An Assessment of Predicted and Measured Ionospheric Total Electron Content Using a Regional GPS Network

The signals from the satellites of the Navstar Global Positioning System (GPS) must travel through the earth's ionosphere on their way to GPS receivers on or near the earth's surface. To achieve the highest possible positioning accuracies from GPS, one must correct for the carrier phase advance and pseudorange group delay imposed on the signals by the ionosphere. Whereas these effects may be considered a nuisance by most GPS users, they will provide the ionospheric community with an opportunity to use GPS as a tool to better understand the plasma surrounding the earth. The dispersive nature of the ionosphere makes it possible to measure its total electron content (TEC) using dualfrequency GPS observations collected by permanent networks of receivers. One such network is that of the International GPS Service for Geodynamics (IGS). We have used dual-frequency GPS pseudorange and carrier phase observations from six European stations in this network to derive regional TEC values. In this research, we investigated the effect of using different elevation cutoff angles and ionospheric shell heights on TEC estimates and satellite-receiver instrumental biases. We found that using different elevation cutoff angles had an impact on TEC estimates at the 2 TEC unit (TECU) level. We also discovered that using different ionospheric shell heights has an effect on the ionospheric TEC estimates at about the 2 TECU level depending on geographic location and time of the day. We found no significant changes in the bias estimates using different elevation cutoff angles. We compared our TEC estimates with TEC predictions obtained by using the International Reference Ionosphere 1990 (IRI90) model. The results of this comparison are similar to those of other studies that were conducted using data sets at low solar activity times. After processing the data from the 6 European stations collected over a 7 day period, we were able to follow highly varying ionospheric conditions associated with geomagnetic disturbances.