Two-line element set: Difference between revisions

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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.

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 tracedwas backeventually determined to be caused by a change in the lackrepresentation of the plus character (+) on thepunched [[Teleprinter|teletype]]cards machineswhen usedNORAD atupgraded NASA,their which[[UNIVAC ultimately1100/2200 turnedseries|UNIVAC]] outcomputers to be a problem fromuse the punch card era that occurred when NORAD updated from the BCD to [[EBCDIC]] [[character set]] onrather thethan computer[[BCD sending(character outencoding)| the updatesBCD]]. 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>

The SGP4 model was later extended with corrections for deep space objects, creating SDP4, which used the same TLE input data. Over the years a number of more advanced prediction models have been created, but these have not seen widespread use. This is due to the TLE not containing the additional information needed by some of these formats, which makes it difficult to find the elements needed to take advantages of the improved model. More subtly, the TLE data is massaged in a fashion to improve the results when used with the SGP series models, which may cause the predictions of other models to be less accurate than SGP when used with common TLEs. The only new model to see widespread use is SGP8/SDP8, which were designed to use the same data inputs and are relatively minor corrections to the SGP4 model.