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Marc Kupper (talk | contribs) →Line 2: Update as we are now well past the year 2020 |
Corrected one hyphen in year range 1957-1999 to a dash 1957–1999. Add author name to CelesTrak refs. |
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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 was eventually determined to be caused by a change in the representation of the plus character (+) on punched cards when NORAD upgraded their [[UNIVAC 1100/2200 series|UNIVAC]] computers to use the [[EBCDIC]] character set rather than [[BCD (character encoding)| BCD]]. 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.
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.
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||2||03–07||[[Satellite Catalog Number|Satellite catalog number]]||25544
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||3|| 08 ||Classification (U: unclassified, C: classified, S: secret)
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||4||10–11||[[International Designator]] (last two digits of launch year)||98
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||8||21–32||[[Epoch (astronomy)|Epoch]] (day of the year and fractional portion of the day)||264.51782528
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||9||34–43||First derivative of [[mean motion]]; the ballistic coefficient
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||10||45–52||Second derivative of [[mean motion]] (decimal point assumed)
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||11||54–61||[[BSTAR|''B''*]], the drag term, or radiation pressure coefficient (decimal point assumed)
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||12||63–63||Ephemeris type (always zero; only used in undistributed TLE data)
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||13||65–68||Element set number. Incremented when a new TLE is generated for this object.<ref name="nasahelp"/>||292
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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 km within a few days of the epoch of the element set.<ref>{{cite web|
Two-digit Epoch Years from 57 to 99 correspond to
The activation of the [[Space Fence]] in 2020 resulted in a great increase in the number of objects being tracked as the Space Fence's [[S-band]] RADAR is more sensitive than the radars used by the [[Air Force Space Surveillance System]] that previously had generated the raw data used for TLEs.<ref name="CelesTrak"/> The increase in the number of objects tracked meant that [[Satellite Catalog Number]]s may no longer fit in the five-digit field available in traditional TLEs. A secondary cause for the increase in Satellite Catalog Numbers was the commercialization of space and break-up events and collisions that have created debris objects.<ref name="CelesTrak"/> Adaptations of the TLE were considered to extend the number of encodable Satellites within the TLE but instead a new format, the [[CCSDS]] OMM (Orbit Mean-Elements Message), started being used in 2020 that supports nine-digit Satellite Catalog Numbers. The legacy TLE format will continue to use five-digit Satellite Catalog Numbers.<ref name="CelesTrak">{{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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