Abstract: The six COSMIC microsatellites, scheduled to be launched in mid-2005, will pick up radio signals from 28 existing GPS satellites as they [the signals, not the satellites, Ed.] pass through the Earth's atmosphere. The microsatellites will observe the refraction (or bending) of the signals, and infer information about atmospheric density from it, at all altitudes. From the density data, researchers will be able to deduce the pattern of pressure and temperature.
"We can also calculate atmospheric moisture near the surface, construct pressure contours, and derive wind fields (…)"'
- Array System Promises Global Atmospheric Monitoring, David Cyranoski, Nature 411, 228 (2001)