How to Read a TLE, Line by Line

This guide assumes you know roughly what a TLE is for. If not, the companion guide on how satellite tracking works covers the concepts; this one is about reading the actual characters.

Here is a well-formed element set for a Starlink satellite, in the standard three-line form with the name line included.

STARLINK-1007
1 44713U 19074A   26235.51782528  .00002182  00000-0  16401-3 0  9994
2 44713  53.0534 143.0243 0001362  90.5539 269.5622 15.06392384345679

The rules that apply everywhere

Three properties govern the whole format, and every parsing bug comes from ignoring one of them.

• Fixed columns — every field is at a specific character position. Spaces are meaningful; you cannot split on whitespace
• Exactly 69 characters per line, always, including trailing content
• The last character of each line is a checksum digit, not data

Line 1, field by field

Line 1 carries identity, timing and drag information.

1 44713U 19074A   26235.51782528  .00002182  00000-0  16401-3 0  9994
^ ^     ^ ^      ^              ^          ^        ^        ^ ^
| |     | |      |              |          |        |        | element set no.
| |     | |      |              |          |        |        ephemeris type
| |     | |      |              |          |        BSTAR drag term
| |     | |      |              |          2nd derivative of mean motion
| |     | |      |              1st derivative of mean motion
| |     | |      epoch (year + fractional day)
| |     | international designator
| |     classification
| catalogue number
line number

Reading those values

The catalogue number 44713 is the satellite's permanent NORAD identifier. It never changes, which makes it the reliable way to refer to a specific object — names get reused and revised, catalogue numbers do not.

The classification U means unclassified. The international designator 19074A decodes as: launched in 2019, the 74th launch of that year, and object A from that launch. It encodes launch provenance in a way the catalogue number does not.

The epoch 26235.51782528 is the timestamp, and it uses a two-digit year with a fractional day-of-year. So: year 2026, day 235 — 23 August — and 0.51782528 of a day elapsed, which is about 12:25:40 UTC. Everything the element set describes was true at that instant.

The value .00002182 is the first derivative of mean motion divided by two, in revolutions per day squared. It is positive here, meaning the orbit is slowly decaying — the satellite is completing marginally more orbits per day as it loses altitude, which is what drag does.

The two notational tricks

Here is where first-time readers go wrong.

The first is the compressed exponent. The BSTAR field reads 16401-3, which is not a subtraction. It means 0.16401 times ten to the power of minus three — the exponent's sign and digit are simply appended, with the leading decimal point implied. So BSTAR here is 0.00016401. The same notation applies to the second-derivative field, where 00000-0 means zero.

The second is the implied decimal point in eccentricity on line 2. That field reads 0001362 and means 0.0001362 — a leading '0.' is assumed and never written. This is an extremely circular orbit, as expected for Starlink.

BSTAR itself is worth understanding: it is a drag coefficient that bundles the satellite's ballistic properties with an atmospheric density assumption. A higher BSTAR means the object decays faster. It is not a physical quantity you could measure directly on the spacecraft — it is a fitted parameter that makes SGP4 reproduce the observed decay.

16401-3 means 0.16401 × 10⁻³, and 0001362 means 0.0001362. Neither is a typo, and both break naive parsers.

Line 2, field by field

Line 2 is the orbit's geometry — the six elements that define where the satellite goes.

2 44713  53.0534 143.0243 0001362  90.5539 269.5622 15.06392384345679
^ ^      ^       ^        ^       ^       ^        ^          ^
| |      |       |        |       |       |        |          revolution no.
| |      |       |        |       |       |        mean motion (rev/day)
| |      |       |        |       |       mean anomaly (deg)
| |      |       |        |       argument of perigee (deg)
| |      |       |        eccentricity (decimal point implied)
| |      |       right ascension of ascending node (deg)
| |      inclination (deg)
| catalogue number (repeated)
line number

What those numbers say about this satellite

Inclination 53.0534° places this satellite squarely in Starlink's main shell, and tells you immediately that it never travels north of 53° N or south of 53° S.

Right ascension of the ascending node, 143.0243°, is the orientation of that orbital plane in space. Satellites in the same shell but different planes share an inclination and differ in this value — it is the field that distinguishes one orbital plane from another.

Eccentricity 0.0001362 is essentially zero: a circular orbit. Because the orbit is circular, the argument of perigee 90.5539° is close to meaningless — there is no meaningful 'lowest point' in a circle, so this value drifts freely and carries little information.

Mean motion 15.06392384 revolutions per day is the one to reach for if you want altitude. Fifteen orbits a day means a period of about 95.6 minutes, which for a circular orbit corresponds to roughly 550 km. This is how a tracking site derives an altitude it was never given directly.

Checking the line survived transit

The final character of each line is a modulo-10 checksum. Sum every digit in the first 68 columns, counting each minus sign as 1 and ignoring letters, spaces, decimal points and plus signs. The last digit of that sum is the checksum.

It is a weak check by modern standards — it catches single-character corruption reliably and not much else — but it is free and it catches the common failure mode of a TLE mangled by copy-paste or line wrapping.

If a TLE will not parse, check three things in order: is each line exactly 69 characters, has a line been wrapped or had trailing spaces stripped, and does the checksum match. That covers the overwhelming majority of broken element sets.

From TLE to a point in your sky

Reading the fields is one step. Turning them into an answer takes SGP4, which uses these values plus a target time to produce a position and velocity in an inertial reference frame — then a rotation into Earth-fixed coordinates, then a conversion to the elevation and azimuth an observer at your location would measure.

Every satellite page on this site shows the result of exactly that chain, recomputed continuously in your browser from current element sets. The pass predictions run the same calculation forward over the next 24 hours and filter for the moments when the satellite is both above your horizon and sunlit.