Two-Line Element Sets (TLE)

A two-line element set (TLE/2LE) or three-line element set (3LE) are lists of Keplerian (Orbital) Elements for satellites orbiting the earth at a given point in time, the epoch.

The TLE, alongside a suitable prediction formula, can be utilised to calculate the position and velocity of a satellite at any point in the past or future.

Format

Take the following two-line element set for the International Space Station (ISS):

ISS (ZARYA)
1 25544U 98067A   08264.51782528 -.00002182  00000-0 -11606-4 0  2927
2 25544  51.6416 247.4627 0006703 130.5360 325.0288 15.72125391563537

Considering the above example, the TLE can be broken down line-by-line. First, an optional title line is present, in our instance ISS (ZARYA). Following this, the first line can be broken down as follows:

Column	Description	Example Value
01	Line number	1
03-07	Satellite number	25544
08	Classification (U=Unclassified, C=Classified, S=Secret)	U
10-11	International designator (Last two digits of launch year)	98
12-14	International designator (Launch number of the year)	067
15-17	International designator (Piece of the launch)	A
19-20	Epoch year (last two digits of year)	08
21-32	Epoch (Day of the year and fractional portion of the day)	264.51782528
34-43	First time derivative of the mean motion	-.00002182
45-52	Second time derivative of the mean motion	00000-0
54-61	BSTAR drag term	-11606-4
63	Ephemeris type	0
65-68	Element number	292
69	Checksum	7

Finally, the second line can be broken down to the following:

Column	Description	Example Value
01	Line number	2
03-07	Satellite number	25544
09-16	Inclination (Degrees)	51.6416
18-25	Right ascension of the ascending node (Degrees)	247.4627
27-33	Eccentricity	0006703
35-42	Argument of perigee (Degrees)	130.5360
44-51	Mean anomaly (Degrees)	325.0288
53-63	Mean motion (Revolutions per day)	15.72125391
64-68	Revolution number at epoch	56353
69	Checksum	7

TLE Datasets

There are several websites which provide two-line element set data for free, updated daily.

• https://www.space-track.org/

• https://celestrak.org/

Further Reading

Two-line element setWikipedia

CelesTrak: NORAD Two-Line Element Set Formatcelestrak.org