How to read a TLE

Concepts library · Updated July 5, 2026

A TLE (two-line element set) is the compact, fixed-width text format tracking networks publish to describe a satellite's orbit. Two 69-character lines carry the object's identity, the epoch the data was fitted at, drag information, and six orbital elements — everything an SGP4 propagator needs to predict positions.

The format at a glance

Here is a real TLE for the International Space Station (this example is from December 2025 — see why that date matters):

1 25544U 98067A   25352.21621920  .00008098  00000-0  15154-3 0  9994
2 25544  51.6313 115.3593 0003170 265.6107  94.4519 15.49648163543704

Every field lives at fixed column positions — parsers read character ranges, not whitespace-separated tokens. Values that would waste columns on punctuation use two shorthand conventions:

Implied decimal point: eccentricity 0003170 means 0.0003170.
Compact exponent: 15154-3 means 0.15154 × 10⁻³ (a leading decimal point is implied, then a power of ten).

Line 1 — identity, epoch, drag

Field	Example	Meaning
Catalog number	`25544`	The satellite's number in the U.S. space catalog (25544 = ISS). Appears on both lines.
Classification	`U`	`U` = unclassified. Published TLEs are always `U`.
International designator	`98067A`	Launch year (1998), launch number of that year (067), and piece (`A` = the primary payload).
Epoch	`25352.21621920`	When the elements were fitted: two-digit year (25 → 2025) plus fractional day of year. Day 352.21621920 of 2025 is December 18, 2025 at about 05:11 UTC. Years 57–99 mean 1957–1999; 00–56 mean 2000–2056.
Mean-motion 1st derivative ÷ 2	`.00008098`	Rate of change of mean motion (rev/day², halved by convention). Legacy field; SGP4 does not use it for propagation.
Mean-motion 2nd derivative ÷ 6	`00000-0`	Second derivative in compact-exponent form (here zero). Also unused by SGP4.
B* (“B-star”)	`15154-3`	The drag term SGP4 actually uses: 0.15154 × 10⁻³ per Earth radius. Encodes how strongly atmospheric drag decelerates the object; higher B* = faster decay.
Ephemeris type	`0`	Always 0 in distributed TLEs (data fitted for SGP4).
Element set number	`999`	Counter incremented each time new elements are published for this object.
Checksum	`4`	Last digit of each line: the sum of all digits on the line (minus signs count as 1) modulo 10. Catches copy/paste corruption.

Line 2 — the orbit itself

Field	Example	Meaning
Inclination, i	`51.6313`	Tilt of the orbital plane relative to the equator, in degrees. 51.63° is the ISS signature.
RAAN, Ω	`115.3593`	Right ascension of the ascending node, degrees — where the orbit crosses the equator heading north.
Eccentricity, e	`0003170`	Orbit shape with an implied leading decimal: 0.0003170 — nearly circular.
Argument of perigee, ω	`265.6107`	Angle from the ascending node to the orbit's lowest point, degrees.
Mean anomaly, M	`94.4519`	Where the satellite is along the orbit at epoch, degrees (in mean-motion time, not geometric angle).
Mean motion, n	`15.49648163`	Revolutions per day. ~15.5 rev/day ≈ a 93-minute period — a low Earth orbit. Orbit size (semi-major axis) is derived from this.
Revolution number	`54370`	Orbit count since launch, at epoch.
Checksum	`4`	Same modulo-10 rule as line 1.

Why TLE age matters

A TLE is a snapshot: its elements are fitted to tracking observations at the epoch, and SGP4's predictions degrade as you move away from that time — drag, maneuvers, and unmodeled forces accumulate. That is why tools surface the epoch age. The ExoAtlas Orbit Visualizer, for example, shows an epoch-age chip beside every catalog entry and flags a TLE as stale once its epoch is more than 7 days old.

The example above is from December 2025, so today it will draw a geometrically correct ISS orbit whose predicted position along that orbit is significantly off. For anything time-sensitive — pass prediction, conjunction screening — always fetch a fresh TLE first.

Where TLEs come from

TLEs are fitted and published by the U.S. Space Force's 18th Space Defense Squadron. The two standard distribution points are Space-Track.org (the authoritative source; free account required) and CelesTrak (curated public mirrors, grouped by constellation and category). The Orbit Visualizer's built-in catalog is refreshed from Space-Track data.

Mean elements — not the orbit you'd measure

TLE elements are mean elements: averaged quantities produced by the SGP4 fitting process, with short-period wiggles deliberately smoothed out. The osculating elements — the instantaneous ellipse you would compute from a position and velocity measurement — differ from them, for the ISS by kilometers-scale amounts. Two practical consequences:

TLE elements belong to SGP4. Feeding them into a different propagator as if they were osculating introduces error from the first timestep — the Orbit Visualizer warns exactly this if you propagate a TLE object with the two-body model: “TLE mean elements propagated as osculating — expect divergence.”
The reverse also holds: classical elements you typed in are osculating, so the app disables SGP4 for them (“SGP4 requires TLE mean elements; classical elements are osculating.”).

More on what each model does: Propagation models.

See it live

This link loads the example TLE above into the Orbit Visualizer via its URL API — with SGP4 selected automatically, as for any TLE input:

▶ Open this TLE in the Orbit Visualizer

Remember: this example TLE is months old, so treat the result as geometry, not a live position. For the current ISS, use the app's Load from Catalog button instead.

Mini-FAQ

Can I edit a TLE by hand?

You can, but each line's final digit is a checksum (digit sum mod 10, minus signs counting as 1), so most parsers will reject an edited line unless you recompute it. Fields are also column-positioned — inserting or deleting characters breaks the whole line.

Why doesn't a TLE list the semi-major axis?

The format carries mean motion (revolutions per day) instead; the semi-major axis follows from it via Kepler's third law. Publishing the observable quantity is a convention from the era when TLEs were distributed on punch cards.