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{{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 [[
{{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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==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
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 techreport |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.
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||12||63–63||Ephemeris type (always zero; only used in undistributed TLE data) <ref>{{Cite web|url=https://celestrak.com/columns/v04n03/|title=CelesTrak: "FAQs: Two-Line Element Set Format"|website=celestrak.com|access-date=2019-09-03}}</ref>||0
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||13||65–68||Element set number. Incremented when a new TLE is generated for this object.
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||14||69||[[Checksum]] ([[modulo operation|modulo]] 10)||7
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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 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|access-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
The maximum number of Satellite Catalog Numbers that can be encoded in a TLE is rapidly being approached with the recent commercialization of space and several key break-up events that have created a massive number of debris objects. Future adaptations of the TLE have been imagined to extend the number of encodable Satellites within the TLE.<ref>{{Cite web|title=CelesTrak: A New Way to Obtain GP Data|url=https://celestrak.com/NORAD/documentation/gp-data-formats.php|access-date=2020-07-29|website=celestrak.com}}</ref>
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