In the context of the International GNSS Service (IGS), several IGS Ionosphere Associated Analysis Centers have developed different techniques to provide global ionospheric maps (GIMs) of vertical ...total electron content (VTEC) since 1998. In this paper we present a comparison of the performances of all the GIMs created in the frame of IGS. Indeed we compare the
classical
ones (for the ionospheric analysis centers CODE, ESA/ESOC, JPL and UPC) with the new ones (NRCAN, CAS, WHU). To assess the quality of them in fair and completely independent ways, two assessment methods are used: a direct comparison to altimeter data (VTEC-altimeter) and to the difference of slant total electron content (STEC) observed in independent ground reference stations (dSTEC-GPS). The main conclusion of this study, performed during one solar cycle, is the consistency of the results between so many different GIM techniques and implementations.
A summary of the main concepts on global ionospheric map(s) hereinafter GIM(s) of vertical total electron content (VTEC), with special emphasis on their assessment, is presented in this paper. It is ...based on the experience accumulated during almost two decades of collaborative work in the context of the international global navigation satellite systems (GNSS) service (IGS) ionosphere working group. A representative comparison of the two main assessments of ionospheric electron content models (VTEC-altimeter and difference of Slant TEC, based on independent global positioning system data GPS, dSTEC-GPS) is performed. It is based on 26 GPS receivers worldwide distributed and mostly placed on islands, from the last quarter of 2010 to the end of 2016. The consistency between dSTEC-GPS and VTEC-altimeter assessments for one of the most accurate IGS GIMs (the tomographic-kriging GIM ‘UQRG’ computed by UPC) is shown. Typical error RMS values of 2 TECU for VTEC-altimeter and 0.5 TECU for dSTEC-GPS assessments are found. And, as expected by following a simple random model, there is a significant correlation between both RMS and specially relative errors, mainly evident when large enough number of observations per pass is considered. The authors expect that this manuscript will be useful for new analysis contributor centres and in general for the scientific and technical community interested in simple and truly external ways of validating electron content models of the ionosphere.
In the context of the International GNSS Service (IGS), several IGS Ionosphere Associated Analysis Centers (IAAC) have developed different techniques to provide Global Ionospheric Maps (GIMs) of ...Vertical Total Electron Content (VTEC) since 1998. In this paper we present a comparison of the performances of all the GIMs created in the frame of IGS. Indeed we compare the classical ones (for the ionospheric analysis centers CODE, ESA/ESOC, JPL and UPC) with the new ones (NRCAN, CAS, UWH). To assess the qual- ity of them in fair and completely independent ways, two assessment meth- ods are used: a direct comparison to altimeter data (VTEC-altimeter) and to the difference of slant total electron content (STEC) observed in independent ground reference stations (dSTEC-GPS). The main conclusion of this study, performed during one solar cycle, is the consistency of the results between so many different GIM techniques and implementations.
O GLONASS (GLObal NAvigation Satellite System) e o GPS (Global Positioning System) são os sistemas pioneiros de navegação global por satélite. A utilização combinada de dados GPS e GLONASS despertou ...interesse inicialmente, mas as pesquisas com investigação de ambos os sinais diminuíram muito no final da década de noventa devido à rápida degradação que o GLONASS sofreu. Porém, após um plano de restabelecimento do sistema, em 2011 o GLONASS voltou a contar com constelação completa e cobertura global. Além do restabelecimento, o sistema passou ainda por um processo de modernização, com novas gerações de satélites desenvolvidas, refinamentos dos sistemas de tempo e referencia e novas estações de controle instaladas. Além do uso de dados combinados, outros fatores que influenciam a qualidade do posicionamento são os métodos empregados e os erros aos quais os sinais transmitidos estão sujeitos. Em relação aos erros, a ionosfera é uma importante fonte, especialmente para usuários de receptores de simples frequência. Com este cenário, o presente trabalho buscou realizar um estudo sobre o sistema GLONASS, bem como analisar as melhorias apresentadas no posicionamento utilizando seus dados combinados aos dados GPS em diferentes métodos de posicionamento, considerando diferentes condições ionosféricas. De modo geral, foi possível verificar a viabilidade do uso de dados GLONASS combinados ao GPS em todos os métodos de posicionamento avaliados, com melhoria em mais de 97% dos casos analisados para o posicionamento por ponto e em torno de 70% dos casos com posicionamento relativo.
