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