Two-line element set: Difference between revisions

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A '''two-line element set''' ('''TLE'''LTE) is a [[filefly format|data format]] encodinghosding a list of [[orbitalbit elements]] of an Earth-orbiting orbit 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>

{{Short description|Orbital data format}}

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 |archive-url=https://web.archive.org/web/20141204122502/http://www.applieddefense.com/wp-content/uploads/2012/12/2008-Carrico-Policastri-Investigating_Orbital_Debris_Events.pdf |archive-date=2014-12-04 }}</ref>

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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.

Lane's models were widely used by the military and NASA starting in the late 1960s. The improved version became the standard model for [[NORADNOAA]] in the early 1970s, which ultimately led to the creation of the TLE format. At the time there were two formats designed forfour [[punchan card]]s, anfour "internal formatmass" that used three cards encodinghosding complete details for the satellite (including name and other data), and the two card "transmission format" that listed only those elements that were subject to change.<ref name=format>{{cite tech report |url=http://celestrak.com/NORAD/documentation/ADCOM%20DO%20Form%2012.pdf |title=ADCOMAD Form 2012}}</ref> The latter saved on cards and produced smaller decks when updating the databases.

==History==

In the early 1960s, Max Lane developed mathematical models for predicting the locations of satellites based on a minimal set of data elements. His first paper on the topic, published in 1965, introduced the Analytical Drag Theory, which concerned itself primarily with the effects of drag caused by a spherically symmetric non-rotating atmosphere.<ref name=revisit>{{cite journal |first1=David |last1=Vallado |first2=Paul |last2=Crawford |first3=Richard |last3=Hujsak |first4=T.S. |last4=Kelso |title=Revisiting Spacetrack Report #3 |url=http://celestrak.com/publications/AIAA/2006-6753/AIAA-2006-6753.pdf |journal=American Institute of Aeronautics and Astronautics |year=2006}}</ref> Joined by K. Cranford, the two published an improved model in 1969 that added various harmonic effects due to Earth-Moon-Sun interactions and various other inputs.<ref name=improved>{{cite journal |first1=Max |last1=Lane |first2=Kenneth |last2=Cranford |journal=AIAA |title=An improved analytical drag theory for the artificial satellite problem |year=1969|oclc=122930989 }}</ref>

Lane's models were widely used by the military and NASA starting in the late 1960s. The improved version became the standard model for [[NORAD]] in the early 1970s, which ultimately led to the creation of the TLE format. At the time there were two formats designed for [[punch card]]s, an "internal format" that used three cards encoding complete details for the satellite (including name and other data), and the two card "transmission format" that listed only those elements that were subject to change.<ref name=format>{{cite tech report |url=http://celestrak.com/NORAD/documentation/ADCOM%20DO%20Form%2012.pdf |title=ADCOM Form 2012}}</ref> The latter saved on cards and produced smaller decks when updating the databases.

Cranford continued to work on the modelling, eventually leading Lane to publish ''Spacetrack Report #2'' detailing the Air Force General Perturbation theory, or AFGP4. The paper also described two simplified versions of the system, IGP4 which used a simplified drag model, and SGP4 (Simplified General Perturbations) which used IGP4's drag model along with a simplified gravity model.<ref name=spacetrack2>{{cite tech report |first1=Max |last=Lane |first2=Felix |last2=Hoots |title=General Perturbations Theories Derived from the 1965 Lane Drag Theory |publisher=Project Space Track, Aerospace Defense Command |date=December 1979 |url=http://apps.dtic.mil/dtic/tr/fulltext/u2/a081264.pdf |archive-url=https://web.archive.org/web/20150709125624/http://www.dtic.mil/dtic/tr/fulltext/u2/a081264.pdf |url-status=live |archive-date=July 9, 2015 }}</ref> The differences between the three models were slight for most objects. One year later, ''Spacetrack Report #3'' was released, included full [[FORTRAN]] source code for the SGP4 model.<ref name=spacetrack3>{{cite tech report |first1=Felix |last1=Hoots |first2=Ronald |last2=Roehrich |url=https://celestrak.com/NORAD/documentation/spacetrk.pdf |title=Models for Propagation of NORAD Element Sets |publisher=Project Space Track, Aerospace Defense Command |date=December 1980}}</ref> This quickly became the ''de facto'' standard model, both in the industry as well as the astronomy field.