We have studied the use of Global Positioning System (GPS) ground and flight tracking data to measure short-period Earth orientation variations and changes in Earth's center of mass location (the geocenter). Ground-based GPS estimates of daily and subdaily changes in Earth orientation presently show centimeter-level precision. Comparisons between GPS-estimated Earth rotation variations (UT1-UTC) and those calculated from ocean tide models suggest that observed subdaily variations in Earth rotation are dominated by oceanic tidal effects. Our prliminary GPS estimates for geocenter location (from a 3-week experiment) agree with an independent satellite laser ranging estimates to 10--15 cm. Covariance analysis predicts that temporal resolution of GPS estimates for Earth orientation and geocenter improves significantly when data collected from low Earth-orbiting satellites as well as from ground sites are combined. The low-Earth GPS tracking data enhance the accuracy and resolution for measuring high-frequency global geodynamical signals over time scales less than 1 day. Low-Earth orbiters such as Topex/Poseidon carrying GPS flight receivers will provide opportunities to demonstrate these enhancments. ¿American Geophysical Union 1993