1,720,974 research outputs found
Study on GPS PPP precision for short observation sessions
No full textPrecise point positioning is increasingly being used in geodetic applications that in many cases are based on static 24-hour RINEX files. Since there are many applications where sub-centimeter position accuracy is not required and users wish to use a single receiver and not be dependent on differential correction, we will evaluate PPP performance for static positioning with 12-, 6-, 3-, 1- and Â1⁄2-h observations. We have, therefore, considered a dataset for the year 2013 from 14 European GNSS stations. The data were analyzed using GIPSY-OASIS II software package and evaluated in terms of repeatability of the coordinates and of coherence with the formal error indicated for each PPP solution. Particular attention was paid to solutions showing large discrepancies in coordinates. The test shows that PPP precision for the 24-h files is below 5Â mm, but decreases slightly for the 12-, 6- and 3-h observation sets. For the 1-h and the Â1⁄2-h RINEX files, precision is within 5 and 10Â cm, respectively. The analysis is completed with a discussion on the impact of the ambiguity resolution that shows how it significantly improves only the easting component and moreover has a higher influence on the formal error rather than on the solutions. Lastly, the study contains an investigation into the reliability of the formal error associated with the PPP solutions. We show that the formal error can be used to identify incorrect solutions, but is not suitable to represent the real accuracy. For that reason, we propose to use the formal error given for the float solutions even for the ones with fixed ambiguities
Impact of Multiconstellation on Relative Static GNSS Positioning
Full text linkUntil a few years ago, a precise survey was only possible using global positioning system (GPS) and Global'naja Navigacionnaja Sputnikovaja Sistema (GLONASS) constellations, but the result was not guaranteed under conditions of poor sky visibility, as in urban canyons. Currently, the number of Global Navigation Satellite System (GNSS) satellites in orbit has strongly increased thanks to the great evolution of the Galileo and the Beidou constellations. In this paper, we investigate the impact of using different constellations and their combinations, in static positioning with the classical differencing approach. For this purpose, two distinct baselines of different lengths (10 and 60 km) were processed using commercial software over a period of one year (2018.24-2019.24). Data were acquired by permanent stations belonging to the European Permanent Network (EPN) network providing 24-h observing sessions. Two datasets were tested, one consisting of 24-h Receiver Independent Exchange Format (RINEX) files and the other considering only 2-h sessions of data acquisition. In both cases, a one-year-long time span has been considered. The baselines were processed considering each of the four GNSS constellations and a series of combinations, for a total of eight solutions. Results have been evaluated looking at the accuracy and repeatability of the coordinates, together with the main constellation parameters. During the analyzed period the number of contemporary visible satellites of the BeiDou constellation was still too poor over the considered area, and therefore this constellation did not provide comparable precisions in respect to the others. Positioning precision provided by the Galileo constellation has shown to be very close to those given by GPS or GLONASS, with a significant difference only on the height component, especially in the case of processing 2-h data. As for 24-h observing sessions, the use of multiconstellation observables actually leads to small improvements in precision with respect to the use of GPS data only, mainly appreciable considering the vertical component. The GPS-Galileo combination gives quite the same performances of the GPS-GLONASS one, but it can potentially take advantage of the integrity message provided by the European constellation
A strategy for the monitoring of tall structures in urban area using GNSS technology
No full textGNSS technology has become widely used for monitoring purposes. The high precisions nowadays available have made the technique suitable also for the monitoring of structures that are usually affected by very small displacements. In this work we investigate the issue concerning the monitoring of a tall structure such as the Garisenda tower, which lay in the Bologna city centre, by using GNSS data gathered by a permanent station placed on its top. We consider the need to investigate the variations in the leaning of the structure, therefore also the position of the ground at the bottom of the structure should be known. Unfortunately it is not possible to place a GNSS receiver under a tall structure in urban context because of the too poor sky visibility. A solution would be to choose another permanent station located as close as possible assuming its behaviour coherent with the ground under the monitored structure. This hypothesis has proven not to be verified in the analysed case, where four permanent stations located within few kilometres far from the Garisenda tower were available. Therefore a strategy to combine data from the five permanent stations using a uniform strain model was developed in order to define a reference to which compare the positions given by the GNSS sensor placed on the top of the tower. The impact of such strategy will be shown and discussed in terms of mean variation of the leaning of the tower over a period of about four years
Unconventional methods for offshore subsidence monitoring
No full textThis study was carried out by the SEADOG Research Center at Politecnico di Torino (Italy). The pur-
pose of this work was to evaluate which complexity degree would be required to reliably approach
a subsidence study for different scenarios. The study was based on sensitivity analyses which were
performed using a series of 3D synthetic numerical models of which the structural characteristics
and geological and mechanical properties were based on available public data of onshore and
offshore hydrocarbon fields in Italy. An array of simulations, both one-way and two-way coupled,
were carried out to assess the magnitude and extension of subsidence potentially induced by hydro-
carbon production. The results allowed the calculation of subsidence indices defined as the rate
of compaction propagation (i.e., the ratio between the maximum surface displacement and the
maximum reservoir compaction) and as the rate of volume loss (i.e. the ratio between the volume
of the subsidence bowl or cone and the volume variation of the reservoir). These indices together
with the degree of the underground systems’ heterogeneity led to the definition of the Intact Rock
Qualitative Subsidence Index (IRQSI), upon which the needed complexity degree of a subsidence
study can be discerned
Advancing InSAR analysis: TimeSAPS for linear and nonlinear displacement modeling
Full text linkThe Synthetic Aperture Radar Interferometry (InSAR) technique enables precise monitoring of ground displacements over extensive areas based on radar data. While several open-source software packages have been developed for SAR data processing, most retrieve the average velocity of Persistent Scatterers (PS) clusters under the assumption of linear behavior, limiting their application in complex scenarios.
