Transponder 4 levels

There are 4 levels of transponders:
Level 1 transponders only have basic surveillance functions: they can identify UF0, UF4, UF5 and UF11 inquiries and reply accordingly. Transponders at this level do not have the ability to transmit data via data link.
Level 2 transponders are based on level 1 and can also recognize UF16, UF20 and UF21 and can make corresponding replies. Most of the transponders are level 2.
Level 3 is based on level 2 and can also receive UF24 inquiries, but can’t reply, which means they can’t send DF24 replies.
Level 4 is based on level 3 and can send DF24 replies.

Mode-S inquiries:
There are 24 different inquiries. The ones defined are:
UF=00 and UF=16 are used for TCAS systems.
UF=04 and UF=05 are used to inquire the altitude and identity of the aircraft.
UF=20 and UF=21 are Comm-A inquiries.
UF=24 is a Comm-C inquiry.
UF=11 is an all-call inquiry.

Mode-S replies:
The reply messages are in 24 different forms. The ones defined are:
DF=00 and DF=16 are used by TCAS systems.
DF=04 and DF=05 are used to transmit the altitude and identity of the aircraft.
DF=20 and DF=21 are Comm-B replies, corresponding to Comm-A inquiries.
DF=24 is Comm-D reply.
DF=11 is all-call reply.
DF=17~19 is ADS-B broadcast

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