Revisit the calibration errors on experimental slant total electron content (TEC) determined with GPS

This is a pre-print of an article published in GPS Solutions. The final authenticated version is available online at: https://doi.org/10.1007/s10291-018-0753-7. The study is funded by National Key Research and Development Program of China (2016YFB0501902), National Natural Science Foundation of China (41574025, 41574013, 41731069), Spanish Ministry of Science and Innovation project (CGL2015-66410-P), The Hong Kong RGC Joint Research Scheme (E-PolyU501/16) and State Key Laboratory of Geo-Information Engineering (SKLGIE2015-M-2-2).The calibration errors on experimental slant total electron content (TEC) determined with global positioning system (GPS) observations is revisited. Instead of the analysis of the calibration errors on the carrier phase leveled to code ionospheric observable, we focus on the accuracy analysis of the undifferenced ambiguity-fixed carrier phase ionospheric observable determined from a global distribution of permanent receivers. The results achieved are: (1) using data from an entire month within the last solar cycle maximum, the undifferenced ambiguity-fixed carrier phase ionospheric observable is found to be over one order of magnitude more accurate than the carrier phase leveled to code ionospheric observable and the raw code ionospheric observable. The observation error of the undifferenced ambiguity-fixed carrier phase ionospheric observable ranges from 0.05 to 0.11 total electron content unit (TECU) while that of the carrier phase leveled to code and the raw code ionospheric observable is from 0.65 to 1.65 and 3.14 to 7.48 TECU, respectively. (2) The time-varying receiver differential code bias (DCB), which presents clear day boundary discontinuity and intra-day variability pattern, contributes the most part of the observation error. This contribution is assessed by the short-term stability of the between-receiver DCB, which ranges from 0.06 to 0.17 TECU in a single day. (3) The remaining part of the observation errors presents a sidereal time cycle pattern, indicating the effects of the multipath. Further, the magnitude of the remaining part implies that the code multipath effects are much reduced. (4) The intra-day variation of the between-receiver DCB of the collocated stations suggests that estimating DCBs as a daily constant can have a mis-modeling error of at least several tenths of 1 TECU.Peer ReviewedPostprint (author's final draft

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

Statistical framework for estimating GNSS bias

We present a statistical framework for estimating global navigation satellite system (GNSS) non-ionospheric differential time delay bias. The biases are estimated by examining differences of measured line integrated electron densities (TEC) that are scaled to equivalent vertical integrated densities. The spatio-temporal variability, instrumentation dependent errors, and errors due to inaccurate ionospheric altitude profile assumptions are modeled as structure functions. These structure functions determine how the TEC differences are weighted in the linear least-squares minimization procedure, which is used to produce the bias estimates. A method for automatic detection and removal of outlier measurements that do not fit into a model of receiver bias is also described. The same statistical framework can be used for a single receiver station, but it also scales to a large global network of receivers. In addition to the Global Positioning System (GPS), the method is also applicable to other dual frequency GNSS systems, such as GLONASS (Globalnaya Navigazionnaya Sputnikovaya Sistema). The use of the framework is demonstrated in practice through several examples. A specific implementation of the methods presented here are used to compute GPS receiver biases for measurements in the MIT Haystack Madrigal distributed database system. Results of the new algorithm are compared with the current MIT Haystack Observatory MAPGPS bias determination algorithm. The new method is found to produce estimates of receiver bias that have reduced day-to-day variability and more consistent coincident vertical TEC values.Comment: 18 pages, 5 figures, submitted to AM

Consolidation and assessment of a technique to provide fast and precise point positioning (Fast-PPP)

