Understanding TLE
Anatomy of the two lines
Despite the name, a TLE usually spans three lines: an optional 'line 0' holding the object's name, followed by two 69-character data lines. Line 1 carries the object's identity and housekeeping information; line 2 carries the pure geometry of the orbit. Every character position is fixed, and each line ends in a modulo-10 checksum digit so software can detect corruption.
| Field | Line | What it describes |
|---|---|---|
| Catalogue number (NORAD ID) | 1 & 2 | Unique object identifier |
| Int'l designator (COSPAR ID) | 1 | Launch year, number and piece |
| Epoch | 1 | Reference time (year + fractional day) |
| BSTAR drag term | 1 | Models atmospheric drag on the orbit |
| Inclination | 2 | Tilt of the orbital plane (°) |
| RAAN | 2 | Swivel of the plane around Earth (°) |
| Eccentricity | 2 | Orbit shape (0 = circle); decimal implied |
| Argument of perigee | 2 | Orientation of the ellipse (°) |
| Mean anomaly | 2 | Position along the orbit at epoch (°) |
| Mean motion | 2 | Revolutions per day (sets the orbit size) |
Why a TLE only works with SGP4
A common mistake is to plug TLE numbers into a textbook two-body orbit calculator. That fails, because TLE values are 'mean' elements deliberately fitted to work with the SGP4 model — they are averaged so that SGP4's own corrections for drag and Earth's oblateness reproduce the real orbit, and they are not the instantaneous (osculating) elements of classical mechanics. SGP4 handles near-Earth orbits with periods under 225 minutes; its sibling SDP4 takes over above that threshold, adding lunar and solar gravitational effects for deep-space orbits. Pair a TLE with the matching propagator and you get accurate position and velocity; use it any other way and the answer can drift by hundreds of kilometres.
How accurate is a TLE, and how long does it last?
A fresh TLE typically places an object within about a kilometre of its true position at the epoch, but that error grows as the prediction runs forward — very roughly one to three kilometres per day for a low-orbit satellite, and faster during periods of high solar activity that puff up the upper atmosphere and increase drag. For this reason TLEs carry a limited shelf life: operators refresh them every few days, and Orbital Radar re-fetches the latest sets from Space-Track so the globe never propagates a stale orbit too far past its epoch. The data is freely published by the US Space Force and mirrored by CelesTrak.
A 1960s format meets modern traffic
The TLE's fixed 69-character layout dates from the era of punch cards, and it is beginning to show its age. The satellite catalogue number is only five digits, capping the scheme at 99,999 objects — a ceiling the fast-growing catalogue is approaching. As a stopgap, the 'Alpha-5' convention lets the first character become a letter to stretch capacity, while the community increasingly favours the modern Orbit Mean-Elements Message (OMM), which carries the same SGP4-compatible data in JSON, XML or KVN without the rigid column limits. For now, though, the humble TLE remains the lingua franca of satellite tracking.