196,053 research outputs found
GPS as a geodetic tool for geodynamics in northern Victoria Land, Antarctica
No full textThe 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
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
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
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
STATIC GPS OBSERVATIONS ON THE ANTARCTIC PLATEAU FOR MOVEMENT DETECTION AND METEREOLOGICAL PURPOSES
No full textStarting from the past decades geodetic community has paid increasing attention to the use of long time series coming from geodetic techniques; nowadays, Very Long Baseline Interferometry, GPS, Satellite Laser Ranging, DORIS are routinely used for measuring global geodynamics. In Antarctica the deployment of permanent and/or semi-permanent instruments in inaccessible regions (E.G. the Antarctic plateau), is strongly constrained by environmental conditions and logistics; as a result, control networks are frequently characterized by a campaign-style repetitions of measurements. With the aim to study the velocities of the ice, whenever speeds are at few cm/y level, it is necessary to be able to discriminate between relative movements due to the local morphology and those absolute induced from the tectonic effects of continental drift.
Long-term GPS data acquisitions on the Antarctic plateau are rare, but they are quite useful for the connection between the local control networks and the global reference frame; the GPS strain networks installed around the deep ice coring sites at Dome C (DC, EPICA Project) and Talos Dome (TD, TALDICE Project) can be cited as examples
Moreover in the framework of the Italian contribution to the ITASE Project, several GPS static sessions were acquired on the Antarctic plateau in order to realize control networks for ice surface monitoring, reference stations for geophysical prospecting, etc. (Frezzotti et al. 2004; Vittuari et al. 2004). We analyse here the feasibility to use these short GPS time series, characterised by continuous acquisitions recorded on periods of some weeks or few months and we point out our attention in order to evaluate, day by day along with time series, the stability in positioning and moreover, the estimation of tropospheric parameters.
Corresponding author: [email protected]
Radiotelescopi di Noto e Medicina: deformazioni gravitazionali valutate tramite un’indagine laser terrestre
No full textI radiotelescopi VLBI (Very Long Baseline Interferometry) subiscono delle deformazioni
gravitazionali al variare della loro elevazione che tendono a modificare la posizione del punto di
riferimento. Tale punto normalmente coincide, per i telescopi VLBI, con il “tracking point” ITRF
utilizzato per la realizzazione dell’ITRS (International Terrestrial Reference System).
La determinazione delle deformazioni è pertanto di importanza primaria e, a tutt’oggi, gli studi
effettuati sull’argomento ed i relativi metodi di indagine risultano essere scarsi.
Per affrontare questa complessa problematica, si è applicata alle antenne di Noto e Medicina la
metodologia del laser scanning terrestre.
La scansione delle parabole è stata possibile ponendo il laser all’interno della struttura, su due
supporti stabili fissati in prossimità del fuoco secondario. Tali supporti sono stati appositamente
realizzati per questo rilievo. Non era possibile, infatti, utilizzare le alte piattaforme esterne,
impiegate solitamente nei rilievi fotogrammetrici, a causa della loro instabilità.
Il laser ad impulsi utilizzato per questa indagine possiede una risoluzione di circa 1,5 mm a 50m e
ha permesso di ottenere nuvole di punti, rappresentanti le parabole scansionate, in un sistema di
riferimento solidale allo strumento. Al fine di inserire le nuvole di punti acquisite dal laser in un
sistema di riferimento topocentrico esterno sono stati utilizzati target sferici collegati a prismi
retrodirettivi che sono stati osservati sia dal laser, sia tramite un rilievo terrestre eseguito con una
coppia di total stations di alta precisione
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
Un'indagine sulla stima dei vettori eccentricità tra strumenti geodetici co-locati nei siti ITRF
No full textLa stima dei vettori eccentrici negli osservatori geodetici co-locati è d’importanza cruciale nelle
procedure di combinazione e calcolo di Terrestrial Reference Frames (TRF) che scaturiscono dalla
integrazione di osservabili spazio-geodetiche di varia natura. In effetti la disponibilità di local ties
dotate di informazione statistica esaustiva rappresenta l’unico mezzo per poter integrare osservabili
geodetiche che derivano da tecniche differenti, consentendo, al tempo stesso, di valutare la coerenza
dei singoli TRF “monotecnica” nelle procedure di combinazione. Tuttavia, l’alto grado di
conoscenze richieste per la produzione di eccentricità, sia sul fronte operativo sia su quello teorico,
ne inficia l’ampia diffusione e ne inibisce l’impiego. E’ inoltre opportuno sottolineare che una delle
maggiori difficoltà nel processo di produzione delle local ties è connessa all’inquadramento del
vettore eccentricità calcolato con tecniche terrestri nell’ITRF. A tal proposito, la possibilità di
ottenere eccentricità attraverso un approccio GPS potrebbe facilitare l’intero processo produttivo: il
vettore eccentricità sarebbe naturalmente inquadrato nel sistema globale di interesse (ITRF). Questo
lavoro approfondisce gli aspetti connessi all’impiego di un approccio GPS nella produzione di
eccentricità VLBI-GPS nel sito co-locato dell’Istituto Nazionale di Astrofisica di Medicina (BO)
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
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