An investigation of new ionospheric models using multi-source measurements and neural networks

Ionosphere is one of the atmospheric layers that has a major impact on human beings since it significantly affects the radio propagation on Earth, and between satellites and Earth (e.g., Global Navigation Satellite Systems (GNSS) signal transmission). The variation of the electrons in the ionosphere is strongly influenced by the space weather due to solar and cosmic radiation. Hence, the short/long-term trend of the free electrons in the ionosphere has been regarded as very important information for both space weather and GNSS positioning. On the other hand, precisely quantifying the distribution and variation of free electrons at a high spatio-temporal resolution is often a challenge if the number of the electrons (electron density) is detected only from the traditional ionospheric sensors (e.g., ionosonde and topside sounder and Incoherent Scatter Radar (ISR)) due to their low spatio-temporal coverage. This disadvantage is also inherited from the empirical ionospheric model developed based on these data sources. Nowadays, the availability of advanced observation techniques, such as GNSS Radio Occultation (RO) and satellite altimetry, for the measurement of Electron Density (Ne) and related parameters (e.g., hmF2, NmF2, Vertical Scale Height (VSH), Electron Density Profile (EDP) and Vertical Total Electron Content (VTEC)) in the ionosphere has heralded a new era for space weather research in the upper atmosphere. The new sources of data for ionospheric modelling can improve not only the accuracy but also the reliability of the model (such as[96] for hmF2 and [28] for VTEC). In this study, Helmert Variance Component Estimation (VCE) aided Weight Total Least Squares (WTLS) is selected for modelling global VTEC using International GNSS Service stations, satellite altimetry and GNSS-RO measurements. The results show that the new VTEC model outperforms the traditional global ionospheric VTEC Model by at least 1.5 Total Electron Content Unit (TECU) over the ocean. This improvement is expected to be significant in the refinement of global ionospheric VTEC Model development. As is well known, the most traditional models developed are prone to the effects of inherent assumptions (e.g. for the construction of the base functions in the models) which may lead to large biases in the prediction. In this study, an innovative machine learning technique (i.e. Neural Network (NN)) is investigated as the modelling method to address this issue. Different from the traditional modelling method, neither the observation equations (or the so called `design matrix'), nor apriori knowledge of the relationship (both of them can be considered as the source of the aforementioned assumptions) is required in the modelling process of a NN. This network system can automatically construct an optimal regression function based on a large amount of sample data and the designed network [43]. In this study, Deep Neural Network (DNN), which is an advanced Artificial Neural Network (ANN) (with more than one hidden layer), is investigated for their usability of VSH and topside EDP modelling, as well as the relationship between Ne and electron temperature. The results reveal that the new VSH model agrees better than the traditional model with regards to either out-of-sample measurements or the external reference (i.e. ISR data). In addition, the new model can represent the characteristic of VSH in the equatorial region better than that of traditional approaches during geomagnetic storms. The relationship between Ne and Electron Temperature (Te) investigated from ISR data can be used to improve the performance of the current Te model. The local time-altitude variation of the model outputs agrees well with that from a physical model (i.e., Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM)). The new topside EDP model takes hmF2 and NmF2 into consideration as part of the variable set. Comparing with the reference data (i.e., out- of-sample COSMIC data, GRACE and ISR data), the new model agrees much better than the International Reference Ionosphere (IRI)-2016 model. In addition, an advanced NN technique, Bidirectional Long Short-Term Memory (Bi-LSTM), is utilised to forecast hmF2 by using the hmF2 measured by Australian ionosondes in the five hours prior. The forecast results are better than the results from real-time models in the next five hours. The new model performs also better than the current hmF2 model (i.e., AMTB [2] and shubin [96] models, which is used inside IRI-2016 model) by at least 10km in most ionosonde stations. Overall, the neural network technique has a great potential in being utilised in the ionospheric modelling. In addition to the accuracy improvement, the physical mechanism can be observed from the model outputs as well. In future work, the neural network is expected to be further applied in some other space weather studies (e.g., Dst, solar flare, etc)

Contributions to ionospheric modeling with GNSS in mapping function, tomography and polar electron

