Estimativa e análise de perfis de densidade de elétrons para a região brasileira: integração de rádio ocultação GNSS e de informações ionosféricas provenientes de estações GNSS terrestres
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Data
2021-10-08
Autores
Jerez, Gabriel Oliveira [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
A rádio ocultação começou a ser utilizada para a sondagem da atmosfera de outros planetas na década de sessenta. Com o desenvolvimento dos Sistemas Globais de Navegação por Satélite (GNSS - Global Navigation Satellite Systems) e de missões com satélites de baixa órbita (LEO - Low Earth Orbiting) surgiram novas possibilidades de investigações da atmosfera terrestre, entre elas a aplicação da técnica de rádio ocultação (RO) GNSS. A influência devido à atmosfera terrestre sofrida pelo sinal transmitido pelo satélite GNSS permite a recuperação dos índices de refração, os quais possuem informações da composição atmosférica. A inversa de Abel é comumente empregada para a recuperação de perfis, porém, para sua aplicação é imposta a condição de simetria esférica da atmosfera. Com isso a técnica tem seu desempenho degradado para alguns casos como, por exemplo, na recuperação de perfis de densidade de elétrons, principalmente em regiões com elevada variabilidade da densidade eletrônica. Visando melhores resultados diversas pesquisas vêm propondo abordagens alternativas. Dentre essas, a utilização da inversa de Abel com o auxílio de mapas ionosféricos se mostrou promissora, no entanto, algumas limitações foram observadas, especialmente em regiões com maior variabilidade na distribuição de elétrons, como é o caso do Brasil. Neste contexto, essa pesquisa teve por objetivo desenvolver uma metodologia para realizar a recuperação de perfis de densidade de elétrons utilizando dados de RO empregando a inversa de Abel com o auxílio de mapas ionosféricos e informações ionosféricas regionais. A metodologia desenvolvida foi implementada no software utilizado pela University Corporation for Atmospheric Research (UCAR) para processamento dos dados da missão Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) e de outras missões. Para o desenvolvimento da metodologia foram realizadas diversas avaliações de mapas ionosféricos disponíveis para a região brasileira, considerando produtos regionais, globais e híbridos. Foi proposta uma nova metodologia de avaliação de mapas ionosféricos por meio de ionossondas, sendo os produtos CODG e UQRG os que apresentaram os melhores resultados entre os produtos analisados. Além disso, foi realizada uma avaliação do impacto do uso das incertezas do Conteúdo Total de Elétrons na Vertical (VTEC - Vertical Total Electron Content) no domínio da ionosfera e do posicionamento. O uso das incertezas dos mapas ionosféricos não influenciou significativamente a interpolação dos valores de VTEC, porém, no domínio do posicionamento o impacto do uso das incertezas ficou evidenciado, principalmente para estações em regiões com maior variabilidade da densidade de elétrons em períodos de intenso fluxo solar. As metodologias de recuperação de perfis de densidade de elétrons a partir de RO foram testadas considerando mapas globais e comparadas com o desempenho de outros produtos COSMIC. As avaliações consideraram medidas in situ e dados de ionossondas. Com as avaliações com medidas in situ o produto proposto obteve melhor desempenho no cenário de baixo fluxo solar, enquanto para dois cenários de intenso fluxo solar o produto oficial COSMIC (ionPrf) obteve melhores resultados. Para as avaliações com dados de ionossondas o produto derivado da metodologia proposta obteve melhor desempenho considerando as ocultações com ocorrência mais próxima das ionossondas de referência (janelas de busca mais restritas).
