The apogee and perigee of the orbit, as heights above the Earth's
equatorial radius. SGP4 works both out when it initialises its record
from the Elements — the semi-major axis from the mean motion once the
Kozai term is taken out, then a(1 ± e) less one Earth radius — so
they are read off the record rather than worked out a second time from
Kepler's law, which would differ by the un-Kozai correction. The radius
is the one SGP4's own constants use (WGS72), the same the propagation
already runs on.
The apogee and perigee of the orbit, as heights above the Earth's equatorial radius. SGP4 works both out when it initialises its record from the Elements — the semi-major axis from the mean motion once the Kozai term is taken out, then
a(1 ± e)less one Earth radius — so they are read off the record rather than worked out a second time from Kepler's law, which would differ by the un-Kozai correction. The radius is the one SGP4's own constants use (WGS72), the same the propagation already runs on.