Determination of tidal displacements using the global positioning system

Three-dimensional tidal displacements of eight semidiurnal and diurnal constituents at 12 sites in Hong Kong and at 125 IGS05 reference sites were obtained using 3--11 years of continuous GPS observations. Tidal displacement residuals after removal of a priori modelled solid Earth tides were estimated using a modified precise point positioning (PPP) technique on a daily basis, and then combined. The results were compared with predictions from seven latest global ocean tide models. Tidal gravity measurements in Hong Kong and global Very Long Baseline Interferometry (VLBI) estimates were also used to validate the GPS results.;The regional results in Hong Kong shows that both the GPS and gravity estimates agree best with the GOT4.7 and NAO99b models, when the K1 and K2 constituents are excluded. The GPS/model agreements are generally at sub-millimetre level, except for S2, K2 and K1 with relatively larger errors. After removing the systematic biases between the GPS estimates and model predictions, the misfits of M 2, S2, N2, O1, P1 and Q 1 at all the sites are within 0.5 and 1.0 mm in the horizontal and vertical components, respectively, while K1 and K2 show relatively larger misfits of up to 2.5 mm.;The global results show that GPS can determine tidal displacements with accuracy superior to VLBI for all the constituents except K2. The AG06a model gives anomalous results compared with both the GPS and VLBI estimates, while the most recent four models (FES2004, TPXO7.1, GOT4.7 and EOT08a) fit the GPS estimates equally well at the global scale. The GPS/model residuals show large-scale spatial coherence for all the constituents except K 2.;The effects of GPS tidal displacement coefficients on long-period signals in position time-series are also investigated. The GPS tidal displacement coefficients with the sub-millimetre accuracy for all the constituents except K1 and K2 have proved to effectively reduce spurious long-period signals in position time-series. The K1 and K 2 coefficients with poorer accuracy should be treated with caution. Including K2 corrections, secular variations of more than 1 mm/yr are evident at some sites.