Influence of ionospheric perturbations in GPS time and frequency transfer

The stability of GPS time and frequency transfer is limited by the fact that GPS signals travel through the ionosphere. In high precision geodetic time transfer (i.e. based on precise modeling of code and carrier phase GPS data), the so-called ionosphere-free combination of the code and carrier phase measurements made on the two frequencies is used to remove the first-order ionospheric effect. In this paper, we investigate the impact of residual second- and third-order ionospheric effects on geodetic time transfer solutions i.e. remote atomic clock comparisons based on GPS measurements, using the ATOMIUM software developed at the Royal Observatory of Belgium (ROB). The impact of third-order ionospheric effects was shown to be negligible, while for second-order effects, the tests performed on different time links and at different epochs show a small impact of the order of some picoseconds, on a quiet day, and up to more than 10 picoseconds in case of high ionospheric activity. The geomagnetic storm of the 30th October 2003 is used to illustrate how space weather products are relevant to understand perturbations in geodetic time and frequency transfer.Comment: 25 pages, 10 eps figures, 1 table, accepted in Journal of Advances in Space Research, Special Issue "Recent advances in space weather monitoring, modelling and forecasting

Satellite Positioning

Satellite positioning techniques, particularly global navigation satellite systems (GNSS), are capable of measuring small changes of the Earths shape and atmosphere, as well as surface characteristics with an unprecedented accuracy. This book is devoted to presenting recent results and development in satellite positioning technique and applications, including GNSS positioning methods, models, atmospheric sounding, and reflectometry as well their applications in the atmosphere, land, oceans and cryosphere. This book provides a good reference for satellite positioning techniques, engineers, scientists as well as user community

New applications and challenges of GNSS variometric approach

Global Navigation Satellite Systems (GNSS) are nowadays widely used in several technical and scientific activities. Since the early stages of development (mid 1980 s), given the high level of accuracy achieved in determining the coordinates of the receiver, it became clear that the extensive deployment of GPS stations all over the world would have improved many tasks in geodesy and geodynamics. The use of GNSS signals is now not only limited to the estimation of the receiver's position, but it has eventually become a key instrument for ionospheric and tropospheric remote sensing studies, and for soil features (GNSS reflectometry). In particular, GNSS can be used to monitor the ionosphere at different time and space scales. On a global scale, GNSS signals are used to generate Global Ionosphere Maps (GIM) by measuring the total electron content from stations located around the world. On a regional scale, the same signals can be used to detect fast ionospheric disturbances, including those generated by natural hazards, such as tsunami and earthquakes. %For these reasons, real-time GNSS applications became particularly relevant in a number of different scientific fields. The Variometric Approach is a processing algorithm for GNSS observations which allow a GNSS receiver to provide valuable real-time information in a stand-alone operative mode. This approach is based on single time differences of suitable linear combinations of GNSS carrier-phase measurements, using a stand-alone GNSS receiver and standard GNSS broadcast products (orbits and clocks corrections) that are available in real-time. This thesis investigates the possibility to apply the Variometric Approach to the monitoring of the ionosphere, in order to detect in real-time ionospheric disturbances generated by tsunami. The first chapter of this thesis will serve as a preface to define fundamental concepts that we will refer to throughout the rest of this work. The rest of this thesis is divided into two main parts. In the first part (chapter~\ref{sec:VADASE}), we present some advances and applications of the VADASE (Variometric Approach for Displacements Analysis Standalone Engine) algorithm to estimate in real time ground velocities and displacements using stand-alone GNSS receivers. This algorithm was eventually appointed as an effective strategy to contribute to GNSS seismology. In this section we used the 2016 Meinong earthquake occurred in Taiwan as a case study and we estimated coseismic displacements and propagation properties of the surface waves in a real-time scenario using low-cost GNSS receivers. The second part of this work (chapters \ref{sec:VARION}, \ref{sec:rtscenario}, and \ref{sec:VARIONimpementation}) is devoted to a new GNSS processing algorithm, VARION (Variometric Approach for Real-Time Ionosphere Observation), which is capable of estimating changes in the ionosphere's Total Electron Content (TEC) using stand-alone GNSS receivers in real time. In chapter~\ref{sec:rtscenario}, the effectiveness of VARION was proven on the following study cases: 2012 Haida Gwaii earthquake and tsunami event, 2015 Chile earthquake and tsunami event, 2013 U.S. East Coast meteotsunami event, and 2017 Mexico tsunami and geomagnetic storm events. Finally, some conclusions and relevant prospects for future VARION developments are outlined. VARION may represent a significant contribution to science because the ionosphere is strongly coupled to the dynamics of the Earth's surface, neutral atmosphere, and geomagnetic field. In particular, these ionospheric perturbations can be used to detect in real time detection atmospheric gravity waves due to tsunamis. During the NASA funded GNSS Tsunami Early Warning System 2017 workshop held in Sendai, Japan, July 25-27 2017, the VARION algorithm was appointed as the first real-time GNSS tsunami tracking and warning system based upon NASA's Global Differential GPS system

