Fast ionospheric correction using Galileo Az coefficients and the NTCM model

Kurzfassung

The European Global Navigation Satellite System (GNSS) Galileo uses the NeQuick-G model to compensate for ionospheric range errors in single frequency operations. The NeQuick-G model is operated by three coefficients broadcasted in Galileo navigation messages. The NeQuick-G model is a 3D electron density model whose vertical profile is formed by several Epstein layers. The required electron peak density and height parameters are derived using spatial and temporal interpolation of numerous related global maps. This makes NeQuick-G computationally expensive in terms of time and processing power when compared with the GPS Klobuchar model. We investigated here an alternative ionospheric correction approach for single frequency Galileo users. In the proposed approach the same broadcasted coefficients are used to drive another ionospheric model called Neustrelitz Total Electron Content (TEC) Model (NTCM) instead of NeQuick-G. The investigation shows that the new approach performs better than NeQuick-G when compared with the reference global Vertical Total Electron Content (VTEC) data. It is found that the Root Mean Squares (RMS), mean and Standard Deviations (STD) of residuals are approximately 9.6, -3.3 and 9.0 TECU (1 TECU = 1016 electrons/m2 ) for NeQuick-G whereas for NTCM they are 7.8, -1.5, 7.7 TECU in 2014. A comparison with slant TEC data from about 200 worldwide reference stations shows that the RMS and STD residuals are approximately 10.0 and 9.5 TECU for both models. When comparing the computation time it is found that NTCM is in average 65 times faster than NeQuick-G. The results prove that the Galileo Az parameters can be used to drive another ionosphere model independent of NeQuick-G. Certain real-time Safety of Life (SoL) operations would certainly benefit from the reduced complexity of the algorithm that greatly facilitates certification for aviation users. The compact NTCM algorithm is also favorable for "standard" users. It is assumed that most mass market and geodetic receiver manufacturers would favor a compact algorithm. In this paper we show how the proposed models reduce the algorithmic and runtime complexity, while at the same time reducing the ranging error due to ionospheric activity.