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What's the minimum requirement for TCAS II RAs to be generated? Will we get RA if we have mode C transponder and the other aircraft also has mode C? Will mode A work as well?

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TCAS II is based on the Mode S protocol.Aircraft with TCAS II on board are equipped with a Mode S transponder.

If the other aircraft is Mode A only (i.e. no altitude reporting) TCAS II will track the range and bearing of the traffic an will only provide Traffic Alerting (TA) for that traffic. Since the relative altitude of the traffic is not known, Resolution Advisories (RA) cannot be generated.

If the other aircraft is equipped with Mode A/C but not mode S, TCAS will track range bearing and altitude of the traffic and provide TA and Resolution Advisories (RA) for that traffic.

Reference material: FAA TCAS II v 7.1 introduction booklet

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