The influence of the moisture gradient on the accuracy of precipitable water derived from GPS data

One of the major sources of the error in estimating precipitable water deri ved from GPS data is horizontal gradient of the moisture. Several character istics of the error were investigated in comparison with water-vapor radiom eter measurements. The GPS-precipitable water (GPS PW) exhibited good agreement with precipita ble 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 bas ed on the time scale of the variation. The first components of the error ha s low frequency with the time scale of several days, with an amplitude of a bout 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 w as overestimated under the condition in which water vapor increased southwa rd. Another component of the error has high frequency with the time scale of se veral hours, having an amplitude of at most 5.0 mm. The high frequency comp onent of the error seems to depend on rapid changes of the GPS-PW gradient.