I am a software developer in the aviation industry, so I do apologize if my terminology is not correct. What I want to know is - are there any cases in which the aircraft's Mode A is switched to OFF or STANDBY and the aircraft is still able to transmit Mode C replies? Or will switching Mode A to OFF/STANDBY also automatically switch Mode C to OFF/STANDBY? Would there be any differences in this behavior in the US versus in Europe?
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1$\begingroup$ Who is writing your specifications? $\endgroup$– DJClayworthCommented Oct 15, 2023 at 23:44
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No. Mode A/C ATCRBS transponders have to comply with local regulatory standards (TSO-C74d/RTCA DO-144A in the US, equivalent to JAA/EURACAE standards in the EU, or other local standards complying with ICAO Annex 10, Vol IV).
Basic operating modes for ATCRBS transponders are OFF/StandBy, Mode A, and ALT or Mode A/C. OFF is self explanatory. StandBy is a holdover from the days when vacuum tubes were used in the power amplifiers. Standby is functionally OFF, but provided power to the tubes for them to warm up so there was no delay when it was switched on. Mode A provides responses to only Mode A interrogations and does not reply to Mode C. The ALT mode is sometimes labeled Mode A/C as it provides both Mode A and Mode C replies. It is not possible to provide Mode C replies without responding to Mode A.
The real question would be why look at a Mode A/C transponder. Other than some small aircraft, most aircraft today are equipped with ATCRBS/Mode S transponders. They follow RTCA DO-181F or the equivalent EUROCAE or ICAO Annex 10 standard. Mode S provides support for TCAS (ACAS) as well as ADS-B out using the Mode S squitter.
