The Westford Water Vapor Experiment: Use of GPS to Determine Total Precipitable Water Vapor.

The Westford Water Vapor Experiment WWAVE was designed to measure the temporal and spatial variability of the total precipitable water vapor PWV over an area defined by an approximate 25 km radius centered on the Haystack Observatory in Westford, MA. PWV is defined as the height of liquid water that would result from condensing all the water vapor in a column from the Earth surface to the top of the atmosphere. The main experiment was conducted from 15 - 30 August 1995, and a variety of different techniques were used to measure the water vapor, including radiosondes, launched two to three times daily from one location a water vapor radiometer and 11 Global Positioning System GPS receivers separated by 0.5 to 35 km. The data were either collected by A.O.A. Turbo Rogue or Ashtech Z12 GPS receivers, and nine sites used choke ring antennas. The WWAVE analysis showed that GPS estimates of zenith wet delay ZWD agreed with measurements by the WVR and radiosondes to within 6-12 mm, corresponding to 1-2 mm of PWV. The precision of the GPS measurement of ZWD is better than 6 mm 1 mm of PWV as shown by the agreement of three closely spaced receivers.