This paper reports results from an investigation into possible total electron content (TEC) on ray paths between GPS satellites and satellites in geostationary orbit. The Sheffield University Plasmasphere Ionosphere Model (SUPIM) has been used to estimate the TEC for conditions appropriate to solar maximum as a first step to assessing possible errors in the potential use of the navigation satellites for station keeping in the geostationary orbit. It has been found that in general, for paths through the plasmasphere, where the satellite-to-satellite geometry is far removed from eclipse by the Earth's horizon, the TEC is usually less than 20 TECU (total electron content units, 1 TECU = 1016 el m-2) with resultant errors in single-frequency GPS positioning of a only a few meters. However, when the rays cross the equatorial F2 ionosphere at near-grazing incidence close to the layer peak, TEC of up to some 2000 TECU have been found, with consequent potential single-frequency GPS positioning uncertainties of hundreds of meters. The exact magnitudes of the TEC are very dependent on the precise geometry and vary significantly with time, so they would be difficult to simulate in any real-time, model-based mitigation system.