Two-line element set: Difference between revisions

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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 techreporttech 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 techreporttech 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 techreporttech 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.

Shortly after the publication of ''Report #3'', NASA began posting elements for a variety of visible and other well known objects in their periodic ''NASA Prediction Bulletins'', which consisted of the transmission format data in printed form. After trying for some time to convince NASA to release these in electronic form, T.S. Kelso took matters into his own hands and began manually copying the listings into text files which he distributed through his [https://celestrak.com CelesTrak] [[bulletin board system]]. This revealed a problem in NASA's [[checksum]] system, which traced back to the lack of the plus character (+) on the [[Teleprinter|teletype]] machines used at NASA, which ultimately turned out to be a problem from the punch card era that occurred when NORAD updated from the BCD to [[EBCDIC]] [[character set]] on the computer sending out the updates. This problem went away when Kelso began to receive data directly from NORAD in 1989.<ref name=controversy>{{cite web |first=Ted |last=Kelso |title=Two-Line Element Set Checksum Controversy |url=http://celestrak.com/NORAD/documentation/checksum.asp |website=CelesTrak |date=January 1992}}</ref>