The proposed investigation includes an analysis of 5 years of global GPS tracking data. First, it will improve state of the art GPS analysis by analyzing a consistent data set and varying models and estimation strategies. Second, it will examine the impact of analysis strategies on the ability of GPS to maintain a consistent reference frame and continue intercomparisons with SLR and VLBI. Finally, it will investigate different ways to represent geophysical signals in the global GPS data by estimating spherical at subdaily periods. By studying Earth rotation we can learn about the interaction of the solid Earth, oceans, and atmosphere at time scales less than a day out to several weeks. Spherical harmonics are a natural parameterization for geophysical processes. This approach is not currently utilized by IGS analysis centers.