Basic Measurement Principle

Orbiting Global Positioning Satellites are used to precisely determine the positions of the receiving site relative to a reference site with known coordinates. Averaging of many estimates of this position provides a precision of less than 1 cm.
 
 

Assuming now that the precise positions of the sites and the satellite orbits are known, the time delays of the radio waves compared to the case with no atmosphere is calculated. The amount of water vapor present in the atmosphere may be deduced from the magnitude of this delay.