1,720,988 research outputs found
Terra Nova Bay GPS permanent station (Antarctica): data quality and first attempt in the evaluation of regional displacement
No full textThe paper discusses the long time series analysis of GPS permanent station located at Terra Nova Bay, East Antarctica (Victoria Land). The station (TNB1) was monumented during the 1997–1998 Italian expedition in Antarctica and the data collection started on January 1998. TNB1 is included in the Victoria Land network for deformation control (VLNDEF) network that was established with the aim to study the horizontal and vertical displacements in the region.
The good-quality data obtained from this station allow the computation of long time series performed both by using a network approach with Bernese V4.2, and precise point positioning technique with GIPSY–OASIS II. In a first step, a subset of data was analysed to define the best processing strategy. Moreover, in the network approach, international GPS service (IGS) permanent stations (intra- and extra-plate) were used.
Results obtained from the two approaches are comparable in terms of both repeatability and linear trend. Finally, velocities estimated for all Antarctic stations were compared to the values provided by ITRF2000. In most cases horizontal velocities agree with the model, while significant differences are present along the vertical components
Inquadramento delle reti regionali VLNDEF e TAMDEF (Antartide) nel sistema di riferimento globale
No full textL’obiettivo del presente lavoro è di definire le strategie più opportune per l’inquadramento delle reti antartiche di stazioni permanenti ed episodiche in un sistema di riferimento globale. Lo studio è stato focalizzato in particolare sulla rete antartica di stazioni permanenti e sulle sottoreti regionali VLNDEF e TAMDEF, allo scopo di ottenere informazioni circa la cinematica superficiale della Terra Vittoria (Antartide). L’approccio scelto per l’inquadramento è stato sviluppato in tre fasi: inquadramento delle stazioni permanenti presenti nella regione oggetto di studio, connessione dei siti delle due reti regionali alle stazioni permanenti precedentemente inquadrate, elaborazione di tutte le informazioni a livello di equazioni normali. Per l’analisi dei dati è stato utilizzato il Bernese GPS software v5.0. Per l’elaborazione dei dati sono stati utilizzati parametri e modelli standard, eseguendo per alcuni di essi dei test per verificare quanto essi incidessero sull’analisi del dato e quali fossero le scelte migliori per la soluzione finale. L’analisi delle serie storiche ha evidenziato quali stazioni permanenti fossero affidabili per la connessione delle reti regionali. L’analisi spettrale delle stesse ha messo in rilievo la presenza di segnali periodici che influenzano la determinazione delle velocità, in particolar modo per la componente verticale. I risultati ottenuti mostrano un andamento ben determinato per quanto riguarda le velocità planimetriche, mentre, sono ancora incerte, in termini di spostamento verticale assoluto, a seconda del sistema di riferimento utilizzato, ITRF2000 o ITRF2005. Questo lascia ancora irrisolto il problema dell’interpretazione dei risultati da utilizzare come condizioni al contorno per i modelli geodinamici delle zone oggetto di studio
Local effects of redundant terrestrial and GPS-based tie vectors in ITRF-like combinations
No full textTie vectors (TVs) between co-located space geodetic
instruments are essential for combining terrestrial reference
frames (TRFs) realised using different techniques. They
provide relative positioning between instrumental reference
points (RPs) which are part of a global geodetic network
such as the international terrestrial reference frame (ITRF).
This paper gathers the set of very long baseline interferometry
(VLBI)–global positioning system (GPS) local ties
performed at the observatory of Medicina (Northern Italy)
during the years 2001–2006 and discusses some important
aspects related to the usage of co-location ties in the combinations
of TRFs. Two measurement approaches of local
survey are considered here: a GPS-based approach and a
classical approach based on terrestrial observations (i.e.
angles, distances and height differences). The behaviour of terrestrial local ties, which routinely join combinations of
space geodetic solutions, is compared to that of GPS-based
local ties. In particular, we have performed and analysed different
combinations of satellite laser ranging (SLR), VLBI
and GPS long term solutions in order to (i) evaluate the
local effects of the insertion of the series of TVs computed
at Medicina, (ii) investigate the consistency of GPS-based
TVs with respect to space geodetic solutions, (iii) discuss
the effects of an imprecise alignment of TVs from a local to
a global reference frame. Results of ITRF-like combinations
show that terrestrial TVs originate the smallest residuals in
all the three components. In most cases, GPS-based TVs fit
space geodetic solutions very well, especially in the horizontal
components (N, E). On the contrary, the estimation of
the VLBI RP Up component through GPS technique appears
to be awkward, since the corresponding post fit residuals
are considerably larger. Besides, combination tests including
multi-temporal TVs display local effects of residual redistribution,
when compared to those solutions where Medicina
TVs are added one at a time. Finally, the combination of
TRFs turns out to be sensitive to the orientation of the local
tie into the global frame
Determination of vertical motion from levelling data in the wider area of Medicina and the Apennine foothills
No full textThe vertical motion of the Medicina Radio Telescope with respect to the north-central European ”stable platform” has been determined from both VLBI and GPS with a clear tendency towards subsidence on the order of 3 to 4 mm/yr. Here, we present investigations of the local motion of the site in the context of groundwater level evolution in the southeastern Po-Plain. From levelling data which are connected to the foothills of the Apennine it appears that the subsidence is indeed closely related to groundwater withdrawal in the region
Medicina and Noto VLBI Radiotelescopes: gravitational deformations evaluated with terrestrial laser scanning
Get PDFThe 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
Vertical motions in Northern Victoria Land inferred from GPS: A comparison with a glacial isostatic adjustment model
No full textRecent development of italian geodetic reseacrh related to the comparison of defromation obtained with GPS measurements with data obtained from GIA model
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Terra Nova Bay GPS permanent station
No full textDuring the IV Italian expedition in Antarctica, the geodetic activity started with the installation of the geodetic network in the area of Terra Nova Bay. The network was established in order to provide a common reference frame to several scientific activities (photogrammetry, cartography, geology) and to study the geodynamics of an area including some interesting tectonic and geologic features. The TNB1 (official name) GPS permanent station was installed during the XIII Italian expedition in 1998, over a granitic hill close to the Italian station of Terra Nova Bay. Following the recommendations for a stable and well-suited site location for the GPS
permanent station a concrete-made pillar was monumented in an area far away from radio frequency disturbs which also guaranteed a complete satellite visibility. The location of the GPS permanent station site with respect to Terra Nova Bay station and a particular of the
monumentation are shown in figure 1.
The geographic location of Terra Nova Bay covered a gap in the distribution of GPS
permanent stations in East Antarctica and, more precisely, along northern Victoria Land. Fifteen GPS permanent stations have been installed in Antarctica until today, and seven of them are IGS (International GPS Service for Geo-dynamics) stations. The non-homogeneous geographic distribution of stations, the long distances
involved and their prevalent settlement on the coastal area leads to some difficulties in a precise satellite ephemeris determination and in GPS data processing for regional geodynamic purposes phenomena with an accuracy at the 5 mm level achieved in static relative positioning
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