Semi-diurnal and diurnal variation of errors in GPS precipitable water vapor at Tsukuba, Japan caused by site displacement due to ocean tidal loading

Simultaneous GPS and water vapor radiometer (WVR) observations were carried out at Tsukuba, Japan from May 1 to June 30, 1998. The precise point posit ioning method of the GIPSY/OASIS-II software package (GIPSY) was used to re trieve precipitable water vapor (GPS_PWV) from GPS data, which was then com pared with precipitable water vapor observed by WVR (WVR_PWV). They agreed quite well with the root mean square difference of less than 1.5 mm. Howeve r, periodic variations were found in the difference between GPS_PWV and WVR _PWV (dPWV). It was also found that semi-diurnal or diurnal components of t hese variations had a positive correlation with site displacement due to Oc ean Tidal Loading (OTL). Two months of dPWV data were decomposed by the per iod of a component of OTL, and then composite time series data with a perio d equal to that of the component were made. This process was performed for K-1, O-1, M-2, and S-1 components of OTL. In each component, a periodic var iation in dPWV appeared which was similar to those of the simulated GPS_PWV errors from OTL effects calculated with 'GOTIC' (Sato and Hanada, 1984), a program for the computation of OTL effect. Inclusion of OTL effects into G IPSY analysis reduced dPWV. In the M-2 component, the amplitude of the dPWV was reduced by about 80%. This suggests that the OTL components calculated by the GOTIC succeeded in simulating the actual site displacement by OTL e ffects in Japan. On the other hand, in K-1 components, the amplitude of dPW V without OTL in GIPSY is 1.5 times larger than the simulated GPS_PWV error , with considerable error remaining even in the case of GIPSY analysis with OTL. The error may be due to multi-path effect, temperature dependency on conversion from Zenith Wet Delay to PWV, or instrument dependency of WVR on temperature. Analysis utilizing much longer data periods than the present two months is required to overcome these difficulties.