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Mode A transponders are sometimes referred to as Mode 3/A transponders. According to NATO STANAG 4193 (not publicly available, summary from Wikipedia), the "3" in that description refers to the mode in the Identification of Friend or Foe ("IFF") system:

  1. Mode 1 – 2-digit octal "mission code" that identifies the aircraft type or mission.
  2. Mode 2 – 4-digit octal unit code or tail number
  3. Mode 3/A – 4-digit octal identification code for the aircraft, assigned by the air traffic controller (the only mode shared with civilian aircraft)
  4. Mode 4 – 3-pulse reply, delay is based on the encrypted challenge
  5. Mode 5 – Cryptographically secured version of Mode S and ADS-B GPS position.

In flight school I was taught that Mode C was an improvement on Mode A through the addition of data fields that provide aircraft altitude. I extrapolated that to Mode S being an improvement on Mode C through the addition of yet other data fields and making the interrogation targeted at a specific aircraft, and Mode ES/ADS-B being a further improvement through automatic broadcasting.

I have, however, never seen Mode S referred to as Mode 3/S. So, is Mode S a variation within the Mode 3 "protocol" or is it a stand-alone mode outside of the five IFF modes?

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Modes 3/A and C are very similar, but the interrogation pulses are timed differently and the responses mean different things.

If you send a mode 3/A pulse, you get a mode 3/A reply (or nothing). The response is a 12-bit number called the squawk code, which is conventionally written as four octal digits, plus an optional ident bit.

If you send a mode C pulse, you get a mode C reply (or nothing). The response is a 12-bit redundant encoding of the pressure altitude between -1200 and +126700 feet, in 100 foot increments.

Note that mode C does not include a squawk code; it is only the altitude. In practice, though, every mode C radar and transponder is also capable of mode 3/A. Likewise, every "mode C" radar always sends both types of pulses and internally combines the two responses, so it looks like a single system. (TCAS only sends mode C pulses since it doesn't care about squawk codes.)

Mode S is almost a completely different system. It includes selective addressing, data frames, message types, and much longer messages (56 bits). It also uses a different, more efficient encoding to allow moving all that extra data without losing efficiency. Of course, for backward compatibility, all mode S transponders can also do mode 3/A and C using the old 12-bit system if needed. Likewise, Mode S radars also send mode 3/A and C pulses, but they're modified very slightly so that mode S transponders will know to ignore them.

1090 Extended Squitter is a relatively minor extension of mode S. A "squitter" means a transponder that sends replies without being interrogated, and "extended" means it added some new message types with 112 bits per message (to hold ADSB data) instead of the previous 56. That's it.

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  • $\begingroup$ Very nice explanation. So are More C and S still grouped under Mode 3? $\endgroup$ – nodapic Dec 24 '19 at 23:26
  • $\begingroup$ @nodapic Not in my view. $\endgroup$ – StephenS Dec 25 '19 at 3:18
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In simplistic terms you can view Mode S as an "enhancement" of the Mode 3/C/A. Mode S changed the way transponders were interrogated by allowing the radar to interrogate one target at a time. This prevented overlapping returns from multiple closely spaced aircraft that caused positioning errors on the radar operators display. Mode S is the same format as the Mode C format except that an interrogation code (the Mode S hex code) is transmitted for each interrogation, letting the radar know what aircraft transmitted the return.

Source: Wikipeida

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