The characterization of the accuracy of ionospheric models currently used in global navigation satellite systems (GNSSs) is a long-standing issue. The characterization remains a challenging problem ...owing to the lack of sufficiently accurate slant ionospheric determinations to be used as a reference. The present study proposes a methodology based on the comparison of the predictions of any ionospheric model with actual unambiguous carrier-phase measurements from a global distribution of permanent receivers. The differences are separated as hardware delays (a receiver constant plus a satellite constant) per day. The present study was conducted for the entire year of 2014, i.e. during the last solar cycle maximum. The ionospheric models assessed are the operational models broadcast by the global positioning system (GPS) and Galileo constellations, the satellite-based augmentation system (SBAS) (i.e. European Geostationary Navigation Overlay System (EGNOS) and wide area augmentation system (WAAS)), a number of post-process global ionospheric maps (GIMs) from different International GNSS Service (IGS) analysis centres (ACs) and, finally, a more sophisticated GIM computed by the research group of Astronomy and GEomatics (gAGE). Ionospheric models based on GNSS data and represented on a grid (IGS GIMs or SBAS) correct about 85 % of the total slant ionospheric delay, whereas the models broadcasted in the navigation messages of GPS and Galileo only account for about 70 %. Our gAGE GIM is shown to correct 95 % of the delay. The proposed methodology appears to be a useful tool to improve current ionospheric models.
Los resultados de estudios previos sobre la variabilidad ionosférica han demostrado que la ionosfera, especialmente
sobre la región ecuatorial y de baja latitud, sufre cambios esporádicos en la ...estructura de densidad de electrones que
provocan efectos nocivos en las señales de radio satelital de alta frecuencia que pasan por la región. Este documento
intenta revisar el trabajo anterior sobre los fenómenos que dominan la ionosfera ecuatorial y de baja latitud y resaltar
los problemas aún no resueltos. Hay varios intentos de científicos anteriores para estudiar y comprender la estructura
dinámica de la ionosfera en diferentes latitudes y longitudes utilizando observaciones terrestres, mediciones satelitales
in situ y modelos ionosféricos. Estos científicos han presentado informes de diferentes fenómenos anómalos que
dominan la región ecuatorial y de baja latitud. Se han destacado algunos fenómenos como la propagación-F, la corriente
de electrochorro ecuatorial y la anomalía ecuatorial. Además de todos los informes interesantes documentados hasta
ahora, todavía hay algunas áreas grises que aún necesitan más investigación. Por ejemplo, el acoplamiento ionosferatermosfera en latitudes bajas y ecuatoriales durante eventos solares extremos y desastres sísmicos sigue siendo un tema
de gran preocupación.
The results from previous studies on ionospheric variability have shown that the ionosphere, especially over the
equatorial and low latitude region undergoes sporadic changes in the electron density structure which causes harmful
effects to high frequency satellite radio signals passing through the region. This paper attempts to review past work on
the phenomena dominating the equatorial and low latitude ionosphere and to highlight yet unsolved problems. There are
several attempts by past scientist to study and understand the dynamic structure of the ionosphere across different
latitudes and longitudes using ground-based observations, in-situ satellite measurements and ionospheric models. These
scientists have come up with reports of different anomalous phenomena dominating the equatorial and low latitude
region. Some phenomena like the spread-F, equatorial electrojet current and equatorial anomaly have been highlighted.
Apart from all the interesting reports so far documented, there are yet some grey areas that still needs further research.
For example, the ionosphere-thermosphere coupling at equatorial and low latitude during extreme solar event and
earthquake disaster is still a subject of major concern.
The book aims to explain the variations of near-Earth plasma observed over seismically active areas several days/hours before strong seismic shocks. It demonstrates how seismo-ionospheric coupling is ...part of the global electric circuit and shows that the anomalous electric field appearing in active seismic areas is the main carrier of information from the earth into the ionosphere. The discussion of physical mechanisms is based on experimental data. The results can be regarded as the basis for future applications such as short-term earthquake prediction. It proceeds to describe existing complex systems of space-born and ground-based monitoring for electromagnetic and ionospheric precursors of earthquakes, as well as those still under construction. It is an excellent text for courses and contains a wealth of information for those scientists working in the field of natural disaster reduction.
