GPS Meteorology - 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 or elevation 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 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 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.