Tesi per compendi de publicacions. La consulta íntegra de la tesi, inclosos els articles no comunicats públicament per drets d'autor, es pot realitzar prèvia petició a l'Arxiu de la UPCPremi extraordinari doctorat UPC curs 2015-2016, àmbit de CiènciesThe research of this paper-based dissertation is focused on the Fast Precise Point Positioning (Fast-PPP) technique. The novelty relies on using an accurate ionosphere model, in combination with the standard precise satellite clock and orbit products, to reduce the convergence time of state-of-the-art high-accuracy navigation techniques from approximately one hour to few minutes. My first contribution to the Fast-PPP technique as a Ph.D. student has been the design and implementation of a novel user navigation filter, based on the raw treatment of undifferenced multi-frequency code and carrier-phase Global Navigation Satellite System (GNSS) measurements. The innovative strategy of the filter avoids applying the usual ionospheric-free combination to the GNSS observables, exploiting the full capacity of new multi-frequency signals and increasing the robustness of Fast-PPP in challenging environments where the sky visibility is reduced. It has been optimised to take advantage of the corrections required to compensate the delays (i.e., errors) affecting the GNSS signals. The Fast-PPP corrections, and most important, their corrections uncertainties (i.e., the confidence bounds) are added as additional equations in the navigation filter to obtain Precise Point Positioning (PPP) in few minutes. A second contribution performed with the new user filter, has been the consolidation of the precise ionospheric modelling of Fast-PPP and its extension from a regional to a global scale. The correct use of the confidence bounds has been found of great importance when navigating in the low-latitude areas of the equator, where the ionosphere is difficult to be accurately modelled. Even in such scenario, a great consistency has been achieved between the actual positioning errors with respect to the formal errors, as demonstrated using similar figures of merit used in civil aviation, as the Stanford plot. A third contribution within this dissertation has been the characterisation of the accuracy of different ionospheric models currently used in GNSS. The assessment uses actual, unambiguous and undifferenced carrier-phase measurements, thanks to the centimetre-level modelling capability within the Fast-PPP technique. Not only the errors of the ionosphere models have been quantified in absolute and relative terms, but also, their effect on navigation.La investigació d'aquesta Tesi Doctoral per compendi d'articles es centra en la tècnica de ràpid Posicionament de Punt Precís (Fast-PPP). La novetat radica en l'ús d'un model ionosfèric precís que, combinat amb productes estàndard de rellotge i de l'òrbita de satèl·lit, redueix el temps de convergència de les actuals tècniques de navegació precisa d'aproximadament una hora a pocs minuts. La meva primera contribució a la tècnica Fast-PPP com a estudiant de Doctorat ha estat el disseny i la implementació d'un filtre de navegació d'usuari innovador, basat en el tractament de múltiples freqüències de mesures de codi i fase sense diferenciar (absolutes). La estratègia del filltre de navegació evita l'aplicació de l'habitual combinació lineal lliure de ionosfera per a aquests observables. Així, s'explota la capacitat completa dels senyals multi-freqüència en el nous Sistemes Globals de Navegació per Satèl·lit (GNSS) i s'augmenta la robustesa del Fast-PPP en entorns difícils, on es redueix la visibilitat del cel. S'ha optimitzat per tal de prendre avantatge de les correccions necessàries per a compensar els retards (és a dir, els errors) que afecten els senyals GNSS. Les correccions de Fast-PPP i més important, les seves incerteses (és a dir, els intervals de confiança) s'afegeixen com a equacions addicionals al filltre per aconseguir Posicionat de Punt Precís (PPP) en pocs minuts. La segona contribució ha estat la consolidació del modelat ionosfèric precís de Fast-PPP i la seva extensió d'un abast regional a una escala global. La correcta determinació i ús dels intervals de confiança de les correccions Fast-PPP ha esdevingut de gran importància a l'hora de navegar en zones de baixa latitud a l'equador, on la ionosfera és més difícil de modelar amb precisió. Fins i tot en aquest escenari, s'ha aconseguit una gran consistència entre els errors de posicionament reals i els nivells de protecció dels usuaris de Fast-PPP, tal com s'ha demostrat amb figures de mèrit similars a les utilitzades en l'aviació civil (els diagrames de Stanford). La tercera contribució d'aquesta Tesi Doctoral ha estat la caracterització de l'exactitud dels models ionosfèrics utilitzats actualment en GNSS. L'avaluació utilitza mesures de fase, sense ambigüitats i sense diferenciar, gràcies a la capacitat de modelatge centimètric emprat a la tècnica de Fast-PPP. No només els errors dels models de la ionosfera han estat quantificats en termes absoluts i relatius, sinó també, el seu efecte sobre la navegacióLa investigación de esta Tesis Doctoral, por compendio de artículos, se centra en la técnica de rápido Posicionamiento de Punto Preciso (Fast-PPP). La novedad, radica en el uso de un modelo ionosférico preciso que, combinado con productos estándard de reloj y órbita de satélite, reduce el tiempo de convergencia de las actuales técnicas de navegación precisa de una hora a pocos minutos.Mi primera contribución a la técnica Fast-PPP como estudiante de Doctorado ha sido el diseño y la implementación de un filtro de navegación de usuario innovador, basado en el tratamiento de múltiples frecuencias de medidas de código y fase sin diferenciar (absolutas). La estrategia del filtro de navegación evita la aplicación de la habitual combinación lineal libre de ionosfera para dichos observables. Así, se explota la capacidad de la señal multi-frecuencia en los nuevos Sistemas Globales de Navegación por Satélite (GNSS) y se aumenta la robustez del Fast-PPP en entornos difíciles, donde se reduce la visibilidad del cielo. Se ha optimizado para tomar ventaja de las correcciones necesarias para compensar los retardos (es decir, los errores) que afectan las señales GNSS. Las correcciones de Fast-PPP y más importante, sus incertidumbres (es decir, los intervalos de confianza) se añaden como ecuaciones adicionales al filtro para conseguir Posicionamiento de Punto Preciso (PPP) en pocos minutos. La segunda contribución ha estado la consolidación del modelado ionosférico preciso de Fast-PPP y la extensión de su cobertura regional a una escala global. La correcta determinación y uso de los intervalos de confianza de las correcciones Fast-PPP ha sido de gran importancia a la hora de navegar en zonas de latitudes ecuatoriales, donde la ionosfera es más difícil de modelar con precisión. Incluso en dicho escenario, se ha conseguido una gran consistencia entre los errores de posicionamiento reales y los niveles de protección de los usuarios de Fast-PPP, tal como se ha demostrado con figuras de mérito similares a las utilizadas en la aviación civil (los diagramas de Stanford).La tercera contribución de esta Tesis Doctoral ha sido la caracterización de la exactitud de los modelos ionosféricos utilizados actualmente en GNSS. El método usa medidas de fase, sin ambigüedad y sin diferenciar, gracias a la capacidad de modelado centimétrico empleado en la técnica de Fast-PPP. No solo los errores de los modelos de la ionosfera han sido cuantificados en términos absolutos y relativos, sino también, su efecto sobre la navegación.Award-winningPostprint (published version

