GPS meteorology

GPS meteorology refers to the use of the effect of the atmosphere on thepropagation of the Global Positioning System's (GPS) radio signals to deriveinformation on the state of the (lower, neutral) atmosphere.

There are currently two main operational techniques in use in GPS meteorology:GPS limb sounding from orbit, and GPS water vapour monitoring.

GPS water vapour monitoring

It is well known that the propagation delay caused by the atmosphere onmicrowaves is some 20 times stronger for water vapour as for all other ("dry")constituents of the atmosphere, like nitrogen, oxygen, argon or carbon dioxide.This is due to the polar nature of the water molecule.

As a result, if it is possible to determine the total atmospheric delay by GPS,one can subtract out the calculated contribution by the well-mixed "dry" gassesfrom the measured air pressure at the surface, and obtain a measure for theabsolute water vapour content of the atmosphere, integrated from surface tospace. This is also referred to as "total precipitable water vapour".

What makes it possible to determine the total atmospheric delay, is its knowndependence of the zenith or elevation angle of the satellite. If $z$is the zenith angle, the propagation path delay is proportional to$frac\{1\}\{cos\; z\}$. This unique signature makes it possible to solveseparately for the "zenith delay" in GPS computations also solving forstation coordinates and receiver clock delays.

Nowadays water vapour estimates are generated routinely in real time (latencymeasured in hours) by permanent geodetic GPS networks existing inmany parts of the world.

Water vapour is a very important gas for meteorological and climatologicalstudies, because of the latent heat it carries in transport. Additionallyit is a powerful greenhouse gas. The GPS technique is especially valuablebecause it measures "absolute" water vapour content or partial pressurerather than relative humidity, which corresponds to water vapour contents thatare strongly dependent on the often not precisely known temperature.

GPS limb sounding from orbit

One can receive on a low flying satellite the signals from the much higherorbiting (20 000 km) GPS satellite constellation. As the low flying satelliteorbits the Earth in 1.5 hours, many of the GPS satellites will "rise" and "set"during the time of the orbit. When they do, their signal will traverse the atmosphere.

A signal delay is produced which grows or decays exponentially with time, justas the atmospheric density is an exponential function of height above theEarth's surface. In fact, this so-called limb sounding technique allowsus to determine the scale height, the constant describing the steepness ofthis atmospheric density decay. This makes the technique extremely valuable forclimatological studies, as the scale height is directly related to thetemperature in the upper atmosphere, where the limb sounding signals do theirsensing. The technique works best in the lower stratosphere and uppertroposphere; it breaks down close to the Earth surface especially in thetropics, due to water vapour extinction.

Getting a precise measure for the mean atmospheric temperature by an absolutemethod which is truly global and not affected by the effects of either thenon-random (Euramerican-centred, land biased) global distribution of weatherstations, or the effects of urban [heat island] s, is the holy grail of climatechange studies. Unfortunately the techniques has only been in use for a small number of years now, offering only a short time base.

Satellites involved in GPS limb sounding have been: METSAT, OERSTED (Danish),and several others.

ee also

*GPS Radio occultation

Links and references

* [http://www.paroscientific.com/gpsmet.htm GPS meteorology]