Radio occultation (RO) is a remote sensing technique used for measuring the physical properties of a planetary atmosphere. It relies on the detection of a change in a radio signal as it passes through the planet's atmosphere i.e. as it is occulted by the atmosphere. When electromagnetic radiation passes through the atmosphere it is refracted. The magnitude of the refraction depends on the gradient of refractivity normal to the path, which in turn depends on the gradients of density and the water vapour content. The effect is most pronounced when the radiation traverses a long atmospheric limb path. At radio frequencies the amount of bending cannot be measured directly; instead the bending can be calculated using the Doppler shift of the signal given the geometry of the emitter and receiver. The amount of bending can be related to the refractive index by using an Abel transform on the formula relating bending angle to refractivity. In the case of the neutral atmosphere (below the ionosphere) information on the atmosphere's temperature, pressure and water vapour content can be derived, giving radio occultation data applications in meteorology.
When current radio occultation missions rely on radio signals from GPS (Global Positioning System) satellites, the technique is then known as GPSRO. The GPS signals can be received on low earth orbit (LEO) satellites. GPSRO observations can also be conducted from aircraft or on high mountaintops.
 Radio occultation missions
Current missions include:
- Melbourne et al. 1994. The application of spacebourne GPS to atmospheric limb sounding and global change monitoring. Publication 94-18, Jet Propulsion Laboratory
- Kursinski et al. 1997. Observing the Earth's atmosphere with radio occultation measurements using the Global Positioning System. J. Geophys. Res. 102:23.429-23.465.
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