What is a transponder

A transponder will send an identifying coded signal in response to a transmitted interrogation from a ground-based radar station . An air traffic controller can then view the identified blip on a screen and know who it is and provide direction to the flight crews maintaining adequate separation with other blips.

Basically, there are two types of transponders. Those are Bent pipe transponders and Regenerative transponders .

A transponder is a wireless communications, monitoring, or control device that picks up and automatically responds to an incoming signal . The term is a contraction of the words transmitter and responder.

Each of this transponder occupy part of the total satellite bandwidth (BW) i.e. 500 MHz . Figure-1 depicts transponder channels with each utilizing 36 MHz of bandwidth from the total BW of 500 MHz. The guard band of 4 MHz is provided between transponders to avoid interference.