AOS stands for Acquisition of Signal (or Satellite). AOS is the time that a satellite rises above the horizon of an observer.
TCA stands for Time of Closest Approach. This is the time when the satellite is closest to the observer and when Doppler shift is zero. This usually corresponds to the time that the satellite reaches maximum elevation above the horizon.
LOS stands for Loss of Signal (or Satellite). LOS is the time that a satellite passes below the observerís horizon.