'Piercing points' shown where signals cross the ionosphere

The figure shows California GPS receivers location and 'piercing points' for ionospheric measurements from all receivers in California (SCIGN + BARD + IGS GPS networks). Different satellites are shown in different colours. The black star shows the epicentre location for the Denali earthquake. The piercing point is the intersection point between the GPS ray path (receiver-satellite) and a spherical shell with infinitesimal thickness at the mean altitude of the peak in the electron density profile.