1,720,979 research outputs found
Improved PPP performance in regional networks
No full textIt has been demonstrated that precise point positioning (PPP) is a powerful tool in geodetic and geodynamic applications. As is known, it provides solutions in the reference system of the satellite orbits. We focuses on the strategy to transform PPP solutions into the International Terrestrial Reference System (ITRS) by applying a set of local Helmert transformation parameters obtained from a regional network rather than using global parameters.
In order to carry out this test, a regional network composed of 14 stations was analyzed using GIPSYOASIS II software, over a period of 6 years. Two solutions differently aligned to the ITRS were compared in terms of accuracy, scattering, frequency content and local movements.
One solution is aligned to IGb08 through the X-files provided by JPL, while the other is aligned to the European reference frame densification of IGb08 using customized regional X-files. Therefore, both are updated realizations of the ITRS. The test shows that a regional, instead of a
global, alignment to the ITRS can significantly improve the repeatability of the solutions. A small improvement can also be found in terms of agreement with the regional densification of IGb08. The analysis of the signal content in the differently aligned time series allowed some differences to be found, in terms of both frequency and magnitude. These differences are mainly due to an evident common signal that is defined for the whole area and which is removed when using regional alignment. Finally, residual scattering was calculated after removing the modeled signals from each time series, which results in a scatter being significantly smaller for the regional solution than for the global solution. In order to obtain these results, the choice of the reference stations is a major question and therefore discussed in detail
Low-cost GNSS sensors for monitoring applications
Full text linkAmong several instruments and techniques that can be used for monitoring purposes, GNSS technology has undergone a fast evolution and provides a large choice of solutions. Despite the best performances can be achieved by using double frequency geodetic receivers, capable to maintain high precisions even for wide-scale monitoring, in the recent years, several interesting solutions were presented in the low-cost market. In this work, a monitoring system based on a couple of low-cost GNSS receivers has been developed and tested in the field (Ponte Motta in Cavezzo - Modena, Italy). The two receivers were positioned about 50 m from each other and data were acquired at 1-Hz frequency. The position solutions were calculated both in post processing, through two different free and open-source software packages, and using an embedded RTK processor. The analysis aims to assess on one hand the capability of the system to perform the monitoring of slow displacements with the best possible precision, and, on the other hand, the performances of the real-time solutions that can be used for early warning purposes. The precisions evidenced by the tests show that such low-cost instrumentation can be used for many monitoring purposes, especially considering the cost that is about a tenth of geodetic instruments
Monitoring of the Garisenda Tower through GNSS using advanced approaches toward the frame of reference stations.
Full text linkStudy on the accuracy of Multi-GNSS PPP for different observing sessions time spans using PRIDE PPP-AR open-source software package
Full text linkThe Precise Point Positioning (PPP) approach to GNSS observables is widely used for processing data from permanent stations, providing highly precise coordinates. However, the performance of PPP for observation sessions shorter than 24 h has not yet been thoroughly investigated in the case of multi-constellation acquisitions. In recent years, the PRIDE PPP-AR software package has been made freely available. Since it includes a graphical user interface (GUI) version that runs under Windows, it can also be easily used by technical surveyors aiming to process data acquired from a single GNSS receiver. This is particularly valuable for surveys conducted in areas lacking dense geodetic infrastructures or reliable augmentation services. In this paper, based on a wide and consistent dataset, the coordinate precision obtained from observation sessions ranging from 30 min to 24 h processed with PRIDE PPP-AR is analyzed. In addition to multi-constellation GNSS data (GPS + Galileo + GLONASS + BeiDou), independent GPS-only and Galileo-only processing was also evaluated. Furthermore, the reliability of the formal errors provided by the software was examined, as these represent the only available information for assessing coordinate quality in surveys that lack geometric redundancy. While several online PPP services already exist, PRIDE PPP-AR overcomes common limitations related to the number of processed files and the choice of GNSS constellations. The results show that two-hour observation sessions can reliably achieve horizontal coordinate accuracy within 2 cm and vertical accuracy within 5 cm, whereas 30-minute sessions are suitable for applications requiring 5–10 cm accuracy
Metric Accuracy of Digital Elevation Models from WorldView-3 Stereo-Pairs in Urban Areas
Full text linkWorldView-3 satellite is providing images with an unprecedented combination of high spatial and spectral resolution. The stereo capabilities and the very high resolution of the panchromatic band (0.31 m) have been fostering new applications in urban areas, where the complexity of the morphology requires a higher level of detail. The present technical note aims to test the accuracy of digital elevation models that can be obtained by WorldView-3 stereo-pairs in these particular contexts, with an operational state-of-the-art algorithm. Validation is performed using check points and existing models of the area (from LiDAR data and oblique aerial images). The experiments, conducted over the city of Bologna (Italy) with six images, proved that roof surfaces and open spaces can be reconstructed with an average error of 1–2 pixels, but severe discrepancies frequently occur in narrow roads and urban canyons (up to several metres in average). The level of completeness achievable with only one pair is extremely variable (ranging from 50% to 90%), due to the combined effect of the geometry of acquisition and the specific urban texture. Better results can be obtained by using more than one pair. Furthermore, smaller convergence angles can be beneficial for the reconstruction of specific urban structures, such as soaring towers
Precision of PPP as a Function of the Observing-Session Duration
No full textOver the past decade, precise point positioning (PPP) has become a tool widely used in many Global Navigation Satellite Syste applications and the performance levels of the method are often close to those that can be achieved through a differenced approach. The aim of this paper is to analyze the dependency of the precision of a PPP solution output from the GIPSY-OASIS II software on the observing-session durations. In detail, starting from real data acquisitions spanning 24, 12, 6, 3 h, 1 h, and 1⁄2 an hour, all processed through the PPP, a continuous function has been defined and the related coefficients have been estimated. The original dataset consists of daily RINEX files provided from 44 sites of both International GNSS Service (IGS) and European Permanent Network (EPN) permanent networks acquired over one year. Each RINEX file was split into several shorter files according to the above listed time spans and processed by using GIPSY-OASIS II together with jet propulsion laboratory (JPL) precise post-processed products. The uncertainty of the proposed function was also estimated and a complete analysis of its compliance with the data sample has been provided. The estimated model is demonstrated to reach a millimeter accuracy level within a statistical confidence level of 99% using the experimental data
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
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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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