When using ADS-B are squawk codes still necessary or does the unit provide a discrete code (like a tail number or something) that is recognized by ATC?

  • $\begingroup$ Are you asking if a transponder is still required? The answer to that question is absolutely yes. Having ADS-B out doesn't remove the requirements for certain airspaces where you need a transponder. Sounds redundant but makes sense when you think of all the existing TCAS systems that rely on it. $\endgroup$
    – Ron Beyer
    Apr 7, 2016 at 19:03
  • $\begingroup$ @RonBeyer I may be wrong about this but I thought the goal in the US was for ADS-B to replace mode c/s in the near future. But the question is more about if the system uses squawk codes. $\endgroup$
    – TomMcW
    Apr 7, 2016 at 19:06
  • $\begingroup$ ADS-B uses a unique code assigned to the airframe for it's life. Is that what you are asking about? $\endgroup$
    – Simon
    Apr 7, 2016 at 19:12
  • $\begingroup$ @Simon Does the atc system use that code to recognize you? $\endgroup$
    – TomMcW
    Apr 7, 2016 at 19:16
  • $\begingroup$ ADS-B does not use squawk codes, and squawk codes are not going away anytime soon, see FAA ADS-B FAQ, #21 - Will Mode-C Transponders be required indefinitely? Its possible in the future that ADS-B will remove the transponder requirement, but not in the near future. So the system will continue to use transponder codes as far as I'm aware. The FAQ page I linked has some good info. $\endgroup$
    – Ron Beyer
    Apr 7, 2016 at 19:20

1 Answer 1


Yes, squawk codes are still used and required with ADS-B.
It's possible that in the future this may change, but because of the bifurcated implementation the FAA has chosen for ADS-B I don't think it's likely.

ADS-B operates by giving additional information beyond the Mode A (squawk) code - this is either encoded on the 1090MHz transponder frequency (Mode S "Extended Squitter" - what most of the world uses) or sent on a separate frequency (the 987MHz "Universal Access Transceiver" frequency, which is a US/FAA only thing mainly intended for light GA aircraft).

The Mode S and Extended Squitter data includes things like the aircraft's unique Mode S address (a 24-bit number assigned to the aircraft by its national registry, in the US this is linked to the N number), and may also provide the tail number or flight number. The ATC radar system will accept and display this information for the controller.
At the moment the Mode S/ES data is not used as a substitute for the Mode A transponder code, but because Mode S data is coming through the ATC secondary radar system it's all immediately available with the secondary radar return the same way a Mode A squawk code is, and it's possible that the FAA may use the Mode S ID in place of the Mode A squawk code for these aircraft in the future.

The 987MHz UAT transmissions also include the Mode S address (because it's a useful unique identifier for a given aircraft) and flight ID/tail number can be sent as well. Information from UAT transmitters is received through separate ground stations and recombined with the radar return in the ATC radar back-end.
There's some magic to that recombining which is out of my depth, but my limited understanding is that since the UAT data isn't directly correlated with a radar return squawk codes are part of how the two are matched up (the radar computer knows your N number by your discrete squawk code, the UAT ground station knows it from your Mode S ID - if they match and the position data is within reason the ADS-B information can be associated with your radar data block).

  • $\begingroup$ The ADS-B data format actually includes a provision for broadcasting the Mode A code, so no fanciful correlation with SSR data is necessary to display it. $\endgroup$ Apr 7, 2016 at 23:17
  • $\begingroup$ @HenningMakholm Do you have a reference for that? I thought there was something for it (it makes sense!) but I couldn't find anything in a quick search, and all the stuff I've been looking at is 1090ES so I'm not too familiar with the details of ADS-B Out on UAT hardware. $\endgroup$
    – voretaq7
    Apr 8, 2016 at 7:56
  • $\begingroup$ What I looked at was DO-260B, which is 1090ES, but I (perhaps wrongly) assumed that it's the same message formats that are being tunneled over the UAT channel. (For 1090ES, the draft of DO-260B that I was able to read without paying through the nose adds the Mode A code to the DF=17, Type=28, Subtype=1 message). $\endgroup$ Apr 8, 2016 at 9:12
  • $\begingroup$ And now I discover that UAT actually uses a completely different message format, so I actually don't know what I thought I knew. :-( $\endgroup$ Apr 8, 2016 at 9:43

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