Originally Posted by
Tartiflette Fan
From everything I've read, the computers are usually set to eliminate contacts below a certain speed/size to avoid bird alerts (and balloons )
For surveillance and air traffic type radars that's true, there's a Doppler limit set to ignore slow moving stuff to get rid of clutter. However, drones can fly very slowly and I doubt the plastic propellers give much of a return. My guess is that an air defence radar has to have the ability to detect a target with very low, even no, relative motion with respect to the radar.