Evaluation of an Ionospheric GNSS model performance in Southern mid-latitudes during geomagnetic storm

Abstract:

The performance of the two versions of the NeQuick model is investigated in the Southern mid-latitude, under quiet and disturbed magnetic conditions. The NeQuick is used among others for the European Geostationary Navigation Overlay Service (EGNOS), developed to supplement the GNSSs systems by reporting on the reliability and accuracy of the positioning data. The model is adapted to local and storm-specific response by using the critical frequency of the F 2 layer (foF2) and the propagation factor (M (3000)F 2) derived from three South African ionosonde measurements, Hermanus (34.40⁰S ; 19.20⁰E), Grahamstown (33.30⁰S ; 26.50⁰E) and Louisvale (28.50⁰S ; 21.20⁰E). The total electron content (TEC) derived from the adapted NeQuick version is compared with observed TEC derived from GNSS data from co-located or nearby GNSS dual frequency receivers. The Hermanus K-index is used to select all the disturbed days (K-index> 4) upon moving from low to high solar activity (from 2009 to 2012). For each disturbed day, the TEC is compared to the TEC values of a quiet reference day of the same month. The study reveals that the NeQuick model shows similar reliability for both magnetic quiet and disturbed conditions but its accuracy is affected by the solar activity. The model is much better for moderate solar activity epochs (2009 and 2010) while it exhibits a discrepancy with observations during high solar activity epochs. For instance in Hermanus, ∆TEC is generally lower than 10 TECu in 2009 and it reaches sometimes 20 TECu in 2011 and 2012. It is also noticed that NeQuick 2 is more accurate than NeQuick 1 with an improvement of TEC estimation more significant for the high solar activity epochs. The improvement realized in the latest version of NeQuick is more than 15% and reaches sometime 50%.