8
$\begingroup$

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?

$\endgroup$
  • $\begingroup$ 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 '16 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 '16 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 '16 at 19:12
  • $\begingroup$ @Simon Does the atc system use that code to recognize you? $\endgroup$ – TomMcW Apr 7 '16 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 '16 at 19:20
6
$\begingroup$

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).

| improve this answer | |
$\endgroup$
  • $\begingroup$ 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. $\endgroup$ – hmakholm left over Monica Apr 7 '16 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 '16 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). $\endgroup$ – hmakholm left over Monica Apr 8 '16 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. :-( $\endgroup$ – hmakholm left over Monica Apr 8 '16 at 9:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.