Software-defined radio technology for GNSS scintillation analysis: bring Antarctica to the lab

Global navigation satellite systems (GNSSs) are widely used to support logistics, scientific operations, and to monitor the polar ionosphere indirectly, which is a region characterized by strong phase scintillation events that severely affect the quality and reliability of received signals. Professional commercial GNSS receivers are widely used for scintillation monitoring; on the contrary, custom-designed solutions based on data grabbers and software receivers constitute novelty. The latter enables a higher level of flexibility and configurability, which is important when working in remote and severe environments. We describe the scientific, technological, and logistical challenges of installing an ionospheric monitoring station in Antarctica, based on a multi-constellation and multi-frequency GNSS data grabber and a software-defined radio receiver. Having access to the full receiver chain and to intermediate signal processing stages allows a deep analysis of the impact of scintillation and, in turn, a better understanding of the physical phenomenon. The possibility to process high-resolution raw intermediate frequency samples of the signal enables not only the computation of scintillation indexes with the same quality as professional devices but also the design and test of innovative receiver architectures and algorithms. Furthermore, the record and replay approach offers the possibility to process in the lab the signals captured on site, with high fidelity level. It is like being in Antarctica again, but with an unlimited set of receivers and higher computational, storage, and bandwidth resources. The main advantages and disadvantages of this approach are analyzed. Examples of monitoring results are reported, confirming the monitoring capabilities, showing the good agreement with commercial receiver outputs and confirming the validity of post-processing and re-play operations

Vulnerability analysis of satellite-based synchronized smart grids monitoring systems

The large-scale deployment of wide-area monitoring systems could play a strategic role in supporting the evolution of traditional power systems toward smarter and self-healing grids. The correct operation of these synchronized monitoring systems requires a common and accurate timing reference usually provided by a satellite-based global positioning system. Although these satellites signals provide timing accuracy that easily exceeds the needs of the power industry, they are extremely vulnerable to radio frequency interference. Consequently, a comprehensive analysis aimed at identifying their potential vulnerabilities is of paramount importance for correct and safe wide-area monitoring system operation. Armed with such a vision, this article presents and discusses the results of an experimental analysis aimed at characterizing the vulnerability of global positioning system based wide-area monitoring systems to external interferences. The article outlines the potential strategies that could be adopted to protect global positioning system receivers from external cyber-attacks and proposes decentralized defense strategies based on self-organizing sensor networks aimed at assuring correct time synchronization in the presence of external attacks

Explotación de nuevas oportunidades científicas de los sistemas de posicionamiento global por satélite (GNSS) desde una perspectiva intensiva en datos

