Assessment of the NeQuick Model at Mid-latitudes using GPS TEC and Ionosonde Data

The ionosphere plays a crucial role in GNSS accuracy. In extreme cases, this electrically charged part of the atmosphere can lead to errors in positioning exceeding 100m. At first approximation, the ionospheric effect depends on the frequency of the incident signal and on the total content in free electrons of the ionosphere (“total electron content”, TEC) which is the integral of the electron density on the path between the satellite and the receiver.
The modelling of this parameter reveals then itself to be critical in particular for single-frequency receivers, the most common ones constituting the mass market, but also for multiple-frequency devices which will comprise a fallback mode in single frequency within the framework of critical applications such as civil aviation where the level of precision must be guaranteed in all circumstances.
The NeQuick model, which has been chosen for correcting the ionospheric error contribution in GALILEO single frequency users, calculates 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. The NeQuick model depends on a parameter Az (“effective ionization level”) which will be daily updated by the GALILEO ground stations to give the solar activity information to the model.
In order to reach the ionosphere error correction level objective (70% or 20 TECu whichever is larger), the model itself as well as its use for GALILEO are investigated. Different situations have to be considered: different latitude regions (space conditions), different hours, seasons and years (time conditions) and specific phenomena appearance (magnetic storms, Travelling Ionospheric Disturbances – TIDs). In addition the results can be compared to different data sets among which GPS slant or vertical TEC measurements, Global Ionospheric Maps, ionosonde profiles, topside soundings but also other ionosphere models results such as IRI.
As a first step in a thorough comparison process, we take benefit of various ionosphere data from the Dourbes Observatory (Belgium) where ionosonde and GPS TEC data are available on a period of more than one solar cycle, to study the mid-latitudes. We first investigate the difference between GPS-derived vTEC for Dourbes station and vTEC values from NeQuick for the latest years (between solar maximum in 2000 and minimum in 2006) in order to observe the temporal dependencies towards Universal Time, season and solar activity. The paper analyses the different situations when NeQuick fails to represent TEC in an adequate way to provide an efficient correction for navigation. Ionosonde measurements will help in the interpretation of these situations.