The Dundee SGP4 code
There are numerous orbital
models in use, but the most commonly used (mostly due to the
continued availability of matching orbital elements) are the
SGP4/SDP4 models that were developed by NORAD/AFSPC for the space
catalogue use in the 1970s. This was officially documented in Project
SpaceTrack
Report #3 published in 1980 in response to the unacceptably
wide discrepancies found in the usage of the Two-Line Elements
(TLE) that are now distributed by http://www.celestrak.com/ and
by http://www.space-track.org/
However, the original FORTRAN code shows its 1980's legacy in style
(difficult to compile or translate) and over the years it has been
improved and numerous small bugs fixed. Unfortunately most of this
has never been the subject of official publications so we are left
guessing at the current state of the 'official' code used to
generate TLE. There have been some releases, intentional or
otherwise, that have helped and we (Paul Crawford & Andrew
Brooks) have made available the merged
SGP4 code in 'C' which is thought to be very close to the
current practice.
It is VERY IMPORTANT to remember that all orbital models are
different, and for accuracy you must use the same theory to
propagate the orbital elements as was used to generate them! This is
why the changes made to the Report #3 code matter.
This is the subject of a paper by David A. Vallado, Paul Crawford,
Richard Hujsak and T. S. Kelso "Revisiting SpaceTrack Report #3",
AIAA 2006-6753 for the American Institute of Aeronautics and
Astronautics. You can download
the
paper from here to help explain the changes from the original
Report #3 version, and to get test cases for verification.
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(c) Paul Crawford, 23rd Sep 2020