Originally Posted by
Specaircrew
Thanks SWH that's a very good explanation of the technical aspects of transponder replies.
So, in pilot terminology, when a TCAS II mode S transponder shouts 'is anybody there' an obsolete mode A only transponder won't hear the call so won't respond whereas a nearly obsolete Mode A/C capable transponder with no altitude encoder will say 'I'm here but I've no idea of what my altitude is'?
Mode A transponders are to conform to a minimum of TSO-C74b, ever since the previous 1967 version of this standard (TSO-C74a) it has been a requirement under that “2
.4 Interrogation. The equipment must accept and reply to interrogations on at least Modes 3/A and C”
So that any Mode A transponder SHOULD accept Mode C uplinks, very very very old transponders that predate this TSO are technically illegal even for VFR use unless they have been modified to meet the minimum Mode A standard. Those transponders are so old, I doubt they would still be serviceable.
TSO-C74a
https://drs.faa.gov/browse/excelExte...300669076.0001 TSO-C74b
https://drs.faa.gov/browse/excelExte...107150505.0001