Definitive Ephemeris (DE) is used for geometrically correcting Landsat 5 and Landsat 7 data.
An ephemeris is a set of data that provides the assigned places of a celestial body (including a manmade satellite) for regular intervals. DE shows the position and velocity of the spacecraft at the time imagery is collected, and shows the position and velocity of the satellites in one-minute intervals.
Newly-acquired Landsat 7 scenes are initially processed with predicted ephemeris, and are reprocessed with DE applied within 1 to 2 days of acquisition.
There is no DE for Landsat 8, since the Payload Correction Data (PCD) for the satellite is very accurate. Telemetry information in the form of Two-Line Element (TLE) Set — which lists the orbital elements that describe the state (position and velocity) of the spacecraft — is listed on http://www.n2yo.com/satellite/?s=39084.