Simple explanation of 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, building upon Level 1, can also recognize UF16, UF20 and UF21 and can make corresponding replies. Most of the transponders are level 2. 
Level 3 transponders, an advancement from Level 2, can recognize UF24 inquiries but lack the ability to reply, which means they can’t send DF24 replies. 
Level 4 transponders, based on Level 3, possess the capability to send DF24 replies.


Regarding Mode-S inquiries,  there are 24 different types. The ones already defined are:
· UF=00 and UF=16 are utilized by TCAS systems.
· UF=04 and UF=05 are employed to inquire about aircraft altitude and identity.
· UF=20 and UF=21 represent Comm-A inquiries.
· UF=24 is designated as a Comm-C inquiry.
· UF=11 serves as an all-call inquiry.


Mode-S reply messages also exist in 24 different forms. The ones already defined are:
· DF=00 and DF=16 are used by TCAS systems.
· DF=04 and DF=05 are utilized for transmitting aircraft altitude and identity.
· DF=20 and DF=21 represent Comm-B replies, corresponding to Comm-A inquiries.
· DF=24 signifies a Comm-D reply.
· DF=11 serves as an all-call reply.
· DF=17~19 are dedicated to ADS-B broadcasts.

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