Explainers

What Is a TLE? Two-Line Elements and NORAD IDs Explained

Updated September 19, 2026 · 6 min read

Every satellite tracker — this site included — runs on TLEs: two-line element sets. A TLE is the compact, standard way of describing an orbit, published as two lines of fixed-width text. If you have ever seen a satellite’s NORAD identifier, you’ve already met the first field of a TLE.

You don’t need to read raw TLEs to use a tracker, but understanding the format makes it easy to see two things: why every "live position" is really a short-lived prediction from a snapshot, and why trackers have to keep fetching fresh elements.

A TLE in one sentence

A TLE is a machine-readable snapshot of an orbit at one exact moment — the epoch. It describes the orbit’s size, shape, orientation in space, and where on that orbit the satellite sits at the epoch. Everything a tracker shows you later is an extrapolation from that snapshot.

The format dates back to the early days of the United States space-tracking programme, and it is still maintained by NORAD and distributed to the public through CelesTrak — the de facto source for live element sets.

Two lines, a lot of information

Line 1 carries launch and drag information; line 2 carries the orbital shape and orientation. The layout, with every field in a fixed column position, looks like this (values replaced with placeholders — a live ISS TLE uses real numbers):

1 NNNNNU YYNNNA   YYDDD.DDDDDDDD  .00000000  00000-0  00000-0 0  9995
2 NNNNN  II.IIII LLL.LLLL NNNNNNN PPP.PPPP MMM.MMMM NN.NNNNNNNNNNNN

The pieces trackers actually use day to day are catalog number, inclination, mean motion, eccentricity, and epoch. The full field map:

Field	Line	What it describes
Catalog (NORAD) number	1 & 2	Unique catalogue identifier for the object. The ISS is 25544; China’s Tiangong is 48274.
International designator	1	Which launch created the object — launch year plus a designator.
Epoch	1	The exact instant in UTC that this element set describes.
BSTAR drag term	1	How much atmospheric drag to assume, and which mathematical model to use.
Element set number / checksum	1	Edition of this element set, plus check digits for data integrity.
Inclination	2	Tilt of the orbit to the equator, in degrees.
Right ascension of ascending node	2	Where the orbit crosses the equator heading north.
Eccentricity	2	How circular the orbit is (0.0000000 = perfectly circular).
Argument of perigee	2	Where the low point of the orbit sits along it.
Mean anomaly	2	Where the satellite is around the orbit at the epoch.
Mean motion	2	How many revolutions the object makes per day.
Revolution number / checksum	2	How many laps the object has completed, plus check digits.

Two things hide in plain sight here. The catalogue number identifies the object, so "25544" is a reliable way to ask a tracker for the ISS. And the epoch tells you how fresh the data is — an element set is only as good as the moment it was measured.

Why a TLE must be combined with a model

A TLE is not a prediction and not a live position — it is a starting point. To get "where is the ISS right now?", the tracker feeds the TLE into a propagation model. For NORAD element sets that model is SGP4, the algorithm that carries the orbit forward and backward in time while correcting for drag and Earth’s uneven gravity. The what is SGP4? page explains that step in detail.

That two-step pipeline — a TLE plus a propagator — is exactly how this site, CelesTrak’s own display, and virtually every other public tracker turn static text into a moving map dot. The two pages work together: how ISS tracking works is the overview, and this page and the SGP4 page are the deep dives.

Why TLEs go stale so quickly

An element set is most accurate at its epoch and degrades from there. For a big satellite at 400 km the reasons are concrete:

Atmospheric drag continuously slows the ISS and lowers its orbit, changing the shape described by the old element set.
Reboost burns deliberately move the station back up, instantly making the old numbers out of date.
Collision-avoidance manoeuvres are occasional, unscheduled, and can shift a prediction by minutes.

Operators therefore publish updated element sets for crewed stations several times a week, and trackers mirror them. This site fetches the latest elements from CelesTrak every few minutes, and the Live Telemetry panel shows the current epoch and age of the data so you can judge freshness yourself.

Reading a real ISS entry

Put it together and a real ISS TLE line 2 reads roughly like: inclination about 51.64°, eccentricity near 0.0005 (very nearly circular), mean motion about 15.5 revolutions per day. Those three numbers alone — tilt, roundness, and rate — summarise almost everything you read on the orbit page.

You can pull the current numbers any time from CelesTrak — the same source feeding this tracker — and compare them against the live telemetry readout on the live position page.

A TLE is useless without a model and a clock. Next, the SGP4 propagation model shows how the snapshot becomes a second-by-second position.

See two-line elements turned into a live map, with the current TLE’s epoch shown in telemetry.

Open the live tracker