Measurements of Precipitable Water Vapor by GPS, Radiosondes, and a Microwave Water Vapor Radiometer

Results from the Westford WAter Vapor Experiment (WWAVE) will be discussed. This experiment was designed to measure the temporal and spatial variability of the total precipitable water vapor (PWV) over an area of roughly 25 km radius around the Haystack Observatory in Westford, MA. The main experiment was conducted from August 15 to August 30, 1995, and a variety of techniques were used to measure the water vapor, including radiosondes launched two to three times daily from the Westford site; a water vapor radiometer (WVR) located at the site; and eleven GPS receivers arranged within a 25 km radius around the site (with three receivers located within one km of each other at the site). Surface meteorological monitoring units were collocated at eight of the GPS sites. In addition, estimates of the precipitable water vapor were obtained with the Westford VLBI antenna as part of a 6-station network which included antennas in Alaska, Hawaii, Germany, Sweden, and Norway. Discrepancies on the order of 10-30 mm of zenith wet delay (1-5 mm of PWV) are seen in the measurements of precipitable water vapor by the Haystack radiosonde launches as compared to those from the nearby NWS radiosonde sites (Grey, Maine, Chatham, Massachusetts, and Albany, New York). Possible explanations are differences in geographical locations, in humidity sensors, and/or in the processing algorithms. A comparison of the collocated Haystack WVR and radiosonde estimates of the precipitable water vapor also indicates differences on the order of 10 mm zenith delay (1-2 mm of PWV). Finally, systematic differences in the GPS determination of PWV are observed that depend on the elevation cutoff used in the GPS analysis. These differences are not related to the type of GPS antenna and receiver, and are not seen at all sites. The discrepancies are consistent with the effects of near field scattering seen in geodetic GPS measurements and indicate that GPS antenna mounts should be considered in designing water vapor retrieval systems based on GPS.