Impact of Ocean Tide Models on GPS Observations

Ocean tides are responsible for periodic deformations of the earth's surface, an effect known as ocean tide loading. The phenomenon induces horizontal and vertical displacements that may reach a few centimeters, for coastal sites and, therefore, a correction should be considered in data analysis. In the last few years different ocean tides models have been developed and implemented in GPS data processing software. In this study we consider 4 different ocean tide models available in the GAMIT/GLOBK software package: the NAO.99b, a hydrodynamic model with TOPEX/Poseidon data assimilated into it that uses interpolation to fit the tide gauges, with a 0.5 degrees x 0.5 degrees grid; the FES99, a pure hydrodynamic model that has assimilated a set of selected tide gauges into it with TOPEX/Poseidon data integrated, computed on a global grid, with a 0.25 degrees x 0.25 degrees grid; the GOT00, a long wavelength adjustments of old FES94.1 using TOPEX/Poseidon data with a 0.5 degrees x 0.5 degrees grid; and the CSR4.0 model, which represents long wavelength adjustments of old FES94.1 using TOPEX/Poseidon data, with a 0.5 degrees x 0.5 degrees grid. In order to compare the different models, we have used data from 57 IGS stations, distributed worldwide, with a total of similar to 240 days spanning the year 2005. The effect of not considering any ocean loading model in data analysis is also investigated. We present an analysis of the ocean loading correction of the station coordinates for each model, the repeatabilities for station coordinates, and an analysis of the dependency of the ocean tide loading on geographic location.