The ionosphere is the single largest error source in point positioning after the application of precise GNSS orbit and clock products, and there are a number of mathematical models that have been proposed to mitigate its effects. Of these, the model developed by John A Klobuchar is used by the GPS system and broadcast by every satellite.
The Klobuchar model is something of a compromise between computational complexity and accuracy. However, it is reckoned to be capable of correcting up to 70% of actual ionospheric delay, mainly during quiet space weather conditions. Unfortunately, it's performance is less than perfect during severe space weather, geomagnetic and ionospheric disturbances.
The International GNSS Service has been providing the total electron content of ionosphere on a global scale since 1998. This model, known as the Global Ionospheric Model, is reputed to provide better results than the Klobuchar model using the same GPS dataset and ephemeris.
Galileo will use the NeQuick model, which is currently used by the European Geostationary Navigation Overlay Service (EGNOS) for system assessment analysis.
Both of these models are incorporated into Spirent’s SimGEN™ based constellation simulators. Find out more about GPS simulation here.