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Signal Formats

In order to make Mode S signals transparent, Lincoln Labs researchers used sidelobe suppression to disable ATCRBS transponders while Mode S signals are sent.

Mode S Interrogations

A Mode S interrogation includes the sidelobe suppression preamble (P2 is equivalent or stronger than P1), followed by a data block containing either 56 or 112 bits of data.

Mode S interrogation format (DABS: A Functional Description (ATC-42), P. R. Drouilhet)

All-Call Interrogations

ATCRBS ground station sensors typically send "All-Call" interrogations to aircraft in their vicinity. Planes that receive these interrogations respond with either plane identification or altitude information. In the original ATCRBS system, all-call interrogations had two pulses (P1, P3).

When designing Mode S, researchers used a similar format for the signal that is sent by Mode S sensors that requests responses from only Mode S equipped aircraft. This format has an additional pulse added (P4).

Aircraft with ATCRBS transponders ignore the last pulse, responding with a normal ATCRBS all-call reply. Mode S transponders see the additional pulse and recognize the signal as a Mode S interrogation, and respond with a Mode S reply.

Without this technique, Mode S-equipped aircraft cannot distinguish between an ATCRBS and Mode S interrogation. They will not know which reply to send, and thus cannot interoperate with the different sensors on the ground.

Mode S Replies

When a Mode S transponder receives either an all-call or normal interrogation, it replies with the same signal. This signal has a four pulse preamble and contains a data block with either 56 or 112 bits of information. The four pulse preamble is what identifies this as a Mode S reply.

(Information based on ATC-42, "DABS: A System Description", P. R. Drouilhet, November 18, 1974)