Recurrent Neural Networks Applied to GNSS Time Series for Denoising and Prediction

Global Navigation Satellite Systems (GNSS) are systems that continuously acquire data and provide position time series. Many monitoring applications are based on GNSS data and their efficiency depends on the capability in the time series analysis to characterize the signal content and/or to predict incoming coordinates. In this work we propose a suitable Network Architecture, based on Long Short Term Memory Recurrent Neural Networks, to solve two main tasks in GNSS time series analysis: denoising and prediction. We carry out an analysis on a synthetic time series, then we inspect two real different case studies and evaluate the results. We develop a non-deep network that removes almost the 50% of scattering from real GNSS time series and achieves a coordinate prediction with 1.1 millimeters of Mean Squared Error

Definition of the Local Geoid Undulation Using Non-contemporary GNSS-Levelling Data on Subsidence Area: Application on the Adriatic Coastline

The knowledge of the so-called geoid undulation, which represents the height of the geoid above a reference ellipsoid, is a fundamental step to link ellipsoidal heights measured using satellite systems and orthometric heights. Several geoid models are available at the time both at the national or global scale, which can be used for the purpose. Another way to define the geoid undulation is to perform joint measures with GNSS and spirit levelling over common benchmarks. This requires onerous measurements that is seldom possible to perform due to their cost. In this work, we evaluated the possibility to define a local model of the geoid undulation using already available spirit levelling orthometric heights and GNSS ellipsoidal heights measured about 13 years later. The test area is the Emilia-Romagna Adriatic coastline, an area of great interest both from the environmental and economic point of view, which is also undergoing consistent subsidence phenomena. Test results show that the available measurements allow defining a geoid undulation that is coherent with the shape defined by the gravimetric models and also allows to transform ellipsoidal heights into orthometric ones more consistent with the height reference available on the surveyed area. A 7 cm overall bias with respect to the ITALGEO05 was found, whereas ITG2009 and EGM2008 have higher differences. The use of subsidence models to align over time the coordinates used to define the geoid undulation has proven to be a fundamental step. The analysis on the a-priori uncertainty in the geoid height definition shown that the combined use of much more precise GNSS coordinates and contemporary spirit levelling campaign is necessary to significantly improve the resulting geoid height

The monitoring of the geodetic reference frames by means of GNSS: from the global dynamic systems to the networks for real time positioning

I sistemi di riferimento geodetici, ed i frame che li materializzano, costituiscono struttura di base che permette di confrontare coerentemente le rappresentazioni geometriche del territorio acquisite con rilievi diversi. Nell’ultimo secolo si è vista una continua evoluzione nella definizione dei frame geodetici, dovuta principalmente alle nuove esigenze, e possibilità, introdotte dalle tecniche di geodesia spaziale. È cambiata la scala alla quale si affronta il problema, sono cambiate le precisioni delle misure e delle accuratezze richieste, sono cambiati gli approcci e le definizioni. L’esigenza di un sistema di riferimento valido su scala globale ha imposto la definizione di frame dinamici, che seguano le evoluzioni della crosta terrestre nel suo complesso, introducendo così la necessità di una coordinata temporale oltre alle classiche coordinate geometriche. Proprio la natura dinamica della Terra implica delle variazioni nel tempo delle coordinate, espresse nel riferimento globale, troppo elevate per essere gestite nelle applicazioni cartografiche a scala continentale/nazionale, per cui si è resa necessaria la definizione di altri sistemi di riferimento, vincolati alle placche tettoniche. Di fondamentale importanza sono diventati quindi i parametri di conversione delle coordinate che premettono il passaggio da sistemi globali a regionali, e viceversa. Nel percorso di questa tesi ci si è occupati del monitoraggio dei frame di riferimento dinamici, in particolare di quello nazionale realizzato dalla Rete Dinamica Nazionale. È stato affrontato il problema delle velocità residue che caratterizzano il territorio italiano nel sistema di riferimento europeo, definendo un nuovo sistema ad hoc per il territorio nazionale. È stata indagata la tecnica di calcolo Precise Point Positioning, sia dal punto di vista geodetico e dell’inquadramento delle coordinate nel sistema globale, sia dal punto di vista del rilievo tecnico. Infine, ci si è occupati delle reti geodetiche diffuse sul territorio di Regione Emilia-Romagna, e della funzione che queste ancora devono avere nell’era dei GNSS.The geodetic reference systems, and the related frames, constitute the basic structure that allows to compare geometric representations of the earth acquired by different surveys. The last century has seen a continuous evolution in the definition of geodetic frames, mainly due to new needs, and possibilities, introduced by the space geodesy techniques. Are changed the scale of the problem, the precision of the measurements and the required accuracies, and also the approaches and definitions. The requirement of a global reference system has imposed the definition of dynamic frames, which follow the evolution of the earth's crust as a whole, so the need for introducing a time coordinate in addition to usual geometric coordinates. The dynamic nature of the Earth involves variations in time of coordinates, when expressed in the global reference system, too quick to be handled in mapping applications on a continental / national scale. So it was necessary the definition of other reference systems, linked to the tectonic plates. Thus, fundamental are the transformation parameters needed to switch from regional systems to global systems, and vice versa. In this thesis the topic of the monitoring of dynamic reference frames has been addressed, in particular the Italian Rete Dinamica Nazionale. The matter of the high residual ETRS velocities that characterize the Italian peninsula has been addressed, and a new ad hoc reference system was defined. The calculation technique of Precise Point Positioning has been investigated, both from the geodetic point of view, addressing the strategies to align the coordinates in the global system, than from the point of view of technical surveys. Finally, the geodetic networks of the Emilia-Romagna Region were analyzed, evaluating their importance in the age of GNSS