This dissertation focuses on determining the vertical electron content distribution in low and high vertical resolution from ground-based and LEO on board GNSS data and improving the knowledge of ionosphere climatology in northern mid-latitude and polar regions. The novelty is summarized in the following four aspects: The first contribution is to propose a new ionospheric mapping function concept - Barcelona Ionospheric Mapping Function (BIMF), in order to improve STEC (Slant Total Electron Content) conversion accuracy from any given VTEC (Vertical Total Electron Content) model. BIMF is based on the climatic modeling of the VTEC fraction in the second layer - µ2, which is the byproduct of UQRG generated by UPC. The first implementation of BIMF is BIMF-nml for the northern mid-latitudes, where the latitudinal variation of µ2 is neglected. µ2 is modeled as function of date and local time. From the user’s perspective, BIMF is the linear combination of µ2 and the standard ionospheric mapping function, and only needs 41 constant coefficients, making BIMF achieve the simplicity for application. The good performance has been demonstrated in the dSTEC assessment for different IGSGIMs: UQRG, CODG and JPLG. The second contribution is to confirm the capability of UQRG GIMs to detect representative ionospheric features in polar regions through six case studies, including TOI (Tongue of Ionization), trough, flux transfer event, theta-aurora, ionospheric convection patterns and storm enhanced density. The long-term VTEC and µ2 data provide valuable databases for studying the morphology and climatology of polar ionospheric phenomena. The unsupervised clustering results of normalized VTEC distribution show that TOI and polar cap patches exhibit an annual dependence, i.e. most TOI and patches occurring in the North Hemisphere winter and the South Hemisphere summer. The third contribution is to propose a hybrid method - AVHIRO (the Abel-VaryChap Hybrid modeling from topside Incomplete RO data), to solve an ill-posed rank-deficient problem in the Abel electron density retrieval. This work is driven by the future EUMETSAT Polar System 2nd Generation, which provides truncated ionospheric RO data, only below impact heights of 500 km, in order to guarantee a full data gathering of the neutral part. AVHIRO takes advantage of one Linear Vary-Chap model, where the scale height increases linearly with altitude above the F2 layer peak, and uses Powell search to solve the full electron densities, ambiguity term, and four parameters of the Vary-Chap model simultaneously, taking into account the nonlinear interactions between the unknown parameters. The fourth contribution is to take advantage of the geometry brought by combining DORIS, ground-based Galileo, ground-based, LEO-POD and vessel-based GPS data and ingest the multi-source dual-frequency carrier phase measurements into the tomographic model to improve the GIM VTEC estimation precision. The impact of adding each type of measurements, which are Galileo data, vessel-based GPS data, DORIS and LEO-POD GPS data, to ground-based GPS data on GIM product is examined according to two complementing evaluation criteria, JASON-3 VTEC comparison and GPS dSTEC test. This study proves the expected better GIM performance by new data ingestion into tomographic model, which is a successful step forward from conception to initial experimental validation.electrones en resolución vertical baja y alta a partir de medidas GNSS terrestres y a bordo de satélites de órbita baja (LEO), además de utilizar medidas GNSS desde buques y medidas DORIS, además de mejorar el conocimiento de la climatología de la ionosfera en las regiones polares y en latitudes medias del hemisferio norte. Las contribuciones se pueden resumir en los siguientes cuatro aspectos: La primera contribución consiste en proponer un nuevo concepto de función de mapeo ionosférico: la función de mapeo ionosférico de Barcelona (BIMF), con el fin de mejorar la precisión de conversión de STEC (contenido total de electrones inclinado) a partir de cualquier modelo de VTEC (contenido total de electrones vertical). BIMF se basa en el modelado climático de la fracción VTEC en la segunda capa - μ2, que es el subproducto de UQRG generado por UPC. La primera implementación de BIMF es BIMF-nml para las latitudes medias del hemisferio norte. μ2 se modela en función del dia y la hora local. Desde la perspectiva del usuario, BIMF es la combinación lineal de μ2 y la función de mapeo ionosférico estándar, y solo necesita 41 coeficientes constantes, lo que hace que BIMF sea facilmente aplicable. Su buen comportamiento se demostró en la evaluación dSTEC para diferentes IGS GIM: UQRG, CODG y JPLG. La segunda contribución se centró en confirmar la capacidad de los GIM UQRG para detectar características ionosféricas representativas en regiones polares a través de seis estudios de casos, que incluyen lenguas de ionización (TOI), depresión de ionización en forma de canal, sucesos de transferencia de flujo, theta-aurora, patrones de convección ionosférica y densidad aumentada durante tormentas geomagnéticas. Los datos a largo plazo de VTEC y μ2 proporcionan valiosas bases de datos para estudiar la morfología y climatología de los fenómenos ionosféricos polares. Los resultados de agrupamiento no supervisados de la distribución normalizada de VTEC muestran que los TOI y los parches en los casquetes polares exhiben una dependencia anual, es decir, la mayoría de los TOI y parches ocurren en el invierno del Hemisferio Norte y el verano del Hemisferio Sur. La tercera contribución ha consistido en proponer un método híbrido: AVHIRO (el modelo híbrido Abel-VaryChap a partir de datos de RO incompletos en la parte superior), para resolver un problema de rango deficiente en la recuperación de la densidad electrónica con el modelo de Abel. Este trabajo está motivado por el futuro sistema polar EUMETSAT de segunda generación, que proporciona datos truncados de RO ionosférica, sólo por debajo de las alturas de impacto de 500 km, con el fin de garantizar una recopilación completa de medidas de la parte neutra. AVHIRO aprovecha un modelo Linear Vary-Chap, donde la altura de la escala aumenta linealmente con la altitud por encima del pico de la capa F2, y utiliza la búsqueda Powell para resolver las densidades completas de electrones, el término de ambig ¨ uedad y cuatro parámetros del modelo Vary-Chap simultáneamente, teniendo en cuenta las interacciones no lineales entre los parámetros desconocidos. La cuarta contribución es aprovechar la geometría aportada por la combinación de datos GPS DORIS, Galileo en tierra, LEO-POD y en barco, e incorporar las mediciones de la fase de la portadora de doble frecuencia de múltiples fuentes en el modelo tomográfico para mejorar la precisión de estimación de GIM VTEC. El impacto de agregar cada tipo de mediciones, que son datos de Galileo, datos de GPS basados en embarcaciones, datos de GPS DORIS y LEO-POD, a datos de GPS terrestres en productos GIM se examina de acuerdo con dos criterios de evaluación complementarios, comparación con VTEC[JASON-3] y con dSTEC[GPS]. Este estudio demuestra el mejor rendimiento esperado de GIM por la nueva ingesta de datos en el modelo tomográfico, que es un exitoso paso adelante desde la concepción hasta la validación experimental inicial

