[morcheeba]

Good explanation.

A simple GPS receiver will have a generic mathematical model for the ionosphere and use that as a good guess. More advanced ones can measure the delay directly.

The ionosphere affects different frequencies differently, so the GPS satellites transmits additional signals at different frequencies. By measuring the phase of these signals (L1 and L2), the math can be done to get a better estimation of the delay caused by the ionosphere between each satellite and the receiver. Those are the dots we're seeing on this animation. (GPS also uses the L2 signal to transmit encrypted information that lets military receivers get a better fix than civilian receivers).

more info: https://www.e-education.psu.edu/geog862/node/1715