Epoch (Orbital Mechanics)

In orbital mechanics, an epoch is the specific date and time at which a set of orbital parameters (such as a TLE) is valid. Because orbits are constantly perturbed by atmospheric drag, gravitational anomalies, solar radiation pressure, and third-body effects, a satellite's actual position diverges from its predicted position over time. Fresh TLEs with recent epochs are therefore essential for accurate tracking. In TLE format, the epoch is encoded as a two-digit year plus the fractional day of the year (e.g., 25014.51253411 = day 14.51 of 2025). Orbital Radar refreshes TLE data multiple times per day.