Within the validation work of EGNOS for civil aviation, it is interesting to analyze the transmitted corrections individually. In this paper, the ionospheric model of EGNOS is compared to the one determined by an independent organization (CODE). The continuous monitoring of the ionosphere is envisaged in order to analyze the ionospheric corrections in real-time. Two methods based on dual-frequency GPS measurements are described. The first approach computes the ionospheric delays after calibration of the receiver DCB (Differential Code Bias) using phase-smoothed code measurements. The second method is based on spherical harmonic expansions.