GNSS transpolar earth reflectometry exploriNg system (G-TERN): mission concept

The global navigation satellite system (GNSS) Transpolar Earth Reflectometry exploriNg system (G-TERN) was proposed in response to ESA's Earth Explorer 9 revised call by a team of 33 multi-disciplinary scientists. The primary objective of the mission is to quantify at high spatio-temporal resolution crucial characteristics, processes and interactions between sea ice, and other Earth system components in order to advance the understanding and prediction of climate change and its impacts on the environment and society. The objective is articulated through three key questions. 1) In a rapidly changing Arctic regime and under the resilient Antarctic sea ice trend, how will highly dynamic forcings and couplings between the various components of the ocean, atmosphere, and cryosphere modify or influence the processes governing the characteristics of the sea ice cover (ice production, growth, deformation, and melt)? 2) What are the impacts of extreme events and feedback mechanisms on sea ice evolution? 3) What are the effects of the cryosphere behaviors, either rapidly changing or resiliently stable, on the global oceanic and atmospheric circulation and mid-latitude extreme events? To contribute answering these questions, G-TERN will measure key parameters of the sea ice, the oceans, and the atmosphere with frequent and dense coverage over polar areas, becoming a “dynamic mapper”of the ice conditions, the ice production, and the loss in multiple time and space scales, and surrounding environment. Over polar areas, the G-TERN will measure sea ice surface elevation (<;10 cm precision), roughness, and polarimetry aspects at 30-km resolution and 3-days full coverage. G-TERN will implement the interferometric GNSS reflectometry concept, from a single satellite in near-polar orbit with capability for 12 simultaneous observations. Unlike currently orbiting GNSS reflectometry missions, the G-TERN uses the full GNSS available bandwidth to improve its ranging measurements. The lifetime would be 2025-2030 or optimally 2025-2035, covering key stages of the transition toward a nearly ice-free Arctic Ocean in summer. This paper describes the mission objectives, it reviews its measurement techniques, summarizes the suggested implementation, and finally, it estimates the expected performance.Peer ReviewedPostprint (published version