The new radio‐occultation (RO) instrument on board the future EUMETSAT Polar System‐Second Generation (EPS‐SG) satellites, flying at a height of 820 km, is primarily focusing on neutral atmospheric ...profiling. It will also provide an opportunity for RO ionospheric sounding, but only below impact heights of 500 km, in order to guarantee a full data gathering of the neutral part. This will leave a gap of 320 km, which impedes the application of the direct inversion techniques to retrieve the electron density profile. To overcome this challenge, we have looked for new ways (accurate and simple) of extrapolating the electron density (also applicable to other low‐Earth orbiting, LEO, missions like CHAMP): a new Vary‐Chap Extrapolation Technique (VCET). VCET is based on the scale height behavior, linearly dependent on the altitude above hmF2. This allows extrapolating the electron density profile for impact heights above its peak height (this is the case for EPS‐SG), up to the satellite orbital height. VCET has been assessed with more than 3700 complete electron density profiles obtained in four representative scenarios of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) in the United States and the Formosa Satellite Mission 3 (FORMOSAT‐3) in Taiwan, in solar maximum and minimum conditions, and geomagnetically disturbed conditions, by applying an updated Improved Abel Transform Inversion technique to dual‐frequency GPS measurements. It is shown that VCET performs much better than other classical Chapman models, with 60% of occultations showing relative extrapolation errors below 20%, in contrast with conventional Chapman model extrapolation approaches with 10% or less of the profiles with relative error below 20%.
Key Points
A new electron density profile extrapolation technique (VCET), founded on First Principles, is presented showing a high performance
VCET is simple and accurate: it is based on the linear behavior of the topside scale height, performing 6‐20 times better than Chapman model
The accuracy and extrapolation range, illustrated with FORMOSAT‐3/COSMIC measurements, will be important for new radio‐occultation missions like EPS‐SG
Atualmente, a interferência da camada ionosférica nos sinais da banda L, transmitidos pelos satélites GNSS (Sistema Global de Posicionamento por Satélite), é a maior fonte de erro na utilização deste ...sistema. Durante a sua propagação entre as antenas do satélite e do receptor, os sinais GNSS são afetados pelos elétrons livres na camada ionosférica. Estes elétrons provocam alterações na velocidade de propagação, na amplitude e na fase dos sinais. Portanto, a magnitude do erro sistemático devido à interferência ionosférica é diretamente proporcional ao TEC (Conteúdo Total de Elétrons) presente no caminho do sinal transionosférico e inversamente proporcional ao quadrado da frequência do sinal. Por outro lado, as redes GNSS ativas, tais como a RBMC, tornaram-se importantes estruturas de sensores para o monitoramento da ionosfera e do clima espacial. Para corrigir os efeitos ionosféricos sobre os sinais e para o monitoramento da ionosfera em tempo real, no Brasil vem sendo desenvolvido um modelo e um algoritmo para a assimilação dos dados GNSS em tempo real e geração da grade ionosférica e seu respectivo nível de confiança (GIVE – ErroVertical da Grade Ionosférica). Este algoritmo, denominado de GIB (Grade Ionosférica Brasileira) também estima e corrige a influênciadas tendências interfrequências dos satélites e dos receptores, para estimar os valores absolutos do TEC. Este trabalho apresenta os resultados obtidos até o momento com o GIB, bem como os mapas ionosféricos gerados em tempo quase real estruturados em formato IONEX (IONosphere map EXchange).
Single-frequency users of the global navigation satellite system (GNSS) must correct for the ionospheric delay. These corrections are available from global ionospheric models (GIMs). Therefore, the ...accuracy of the GIM is important because the unmodeled or incorrectly part of ionospheric delay contributes to the positioning error of GNSS-based positioning. However, the positioning error of receivers located at known coordinates can be used to infer the accuracy of GIMs in a simple manner. This is why assessment of GIMs by means of the position domain is often used as an alternative to assessments in the ionospheric delay domain. The latter method requires accurate reference ionospheric values obtained from a network solution and complex geodetic modeling. However, evaluations using the positioning error method present several difficulties, as evidenced in recent works, that can lead to inconsistent results compared to the tests using the ionospheric delay domain. We analyze the reasons why such inconsistencies occur, applying both methodologies. We have computed the position of 34 permanent stations for the entire year of 2014 within the last Solar Maximum. The positioning tests have been done using code pseudoranges and carrier-phase leveled (CCL) measurements. We identify the error sources that make it difficult to distinguish the part of the positioning error that is attributable to the ionospheric correction: the measurement noise, pseudorange multipath, evaluation metric, and outliers. Once these error sources are considered, we obtain equivalent results to those found in the ionospheric delay domain assessments. Accurate GIMs can provide single-frequency navigation positioning at the decimeter level using CCL measurements and better positions than those obtained using the dual-frequency ionospheric-free combination of pseudoranges. Finally, some recommendations are provided for further studies of ionospheric models using the position domain method.
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