Radio occultation (RO) is a

remote sensing Remote sensing is the acquisition of information about an physical object, object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring inform ...

technique used for measuring the physical properties of a planetary

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

or ring system. Satellites carrying onboard GNSS-Radio occultation instruments include CHAMP, GRACE and GRACE-FO, MetOp and the recently launched COSMIC-2.

Atmospheric radio occultation

Atmospheric radio occultation relies on the detection of a change in a radio signal as it passes through a planet's atmosphere, i.e. as it is occulted by the atmosphere. When

electromagnetic radiation In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...

passes through the atmosphere, it is refracted (or bent). The magnitude of the

refraction In physics, refraction is the redirection of a wave as it passes from one transmission medium, medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commo ...

depends on the gradient of refractivity normal to the path, which in turn depends on the density gradient. 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 The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described t ...

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 In mathematics, the Abel transform,N. H. Abel, Journal für die reine und angewandte Mathematik, 1, pp. 153–157 (1826). named for Niels Henrik Abel, is an integral transform often used in the analysis of spherically symmetric or axially symmetri ...

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 is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

and water vapor content can be derived, thus giving radio occultation data applications in

meteorology Meteorology is the scientific study of the Earth's atmosphere and short-term atmospheric phenomena (i.e. weather), with a focus on weather forecasting. It has applications in the military, aviation, energy production, transport, agricultur ...

GNSS radio occultation

GNSS radio occultation (GNSS-RO), historically also known as GPS radio occultation (GPS-RO or GPSRO), is a type of radio occultation that relies on radio transmissions from GPS (

Global Positioning System The Global Positioning System (GPS) is a satellite-based hyperbolic navigation system owned by the United States Space Force and operated by Mission Delta 31. It is one of the global navigation satellite systems (GNSS) that provide ge ...

), or more generally from GNSS ( Global Navigation Satellite System), satellites. This is a relatively new technique (first applied in 1995) for performing atmospheric measurements. It is used as a

weather forecasting Weather forecasting or weather prediction is the application of science and technology forecasting, to predict the conditions of the Earth's atmosphere, atmosphere for a given location and time. People have attempted to predict the weather info ...

tool, and could also be harnessed in monitoring

climate change Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...

. The technique involves a low-Earth-orbit satellite receiving a signal from a GNSS satellite. The signal has to pass through the atmosphere and gets refracted along the way. The magnitude of the refraction depends on the temperature and water vapor concentration in the atmosphere. GNSS radio occultation amounts to an almost instantaneous depiction of the atmospheric state. The relative position between the GNSS satellite and the low-Earth-orbit satellite changes over time, allowing for a vertical scanning of successive layers of the atmosphere. GNSS-RO observations can also be conducted from aircraft or on high mountaintops.

Planetary satellite missions

Current missions include REX on

New Horizons ''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institut ...

Satellite missions

* CLARREO * Microlab 1 * FORMOSAT-3/COSMIC * FORMOSAT-7/COSMIC-2 * CHAMP * GRACE * Oceansat * RO instrument onboard Sentinel-6 Michael Freilich * GRAS instrument onboard MetOp satellites * Spire LEMUR cubesats * Yunyao 1

References

9. Alexander, P., A. de la Torre, and P. Llamedo (2008), Interpretation of gravity wave signatures in GPS radio occultations, J. Geophys. Res., 113, D16117, doi:10.1029/2007JD009390.

External links