BDS GNSS for Earth Observation

For millennia, human communities have wondered about the possibility of observing phenomena in their surroundings, and in particular those affecting the Earth on which they live. More generally, it can be conceptually defined as Earth observation (EO) and is the collection of information about the biological, chemical and physical systems of planet Earth. It can be undertaken through sensors in direct contact with the ground or airborne platforms (such as weather balloons and stations) or remote-sensing technologies. However, the definition of EO has only become significant in the last 50 years, since it has been possible to send artificial satellites out of Earth’s orbit. Referring strictly to civil applications, satellites of this type were initially designed to provide satellite images; later, their purpose expanded to include the study of information on land characteristics, growing vegetation, crops, and environmental pollution. The data collected are used for several purposes, including the identification of natural resources and the production of accurate cartography. Satellite observations can cover the land, the atmosphere, and the oceans. Remote-sensing satellites may be equipped with passive instrumentation such as infrared or cameras for imaging the visible or active instrumentation such as radar. Generally, such satellites are non-geostationary satellites, i.e., they move at a certain speed along orbits inclined with respect to the Earth’s equatorial plane, often in polar orbit, at low or medium altitude, Low Earth Orbit (LEO) and Medium Earth Orbit (MEO), thus covering the entire Earth’s surface in a certain scan time (properly called ’temporal resolution’), i.e., in a certain number of orbits around the Earth. The first remote-sensing satellites were the American NASA/USGS Landsat Program; subsequently, the European: ENVISAT (ENVironmental SATellite), ERS (European Remote-Sensing satellite), RapidEye, the French SPOT (Satellite Pour l’Observation de laTerre), and the Canadian RADARSAT satellites were launched. The IKONOS, QuickBird, and GeoEye-1 satellites were dedicated to cartography. The WorldView-1 and WorldView-2 satellites and the COSMO-SkyMed system are more recent. The latest generation are the low payloads called Small Satellites, e.g., the Chinese BuFeng-1 and Fengyun-3 series. Also, Global Navigation Satellite Systems (GNSSs) have captured the attention of researchers worldwide for a multitude of Earth monitoring and exploration applications. On the other hand, over the past 40 years, GNSSs have become an essential part of many human activities. As is widely noted, there are currently four fully operational GNSSs; two of these were developed for military purposes (American NAVstar GPS and Russian GLONASS), whilst two others were developed for civil purposes such as the Chinese BeiDou satellite navigation system (BDS) and the European Galileo. In addition, many other regional GNSSs, such as the South Korean Regional Positioning System (KPS), the Japanese quasi-zenital satellite system (QZSS), and the Indian Regional Navigation Satellite System (IRNSS/NavIC), will become available in the next few years, which will have enormous potential for scientific applications and geomatics professionals. In addition to their traditional role of providing global positioning, navigation, and timing (PNT) information, GNSS navigation signals are now being used in new and innovative ways. Across the globe, new fields of scientific study are opening up to examine how signals can provide information about the characteristics of the atmosphere and even the surfaces from which they are reflected before being collected by a receiver. EO researchers monitor global environmental systems using in situ and remote monitoring tools. Their findings provide tools to support decision makers in various areas of interest, from security to the natural environment. GNSS signals are considered an important new source of information because they are a free, real-time, and globally available resource for the EO community

Near real-time estimation of ionosphere vertical total electron content from GNSS satellites using B-splines in a Kalman filter

