The Radio Occultation technique uses radio signals from Global Positioning System (GPS) satellites as the signals pass through the edge of the atmosphere. The atmosphere bends radio signals which graze the Earth at a shallow angle, when the signals are nearly tangent to the Earth.
A second satellite receives the signals, and can observe slight changes in the time that the signal is received. These timing shifts indicate how much a signal has been bent, which is related to subtle changes in air density, caused by changes in the temperature or moisture of air layers. These satellite measurements of radio refractivity of the atmosphere (or what is equivalent, measurements of the bending angle of the radio signals) complement conventional satellite measurements of temperature, when both are assimilated in numerical weather prediction models.
At the Joint Center for Satellite Data Assimilation, National Weather Service and NESDIS / STAR scientists have developed methods to use these bending angle and refractive index measurements from GPS satellites in the Global Forecast System (GFS) model. Observations from the "German Challenging Satellite Payload" (CHAMP) mission have been used successfully to test these methods. New tests with near-real-time data from the "Constellation Observing System for Meteorology, Ionosphere and Climate" (COSMIC) satellite have been promising.