1,720,983 research outputs found
Performance of Atlas GNSS Global Correction Service for High-Accuracy Positioning
Full text linkIn the last decade, a number of correction services for global navigation satellite systems (GNSS) precise positioning have been developed, mainly for offshore applications, based on a precise point positioning (PPP) real-time processing. These allow receiving corrections without the need for an internet connection or reference benchmarks around the survey area. In the paper, we tested the Atlas correction service implemented in a Stonex S900A machine (Monza, Italy), with the purpose to verify its performances under optimal operational conditions and in the practical case of a land survey on several benchmarks along the Adriatic coast. The data analysis focused on: accuracy with respect to the reference frame, repeatability of the coordinates considering short and long acquisition periods, time to initialize the survey, and reliability of the formal errors provided by the instrument. The system confirmed the declared performances in most cases and is shown to be a viable alternative to other GNSS techniques also for land surveys where no obstacles affect the sky visibility
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
Reliability of Real-Time Kinematic (RTK) Positioning for Low-Cost Drones’ Navigation across Global Navigation Satellite System (GNSS) Critical Environments
Full text linkUAVs are nowadays used for several surveying activities, some of which imply flying close to tall walls, in and out of tunnels, under bridges, and so forth. In these applications, RTK GNSS positioning delivers results with very variable quality. It allows for centimetric-level kinematic navigation in real time in ideal conditions, but limitations in sky visibility or strong multipath effects negatively impact the positioning quality. This paper aims at assessing the RTK positioning limitations for lightweight and low-cost drones carrying cheap GNSS modules when used to fly in some meaningful critical operational conditions. Three demanding scenarios have been set up simulating the trajectories of drones in tasks such as infrastructure (i.e., building or bridges) inspection. Different outage durations, flight dynamics, and obstacle sizes have been considered in this work to have a complete overview of the positioning quality. The performed tests have allowed us to define practical recommendations to safely fly drones in potentially critical environments just by considering common software and standard GNSS parameters
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
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
Gnss and photogrammetric uav derived data for coastal monitoring: A case of study in emilia-romagna, italy
Full text linkGNSS real-time techniques and UAV photogrammetry can be alternative methods for the monitoring of sand beaches. This activity is particularly important in environments such as the Emilia-Romagna coastline. In this paper, two couples of surveys (year 2019 and 2020) performed using GNSS or a low-cost UAV equipment over a common area were compared in order to analyse: point-wise height differences, profile shapes along defined sections, and volumes variations over time. Both surveys were aligned to the same reference benchmark through GNSS measurements. The highest discrepancies between the two surveying methods (tens of cm) were found in vegetated areas and along the shoreline, otherwise, the height differences are mainly within the 10 cm level. In terms of volumes, excluding the most critical areas, differences close to zero can be found. Obtained results show that GNSS and UAV photogrammetry provides similar results, at least for quite flat terrains and when decimetre-level accuracy is required
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
A PPP kinematic application on historical GPS data: the reprocessing of the ITASE98-99 Antarctica mission height profiles
No full textThe analysis of altimetric profiles in Antarctica and their evolution over the years is a sensitive topic for the scientific community since it helps understand the effects of climate change that the continent undergoes. Different geomatic techniques, including the GNSS technology, can be employed to obtain altimetric profiles. However, the GNSS differenced approaches, such as the Post Processing Kinematic, are hardly usable to define long profiles in Antarctica because of the low number of CORS stations. In these conditions, the Precise Point Positioning (PPP) approach is a valid alternative to avoid processing very long baselines. The aim of this article is to define a standard procedure for the processing of historical GPS data, thanks to the availability of a dataset from the International Trans-Antarctic Scientific Expedition, which took place between 1998 and 1999 (ITASE98-99). This expedition focused on mapping the Antarctic territory, subdividing it by nations of influence, using geophysical and geodetic technologies, including GPS. The altimetric profiles had already been calculated in 2002 by the Geomatics group of the University of Bologna using the Gipsy-OASIS II software. In this work, the new version of the JPL software, GipsyX, is used to apply the newly implemented models and reprocessed products. The calibration of the processing parameters leading to the final PPP solution is described in the paper, including details on the implementation of a post-processing filtering procedure. The average a posteriori elevation error is 4.6 cm, while 99% of them are within 27 cm. The comparison of the new results to both the previous processing and the REMA elevation model shown that about double the number of solutions are now available, meter-level elevation spikes have been avoided, and a half meter bias is now reduced to a few centimeters. Given the almost 15 years difference between the 1999.0 expedition epoch and the REMA reference epoch, the obtained results can be used to study accumulation/erosion effects on the Antarctica ice sheet
A Priori Estimation of Radar Satellite Interferometry’s Sensitivity for Landslide Monitoring in the Italian Emilia-Romagna Region
Full text linkThe InSAR technique is known to be a powerful tool for precise monitoring of wide areas in terms of displacements. It is conceivable to also use this technique to monitor landslide areas, but geometrical distortions due to ground morphology and land cover could make InSAR processing ineffective for such applications. Because of the computational burden of InSAR processing, it is important to have preliminary knowledge about the possible suitability of the technique for the inspected area before acquiring and processing the data. This paper aims to perform a preliminary analysis of the InSAR sensitivity for the specific case of landslide monitoring. A new approach is proposed considering aspects specific to landslide displacements, which are basically tangent to the slope direction. Pre-processed coherence maps were used to account for the impact of land cover. The whole analysis can be carried out without acquiring cumbersome SAR datasets and can be used as a preliminary step. The Italian Emilia-Romagna region has been considered as the study area, with landslide areas accounting for more than 12% of its territory. The outcomes show that the inspected area has favourable morphological conditions, mainly thanks to its mild slopes and the limited number of landslides facing north, but the land cover has a strong negative impact on the InSAR sensitivity. Nevertheless, 7.5% of the landslide areas have promising conditions for monitoring using radar interferometry
Studi di fattibilità per il monitoraggio delle deformazioni del suolo. Studio di metodi avanzati per il monitoraggio della subsidenza in ambito off-shore.
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