[EN] With the current GNSS infrastructure development plans, over 120 GNSS satellites (including European Galileo satellites)will provide, already this decade, continuous data, in several frequencies, without interruption and on a permanent basis.This global and permanent GNSS infrastructure constitutes a major opportunity for GNSS science applications. In themeantime, recent advances in technology have contributed "de-facto" to the deployment of a large GNSS receiver arraybased on Internet of Things (IoT), affordable smart devices easy to find in everybody’s pockets. These devices – evolvingfast at each new generation – feature an increasing number of capabilities and sensors able to collect a variety ofmeasurements, improving GNSS performance. Among these capabilities, Galileo dual band smartphones receivers andAndroid’s support for raw GNSS data recording represent major steps forward for Positioning, Navigation and Timing (PNT)data processing improvements. Information gathering from these devices, commonly referred as crowdsourcing, opensthe door to new data-intensive analysis techniques in many science domains. At this point, collaboration between variousresearch groups is essential to harness the potential hidden behind the large volumes of data generated by thiscyberinfrastructure. Cloud Computing technologies extend traditional computational boundaries, enabling execution ofprocessing components close to the data. This paradigm shift offers seamless execution of interactive algorithms andanalytics, skipping lengthy downloads and setups. The resulting scenario, defined by a GNSS Big Data repository with colocatedprocessing capabilities, sets an excellent basis for the application of Artificial Intelligence / Machine Learning (ML)technologies in the context of GNSS. This unique opportunity for science has been recognized by the European SpaceAgency (ESA) with the creation of the Navigation Scientific Office, which leverages on GNSS infrastructure to deliverinnovative solutions across multiple scientific domains.[ES] Con los planes actuales de desarrollo de la infraestructura GNSS, más de 120 satélites GNSS (incluidos los satélites europeos Galileo) proporcionarán, ya en esta década, datos continuos, en varias frecuencias, sin interrupciones y de forma permanente. Esta infraestructura GNSS global y permanente constituye una gran oportunidad para las aplicaciones científicas de GNSS. Mientras tanto, avances recientes han contribuido al despliegue de una red GNSS paralela basada en la Internet de las Cosas (IoT), asequibles dispositivos inteligentes fáciles de encontrar en todos los bolsillos. Estos dispositivos, que evolucionan rápidamente con cada nueva generación, acumulan un número creciente de funcionalidades y sensores capaces de recopilar una gran variedad de mediciones. Entre estas funcionalidades, los receptores de teléfonos inteligentes de banda dual Galileo y el soporte Android para la grabación de datos GNSS sin procesar representan pasos especialmente relevantes. La recopilación de información mediante estos dispositivos, comúnmente conocida como crowdsourcing, abre la puerta a nuevas técnicas de análisis de datos en múltiples dominios científicos. Llegados a este punto, la colaboración entre diversos grupos de investigación resulta esencial para aprovechar el potencial que se esconde en los grandes volúmenes de datos generados por esta ciberinfraestructura. Las tecnologías de Cloud Computing extienden los límites computacionales tradicionales permitiendo la ejecución de componentes de procesamiento cerca de los datos. Este cambio de paradigma ofrece una rápida ejecución de algoritmos y análisis interactivos, omitiendo largas descargas y configuraciones. El escenario resultante, definido por un repositorio GNSS Big Data con capacidades de procesamiento acopladas, establece una base excelente para la aplicación de tecnologías de Inteligencia Artificial / Aprendizaje Automático (ML). Esta oportunidad única para la ciencia ha sido reconocida por la Agencia Espacial Europea (ESA) con la creación de la Oficina Científica de Navegación, que aprovecha la infraestructura GNSS para ofrecer soluciones innovadoras en múltiples dominios científicos.This work was supported by the European Space Agency as part of Research and Development Programmes under Science and Navigation Directorates. The authors would like to thank the GNSS Science Advisory Committee and ESA Navigation Support Office for their support and suggestions. We also thank our Industrial partners, involved in science use cases assessment and implementation. Thanks also to the Science and Operations technical IT Unit at ESAC supporting the deployment of the GSSC Thematic Exploitation Platform. We would like to thank all data collection providers, with special thanks to IGS, ILRS, CDDIS, BKG and IGN for their sustained and remarkable support making possible the creation of the GSSC Repository at the core of this work.Navarro, V.; Ventura-Traveset, J. (2021). A data-intensive approach to exploit new GNSS science opportunities. En Proceedings 3rd Congress in Geomatics Engineering. Editorial Universitat Politècnica de València. 43-53. https://doi.org/10.4995/CiGeo2021.2021.12740OCS435

Direct comparisons of European primary and secondary frequency standards via satellite techniques

partially_open49openRiedel, F; Al-Masoudi, A; Benkler, E; Dörscher, S; Gerginov, V; Grebing, C; Häfner, S; Huntemann, N; Lipphardt, B; Lisdat, C; Peik, E; Piester, D; Sanner, C; Tamm, C; Weyers, S; Denker, H; Timmen, L; Voigt, C; Calonico, D; Cerretto, G; Costanzo, G A; Levi, F; Sesia, I; Achkar, J; Guéna, J; Abgrall, M; Rovera, D; Chupin, B; Shi, C; Bilicki, S; Bookjans, E; Lodewyck, J; Le Targat, R; Delva, P; Bize, S; Baynes, F N; Baynham, C F A; Bowden, W; Gill, P; Godun, R M; Hill, I R; Hobson, R; Jones, J M; King, S A; Nisbet-Jones, P B R; Rolland, A; Shemar, S L; Whibberley, P B; Margolis, H SRiedel, F; Al-Masoudi, A; Benkler, E; Dörscher, S; Gerginov, V; Grebing, C; Häfner, S; Huntemann, N; Lipphardt, B; Lisdat, C; Peik, E; Piester, D; Sanner, C; Tamm, C; Weyers, S; Denker, H; Timmen, L; Voigt, C; Calonico, D; Cerretto, G; Costanzo, G A; Levi, F; Sesia, I; Achkar, J; Guéna, J; Abgrall, M; Rovera, D; Chupin, B; Shi, C; Bilicki, S; Bookjans, E; Lodewyck, J; Le Targat, R; Delva, P; Bize, S; Baynes, F N; Baynham, C F A; Bowden, W; Gill, P; Godun, R M; Hill, I R; Hobson, R; Jones, J M; King, S A; Nisbet-Jones, P B R; Rolland, A; Shemar, S L; Whibberley, P B; Margolis, H

Advanced tracking systems design and analysis

The results of an assessment of several types of high-accuracy tracking systems proposed to track the spacecraft in the National Aeronautics and Space Administration (NASA) Advanced Tracking and Data Relay Satellite System (ATDRSS) are summarized. Tracking systems based on the use of interferometry and ranging are investigated. For each system, the top-level system design and operations concept are provided. A comparative system assessment is presented in terms of orbit determination performance, ATDRSS impacts, life-cycle cost, and technological risk