Spatial and Temporal evolution of the subsidence phenomena in the Italian Peninsula

A large number of continuous GNSS (CGNSS) stations are nowadays available in Italy, this has already allowed an accurate monitoring of the horizontal and vertical kinematic pattern in the Italian peninsula in terms of linear trends. The crustal displacements can be considered as the result of several contributions: global, regional and local tectonic processes, climatic and meteorological phenomena, but also human activities. In particular, the groundwater exploitation for agricultural and industrial purposes and the extractive activities of gas, oil and geothermal fluids can induce displacements that can be greater than the ones due to natural contributions. Human activities could induce rapid changes in the local dynamic of the Earth crust and usually have stronger impact on the vertical component. Therefore, an accurate monitoring of the vertical displacements that takes into account also the spatial heterogeneity of the human activities is a major issue. In order to monitor and study the vertical velocity field in the Italian area, the observation of more than 600 CGNSS sites have been analysed using the GAMIT software package. The interdistances between the considered sites is about 40-50 Km and should allow a fairly good definition of the vertical velocity field and to study the possible spatial evolution of the pattern. The relatively long time interval of data acquisition (2001-2018) provides an important data set that make possible to identify different time evolutions with respect to the linear trend usually adopted in the GNSS time series analysis. The present vertical velocity field in the Italian peninsula and in particular along the coastal areas and neighbour zones will be shown. Preliminary studies about the spatial and temporal evolution of the subsidence phenomena in these areas will be also discussed

Spatial and Temporal evolution of the subsidence phenomena in the Italian Peninsula

A large number of continuous GNSS (CGNSS) stations are nowadays available in Italy, this has already allowed an accurate monitoring of the horizontal and vertical kinematic pattern in the Italian peninsula in terms of linear trends. The crustal displacements can be considered as the result of several contributions: global, regional and local tectonic processes, climatic and meteorological phenomena, but also human activities. In particular, the groundwater exploitation for agricultural and industrial purposes and the extractive activities of gas, oil and geothermal fluids can induce displacements that can be greater than the ones due to natural contributions. Human activities could induce rapid changes in the local dynamic of the Earth crust and usually have stronger impact on the vertical component. Therefore, an accurate monitoring of the vertical displacements that takes into account also the spatial heterogeneity of the human activities is a major issue. In order to monitor and study the vertical velocity field in the Italian area, the observation of more than 600 CGNSS sites have been analysed using the GAMIT software package. The interdistances between the considered sites is about 40-50 Km and should allow a fairly good definition of the vertical velocity field and to study the possible spatial evolution of the pattern. The relatively long time interval of data acquisition (2001-2018) provides an important data set that make possible to identify different time evolutions with respect to the linear trend usually adopted in the GNSS time series analysis. The present vertical velocity field in the Italian peninsula and in particular along the coastal areas and neighbour zones will be shown. Preliminary studies about the spatial and temporal evolution of the subsidence phenomena in these areas will be also discussed

How do cardiologists select patients for dual antiplatelet therapy continuation beyond 1 year after a myocardial infarction? Insights from the EYESHOT Post-MI Study

Background: Current guidelines suggest to consider dual antiplatelet therapy (DAPT) continuation for longer than 12 months in selected patients with myocardial infarction (MI). Hypothesis: We sought to assess the criteria used by cardiologists in daily practice to select patients with a history of MI eligible for DAPT continuation beyond 1 year. Methods: We analyzed data from the EYESHOT Post-MI, a prospective, observational, nationwide study aimed to evaluate the management of patients presenting to cardiologists 1 to 3 years from the last MI event. Results: Out of the 1633 post-MI patients enrolled in the study between March and December 2017, 557 (34.1%) were on DAPT at the time of enrolment, and 450 (27.6%) were prescribed DAPT after cardiologist assessment. At multivariate analyses, a percutaneous coronary intervention (PCI) with multiple stents and the presence of peripheral artery disease (PAD) resulted as independent predictors of DAPT continuation, while atrial fibrillation was the only independent predictor of DAPT interruption for patients both at the second and the third year from MI at enrolment and the time of discharge/end of the visit. Conclusions: Risk scores recommended by current guidelines for guiding decisions on DAPT duration are underused and misused in clinical practice. A PCI with multiple stents and a history of PAD resulted as the clinical variables more frequently associated with DAPT continuation beyond 1 year from the index MI

Monitoring system using low cost GNSS sensors: first experiments and performance evaluation

This work aims to assess the possibility to use a low cost GNSS instrumentation for structural monitoring purposes. Under the assumption that about one centimeter can be the magnitude of the displacements that we aim to detect, several instrumentations were compared at different baseline distances. All the tests have been performed considering one week of observations. Different observing session time spans have been considered, ranging from 1 hour to 24 hours. Tests demonstrate that by using a single frequency receiver is not possible to achieve the requested precision for baselines longer than a km, at least if observing sessions shorter than 6 hours are needed. Nevertheless, for a baseline within a hundred meters is possible to achieve effective precisions by using a couple of low cost stations also for observing sessions of one hour. The only configuration which does not respect such performances is the one using the default "patch" antennas that we discourage for precision purposes