Using PPP Information to Implement a Global Real-Time Virtual Network DGNSS Approach

Global Navigation Satellite Systems (GNSS) provide positioning services for connected and autonomous vehicles. Differential GNSS (DGNSS) has been demonstrated to provide reliable, high quality range correction information enabling real-time navigation with sub-meter or centimeter accuracy. However, DGNSS requires a local reference station near each user, which for a continental or global scale implementation would require a dense network of reference stations whose construction and maintenance would be prohibitively expensive. Precise Point Positioning (PPP) affords more flexibility as a public service for GNSS receivers, but its State Space Representation (SSR) format is not currently supported by most receivers. This article proposes a novel Virtual Network DGNSS (VN-DGNSS) design that capitalizes on the PPP infrastructure to provide global coverage for real-time navigation without building physical reference stations. Correction information is computed using data from public GNSS SSR data services and transmitted to users by Radio Technical Commission for Maritime Services (RTCM) Observation Space Representation (OSR) messages which are accepted by most receivers. The real-time stationary and moving platform testing performance, using u-blox M8P and ZED-F9P receivers, surpasses the Society of Automotive Engineering (SAE) specification (68% of horizontal error $\leqslant$ 1.5 m and vertical error $\leqslant$ 3 m) and shows significantly better horizontal performance than GNSS Open Service (OS). The moving tests also show better horizontal performance than the ZEDF9P receiver with Satellite Based Augmentation Systems (SBAS) enabled and achieve the lane-level accuracy which requires 95% of horizontal errors less than 1 meter.Comment: 14 pages, 8 tables, 4 figures, Code and data are available at https://github.com/Azurehappen/Virtual-Network-DGNSS-Projec

