Earth rotation parameters determined by VLBI within project IRIS

The very long baseline interferometry (VLBI) project for international radio interferometric surveying (IRIS) is presented. It involves three stations in the United States (Westford, MA; Ft. Davis, TX; Richmond, FL) and two radio telescopes in Europe (Wettzell, West Germany, and Onsala, Sweden). The IRIS experiments provide all of the Earth orientation parameters (UT1, polar motion, and nutation), baseline components, extragalactic radio source positions, and parameters of geophysical and astronomical models. For the investigation of the short periods of polar motion, the pole path observed within project IRIS since January 1984 was analyzed in the along-track and cross-track components. Both components contain variations with periods of approximately=13.7 days. A comparison of the phases shows that the fortnightly variation of the cross-track components can be related to the variation of the tidal potential (Earth tides and/or ocean tides with a period of 13.66 days) with a phase lag of about one day. The largest variations were found in the along-track pole components, with an average period of approximately=10.8 days. This confirms the assumption recently made by several authors that the short-period variations in global atmospheric data-with periods of 10-12 days-influence the motion of the pole.>