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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 ...


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", ...


3

SDP4 has a lot of algorithmic nuances to handle objects in 12- and 24-hour orbits. You can get decent (km-level over a week or so) fits to the model. I see no evidence in the SDP4 algorithm that they had higher-orbiting objects in mind. Go much beyond a one-day orbit with SDP4, and you're basically using a hammer as a screwdriver. If you take a look at ...


2

You don't have to implement the propagator. You can download it from space-track.org, just like you download TLEs. Go to space-track, click "Help", then click "SGP4", and arrive at https://www.space-track.org/documentation#/sgp4 . From there, pick Windows or Linux, and download compiled shared libraries of the latest SGP4 as released ...


1

I have no formal, mathematical answer to this. When I have to fit TLEs to a new object, I try SDP4; if it fails, I drop back to SGP4. It's an entirely empirical "gee, SDP4 didn't work here" approach. I also haven't kept particularly careful track of when it happens, and I've only had SDP4 fail on maybe a dozen objects. So my statistics are ...


1

I found Hujsak 1979 -- its title is "A Restricted Four Body Solution for Resonating Satellites Without Drag", but the linked pdf shows it is also Spacetrack Report #1. Four body means Earth, Moon, Sun, and satellite. There is a lot of integration going on: some things are averaged three or four times, over different periods with respect to ...


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