Saw an interesting story today regarding Turkey potentially downing a Russian drone in their airspace. Someone commenting on the story raised an interesting question about how to warn a drone that it was intruding in unauthorized airspace. If the ground control system (GCS) for the drone does not publish or otherwise make available a contact frequency/phone number associated with that drone there is potentially no way to do that. It is possible that the drone has a radio repeater on a guard or other known frequency that the GCS is listening for but the advent of low cost drones may reduce that opportunity due to size/weight/power/cost.