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The TCAS uses an ATCRBS all-call whisper-shout interrogation to look for airplanes that have an ATCRBS transponder. The whisper-shout procedure changes the strength of the interrogation pulse in many steps. This lets TCAS interrogate intruders at different ranges. Intruders that reply are put on the TCAS roll call list and TCAS monitors them.
The airplanes that are near receive the lower power (whisper) interrogations. The airplanes that are far away do not receive these low power signals so they do not respond.
The TCAS transmits a suppression pulse as the interrogation power increases. The airplanes that are farther away receive the higher power (shout) interrogations. The airplanes that are near receive the interrogation and the suppression pulse so they do not reply.
Mode S Interrogation
Mode S transponders transmit a squitter signal once each second. The signals contain a 24-bit airplane address. The TCAS listens for mode S squitter signal transmissions from airplanes that have mode S transponders. When the TCAS receives an address, it puts that airplane on a roll call list. The TCAS then interrogates the mode S transponder of the traffic airplane with the discrete 24-bit airplane address on the roll call list.