# Two-Line Element Sets (TLE)

A two-line element set (TLE/2LE) or three-line element set (3LE) are lists of [keplerian-orbital-elements](../../aerospace-fundamentals/keplerian-orbital-elements "mention") 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:

<table><thead><tr><th width="124">Column</th><th width="477">Description</th><th>Example Value</th></tr></thead><tbody><tr><td>01</td><td>Line number</td><td>1</td></tr><tr><td>03-07</td><td>Satellite number</td><td>25544</td></tr><tr><td>08</td><td>Classification (U=Unclassified, C=Classified, S=Secret)</td><td>U</td></tr><tr><td>10-11</td><td>International designator (Last two digits of launch year)</td><td>98</td></tr><tr><td>12-14</td><td>International designator (Launch number of the year)</td><td>067</td></tr><tr><td>15-17</td><td>International designator (Piece of the launch)</td><td>A</td></tr><tr><td>19-20</td><td>Epoch year (last two digits of year)</td><td>08</td></tr><tr><td>21-32</td><td>Epoch (Day of the year and fractional portion of the day)</td><td>264.51782528</td></tr><tr><td>34-43</td><td>First time derivative of the mean motion</td><td>-.00002182</td></tr><tr><td>45-52</td><td>Second time derivative of the mean motion</td><td>00000-0</td></tr><tr><td>54-61</td><td>BSTAR drag term</td><td>-11606-4</td></tr><tr><td>63</td><td>Ephemeris type</td><td>0</td></tr><tr><td>65-68</td><td>Element number</td><td>292</td></tr><tr><td>69</td><td>Checksum</td><td>7</td></tr></tbody></table>

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

<table><thead><tr><th width="124">Column</th><th width="477">Description</th><th>Example Value</th></tr></thead><tbody><tr><td>01</td><td>Line number</td><td>2</td></tr><tr><td>03-07</td><td>Satellite number</td><td>25544</td></tr><tr><td>09-16</td><td>Inclination (Degrees)</td><td>51.6416</td></tr><tr><td>18-25</td><td>Right ascension of the ascending node (Degrees)</td><td>247.4627</td></tr><tr><td>27-33</td><td>Eccentricity</td><td>0006703</td></tr><tr><td>35-42</td><td>Argument of perigee (Degrees)</td><td>130.5360</td></tr><tr><td>44-51</td><td>Mean anomaly (Degrees)</td><td>325.0288</td></tr><tr><td>53-63</td><td>Mean motion (Revolutions per day)</td><td>15.72125391</td></tr><tr><td>64-68</td><td>Revolution number at epoch</td><td>56353</td></tr><tr><td>69</td><td>Checksum</td><td>7</td></tr></tbody></table>

## TLE Datasets

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

* [https://www.space-track.org/](https://www.space-track.org/)
* [https://celestrak.org/](https://celestrak.org/)

## Further Reading

{% embed url="https://en.wikipedia.org/wiki/Two-line_element_set" %}

{% embed url="https://celestrak.org/NORAD/documentation/tle-fmt.php" %}
