Spatial correlation of transionospheric signal-time-delays

Excess time delays of transionospheric radio signals introduce ranging errors in satellite-navigation and radar systems. The errors introduced by these delays can be compensated for by the prediction of the total electron content (TEC) along the signals: propagation paths. It is envisioned that such predictions should be updated at periodic time-intervals at specific locations. The question arises as to the extent of the geographic area surrounding a station with real-time TEC determination capabilities, within which TEC values could be interpolated within accepted accuracy.