accuracy of eccentricity vectors determined with high precision terrestrial observations strictly depends on the possibility of univocally defining the geodetic instrument’s RP to be surveyed and estimated: technique dependent effects (e.g. gravitational and thermal deformations for VLBI, phase centre variations for GPS, etc) bias RP positions and weaken and perturb the information contained in the eccentricity. The impact on combined geodetic products is remarkable; a proper definition of space geodetic instruments’ RP must therefore account for possible biases that modify its theoretical position. Whether the problem must be directly addressed by each technique-specific Service is still an open issue. Indirect approaches based on high precision terrestrial observations have proved to be additional, accurate and independent tools for determining and monitoring the eccentricities at co-location sites. Nevertheless, a deeper and rigorous investigation on RP location’s variations is at least as important and it is nowadays fundamental for each space geodetic instrument. To this respect, we are presenting the investigations on VLBI telescope’s RP position that were carried out at Medicina and Noto (Italy) on the 32 m antennas: trilateration, triangulation and laser scanning observations were applied and combined to monitor the gravitational deformations which affect the telescope’s structure and to derive an elevation dependent correction function for radio signal path
A Comparative Study of the Applied Methods for Estimating Deflection of the Vertical in Terrestrial Geodetic Measurements
No full textThis 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
Laser scanner and terrestrial surveying applied to gravitational deformation monitoring of large VLBI telescopes’ primary reflector
No full textLaser scanning surveys were performed on the primary mirror of the VLBI (Very Long Baseline Interferometry) telescopes situated at Medicina and Noto observatories, with the specific purpose of investigating (i) gravity deformation patterns of the radio telescopes’ primary reflector and (ii) the magnitude and relative variations of focal length as the antennas are steered in elevation. Both instruments have AZ-EL mounts and have 32 m parabolic mirrors which were surveyed in steps of 15 degrees spanning the 90-15 degrees 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 decreases from 90 deg to 15 deg, the edges of the primary mirror of both telescopes fold in by a couple of cm. A least squares adjustment was applied to point
clouds corresponding to different elevations aimed at estimating the parameters of the rotational paraboloids that better fit the experimental data: this led to estimate the focal length variations induced by the structure deformative behaviour. The focal lengths of the best fit surfaces were compared; their largest variation is found to be 2.5 cm at Medicina, between the 90 deg and the 15 deg positions. The clouds were also used to attempt a direct computation of the incoming radio signal’s path length variation due to primary reflectors’ deformations. Finally, two Leica total stations, a TDA5005 and a TC2003, were used to perform a survey of the local ground control network and of some selected targets placed on the edge of the dish. The comparison of the distances determined with the two terrestrial surveying methods (laser scanner vs. triangulation and trilateration) highlights a statistically significant scale factor of about 1.0005 ± 0.0002, being the laser estimates smaller than those obtained with total stations.
This study proves that laser scanners can be efficiently used to determine gravitational influences on large VLBI telescopes’ primary reflectors: deformation patterns are clearly and reliably depicted, focal length and incoming radio signal path variations are precisely quantified
Medicina and Noto VLBI Radiotelescopes: gravitational deformations evaluated with terrestrial laser scanning
No full textThe 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
Impact of network geometry, observation schemes and telescope structure deformations on local ties: simulations applied to Sardinia Radio Telescope
No full textEffects of illumination functions on the computation of gravity-dependent signal path variation models in primary focus and Cassegrainian VLBI telescopes
No full textThirteen years of integrated precipitable water derived by GPS at Mario Zucchelli Station, Antarctica
No full textSince 1998, the Italian Antarctic Programme has been funding space geodetic activities based on the use of episodic and permanent global positioning system (GPS) observations. As well as their exploitation in geodynamics, these data can be used to sense the atmosphere and to retrieve and monitor its water vapor content and variations. The surface pressure p and temperature Ts at the GPS tracking sites are necessary to compute the zenith hydrostatic delay (ZHD), and consequently, the precipitable water. At sites where no surface information is recorded, the p and Ts values can be retrieved from, e.g., global numerical weather prediction models. Alternatively, the site-specific ZHD values can be computed by interpolation of the ZHD values provided in a grid model (2.5° × 2.0°). We have processed the data series of the permanent GPS site TNB1 (Mario Zucchelli Station, Antarctica) from 1998 to 2010, with the purpose of comparing the use of grid ZHD values as an alternative to the use of real surface records. With these approaches, we estimate almost 7 × 104 hourly values of precipitable water over 13 years, and we find discrepancies that vary between 1.8 (±0.2) mm in summer and 3.3 (±0.5) mm in winter. In addition, the discrepancies of the two solutions show a clear seasonal dependency. Radiosounding measurements were used to derive an independent series of precipitable water. These agree better with the GPS precipitable water derived from real surface data. However, the GPS precipitable water time series is dry biased, as it is ca. 77% of the total moisture measured by the radiosoundings. Both the GPS and radiosounding observations are processed through the most up-to-date strategies, to reduce known systematic errors.</p
