High-frequency variations in Earth rotation from Global Positioning Systemdata

Using the data of the global, dense Global Positioning System (GPS) network established by the International GPS Service a continuous, uninterrupted s eries of subdaily Earth rotation parameters (ERPs) with a time resolution o f 2 hours has been generated at the Center for Orbit Determination in Europ e. The series starts in January 1995 and has a length of more than 3 years. Starting from the 2-hour ERP values of this, to our knowledge, unique time series, the high-frequency variations in Universal Time (UTI) and polar mo tion (PM) due to ocean tides are studied and a set of sine and cosine coeff icients is estimated for all the major tidal terms at nearly diurnal and se midiurnal frequencies. The GPS series is not very homogeneous (various proc essing changes during the 3 years) and still short compared to the length o f very long baseline interferometry (VLBI) and satellite laser ranging (SLR ) data sets. However, the results derived from this series are already of t he same quality as the results from VLBI and SLR. A comparison of the tidal coefficients stemming from all three space-geodetic techniques shows an ag reement on the 1 mus level for UT1 and 10 microarc seconds (mu as) for PM, respectively. The RMS difference between the ocean tide amplitudes estimate d from GPS data and from TOPEX/Poseidon altimeter data amounts to 0.7-0.9 m us in UT1 and 9-13 mu as in PM. The residual spectrum that remains after th e removal of all tidal terms has a noise level of similar to 5-10 mu as in PM and 0.5-1 mus in UT1 and contains nontidal signals (up to 55 mu as in PM and 3 mus in UT1) that might be due to the impact of the satellite orbit m odeling (12-hour revolution period of the satellites) or, alternatively, du e to atmospheric or oceanic normal modes.