The central processor unit (CPU) controls the transponder to reply to a mode A, mode C, or mode S interrogation. The mode format circuits use the identity code, airplane altitude and the unique 24-bit airplane address from the memory to format the data for the reply signals.
The mode A reply contains the airplane identity code.
The mode C reply contains the airplane altitude.
The mode S replies contain a unique 24-bit airplane address, identity code, altitude information, and TCAS information. After the signal processor chooses the correct mode for the reply data, the modulator makes a reply signal in that format. The modulator uses the signal processor output to modulate a 1090 MHz carrier to make the transmission reply signal. The transmission reply signal goes to the power amplifier and then to the diplexer and to the ATC antenna.
The antenna select circuit makes the transmission signal go to the antenna that receives the strongest interrogation signal.