Abstract. Although the number of terrestrial global navigation satellite system (GNSS) receivers supported by the International GNSS Service (IGS) is rapidly growing, the worldwide rather inhomogeneously distributed observation sites do not allow the generation of high-resolution global ionosphere products. Conversely, with the regionally enormous increase in highly precise GNSS data, the demands on (near) real-time ionosphere products, necessary in many applications such as navigation, are growing very fast. Consequently, many analysis centers accepted the responsibility of generating such products. In this regard, the primary objective of our work is to develop a near real-time processing framework for the estimation of the vertical total electron content (VTEC) of the ionosphere using proper models that are capable of a global representation adapted to the real data distribution. The global VTEC representation developed in this work is based on a series expansion in terms of compactly supported B-spline functions, which allow for an appropriate handling of the heterogeneous data distribution, including data gaps. The corresponding series coefficients and additional parameters such as differential code biases of the GNSS satellites and receivers constitute the set of unknown parameters. The Kalman filter (KF), as a popular recursive estimator, allows processing of the data immediately after acquisition and paves the way of sequential (near) real-time estimation of the unknown parameters. To exploit the advantages of the chosen data representation and the estimation procedure, the B-spline model is incorporated into the KF under the consideration of necessary constraints. Based on a preprocessing strategy, the developed approach utilizes hourly batches of GPS and GLONASS observations provided by the IGS data centers with a latency of 1 h in its current realization. Two methods for validation of the results are performed, namely the self consistency analysis and a comparison with Jason-2 altimetry data. The highly promising validation results allow the conclusion that under the investigated conditions our derived near real-time product is of the same accuracy level as the so-called final post-processed products provided by the IGS with a latency of several days or even weeks.Publisher's Versio

GNSS transpolar earth reflectometry exploriNg system (G-TERN): Mission concept

The global navigation satellite system (GNSS) Transpolar Earth Reflectometry exploriNg system (G-TERN) was proposed in response to ESA's Earth Explorer 9 revised call by a team of 33 multi-disciplinary scientists. The primary objective of the mission is to quantify at high spatio-temporal resolution crucial characteristics, processes and interactions between sea ice, and other Earth system components in order to advance the understanding and prediction of climate change and its impacts on the environment and society. The objective is articulated through three key questions. 1) In a rapidly changing Arctic regime and under the resilient Antarctic sea ice trend, how will highly dynamic forcings and couplings between the various components of the ocean, atmosphere, and cryosphere modify or influence the processes governing the characteristics of the sea ice cover (ice production, growth, deformation, and melt)? 2) What are the impacts of extreme events and feedback mechanisms on sea ice evolution? 3) What are the effects of the cryosphere behaviors, either rapidly changing or resiliently stable, on the global oceanic and atmospheric circulation and mid-latitude extreme events? To contribute answering these questions, G-TERN will measure key parameters of the sea ice, the oceans, and the atmosphere with frequent and dense coverage over polar areas, becoming a "dynamic mapper" of the ice conditions, the ice production, and the loss in multiple time and space scales, and surrounding environment. Over polar areas, the G-TERN will measure sea ice surface elevation (&lt;10 cm precision), roughness, and polarimetry aspects at 30-km resolution and 3-days full coverage. G-TERN will implement the interferometric GNSS reflectometry concept, from a single satellite in near-polar orbit with capability for 12 simultaneous observations. Unlike currently orbiting GNSS reflectometry missions, the G-TERN uses the full GNSS available bandwidth to improve its ranging measurements. The lifetime would be 2025-2030 or optimally 2025-2035, covering key stages of the transition toward a nearly ice-free Arctic Ocean in summer. This paper describes the mission objectives, it reviews its measurement techniques, summarizes the suggested implementation, and finally, it estimates the expected performance

The Europa Clipper Gravity and Radio Science Investigation

The primary objective of the Europa Clipper mission is to assess the habitability of Europa, an overarching goal that rests on improving our understanding of Europa’s interior structure, composition, and geologic activity. Here we describe the Gravity and Radio Science (G/RS) investigation. The primary measurement, the gravitational tidal Love number k2 , will be an independent diagnostic of the presence of a global subsurface ocean, but G/RS will make a number of other key measurements related to Europa’s deep interior, silicate mantle-ocean interface, ice shell, ionosphere, and plasma environment. Although radio science is common to many missions, Europa Clipper’s orbit and spacecraft configuration during flybys present special challenges for the design of this experiment. The information obtained through G/RS will be complementary to the measurements by the other instruments onboard Europa Clipper, and their combined analysis will refine the geophysical understanding of Europa necessary to best assess its potential habitability