Estimation of GPS Differential Code Biases Based on Independent Reference Station and Recursive Filter

The differential code bias (DCB) of the Global Navigation Satellite Systems (GNSS) receiver should be precisely corrected when conducting ionospheric remote sensing and precise point positioning. The DCBs can usually be estimated by the ground GNSS network based on the parameterization of the global ionosphere together with the global ionospheric map (GIM). In order to reduce the spatial-temporal complexities, various algorithms based on GIM and local ionospheric modeling are conducted, but rely on station selection. In this paper, we present a recursive method to estimate the DCBs of Global Positioning System (GPS) satellites based on a recursive filter and independent reference station selection procedure. The satellite and receiver DCBs are estimated once per local day and aligned with the DCB product provided by the Center for Orbit Determination in Europe (CODE). From the statistical analysis with CODE DCB products, the results show that the accuracy of GPS satellite DCB estimates obtained by the recursive method can reach about 0.10 ns under solar quiet condition. The influence of stations with bad performances on DCB estimation can be reduced through the independent iterative reference selection. The accuracy of local ionospheric modeling based on recursive filter is less than 2 Total Electron Content Unit (TECU) in the monthly median sense. The performance of the recursive method is also evaluated under different solar conditions and the results show that the local ionospheric modeling is sensitive to solar conditions. Moreover, the recursive method has the potential to be implemented in the near real-time DCB estimation and GNSS data quality chec

GNSS-based global ionospheric maps : real-time combination, time resolution and applications on space weather monitoring

