Global maps of ionospheric total electron content (TEC) are produced in real-time (RT) by mapping GPS observables collected from ground stations. These maps are produced to test real-time data acquisition, monitoring facilities, and mapping techniques. The RT TEC mapping can provide accurate ionospheric calibrations to navigation systems.These maps are also used to monitor ionospheric weather, and to nowcast ionospheric storms that often occur responding to activities in solar wind and Earth's magnetosphere as well as thermosphere.
Besides the GPS satellites, how many GPS stations are we talking about, from this page:
Monitoring Global Ionospheric Irregularities Using the Worldwide GPS NetworkThe current global GPS network contains about 360 GPS stations, and the number of stations is still increasing. Each receiver at these stations is capable of receiving L-band dual frequency signals from 8+ GPS satellites (totally 24) simultaneously in different directions. GPS data are downloaded to JPL through Internet and commercial phone lines on near real-time and daily bases. This network is a potential resource that can be used to achieve the NSWP goals.