- Radio occultation
Radio occultation (RO) is a
remote sensing technique used for measuring the physical properties of a planetaryatmosphere . 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. Whenelectromagnetic radiation passes through the atmosphere it is refracted. The magnitude of therefraction depends on the gradient of refractivity normal to the path, which in turn depends on the gradients of density and the water vapour. 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 theDoppler 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 anAbel transform on the formula relating bending angle to refractivity. In the case of the neutral atmosphere (below the ionosphere) information on the atmosphere'stemperature, pressure and water vapour can be derived, hence radio occultation data has applications inmeteorology .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 are received on low earth orbit (LEO) satellites.Radio occultation missions
Current missions include:
*FORMOSAT-3/COSMIC [http://www.cosmic.ucar.edu/index.html]
*CHAMP [http://www.gfz-potsdam.de/pb1/op/champ/index_CHAMP.html]
*GRACE [http://www.gfz-potsdam.de/pb1/GASP/GASP2/GRACE/index_grace.html]
*GRAS onMetOp [http://www.eumetsat.int/Home/Main/What_We_Do/Satellites/EUMETSAT_Polar_System/Space_Segment/SP_1139327243878]References
*Melbourne et al 1994. "The application of spacebourne GPS to atmospheric limb sounding and global change monitoring." Publication 94-18, Jet Propulsion Laboratory External links
* [http://www.metoffice.gov.uk/research/interproj/grassaf/monitoring/index.html GRAS SAF monitoring]
* [http://grassaf.dmi.dk/index.php GRAS SAF website]
* [http://www.ecmwf.int/products/forecasts/d/charts/monitoring/satellite/gpsro/ ECMWF monitoring]
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