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As usual when we discuss TLE headaches (here, here, here, here, and here, just to name a few), the core issue is keeping track of whether you are working with mean elements or osculating ones. Within the SGP4 ecosystem, apogee and perigee are not osculating quantities, they are mean quantities, so their values cannot easily be derived from osculating ...


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I'm happy to see OP has used Brandon Rhodes' sgp4 python library to obtain periapsis and apoapsis from a TLE. I've always used the skyfield python library for this which is from the same developer and builds on the sgp4 library but has way more features. From https://celestrak.com/NORAD/elements/table.php?tleFile=stations&title=Space%20Stations&...


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To answer my own question, I've finally found an elegant way of calculating apogee and perigee from TLE: from sgp4.api import Satrec line=['0 ISS (ZARYA)', '1 25544U 98067A 21356.62544795 .00006800 00000-0 13125-3 0 9998', '2 25544 51.6428 130.9420 0004657 342.5227 11.5462 15.49048823317794'] sat = Satrec.twoline2rv(line[1],line[2]) print((sat.alta * ...


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