What is a TLE? Reading a satellite's two-line element set
Every tracked satellite's orbit is published as two lines of numbers. What those numbers mean, where they come from, and why predictions get less accurate as they age.
Two lines that describe an orbit
A TLE (two-line element set) is a compact text format, dating from the punched-card era, that describes a satellite's orbit at one moment called the epoch. Feed it to a standard model called SGP4 and you can work out where the satellite is at any time near that moment. It is what most tracking sites, including this one, use.
What is in it
- The catalog number (NORAD ID), e.g. 25544 for the ISS, and the international designator, e.g. 1998-067A: launched in 1998, the 67th launch of the year, object A.
- The epoch: when the orbit was measured.
- Inclination, the right ascension of the ascending node, eccentricity and the argument of perigee: the orientation and shape of the orbit.
- The mean anomaly: where on the orbit the satellite was at the epoch.
- The mean motion: orbits per day, which gives the size of the orbit, plus terms describing how drag slowly changes it.
Where they come from
The US Space Force tracks objects with radar and telescopes and publishes their element sets on Space-Track.org. This site downloads the whole catalog every hour, so the orbits used for every page are at most a few hours old for active satellites.
Why predictions drift
A TLE is a snapshot. Drag, engine burns and the uneven pull of Earth slowly change real orbits, so a prediction made from an old element set drifts, by a few kilometers per day for low satellites. The ISS also raises its orbit every few weeks. That is why pass times are always recalculated from fresh data, and why a prediction a week ahead can move by a minute or so.