Impact of Ionospheric Horizontal Asymmetry on Electron Density Profiles Derived by GNSS Radio Occultation

M.M. Shaikh, R. Notarpietro, R. Romero, F. Dovis

Abstract:	The ‘Onion-peeling’ algorithm is a very common technique used to invert Radio Occultation (RO) data in the ionosphere. Because of the implicit assumption of spherical symmetry for the electron density distribution in the ionosphere, the standard Onion-peeling algorithm could give erroneous concentration values in the retrieved electron density profile. In particular, this happens when strong horizontal ionospheric electron density gradients are present, like for example in the Equatorial Ionization Anomaly (EIA) region during high solar activity periods. In this work, using simulated RO TEC data computed by means of the NeQuick2 ionospheric electron density model and ideal RO geometries, we tried to formulate and evaluate an asymmetry level indicator for quasi-horizontal radio occultation observations. This asymmetry index is based on the electron density variation that a ray may experience along its propagation path (satellite to satellite link) in a RO event. This index is dependent on the occultation geometry and on the azimuth of the occultation plane. Our qualitative assessment shows very high correlation between our asymmetry index and Onion-peeling retrieval errors; i.e. errors produced by Onion-peeling in the retrieval of NmF2 and VTEC are larger at the geographical locations where our asymmetry index indicates high asymmetry in the ionosphere. An analysis of the asymmetry index has also been carried out using real radio occultation data taken from COSMIC mission. This has been done for COSMIC events for which vertical profiles of simulated horizontal-TEC under NeQuick2 background were quite close to the real horizontal-TEC observations; considering the same RO geometry. This has also validated NeQuick2 as a good background ionospheric electron density model to be used in ionospheric asymmetry evaluation. On the basis of the outcomes of our work, for a given geometry of a real RO event and using a suitable ionospheric model, we will try to investigate the possibility to predict ionospheric asymmetry expected for the particular RO geometry considered. We could also try to evaluate, in advance, its impact on the inverted electron density profile, providing an indication of the product quality. Results presented in this paper are initial outcomes based on our asymmetry evaluation algorithm. TRANSMIT - Training Research and Applications Network to Support the Mitigation of Ionospheric Threats is an FP7 Marie Curie Initial Training Network focused on the study of ionospheric phenomena and their effects on systems embedded in our daily life. As final outcome of our participation in the project TRANSMIT, we will try to formulate an improved model aided inversion algorithm to produce more realistic ionospheric inversion products using radio occultation data.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	2463 - 2469
Cite this article:	Shaikh, M.M., Notarpietro, R., Romero, R., Dovis, F., "Impact of Ionospheric Horizontal Asymmetry on Electron Density Profiles Derived by GNSS Radio Occultation," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 2463-2469.
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