Oblivian:joker97: estimated by "primary radar" whatever that thing is
Rotating radar tower sending radio waves with interrogation request and awaiting response.
As for the FL430, I think they threw that out the window as a bung calculation from doppler/estimation
Primary radar works on seeing a reflection of the signal it sends out, it's a pretty low tech system.
Secondary radar, which is predominately used today sends out a signal that triggers a transponder on the aircraft which sends back an encoded signal with aircraft specific information in response.