Transponders return a signal from a radar ping. Even without the transponder returning a signal the radar signal is still returned. The airplane would still be tracked, but the altitude information the transponder returns simply wouldn’t be available,
” Transponders return a signal from a radar ping. “
Transponders return one of several ‘things’ depending on what was *received*’ from the ‘interrogator’ at 1030 MHz (in a “SSR” or Secondary Surveillance radar vs a “PSR” or Primary Surveillance RADAR/’skin paint’ system RADAR system using frequencies in S-band).
Transponder Modes A, C or “S” each have different pulse or interrogation characteristics.
And - anybody here familiar with ADS-B (Automatic Dependent Surveillance - Broadcast) technology?
http://en.wikipedia.org/wiki/Automatic_dependent_surveillance-broadcast
What used to be called “ATCRBS” (At Crabs) back in the day:
http://en.wikipedia.org/wiki/Air_traffic_control_radar_beacon_system