GPS meteorology refers to the use of the effect of the atmosphere on the
propagation of the Global Positioning System's (GPS) radio signals to derive
information 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 on
microwaves 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" gasses
from the measured air pressure at the surface, and obtain a measure for the
absolute water vapour content of the atmosphere, integrated from surface to
space. This is also referred to as "total precipitable water vapour".

What makes it possible to determine the total atmospheric delay, is its known
dependence of the zenith

Zenith

The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...

 or elevation

Elevation

The elevation of a geographic location is its height above a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface ....

 angle of the satellite. If
is the zenith angle, the propagation path delay is proportional to
. This unique signature makes it possible to solve
separately for the zenith delay in GPS computations also solving for
station coordinates and receiver clock delays.

Nowadays water vapour estimates are generated routinely in real time (latency
measured in hours) by permanent geodetic

Geodesy

Geodesy , also named geodetics, a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space. Geodesists also study geodynamical phenomena such as crustal...

 GPS networks existing in
many parts of the world.

Water vapour is a very important gas for meteorological and climatological
studies, because of the latent heat

Latent heat

Latent heat is the heat released or absorbed by a chemical substance or a thermodynamic system during a process that occurs without a change in temperature. A typical example is a change of state of matter, meaning a phase transition such as the melting of ice or the boiling of water. The term was...

 it carries in transport. Additionally
it is a powerful greenhouse gas. The GPS technique is especially valuable
because it measures absolute water vapour content or partial pressure
rather than relative humidity, which corresponds to water vapour contents that
are 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 higher
orbiting (20 000 km) GPS satellite constellation. As the low flying satellite
orbits 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, just
as the atmospheric density is an exponential function of height above the
Earth's surface. In fact, this so-called limb sounding technique allows
us to determine the scale height

Scale height

In various scientific contexts, a scale height is a distance over which a quantity decreases by a factor of e...

, the constant describing the steepness of
this atmospheric density decay. This makes the technique extremely valuable for
climatological studies, as the scale height is directly related to the
temperature in the upper atmosphere, where the limb sounding signals do their
sensing. The technique works best in the lower stratosphere and upper
troposphere; it breaks down close to the Earth surface especially in the
tropics, due to water vapour extinction.

Getting a precise measure for the mean atmospheric temperature by an absolute
method which is truly global and not affected by the effects of either the
non-random (Euramerican-centred, land biased) global distribution of weather
stations, or the effects of urban [heat island]s, is the holy grail of climate
change 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.