For a given pass of a satellite relative to a particular
observer, there are three important times:
Acquisition of Signal (AOS) - the time when the satellite rises above
Loss of Signal (AOS) - the time when the satellite sets below the
Time of Closest Approach (TCA) - the best time for communication, when Doppler
shift is zero and path loss is minimized.
For circular orbits, TCA is usually near the halfway point, and corresponds to
the maximum antenna elevation (highest point in the sky). For elliptical
orbits, it could occur at AOS or LOS. WinOrbit actually reports the
maximum antenna elevation in both cases.