Mode C on the ground?


Throughout my training, I was always told to switch the transponder to “standby” on the ground, however I am noticing more and more airports that request the transponder be left on at all times while on the ground.

Also, I have heard that ATC radar generally filters out transponder replies from ground level on or near airports.

So, is there any reason why I should not just leave the transponder on “Alt” all the time? Any controllers out there care to comment?



More and more larger airports are getting ground radar that, unlike the old ground radar, uses transponders to identify aircraft.


The larger airports are using ASDE, which is a grounding tracking radar. This is to assist ground control and to help prevent runway incursions. You transponder must be activated when on the ground at these airports.


So, basically then leaving in on “Alt” mode is fine everywhere?


At airports that have it noted in the remarks section, yes. The reason for requiring the Mode C is so the ground radar can tell the difference between ground and air traffic. Airports without the ASDE radar may still want you to shut off your transponder.


Unless ATC tells you to Squawk Standby leave it on. I have never put a transponder on standby. Also remember most airports DO NOT have radar on the field so they won’t see your transponder anyway.

Frank Holbert