When the orbit began: the last perigee, fraction of a period ago.
How long until it is back at perigee, in seconds: the rest of the orbit at the mean motion.
When it ends: the next perigee, completesInSeconds from the instant, where the next orbit begins.
How much of the orbit is behind it since the last perigee, in [0, 1).
One orbit at the mean motion, in seconds (orbitalPeriodSeconds).
Where along its orbit the Satellite is at an instant, by mean anomaly.