One can estimate the local refractivity of the atmosphere as a function of altitude.

A method of determining the radio refractivity of the atmosphere as a function of altitude involves processing of data acquired by airborne or mountain-top Global Positioning System (GPS) receivers from particular GPS satellites as those satellites rise above or fall below the horizon. Previously, data of this type ("GPS occultation data" for short) have been gathered from outside the atmosphere by GPS receivers in low orbits around the Earth and used to generate global refractivity profiles. With the help of temperature data from weather analysis, the refractivity profiles can be converted to water-vapor profiles. In contrast, the present method of utilizing data from GPS receivers located within the atmosphere (see figure) makes it possible to obtain refractivity profiles, and thus water-vapor profiles, that are not global averages and, instead, are averaged over smaller geographic regions wherein the GPS receivers are located. Such higher-resolution water-vapor profiles can be used in studies of regional weather.