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.
In fact, this so-called limb sounding technique allows us to determine the scale height, 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.