Comparison of simulated phase scintillation indices (colored circles along the orbit of the Swarm satellite) with the empirical values (colored squares) for L1 radio signals. Simulations are performed using the conventional random phase screen technique (a) and by using the phase gradient screens (b). The empirical values are obtained from several GNSS receivers located in Europe and their geographical locations are given by the position of their ionospheric piercing points. The black arrows correspond to the NeGIX vectors (arbitrary scale), cf. Appendix D for the NeGIX definition. For reference, the contours for the magnitude of the vertical TEC gradient (in units of mm/km on GPS L1) are also shown.