Modelling the Ionosphere over Europe: Investigation of NeQuick Formulation

The modelling of the Total Electron Content (TEC) plays an important role in global satellite navigation systems (GNSS) accuracy, especially for single-frequency receivers, the most common ones constituting the mass market. For the latter and in the framework of Galileo, the NeQuick model has been chosen for correcting the ionospheric error contribution. It has been designed to calculate the electron density at a given point of the ionosphere according to the time conditions and the solar activity. This electron density can be integrated along the path from the receiver to the considered satellite to provide the TEC. For Galileo, a parameter Az (“effective ionisation level”) will be provided to the model as solar activity information and will be daily updated by the ground stations.
Since NeQuick was chosen for Galileo purpose, a new version of the model has been released. It involves simplifications in the representation of the bottomside as well as a unique formula for a key parameter of the topside formulation previously defined through two equations, each one used for six months of the year. Hence we decided to investigate consecutive improvements and remaining weaknesses of this new formulation.
To this extent, we take benefit of various ionosphere data from several European stations (Chilton in UK, Dourbes in Belgium, El Arenosillo and Roquetes in Spain, Pruhonice in Czech Republic) where ionosonde and GPS TEC data are available for different solar activity levels. These data allow us to study NeQuick representation of the ionosphere at mid-latitudes. We investigate the difference between GPS-derived vTEC and corresponding values from NeQuick for the latest years (between solar maximum in 2000 and minimum in 2007) in order to observe the temporal dependencies towards Universal Time, season and solar activity. We use ionosonde data to constrain the model so that we can concentrate on its formulation of the profile only. We especially highlight the improvements from the second version of NeQuick and show the critical importance of the topside formulation.