6

This has to do with ambiguities in the definitions used for certain terms, and software doing strange contortions to avoid breaking backwards compatibility. The USAF's family of Simplified General Perturbations propagators are based on mean element theories of orbital motion. However, there are multiple ways to define and compute mean motion, and ...


4

If a two line element represented the exact state of a satellite at the epoch time of the element set, there would be zero penalty in applying the SGP4 algorithm to cartesian coordinates, which are in the True Equator, Mean Equinox (TEME) frame. From there it would be simple matter of a coordinate transformation to convert those TEME coordinates to something ...


3

I find Ryan C's answer interesting... but I've never actually interpreted the documentation that way. It's difficult to be certain, since all official TLEs have had ephem type 0. But I (and others with whom I've discussed the matter) have assumed that ephem type 0 means "use SGP4 if you've got 6.4 or more revolutions/day; otherwise, use SDP4", ...


1

Edited in response to very constructive criticism from @DavidHammen and @CallMeTom. I agree with them, but I didn't say those things in my initial answer, and I should have. If the only source of data you have is a TLE, then you are starting from a low-quality initial state, which you should expect to be wrong by several kilometers. All a high-quality ...


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