1,720,975 research outputs found
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
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)
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]
An investigation on a GPS-based approach to local tie computation
No full textThe estimation of eccentricity vectors in co-located sites represents a crucial aspect in order to combine reference frames arising from different space-geodetic techniques; thus their role is pivotal as they deeply influence the intrinsic precision of the combined reference frame and, at the same time, allow a strictly coherent datum realization of combined networks. The way the local ties are estimated through an indirect approach strongly reflects upon the precision of the final multi-technique reference frame and its inner consistency (Ray & Altamimi, 2005).
The investigation, here presented, dwells upon the possibility to pursue the local tie estimations through a purely GPS-based survey performed in the ITRF Co-located site of Medicina (Italy).
Each local tie is characterized by an intrinsic precision strictly related not only to the way it has been surveyed but also to the applied conditions during the Least Square Estimation in a rigorous statistically-based procedure. Nevertheless, if a terrestrial survey is executed, it should be emphasized that the eccentricity vector components, finally aligned in ITRF, depend on the transformation performed in order to carry out the transition from the local topocentric reference frame, where the tie was classically surveyed, and the final realization of ITRF (Ray & Altamimi, 2005) .
Processing and obtaining local eccentricities vectors via GPS-based surveys could allow to overcome the difficulties related to the aforementioned reference frame transition. In fact, since the Global Positioning Systems directly operate in a geocentric reference frame, the passage to ITRF naturally comes neglecting the difficulties related to the gauging of the vertical deflection.
The local ties computations are performed through CLEMeNT, an house-made software,which showed satisfactory results in least-squares estimations both for highly precise terrestrial datasets and for simulated and intrinsecally noisy ones ; as far as it is concerned, the accuracies reached in processing noisy datasets have inspired the possibility to perform GPS based local ties by means of a new release of CLEMeNT, modified ad hoc in its structure in order to include a brand new ensemble of constraints which are going to strengthen the definition of the tracking points belonging to the local tie
LA GEOMATICA APPLICATA ALLA CORREZIONE DEGLI EFFETTI DI DEFORMAZIONE GRAVITAZIONALE NEL VLBI GEODETICO GEOMATICS APPLIED TO GRAVITATIONAL DEFORMATION CORRECTIONS IN GEODETIC VLBI DATA ANALYSIS
Get PDFA few recent studies have addressed a number of shortcomings currently embedded in space geodesy observations whose solution has been successfully based on geomatics methods. The tie vectors between co-located space geodetic instruments can be efficiently and accurately measured by means of triangulation, trilateration, levelling and GPS.
In addition, laser tracking and laser scanning have also proved to be central in addressing the deformations of the structures of VLBI telescopes. Their effects on the estimated heights were spotlighted as far as the late 70s.
Nevertheless, only during the last decade some Italian studies have successfully used geomatics to model and remove the effects of gravitational flexures. We will show how the investigations published so far have quantified this effect for the very first time
An Analysis of Local Tie Vectors’ Temporal Evolution and Site Stability at Medicina Observatory through Terrestrial and GPS-based Observations
No full textThe observatory of Medicina (Italy) hosts a co-location between GPS and VLBI, whose eccentricity vector has been re-measured
several times once since 2001.
As a result, the co-located site is now provided with a series of nearly annual local tie vectors produced by means of terrestrial and GPS-based observations.
This work aims at summarising the results of a systematic analysis of the whole set of local ties produced so far and highlighting at the same time their temporal evolution.
As such, the application of algebraic tools focussed on deformation detections can benefit the analysis of the local tie temporal variations and at the same time that one of the “geodynamical” stability of the co-located site.
Nevertheless, just like in any other problem of deformation detection, such an analysis would ask for a reproducible local datum,
according to which the relative displacements among the points have to be referred to.
The application of algebraic projection theory to the local tie oriented-networks will be carried out in order to select a “common and fully reproducible” datum onto which the network solutions will be projected and discuss possible displacements within the re-measured networks
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
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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