Tesi amb una secció retallada per drets d'editor.The research of this paper-based dissertation is focused on the Global Ionospheric Maps (GIMs) based on Global Navigation Satellite System (GNSS) including real-time combination, validation, time resolution and applications. The novelty of these works can be summarized as follows: The first contribution is to connect GIM assessment methods in post-processing and real-time mode including Jason-altimeter Vertical Total Electron Content (VTEC) assessment, GNSS differences of Slant Total Electron Content (dSTEC) assessment and real-time dSTEC (RT-dSTEC) assessment. With the RT-dSTEC assessment, we can assess the accuracy and calculate the weight of different real-time GIMs for combination in real-time mode. The Jason-altimeter VTEC assessment and dSTEC assessment can be used for evaluating GIMs over oceans and continental regions, respectively. In addition, the accurate GIMs shown in the GIM assessment methods can be regarded as reliable representations of global VTEC. The second contribution is to apply the RT-dSTEC assessment in real-time mode for the combination of different International GNSS Service (IGS) real-time GIMs. The IGS combined real-time GIM is generated to provide robust ionospheric corrections for real-time GNSS positioning and reliable global VTEC distribution for earth observations. The current status of IGS real-time GIMs from different centers is summarized and compared. The Jason-altimeter VTEC assessment and dSTEC assessment in post-processing mode are used for the validation of IGS real-time GIMs. The sensibility of real-time weighting technique by RT-dSTEC assessment is also verified. The third contribution is to investigate the influence of temporal resolution on the performance of GIMs. The variation of ionosphere is typically assumed as linear between two consecutive GIM TEC maps in a sun-fixed reference frame for up to few hours. However, the variation of ionospheric TEC is irregular due to the occurrence of space weather events. One and a half solar cycle of the IGS GIM with higher time resolution and accuracy (the UPC-IonSAT Quarter-of-an-hour time resolution Rapid GIM, UQRG) has been taken as a baseline to downsample them to all possible sub-daily temporal resolutions. The performance of the resulting GIMs has been evaluated taking into account the geographical position, solar and geomagnetic activity by Jason-altimeter VTEC assessment and dSTEC assessment. The fourth contribution is to propose a new way of estimating the spatial and temporal components of the VTEC gradient. The determination of ionospheric perturbation degrees can be helpful for guaranteeing the safety level of Satellite-Based Augmentation System (SBAS) and Ground-Based Augmentation System (GBAS) services. In order to estimate the spatial and temporal components of the VTEC gradient on a global scale, the accurate UQRG is selected. The VTEC gradient indices derived from UQRG GIMs (VgUG) allow users to obtain full (non-relative) values of TEC spatial gradients and temporal variations separately. The Regional VTEC spatial Gradient indices, based on UQRG (RVGU) and the Regional Ionospheric Disturbance index based on UQRG (RIDU), are proposed to estimate the regional ionospheric perturbation degree over selected regions. In addition, the spatial and temporal components of VTEC gradient at grid points of UQRG on a global scale are also introduced. The fifth contribution is to define a new ionospheric storm scale. The ionospheric response to high geomagnetic activity, ionospheric storm, can enlarge GNSS positioning errors by the increase of ionospheric electron density and disable high-frequency communications by the decrease of ionospheric electron density. To characterize the ionospheric state on a global scale, reliable global VTEC distribution is essential. According to previous studies, UQRG is one of the most accurate GIM. In this regard, the new Ionospheric storm Scale based on UQRG, IsUG, is proposed.La investigación de esta tesis doctoral se centra en los Mapas Ionosféricos Globales (GIMs) basados en el Sistema Global de Navegación por Satélite (GNSS), incluyendo la combinación en tiempo real, la validación, la resolución temporal y su aplicación. La novedad de los trabajos presentados puede resumirse como sigue: La primera contribución consiste en conectar los métodos de evaluación de los GIM en modo de posprocesamiento y en tiempo real, incluyendo la evaluación VTEC gracias a las medidas de los altímetros Jason, la evaluación del contenido total de electrones diferencial (dSTEC) y la evaluación dSTEC en tiempo real (RT-dSTEC). Con la evaluación RT-dSTEC, podemos evaluar la precisión y calcular el peso de diferentes GIM en tiempo real para su combinación también en tiempo real. La evaluación VTEC del altímetro Jason y la evaluación dSTEC pueden utilizarse para evaluar los GIM sobre los océanos y las regiones continentales, respectivamente. Además, los GIM precisos mostrados en los métodos de evaluación de GIM pueden considerarse como representaciones fiables del contenido total de electrones vertical global (VTEC). La segunda contribución consiste en aplicar la evaluación RT-dSTEC en tiempo real para la combinación de diferentes GIM del Servicio Internacional GNSS (IGS), todo ello en tiempo real. El GIM IGS combinado resultante proporciona correcciones ionosféricas robustas para el posicionamiento GNSS en tiempo real y una distribución global de VTEC fiable para las observaciones terrestres. Se resume y compara el estado actual de los GIM en tiempo real de diferentes centros IGS. La evaluación de VTEC respecto de los altímetros Jason y la evaluación de dSTEC en modo de posprocesamiento también se utilizan para la validación de los GIM en tiempo real del IGS. Y se verifica la sensibilidad de la técnica de ponderación en tiempo real mediante la evaluación RT-dSTEC. La tercera contribución consiste en proponer una nueva forma de estimar las componentes espaciales y temporales del gradiente VTEC. La determinación de los grados de perturbación ionosférica puede ser útil para garantizar el nivel de seguridad de los servicios del Sistema de Aumento Basado en Satélites (SBAS) y del Sistema de Aumento Basado en Tierra (GBAS). Para estimar los componentes espaciales y temporales del gradiente de VTEC a escala global, se selecciona el GIM UQRG debido a su exactitud y resolución temporal. Los índices de gradiente VTEC derivados de los GIM de UQRG (VgUG) permiten a los usuarios obtener valores completos (no relativos) de gradientes espaciales de VTEC y de las variaciones temporales por separado. Los índices de gradiente espacial VTEC regional, basados en UQRG (RVGU) y el índice de perturbación ionosférica regional basado en UQRG (RIDU), se proponen para estimar el grado de perturbación ionosférica regional sobre zonas de interés. Además también se introducen los componentes espaciales y temporales del gradiente VTEC en los puntos de la cuadrícula con valores proporcionados por UQRG a escala global. La cuarta contribución consiste en definir una nueva escala de tormentas ionosféricas. La respuesta ionosférica a la alta actividad geomagnética, la tormenta ionosférica, puede aumentar los errores de posicionamiento del GNSS por el aumento de la densidad de electrones ionosféricos e inhabilitar las comunicaciones de alta frecuencia por la disminución y en general rápida variación de la densidad de electrones ionosféricos. Para caracterizar el estado de la ionosfera a escala global, es esencial contar con una distribución global fiable de VTEC. Según estudios anteriores, el UQRG es uno de los GIM más precisos. En este sentido se propone la nueva Escala de tormentas ionosféricas basada en UQRG, IsUG.Postprint (published version