Height bias and scale effect induced by antenna gravitational deformations in geodetic VLBI data analysis
No full textThe impact of signal path variations (SPVs) caused by antenna gravitational deformations on geodetic very long baseline interferometry (VLBI) results is evaluated for the first time. Elevation-dependent models of SPV for Medicina and Noto (Italy) telescopes were derived from a combination of terrestrial surveying methods to account for gravitational deformations. After applying these models in geodetic VLBI data analysis, estimates of the antenna reference point positions are shifted upward by 8.9 and 6.7 mm, respectively. The impact on other parameters is negligible. To simulate the impact of antenna gravitational deformations on the entire VLBI network, lacking measurements for other telescopes, we rescaled the SPV models of Medicina and Noto for other antennas according to their size. The effects of the simulations are changes in VLBI heights in the range [-3, 73] mm and a net scale increase of 0.3-0.8 ppb. The height bias is larger than random errors of VLBI position estimates, implying the possibility of significant scale distortions related to antenna gravitational deformations. This demonstrates the need to precisely measure gravitational deformations of other VLBI telescopes, to derive their precise SPV models and to apply them in routine geodetic data analysis
Improved geodetic European very-long-baseline interferometry solution using models of antenna gravitational deformation
No full textVery-long-baseline interferometry (VLBI) is used for establishing global geodetic networks where the coordinates attain a 1-mm level of precision. Technique-dependent bias can degrade the VLBI positioning accuracy if it is present and unaccounted for. Among the potential bias, gravitational flexure of VLBI telescopes can vary the path traveled by the incoming radio signal and induce a bias in the height component of the station position. We process here more than 100 European VLBI sessions spanning 1990-2009 with VLBI time delay/Solve software, as the only VLBI analysis package that can be used to correct signal-path variation (SPV) due to gravitational flexure of VLBI telescopes. Currently, SPV models are neglected in VLBI data analysis. To determine the kinematics of the European area over the last 20 years and to assess the effects of telescope gravitational deformation on geodetic VLBI estimates, we perform two VLBI solutions with and without SPV models for telescopes in Medicina (northern Italy) and Noto (southern Italy). The two solutions differ by 8.8 mm and 7.2 mm in their height components, with this bias being one order of magnitude larger than the formal errors of the estimated heights. SPV models impact uniquely on the height component of stations where SPVs are modeled. Velocities are not affected by the use of the Medicina and Noto SPV models, and we show that the crustal kinematics derived from VLBI does not suffer from a lack of information with regard to the flexure of other telescopes.<br /><!--[if gte mso 9]><xml> <w:WordDocument> <w:View>Normal</w:View> <w:Zoom>0</w:Zoom> <w:HyphenationZone>14</w:HyphenationZone> <w:PunctuationKerning /> <w:ValidateAgainstSchemas /> <w:SaveIfXMLInvalid>false</w:SaveIfXMLInvalid> <w:IgnoreMixedContent>false</w:IgnoreMixedContent> <w:AlwaysShowPlaceholderText>false</w:AlwaysShowPlaceholderText> <w:Compatibility> <w:BreakWrappedTables /> <w:SnapToGridInCell /> <w:WrapTextWithPunct /> <w:UseAsianBreakRules /> <w:DontGrowAutofit /> </w:Compatibility> <w:BrowserLevel>MicrosoftInternetExplorer4</w:BrowserLevel> </w:WordDocument> </xml><![endif]--><!--[if gte mso 9]><xml> <w:LatentStyles DefLockedState="false" LatentStyleCount="156"> </w:LatentStyles> </xml><![endif]--><!--[if !mso]><object classid="clsid:38481807-CA0E-42D2-BF39-B33AF135CC4D" id=ieooui></object> <mce:style><! st1:*{behavior:url(#ieooui) } --> <!--[endif]--> <!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 2.0cm 2.0cm 2.0cm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> <!--[if gte mso 10]> <mce:style><! /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Tabella normale"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;} --> <!--[endif]-->
Surveying the GPS-VLBI Eccentricity at Medicina: Methodological Aspects and Practicalities
No full textSummary: This paper describes our experiences in measuring the GPS-VLBI eccentricity at Medicina (Italy). It has been re-measured yearly during three different terrestrial surveys (2001, 2002 and 2003) and corresponding SINEX files with full variance covariance information were produced. The eccentricity vector connecting the GPS and the VLBI reference points has been measured within slightly different local ground control networks and high accuracy has been sought since the first campaign. The methodology has been refined year after year. We describe the choices that concern stationing, selection of instruments, devices and surveying strategies
