# Two-line element set

A two-line element set (TLE) is a data format used to convey sets of orbital elements that describe the orbits of Earth-orbiting satellites. A computer program called a model can use the TLE to compute the position of a satellite at a particular time. The TLE is a format specified by NORAD and used by NORAD and NASA. The TLE can be used directly by the SGP4 model (or one of the SGP8, SDP4, SDP8 models). Orbital elements are determined for many thousands of space objects by NORAD and are freely distributed on the Internet in the form of TLEs.[1] A TLE consists of a title line followed by two lines of formatted text.

## Format

The following is an example of a 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:

Title line
FieldColumnsContentExample
101–24Satellite nameISS (ZARYA)
LINE 1
FieldColumnsContentExample
101–01Line number1
203–07Satellite number25544
308–08Classification (U=Unclassified)U
410–11International Designator (Last two digits of launch year)98
512–14International Designator (Launch number of the year)067
615–17International Designator (Piece of the launch)A
719–20Epoch Year (Last two digits of year)08
821–32Epoch (Day of the year and fractional portion of the day)264.51782528
934–43First Time Derivative of the Mean Motion divided by two [2]−.00002182
1045–52Second Time Derivative of Mean Motion divided by six (decimal point assumed)00000-0
1154–61BSTAR drag term (decimal point assumed) [2]-11606-4
1263–63The number 0 (Originally this should have been "Ephemeris type")0
1365–68Element set number. incremented when a new TLE is generated for this object. [2]292
1469–69Checksum (Modulo 10)7
LINE 2
FieldColumnsContentExample
101–01Line number2
203–07Satellite number25544
309–16Inclination [Degrees]51.6416
418–25Right Ascension of the Ascending Node [Degrees]247.4627
527–33Eccentricity (decimal point assumed)0006703
635–42Argument of Perigee [Degrees]130.5360
744–51Mean Anomaly [Degrees]325.0288
853–63Mean Motion [Revs per day]15.72125391
964–68Revolution number at epoch [Revs]56353
1069–69Checksum (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.

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.[3]

## References

1. ^ Celestrak (http://celestrak.com/)
2. ^ a b c NASA, Definition of Two-line Element Set Coordinate System
3. ^ Kelso, T.S., "Validation of SGP4 and IS-GPS-200D Against GPS Precision Ephemerides," AAS paper 07-127, presented at the 17th AAS/AIAA Space Flight Mechanics Conference, Sedona, AZ, 2007 January 29. http://celestrak.com/publications/AAS/07-127/, browsed 2011-06-08.