On the short-term temporal variations of GNSS receiver differential phase biases

As a first step towards studying the ionosphere with the global navigation satellite system (GNSS), leveling the phase to the code geometry-free observations on an arc-by-arc basis yields the ionospheric observables, interpreted as a combination of slant total electron content along with satellite and receiver differential code biases (DCB). The leveling errors in the ionospheric observables may arise during this procedure, which, according to previous studies by other researchers, are due to the combined effects of the code multipath and the intra-day variability in the receiver DCB. In this paper we further identify the short-term temporal variations of receiver differential phase biases (DPB) as another possible cause of leveling errors. Our investigation starts by the development of a method to epoch-wise estimate between-receiver DPB (BR-DPB) employing (inter-receiver) single-differenced, phase-only GNSS observations collected from a pair of receivers creating a zero or short baseline. The key issue for this method is to get rid of the possible discontinuities in the epoch-wise BR-DPB estimates, occurring when satellite assigned as pivot changes. Our numerical tests, carried out using Global Positioning System (GPS, US GNSS) and BeiDou Navigation Satellite System (BDS, Chinese GNSS) observations sampled every 30 s by a dedicatedly selected set of zero and short baselines, suggest two major findings. First, epoch-wise BR-DPB estimates can exhibit remarkable variability over a rather short period of time (e.g. 6 cm over 3 h), thus significant from a statistical point of view. Second, a dominant factor driving this variability is the changes of ambient temperature, instead of the un-modelled phase multipath

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

Impact and implementation of higher-order ionospheric effects on precise GNSS applications

High precision Global Navigation Satellite Systems (GNSS) positioning and time transfer require correcting signal delays, in particular higher-order ionospheric (I2+) terms. We present a consolidated model to correct second- and third-order terms, geometric bending and differential STEC bending effects in GNSS data. The model has been implemented in an online service correcting observations from submitted RINEX files for I2+ effects. We performed GNSS data processing with and without including I2+ corrections, in order to investigate the impact of I2+ corrections on GNSS products. We selected three time periods representing different ionospheric conditions. We used GPS and GLONASS observations from a global network and two regional networks in Poland and Brazil. We estimated satellite orbits, satellite clock corrections, Earth rotation parameters, troposphere delays, horizontal gradients, and receiver positions using global GNSS solution, Real-Time Kinematic (RTK), and Precise Point Positioning (PPP) techniques. The satellite-related products captured most of the impact of I2+ corrections, with the magnitude up to 2 cm for clock corrections, 1 cm for the along- and cross-track orbit components, and below 5 mm for the radial component. The impact of I2+ on troposphere products turned out to be insignificant in general. I2+ corrections had limited influence on the performance of ambiguity resolution and the reliability of RTK positioning. Finally, we found that I2+ corrections caused a systematic shift in the coordinate domain that was time- and region-dependent and reached up to -11 mm for the north component of the Brazilian stations during the most active ionospheric conditions.Peer ReviewedPostprint (published version