# Two-line element set

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A two-line element (TLE) is a set of two data lines listing orbital elements that describe the state (position and velocity) of an Earth-orbiting object. The TLE data representation is specific to the simplified perturbations models (SGP, SGP4, SDP4, SGP8 and SDP8), so any algorithm using a TLE as a data source must implement one of the simplified perturbations models to correctly compute the state of an object at a time of interest.

The United States Air Force tracks all detectable objects in Earth orbit, creating a corresponding TLE for each object, and makes available TLEs for non-classified objects on the website Space Track.[1][2] The TLE format is a de facto standard for distribution of an Earth-orbiting object's orbital elements.

A TLE set may include a title line proceeding the element data. The title is not required, as each data line includes a unique object identifier code.

## Format

An example TLE for the International Space Station:

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


The meaning of this data is as follows:[3]

Title line
Field Columns Content Example
1 01–24 Satellite name ISS (ZARYA)
LINE 1
Field Columns Content Example
1 01–01 Line number 1
2 03–07 Satellite number 25544
3 08–08 Classification (U=Unclassified) U
4 10–11 International Designator (Last two digits of launch year) 98
5 12–14 International Designator (Launch number of the year) 067
6 15–17 International Designator (piece of the launch) A
7 19–20 Epoch Year (last two digits of year) 08
8 21–32 Epoch (day of the year and fractional portion of the day) 264.51782528
9 34–43 First Time Derivative of the Mean Motion divided by two [4] −.00002182
10 45–52 Second Time Derivative of Mean Motion divided by six (decimal point assumed) 00000-0
11 54–61 BSTAR drag term (decimal point assumed) [4] -11606-4
12 63–63 The number 0 (originally this should have been "Ephemeris type") 0
13 65–68 Element set number. Incremented when a new TLE is generated for this object.[4] 292
14 69–69 Checksum (modulo 10) 7
LINE 2
Field Columns Content Example
1 01–01 Line number 2
2 03–07 Satellite number 25544
3 09–16 Inclination (degrees) 51.6416
4 18–25 Right Ascension of the Ascending Node (degrees) 247.4627
5 27–33 Eccentricity (decimal point assumed) 0006703
6 35–42 Argument of Perigee (degrees) 130.5360
7 44–51 Mean Anomaly (degrees) 325.0288
8 53–63 Mean Motion (revolutions per day) 15.72125391
9 64–68 Revolution number at epoch (revolutions) 56353
10 69–69 Checksum (modulo 10) 7

Where decimal points are assumed, they are leading decimal points. The last two symbols in Fields 10 and 11 of the first line give powers of 10 to apply to the preceding decimal. Thus, for example, Field 11 (-11606-4) translates to -0.11606E-4 (-0.11606×10-4).

The checksums for each line are calculated by adding the all numerical digits on that line, including the line number. One is added to the checksum for each negative sign (−) on that line. All other non-digit characters are ignored.

For a spacecraft in a typical Low Earth orbit, the accuracy that can be obtained with the SGP4 orbit model is on the order of 1 km within a few days of the epoch of the element set.[5]

## Applications

TLEs are widely used as input for projecting the future orbital tracks of space debris for purposes of characterizing "future debris events to support risk analysis, close approach analysis, collision avoidance maneuvering" and forensic analysis.[6]

## References

1. ^ "Introduction and sign in to Space-Track.Org". Space-track.org. Retrieved 28 November 2014.
2. ^ "Celestrak homepage". Celestrak.com. Retrieved 28 November 2014.
3. ^ "Space Track". Space-track.org. Retrieved 28 November 2014.
4. ^ a b c "NASA, Definition of Two-line Element Set Coordinate System". Spaceflight.nasa.gov. Retrieved 28 November 2014.
5. ^ Kelso, T.S. (29 January 2007). ""Validation of SGP4 and IS-GPS-200D Against GPS Precision Ephemerides"". Celestrak.com. Retrieved 28 November 2014. AAS paper 07-127, presented at the 17th AAS/AIAA Space Flight Mechanics Conference, Sedona, Arizona
6. ^ Carrico, Timothy; Carrico, John; Policastri, Lisa; Loucks, Mike (2008). "Investigating Orbital Debris Events using Numerical Methods with Full Force Model Orbit Propagation" (PDF). American Institute of Aeronautics and Astronautics (AAS 08-126).