Landsat 7 ETM+ was designed to provide a geometric accuracy of at least 250 meters (1 sigma) in low-relief areas at sea level. However, the use of definitive ephemeris for L1GS (systematic) correction frequently provides a geometric accuracy within 30-50 meters.
The .MTL field "EPHEMERIS_TYPE" displays either PREDICTED or DEFINITIVE. Predicted ephemeris is applied immediately after data acquisition. Shortly after initial processing (about 1 day), the data are reprocessed using definitive ephemeris.
The Landsat 7 Image Assessment System (IAS) Geometric Algorithm Theoretical Basis Document provides more details about processing and geometric correction for Level-1G data products.