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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 ...


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 ...


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