Radio occultation has been used for probing the atmosphere of other planets since the sixties. With the development of Global Navigation Satellite Systems (GNSS) and Low Earth Orbiting (LEO) satellite missions, new possibilities for investigating the Earth's atmosphere have emerged, including the use of the GNSS radio occultation (RO) technique. The influence due to the Earth's atmosphere suffered by the transmitted signal allows the retrieval of refraction indices, which carry information about the atmospheric composition. The Abel inversion is commonly employed for profiles retrieval, however, for its application it is imposed that the atmosphere is spherically symmetric. Thus, the technique has its performance degraded in some cases, as for instance, in the electron density profiles retrieval, mainly in regions with high variability of the electron density. To overcome this issue, several studies have been proposing alternative approaches. Among those, the use of the Abel inversion aided by ionospheric maps has shown promising results, however, some limitations have been observed, especially in regions with greater variability in the electron distribution, as the Brazilian region. In this context, this research aimed to develop a methodology to perform the electron density profiles retrieval using RO data, employing the Abel inversion aided by ionospheric maps and regional ionospheric information. The developed methodology was implemented in the software used by the University Corporation for Atmospheric Research (UCAR) for processing data from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) mission and other missions. For the development of the methodology, several evaluations of ionospheric maps available for the Brazilian were performed, considering regional, global and hybrid products. A new methodology was proposed for the assessment of ionospheric maps by means of ionosonde data, with the products CODG and UQRG presenting the best results among the analyzed products. Furthermore, an evaluation of the impact of the use of Vertical Total Electron Content (VTEC) uncertainties in the ionospheric and positioning domain was performed. The use of the uncertainties of the ionospheric maps did not significantly influence the interpolation of the VTEC values, however, in the positioning domain the impact of the use of the uncertainties was evidenced, mainly for stations in regions with higher ionospheric variability in periods of high solar flux. The profile retrieval methodologies were tested considering global maps and compared with the performance of other COSMIC products. The evaluations considered in situ measurements and ionosonde data. With the assessment with in situ measurements, the proposed product performed better in the low solar flux scenario, while for two high solar flux scenarios the official COSMIC product (ionPrf) led to better results. For the evaluations with ionosonde data the product derived from the proposed methodology obtained better performance considering the occultations with occurrence closer to the reference ionosondes (more restricted search windows).
Radio occultation has been used for probing the atmosphere of other planets since the sixties. With the development of Global Navigation Satellite Systems (GNSS) and Low Earth Orbiting (LEO) satellite missions, new possibilities for investigating the Earth's atmosphere have emerged, including the use of the GNSS radio occultation (RO) technique. The influence due to the Earth's atmosphere suffered by the transmitted signal allows the retrieval of refraction indices, which carry information about the atmospheric composition. The Abel inversion is commonly employed for profiles retrieval, however, for its application it is imposed that the atmosphere is spherically symmetric. Thus, the technique has its performance degraded in some cases, as for instance, in the electron density profiles retrieval, mainly in regions with high variability of the electron density. To overcome this issue, several studies have been proposing alternative approaches. Among those, the use of the Abel inversion aided by ionospheric maps has shown promising results, however, some limitations have been observed, especially in regions with greater variability in the electron distribution, as the Brazilian region. In this context, this research aimed to develop a methodology to perform the electron density profiles retrieval using RO data, employing the Abel inversion aided by ionospheric maps and regional ionospheric information. The developed methodology was implemented in the software used by the University Corporation for Atmospheric Research (UCAR) for processing data from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) mission and other missions. For the development of the methodology, several evaluations of ionospheric maps available for the Brazilian were performed, considering regional, global and hybrid products. A new methodology was proposed for the assessment of ionospheric maps by means of ionosonde data, with the products CODG and UQRG presenting the best results among the analyzed products. Furthermore, an evaluation of the impact of the use of Vertical Total Electron Content (VTEC) uncertainties in the ionospheric and positioning domain was performed. The use of the uncertainties of the ionospheric maps did not significantly influence the interpolation of the VTEC values, however, in the positioning domain the impact of the use of the uncertainties was evidenced, mainly for stations in regions with higher ionospheric variability in periods of high solar flux. The profile retrieval methodologies were tested considering global maps and compared with the performance of other COSMIC products. The evaluations considered in situ measurements and ionosonde data. With the assessment with in situ measurements, the proposed product performed better in the low solar flux scenario, while for two high solar flux scenarios the official COSMIC product (ionPrf) led to better results. For the evaluations with ionosonde data the product derived from the proposed methodology obtained better performance considering the occultations with occurrence closer to the reference ionosondes (more restricted search windows).
Descrição
Palavras-chave
Rádio ocultação, Recuperação de perfis de densidade de elétrons, Mapas ionosféricos, Radio occultation, Electron density profiles retrieval, Ionospheric maps