ionospheric modeling

The time delay caused by the ionosphere varies with respect to the frequency of the GNSS signals and affects each phase of a signal differently. When dual frequency receivers are used, the carrier phase observations for both frequencies can be used to model and eliminate most of the ionospheric effects. When dual frequency measurements are not available an ionospheric model broadcast by the GNSS satellites can be used to reduce ionospheric affects. The use of the broadcast model, however, is not as effective as the use of dual frequency measurements.