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

GNSS technology for structural and land monitoring

Al giorno d’oggi, la tecnologia GNSS può essere uno strumento utile, non solo in ambito di navigazione, ma anche per applicazioni di posizionamento di precisione come il monitoraggio di strutture o del territorio. Questa tecnica permette un controllo in continuo, anche da remoto, e ha un costo relativamente basso sia per quanto riguarda le strumentazioni che per gli aspetti di materializzazione e mantenimento. Nell’ottica di utilizzare la tecnologia GNSS per applicazioni di monitoraggio, il problema, nella sua interezza, può essere suddiviso in due grandi ambiti: l’ambito tecnologico (strumentazioni e apparati da adottare per un problema di monitoraggio anche in tempo reale) e l’ambito scientifico legato al problema del trattamento del dato che può, o deve, essere separato a sua volta in due aree legate alla tipologia di monitoraggio che si intende compiere. Il monitoraggio di movimenti lenti per analisi di stabilità (volto principalmente allo studio dell’evoluzione del quadro deformativo nel lungo periodo) o il monitoraggio per possibili applicazioni nell’ambito dell’early warning (che richiedono la valutazione di un movimento in tempo reale). Per le sperimentazioni sia di natura tecnologica che di trattamento del dato sono stati individuati due siti su cui sono stati installate stazioni permanenti GNSS di classe geodetica. Il primo sito è collocato sul tetto della scuola di Ingegneria ed Architettura dell’Università di Bologna, il secondo sito è la sommità della Torre Garisenda di Bologna (oggetto di particolare rilevanza culturale). Sul tetto di Ingegneria è stata collocata anche una slitta micrometrica che ha consentito l’imposizione di movimenti calibrati nelle tre direzioni. Per ciascun aspetto sono stati compiuti numerosi test sperimentali volti ad individuare le tecniche migliori per il monitoraggio con GNSS a partire dalle problematiche di trasferimento dati e finendo con lo sviluppo di particolari algoritmi per migliorare le prestazioni del sistema in condizioni di monitoraggio statico.Nowadays, GNSS technology can be a useful tool, not only for navigation and location aspects, but also for precise positioning applications, such as structures and environmental monitoring. This technique allows a continuous control, even remotely, and has a relatively low cost both regarding the instrumentation, the monumentation and the maintenance. In order to use the GNSS technology for monitoring applications, two main problems can be identified: (a) the technological one is represented by instrumentation and equipment that are used in order to provide data in real time. The scientific one (b) is related to the issue of data processing. It can be in turn separated in two fields, which are related to the type of monitoring. In detail, (b1) the monitoring of slow movements for stability analysis (aimed mainly at the study of the long time evolution of these phenomena) or (b2) the monitoring for possible applications in early warning systems. For this study two sites were considered. In each one, a GNSS geodetic class permanent station was installed. The first site is located on the roof of the School of Engineering and Architecture, University of Bologna, while the second one is on the top of the Garisenda Tower in Bologna. Moreover, in the first site, a micrometric sledge which allowed the imposition of calibrated movements in the three directions, has also been placed. For each aspect, several experimental tests have been carried out. They were aimed at identifying the best techniques for GNSS monitoring, starting from the data transfer problems and ending with the development of algorithms to improve system performance in static monitoring conditions

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

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

Improved PPP performance in regional networks

It 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

Precision of PPP as a Function of the Observing-Session Duration

Over 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 \ubd 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