Global positioning system (GPS) technology has also impacted the collection of data for water
resources monitoring. GPSs have wide application for real-time monitoring where personnel and
equipment can be deployed to the field and their locations established. GPS signal processing has also
been used to determine the state of the atmospheric water vapor. NOAA uses existing GPS sites to
receive measurements for the total amount of water vapor above a GPS antenna. Water vapor refracts
radio waves, including GPS signals. When this happens, the apparent distance between a GPS satellite
and a receiver on the ground is a little longer than it would be if the air were completely dry.
Differences between the GPS signals at different times are correlated with the atmospheric water
vapor, and a large GPS water-vapor-sensing system has been developed (http://www.esrl.noaa.gov/
media/2007/gps/).