The sensing of absolute precipitable water vapor (PW) by the Global Positio
ning System (GPS) and a Water Vapor Radiometer (WVR) is presented. The GPS
approach requires a priori knowledge of the relationship between the weight
ed mean temperature of the atmosphere and surface temperature whose regress
ion relationship is derived based on ten-year climatological data observed
by radiosonde and surface meteorological instruments. Similarly, the WVR sc
heme 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 th
e 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 av
erage GPS-derived and WVR-observed PW is 0.27 cm with a bias of -0.04 cm, w
hile 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 lo
wer average humidity.