Two-line element set: Difference between revisions

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Revision as of 10:52, 31 January 2021

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A '''two-line element set''' ('''TLE''') is a [[file format|data format]] encoding a list of [[orbital elements]] of an Earth-orbiting object for a given point in time, the ''epoch''. Using a suitable prediction formula, the [[Orbital_state_vectors|state]] (position and velocity) at any point in the past or future can be estimated to some accuracy. 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 SGP models to correctly compute the state at a time of interest. TLEs can describe the trajectories only of Earth-orbiting objects. 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 (spacecraft)|collision avoidance]] maneuvering" and [[forensic analysis]].<ref name=aiaa2008>

{{cite journal |last1=Carrico |first1=Timothy |last2=Carrico |first2=John |last3=Policastri |first3=Lisa |last4=Loucks |first4=Mike |title=Investigating Orbital Debris Events using Numerical Methods with Full Force Model Orbit Propagation |journal=American Institute of Aeronautics and Astronautics |date=2008 |issue=AAS 08–126 |url=http://www.applieddefense.com/wp-content/uploads/2012/12/2008-Carrico-Policastri-Investigating_Orbital_Debris_Events.pdf |url-status=dead |archiveurlarchive-url=https://web.archive.org/web/20141204122502/http://www.applieddefense.com/wp-content/uploads/2012/12/2008-Carrico-Policastri-Investigating_Orbital_Debris_Events.pdf |archivedatearchive-date=2014-12-04 }}</ref>

The format was originally intended for [[punch card]]s, encoding a set of elements on two [[Punched_card#IBM_80-column_punched_card_format_and_character_codes|standard 80-column cards]]. This format was eventually replaced by [[text file]]s{{when|date=June 2019}} with each set of elements written to two 70-column [[ASCII]] lines. The [[United States Air Force]] tracks all detectable objects in Earth orbit, creating a corresponding TLE for each object, and makes publicly available TLEs for many of the space objects on the website Space Track,<ref>{{cite web|url=https://www.space-track.org/|title=Introduction and sign in to Space-Track.Org|publisher=Space-track.org|accessdateaccess-date=28 November 2014}}</ref><ref>{{cite web|url=http://celestrak.com/|title=Celestrak homepage|publisher=Celestrak.com|accessdateaccess-date=28 November 2014}}</ref> holding back or obfuscating data on many military or [[Classified information|classified objects]]. 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 preceding the element data, so each listing may take up three lines in the file. The title is not required, as each data line includes a unique object identifier code.

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2 25544 51.6416 247.4627 0006703 130.5360 325.0288 15.72125391563537

The meaning of this data is as follows:<ref>{{cite web|url=https://www.space-track.org/documentation#/tle|title=Space Track|publisher=Space-track.org|accessdateaccess-date=28 November 2014}}</ref>

;Title line

[[File:Tle title.jpg|left|TLE title]]

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||8||21–32||[[Epoch (astronomy)|Epoch]] (day of the year and fractional portion of the day)||264.51782528

|-

||9||34–43||First Derivative of [[Mean Motion]] aka the Ballistic Coefficient <ref name="nasahelp">{{cite web|url=http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/SSOP_Help/tle_def.html|title=NASA, ''Definition of Two-line Element Set Coordinate System''|publisher=Spaceflight.nasa.gov|accessdateaccess-date=28 November 2014}}</ref>||-.00002182

|-

||10||45–52||Second Derivative of [[Mean Motion]] (decimal point assumed) <ref name="nasahelp" />||00000-0

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The checksums for each line are calculated by adding 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 body in a typical [[low Earth orbit]], the accuracy that can be obtained with the SGP4 orbit model is on the order of 1&nbsp;km within a few days of the epoch of the element set.<ref>{{cite web|author=Kelso, T.S.|title=Validation of SGP4 and IS-GPS-200D Against GPS Precision Ephemerides|quote=AAS paper 07-127, presented at the 17th AAS/AIAA Space Flight Mechanics Conference, Sedona, Arizona|date=29 January 2007|url=http://celestrak.com/publications/AAS/07-127/|publisher=Celestrak.com|accessdateaccess-date=28 November 2014}}</ref> The term "low orbit" may refer to either the altitude (minimal or global) or orbital period of the body. Historically, the SGP algorithms defines low orbit as an orbit of less-than 225 minutes.

Two-digit Epoch Years from 57-99 correspond to 1957-1999 and those from 00-56 correspond to 2000-2056.<ref>{{cite web |title=Frequently Asked Questions: Two-Line Element Set Format |publisher=CelesTrak |url=https://celestrak.com/columns/v04n03/}}</ref>