AOS/LOS/TCA

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 the horizon.
Loss of Signal (AOS) - the time when the satellite sets below the horizon.
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.