Abstract
One of the major sources of the error in estimating precipitable water derived from GPS data is horizontal gradient of the moisture. Several characteristics of the error were investigated in comparison with water-vapor radiometer measurements. The GPS-precipitable water (GPS PW) exhibited good agreement with precipitable water derived from water-vapor radiometer measurements. The correlation coefficient and rms error for this comparison were determined as 0.991 and 1.93 mm, respectively. The estimation errors for the GPS PW can be divided into two components based on the time scale of the variation. The first components of the error has low frequency with the time scale of several days, with an amplitude of about 2.5 mm. This estimation error was negatively correlated with the north-south component of the GPS-PW gradient derived from GPS data. The GPS PW was overestimated under the condition in which water vapor increased southward. Another component of the error has high frequency with the time scale of several hours, having an amplitude of at most 5.0 mm. The high frequency component of the error seems to depend on rapid changes of the GPS-PW gradient.
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