Due to the gravitation of the earth air molecules are prevented to evaporate into outer space. These components cover the earth. The earth's atmosphere includes all masses which surround the earth. It consists of a nearly constant mixture of different gases like of nitrogen (approx. 78%), oxygen (approx. 21%), minor rates of inert gases (argon, helium, krypton, neon, xenon), carbon dioxide, ozone, and hydrogen.
Especially water vapor is an important component of the atmosphere of the earth, since it has substantial influence on weather and climate and plays also an important in chemistry of the atmosphere. Water vapor is the dominating greenhouse gas and occurs in temporally and spatially very variable concentrations.
Oceanic and continetal surfaces are resources of the water vapor of the climatic system. By means of evaporation and convection water vapor is created and integrated into the atmospheric movements.
Despite its meaning for atmospheric processes in a broad spatial and temporal scale water vapor so far remained as a component which cannot easily determined or handled.
The knowledge of global and regional distributions of water vapor and its temporal variation is fundamental for weather forecast and climatic research.
Since the atmosphere affects GPS observations, its behavior has to be modeled within the parameter estimation. This is normally done using a two-step-concept, which consists of a so-called "A priori prediction model" and a so-called "station-specific auxiliary parameter estimation".
The a priori prediction model is based on specific assumptions concerning the behavior of the neutrosphere (for example temperature gradient, composition, ideal gas law) resulting in an approximation of the actually conditions. This is obtained for example by extrapolation of surface meteorology.
Auxiliary parameter estimation: In order to carry out relative positioning, according to the principle of the least squares on basis of double-differences, improvements of the prediction models are needed and determined in zenithale direction by means of a suitable functional model with limited temporal validity (for example 2 h).
On the contrary, GPS can be used also as tool to monitor the atmospheric water vapor.