Ionospheric Depletion Detection and Analysis Over Africa

M. Cueto, A. Cezón, E. Sardón

Peer Reviewed

Abstract:	GMV has a broad experience in performing ionospheric analysis in equatorial regions (SACCSA project, GAGAN project, studies over Africa…). The objective of this paper is to show some of the most relevant results achieved by GMV on ionospheric plasma depletions, analyzing in particular, with GMV´s tools, their main features in equatorial regions, with special emphasis in the African region. Plasma depletions (or bubbles) are strong reductions in the ionospheric F-region plasma density due to the appearance of a Rayleigh-Taylor instability in the post-sunset, producing severe radio signal disruptions when crossing them. They are mostly aligned to the geomagnetic field with plasma density decreases up to about 3 orders of magnitudes or 99.9% of the background. Most of the plasma depletions are confined on the Appleton Anomaly region, which also shows the presence of strong scintillation activity (having consequently a severe effect on the L band signals propagation). The geomagnetic latitudes of the stations selected for the analysis developed in this paper are frequently affected by the presence of plasma depletions. Satellite Based Augmentation Systems (SBAS) support regional GNSS augmentation through the transmission of additional satellite-broadcast messages. In those systems, a network of double-frequency reference stations is used to estimate the ionospheric delay over the region. This ionospheric information is broadcast to the single-frequency SBAS users through an ionospheric model of grid vertical Total Electron Content (TEC) values, called Grid Ionospheric Vertical Delays (GIVD). In addition, the SBAS system also broadcasts for each grid point a conservative bound on the estimated GIVD accuracy, known as Grid Vertical Ionospheric Error (GIVE). The presence of plasma depletions in equatorial regions represents a potential difficulty for the implementation of a SBAS system in those regions, due to the small scale of the plasma depletion and the limited number of reference stations, making these phenomena difficult to be detected by the SBAS ground systems that mainly provide ionospheric large scale information. The analysis of the main features of the ionospheric depletions in a specific equatorial region is crucial for the subsequent proposal of any technique to reduce their impact in a SBAS system, both at system and receiver levels. This is the main incentive of the analysis performed in this paper. Therefore, in order to analyze the presence and main features of ionospheric depletions in the equatorial regions (and especially in Africa), a depletions´ detection algorithm has been used: magicIONO (GMV-IDAT), developed by GMV. This algorithm uses as input the TEC values estimated using GILION (GPS Information of the Local Ionosphere) algorithm to detect the presence of depletions in the Total Electron Content time series. GILION is currently implemented as a set of UNIX scripts that automatically run a set of FORTRAN programs to estimate the GPS satellite and receiver differential instrumental biases and the TEC values at GPS stations. The detection of plasma depletions will be used to characterize the occurrence of depletions in a set of equatorial stations. In addition to the plasma occurrence, the local time and monthly distribution of the depletions occurrence is analyzed in this paper. This analysis can contribute to propose different operational solutions for a SBAS system in Africa. On the other hand, the main features (depth and width) of the plasma depletions are analyzed as well for the selected stations, contributing also to the design of the system algorithm. Finally, it is broadly known that the behavior of the plasma depletions depends on the geomagnetic longitudinal location of the station under analysis. For instance, the monthly trend of the depletions occurrence depends on the geomagnetic longitudinal location of the station under study. In this paper a comparison with similar ionospheric studies from other regions and epochs will be done to analyze the impact of the specific locations under analysis in the depletions study presented here.
Published in:	Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015) September 14 - 18, 2015 Tampa Convention Center Tampa, Florida
Pages:	3557 - 3564
Cite this article:	Cueto, M., Cezón, A., Sardón, E., "Ionospheric Depletion Detection and Analysis Over Africa," Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015), Tampa, Florida, September 2015, pp. 3557-3564.
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