Originally Posted by
BluSdUp
The transponder is suppressed on ground.
Most types are switched on/ off on nosewheel squat switch.
If you dont believe me watch the guy racing down the rwy on takeoff next time you are on short final!
You can see him but TCAS cant. And voila he lifts, and there he is.
Same on landing behind a fella: Now he is there, now he is gone, ie on the runway, no more mode A and C.
Unfortunately, sometimes aircraft transmit a continuous signal due some problem in the air/ground logic, which then in turn can give false TA/RA when aircraft is taxiing on ground to airborne aircraft.
e.g. on several occasions I have requested ATC to ask an aircraft on ground to switch off their transponder as I could see them on taxiway when on short final, when flying in IMC.