What is a TLE?

What the two lines contain

Each line is exactly 69 characters, and every value lives in a fixed column range rather than being separated by commas. A third line with the object's name is often added on top, which is why you will also see the term "three-line element".

Line	Field	What it tells you
1	Catalogue number	The object's permanent NORAD number
1	International designator	Launch year, launch number and piece
1	Epoch	The moment the elements describe, as year and fractional day of year
1	B* drag term	How strongly the atmosphere slows this object
2	Inclination	Tilt of the orbit against the equator, in degrees
2	RAAN	Where the orbit crosses the equator heading north
2	Eccentricity	How far from circular, with the leading decimal point left out
2	Argument of perigee	Where in the orbit the lowest point sits
2	Mean anomaly	Where the object was along its orbit at the epoch
2	Mean motion	Revolutions per day, which fixes the period and the altitude

Each line ends with a checksum: add up the digits, count every minus sign as 1, and take the last digit of the total. It exists to catch transcription errors from the era when these were sent over teletype.

They only mean something to SGP4

This is the detail that trips up people who try to use TLEs in their own code. The numbers are mean elements fitted specifically for the SGP4 model, not the instantaneous orbit you would get from a physics textbook.

Feed them into a different propagator, or treat them as a plain Keplerian orbit, and positions come out wrong by a large margin. The elements and the model are a matched pair.

Why the age matters

A TLE describes one moment. The further you propagate from its epoch, the more small errors accumulate - above all from atmospheric drag, which changes with solar activity in ways a single B* value cannot capture.

A fresh element set on a high orbit stays reliable for a long time. An old one on a low orbit can be kilometres off. That is why a tracker worth using shows you how old the elements are, and why pass times from two apps using different sets can disagree.

The format is running out of room

The catalogue number field is five characters wide, which caps it at 99999. The catalogue is approaching that. The stop-gap is the "alpha-5" scheme, which replaces the first digit with a letter.

The longer-term replacement is the OMM (Orbit Mean-Elements Message), which carries the same GP elements in XML, JSON or CSV with no fixed-width limits. The information is the same; only the packaging changes.