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Hangar Flying / Passive Collision Avoidance Systems
« on: September 27, 2006, 12:35:00 PM »Quote from: "Chairboy"
When I heard "Passive", I immediately began to imagine what "active" would entail. After a moment of contemplation, I decided that active would be similar to Mode S or primary radar, and not the swarm of protective missiles I had originally imagined.
Correct. Active surveillance broadcasts a query for transponders to respond to, exactally like a ground based radar station would. It will also listen for responces from aircraft that have been queried by an actual ground based radar.
Mode S on the other hand does not listen for other aircraft. It depends on ground based repeaters at the radar station to rebroadcast the traffic information to it. If you fly into an area with no re-transmit coverage you get "Traffic Not Available" with a Mode S system.
Passive surveillance sits and listens for transponder responces that are started by a ground based radar station... if you are in an area without a radar around then you will not see the other traffic. This is normally not an issue as where you really need this service is in busy terminal areas which normally all have a ground radar on site or near by.