To enable more advanced and detailed analysis of InSAR time series, the TimeSAPS
software package has been developed. This tool addresses the limitations of existing open-source packages, which primarily focus on linear approximations of displacement time series, by introducing advanced capabilities for analyzing both linear trends and nonlinear components. TimeSAPS performs a comprehensive analysis of PS derived from InSAR processing, characterizing time series in terms of linear trends, periodic signals, and nonlinear movements. Nonlinear components are modeled as a combination of sinusoids, each defined by its phase, amplitude, and frequency power spectrum. TimeSAPS overcomes the limitations of existing tools by providing advanced methods to recognize and model nonlinear surface movements, even when they are not known a priori.
This paper presents the theoretical foundations of TimeSAPS and demonstrates its capabilities through two case studies based on real InSAR data. These examples showcase the software’s effectiveness in reconstructing nonlinear displacement patterns and identifying periodic trends. The results underline TimeSAPS’s potential to analyze complex ground displacement scenarios, making it a valuable resource for the scientific and engineering communities
Monitoring of the Garisenda Tower through GNSS using advanced approaches toward the frame of reference stations
No full textThe Garisenda tower in Bologna is a symbol of the city and one of the most valuable heritages of the medieval age. The tower is leaning markedly since the XIV century because of a foundation failure and its stability is nowadays under constant monitoring through many sensors. In 2013 a GNSS permanent station was installed on the top of the tower with the aim to test the satellite technology for this particular kind of structural monitoring. Being the leaning of the Garisenda the subject of the investigation and being the sensor placed on its top, one fundamental hypothesis is the stationarity of the ground under the tower with respect to the reference system used for the GNSS measures. This hypothesis has demonstrated to be unreliable considered the high precision of the survey and the Earth crust dynamics, therefore opening interesting issues concerning the reference to be used in such kind of monitoring. The proposed solution rely on a strain model of the area surrounding the Garisenda tower, estimated using data from four other GNSS permanent stations already present in Bologna. The method is described and results are shown in terms of trend over time of the Garisenda's leaning. Nevertheless, the methodology can be generalized for every kind of structural monitoring based on GNSS data for which millimetre level of precision is needed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
GNSS measurements for ground deformations detection around offshore natural gas fields in the Northern Adriatic Region
Full text linkThe study aims to evaluate ground deformations in a vast area characterized by the coexistence of intense anthropic activities and offshore natural gas production. Onshore subsidence can be studied by GNSS, InSAR, high precision leveling and extensometers that provide broad datasets for a fully integrated description of the phenomenon. At present, seafloor subsidence monitoring cannot be carried out by high precision leveling, and GNSS is the only reliable method, implemented by means of permanent stations installed on offshore hydrocarbon production facilities. In the Northern/Central Adriatic Sea gas production platforms, GNSS data are recorded since more than 15 years, allowing to estimate not only the average subsidence of the platform/seafloor, but also possible velocity variations due to underground fluids withdrawal. This study shows the comparison of 22 offshore GNSS permanent stations located in the study area. Raw data have been processed with two different software packages (GIPSY-OASIS and GAMIT-GLOBK) based on different approaches and considering different boundary conditions of geodetic and/or modeling nature. Main results point out the high accuracy of the GNSS technology considering also the impact of data processing. Finally, at selected permanent stations we also performed a comparison of results obtained by GNSS, InSAR and high precision leveling
Variations on the Author
Full text link“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
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