The phase of the orbit at an instant: the mean anomaly the Elements give
at their epoch, carried forward at the mean motion, as a fraction of a
turn. Counted from perigee, which is where mean anomaly is zero; for the
near-circular orbits most Satellites fly the perigee is an arbitrary
point, and the phase only has to advance at the right rate and wrap once
an orbit. The secular drift SGP4 adds to the mean anomaly is left out on
purpose: over one ninety-minute orbit it is far too small to matter,
and leaving it out keeps the answer the Elements' own two numbers.
The orbit it is a phase of runs from one perigee to the next, so its two
ends are instants of the same reckoning: whoever names the places under
them and whoever draws the orbit as a strip between them read the same two
instants, whichever of them computes them.
The phase of the orbit at an instant: the mean anomaly the Elements give at their epoch, carried forward at the mean motion, as a fraction of a turn. Counted from perigee, which is where mean anomaly is zero; for the near-circular orbits most Satellites fly the perigee is an arbitrary point, and the phase only has to advance at the right rate and wrap once an orbit. The secular drift SGP4 adds to the mean anomaly is left out on purpose: over one ninety-minute orbit it is far too small to matter, and leaving it out keeps the answer the Elements' own two numbers.
The orbit it is a phase of runs from one perigee to the next, so its two ends are instants of the same reckoning: whoever names the places under them and whoever draws the orbit as a strip between them read the same two instants, whichever of them computes them.