Ground-based GPS receivers measure the Zenith Total Delay (ZTD) in mm which is the sum of the Zenith Hydrostatic Delay (ZHD) caused by the atmospheres dry components and the Zenith Wet Delay (ZWD) resulting
Atmoshperic water vapor content and zenith wet delay derived from gps (Global Positioning System) measurements by the Geoforschungszentrum Potsdam within the EUMETNET GPS water vapour programme.