Global Positioning System (GPS) satellites have orbital altitudes of about 20,200 km, while satellites in the Navy Ionospheric Monitoring System (NIMS) constellation are in circular orbits at heights of about 1100 km. Independent measurements of the electron content in the ionized atmosphere can be made using the radio signals from both satellite constellations. Differences between the two estimates can be related to the electron content on the GPS ray paths above 1100 km, through the tenuous plasma of the protonosphere. Results are reported from some 21 months of simultaneous observations of both GPS and NIMS transmissions at a European midlatitude station at solar minimum. It is shown that the average differences between the electron contents measured by the two systems are in broad agreement with the predictions from an earlier modeling study of the effects of the protonosphere on GPS total electron content. The expected influence of ray path/flux tube geometry and the rapid depletion and slow refilling of the protonosphere in response to geomagnetic storm activity can be seen in the averaged measurements. ¿ 1999 American Geophysical Union