Stability of direct GPS estimates of ocean tide loading

We observe ocean tide loading (OTL) at diurnal and semi-diurnal periods by directly estimating fixed-period harmonic motions within individual daily GPS analyses. Stacking of multiple solutions allows us to isolate the principal near-diurnal and near-semi-diurnal OTL components. Using data from UK sites where predicted OTL for the M-2 component varies from 3-46 mm in amplitude, we show that our estimates are stable for the principal OTL components when data from at least 90 days are stacked. Our final observations are compared to model predictions (using the FES95.2, FES99, GOT00.2, TPXO.2, TPXO.6, NAO.99b and CSR3 models) and are in agreement with OTL predicted by FES99, but differ significantly at some locations from those predicted by the CSR3 and TPXO.2 tidal models.