The sensing of precipitable water (PW) using the Global Positioning System (GPS) in the near Tropics is investigated. GPS data acquired from the Central Weather Bureau's Taipei weather station in Banchao (Taipei), Taiwan, and each of nine International GPS Service (IGS) stations were utilized to determine independently the PW at the Taipei site from 18 to 24 March 1998. Baselines between Taipei and the other nine stations range from 676 to 3009 km. The PW determined from GPS observations for the nine baseline cases are compared with measurements by a dual-channel water vapor radiometer (WVR) and radiosondes at the Taipei site. Although previous results from other locations show that the variability in the rms difference between GPS-and WVRobserved PW ranges from 1 to 2 mm, a variability of 2.2 mm is found. The increase is consistent with scaling of the variability with the total water vapor burden (PW). In addition, accurate absolute PW estimates from GPS data for baseline lengths between 1500 and 3000 km were obtained. Previously, 500 and 2000 km have been recommended in the literature as the minimum baseline length needed for accurate absolute PW estimation. An exception occurs when GPS data acquired in Guam, one of the nine IGS stations, were utilized. This result is a possible further indication that the rms difference between GPS-and WVR-measured PW is dependent on the total water vapor burden, because both Taipei and Guam are located in more humid regions than the other stations.

The sensing of absolute precipitable water vapor (PW) by the Global Positioning System (GPS) and a Water Vapor Radiometer (WVR) is presented. The GPS approach requires a priori knowledge of the relationship between the weighted mean temperature of the atmosphere and surface temperature whose regression relationship is derived based on ten-year climatological data observed by radiosonde and surface meteorological instruments. Similarly, the WVR scheme needs a priori information of the relationship between sky brightness temperature and PW whose regression relationship is characterized based on the same set of climatological data. GPS-derived PW are compared with those observed by WVR and radiosondes. The GPS and WVR data were collected at the Taipei weather station of Taiwan Central Weather Bureau (CWB) from March 18 to 24, 1998. To obtain the estimates of absolute PW at the Taipei site, GPS data acquired from Tsukuba, Japan, at a distance of 2155 km from Taipei were utilized. It is found that GPS-derived PW agrees reasonably well with observations by the WVR and radiosondes. The average of GPS-derived PW is 3.38 cm with a standard deviation of 0.39 cm. The difference between the average GPS-derived and WVR-observed PW is 0.27 cm with a bias of −0.04 cm, while the difference between the average GPS-derived and radiosonde-observed PW is somewhat larger, 0.36 cm with a bias of −0.42 cm. These differences are larger than differences reported at higher latitudes in regions with lower average humidity.