cjcampbell wrote:

Some transponders have a 'ground' mode. The Garmin G-1000 turns its
transponder on and off automatically.


The Garmin transponders automatically turn on and off (my GTX33 does)
based I suppose on noticing that the encoded altitude has jumped since
it was turned on.

The the SL70 (formerly Apollo) transponder also has an engage mode if
an attached device (like your GPS) says it is now moving at a certain
threshold speed.