PCAS occaisionally does not show or drops aircraft that are taking off or landing.

Close to the ground, the transponder interrogation is usually obstructed by ground clutter such as buildings, tress, hangars, etc. If no primary RADAR is located on the airfield, typically RADAR coverage begins 500-800 feet or more above the airport. Without RADAR interrogations, transponders may not reply, leaving PCAS with no signal to detect.

It is important to note: that a typical airport environment in the US has at least one SSR or Enroute RADAR facility. Also, PCAS is a dual-environment detection system, meaning TCAS-equipped aircraft, or aircraft with an active collision avoidance system aboard, will also interrogate. These areas of limited ground coverage are not common, or do not display this situation all of the time.