Radio occultation
Encyclopedia
Radio occultation 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. 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 can be derived, hence radio occultation data has 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 are received on low earth orbit (LEO) satellites.
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
technique used for measuring the physical properties of a planetary atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
. 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
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
passes through the atmosphere it is refracted. The magnitude of the refraction
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
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 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
Abel transform
In mathematics, the Abel transform, named for Niels Henrik Abel, is an integral transform often used in the analysis of spherically symmetric or axially symmetric functions...
on the formula relating bending angle to refractivity. In the case of the neutral atmosphere (below the ionosphere) information on the atmosphere's temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
and water vapour can be derived, hence radio occultation data has applications in meteorology
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
.
When current radio occultation missions rely on radio signals from GPS (Global Positioning System
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
) 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
- CHAMPCHAMPCHAllenging Minisatellite Payload is a German satellite launched July 15, 2000 from Plesetsk, Russia and was used for atmospheric and ionospheric research, as well as other geoscientific applications, such as GPS radio occultation....
http://www.gfz-potsdam.de/pb1/op/champ/index_CHAMP.html - GRACEGrace-Film:* Grace , a 2009 horror film* States of Grace, a 2005 drama film- Games :* The Game of graces, a 19th century game for young girls* Tales of Graces, a 2009 Tales of game by Namco Tales Studio- Literature :...
http://www.gfz-potsdam.de/pb1/GASP/GASP2/GRACE/index_grace.html - GRAS on MetOpMetOpMetOp is a series of polar orbiting meteorological satellites operated by the European Organisation for the Exploitation of Meteorological Satellites. The satellites are all part af the EUMETSAT Polar System. It is intended to replace the soon to be retired TIROS network...
http://www.eumetsat.int/Home/Main/What_We_Do/Satellites/EUMETSAT_Polar_System/Space_Segment/SP_1139327243878 - REX on New HorizonsNew HorizonsNew Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...
http://pluto.jhuapl.edu/spacecraft/sciencePay.php