Soundings are of refractive index, so must be interpreted in terms of the combined effects of temperature and humidity on refractive index. Properties average over distances that represent of order 100km horizontally, so are different in character from radiosonde soundings. Flights must be planned with known locations of GPS satellites to benefit from occultation during the flight. For a summary of the theory underlying sounding by occultation, see the article by Kursinski et al., 1997, referenced in the "Documentation" section at the end of this page.
The following diagram illustrates how the "bending angle" changes as the aircraft and satellite move and how a profile of bending angles are measured. The refractivity obtained by inversion of the bending angle measurements then provides information on the water vapor profile (if temperature is known) or of temperature (where water vapor is negligible).