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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