Two-Line Orbital Element Set File

Two-Line Element Sets (TLEs) are general perturbation mean elements constructed by a least squares estimation from observations of a satellite's orbit. A derivation of the Two-Line Element set format called the Three-Line Element set exists where the first line contains the name of the satellite. The observation data can be provided from sources such as the Space Surveillance Network or commercial entities; or the data can be generated by using FreeFlyer's simulated observation capabilities. FreeFlyer can use any of these real or simulated observation data sources to generate TLEs.

Most TLEs are publicly available on the internet at space-track.org or CelesTrak.

The mean values are constructed by removing periodic variations in the force model due to the oblateness of the Earth, atmospheric drag, and lunar and solar gravitational effects. In order to obtain good predictions from the TLE, the periodic variations must be reconstructed in exactly the same way they were removed. FreeFlyer's SGP4 propagator type (which implements the SGP4/SGP4-XP algorithms) provides the means of reconstructing the variations, thus enabling the proper propagation of TLE states.

Format

Two-Line Orbital Element Sets are provided in an ASCII formatted text file and each file could contain TLEs for multiple objects. A typical TLE entry can contain the name of the object on the first line (line 0), however this is not a requirement. The next two lines (lines 1 and 2) contain 69 characters which describe the orbit of the object in the SGP4 Mean Element set.

Below is a template of the common format of a Three-Line Element Set entry. In the template shown below, 'A's can represent any character between A-Z or a space and 'N's can represent any number between 0-9 or a space. The last line is not part of a TLE entry; it was added to help identify the individual columns.

Format Definition, Line 0

Line 0 is provided in a Three-Line Element Set file, and is not included in a Two-Line Element Set. The line contains the name of the object, up to 11 characters.

Format Definition, Line 1

Line 1 of a TLE entry contains general descriptive information of the object itself (classification, origin, etc.), along with information describing the force model parameters (drag/solar radiation pressure, time derivatives, etc.).

[1] Note: Replacing the first digit of the 5-digit catalog number with an alphanumeric character provides the ability for the TLE format to represent up to 339,999 objects. This numbering schema implemented by the United States Space Force (USSF) is known as Alpha-5. The letters "I" and "O" are omitted to avoid confusion with the numbers "1" and "0". The catalog number maps to the Alpha5ID and SatelliteNumber properties for the TwoLineElement and SGP4Properties objects. More information is available at https://www.space-track.org/documentation#tle-alpha5.

[2] Note: Although the AstroStds documentation states that Type 0 is used for states generated using the old/deprecated SGP algorithm, we often observe Type 0 being used to label states that were generated using the SGP4 algorithm using Kozai mean motion, and Type 2 being used to label states that were generated using the SGP4 algorithm using Brouwer mean motion. Type 4 indicates that the state elements are calculated and will be propagated using the SGP4-XP algorithm. See Type 4 Ephemerides for details on format changes between Types 0/2 and Type 4.

Format Definition, Line 2

Line 2 of the TLE entry contains the state data of the object.

Type 4 Ephemerides

The Ephemeris Type 4 TLE uses the SGP4-XP algorithm for propagation, which accounts for additional forces and perturbations on top of the standard SGP4 algorithm. This format includes the AGOM (Area times Gamma Over Mass, where Gamma = Cr) term and uses the B term (as defined by USSF) instead of B*. The B term still maps to the Bstar property. These terms are represented on Line 1 of the TLE format in place of the MeanMotionDDot and BStar terms as indicated below:

See Also