Been around a long time but I call it IFF (Identification Friend or Foe). Simply put I acquisition a potential target on my radar and I send out a signal looking for a reply. So I ping the potential target. If there is no reply I launch a radar guided missal and remove the target. Matters not if the target or myself are moving or stationary. IFF has likely been around since WWII. Things like reply pulses are changed regularly.
Side note: When US submarines are running on the surface they, like aircraft, run IFF. This way aircraft can identify them as friend or foe.
You want to enter my perimeter gate and I challenge you. I ask you the password and you either know it or you don't. Only done on a much higher refined scale.
Just develop a basic IFF system with a transponder. Yes, as mentioned similar to Easy Pass with a whole bunch of whistles and bells.
Ron
What mode did you squawk?