A TLE's epoch is the moment in time when the orbital elements were measured. From that moment forward, every prediction is an extrapolation — and accuracy degrades. The primary culprit in LEO: atmospheric drag.
A TLE's epoch is the moment in time when the orbital elements were measured. From that moment forward, every prediction is an extrapolation — and accuracy degrades. The primary culprit in LEO: atmospheric drag.
The epoch is the reference timestamp baked into the TLE. All six orbital elements describe the satellite's state at that exact moment. Propagation forward from the epoch is prediction — not measurement.
Satellites below ~1,000 km experience atmospheric drag — a tiny but persistent force that lowers the orbit over time. The lower the orbit, the stronger the drag.
The relationship is exponential — each 100 km of altitude roughly doubles the orbital lifetime. This is why the 25-year deorbit rule translates to a maximum operational altitude around 600–650 km for satellites without propulsion.
As drag lowers the orbit, the satellite enters denser atmosphere, which increases drag further — a positive feedback loop that accelerates toward re-entry.
Always check the epoch age. A fresh TLE gives you metre-level accuracy. A stale one can be off by kilometres. On Orbital Radar, this is why tracks sometimes 'jump' when a new TLE arrives.