Do non-ADSB capable Mode-S transponders transmit Flight ID?


I am wondering whether non-ADSB capable MODE-S transponders transmit Flight ID information as well as the ICAO 24-bit code?

MLAT DF18 CF=2 messages from Piaware often include flight ID info, which Planeplotter can use to look up route information if they are other than the aircraft tail number. Does such info come from the transponder or other sources available to Flightaware?




Yes, although the consistency/quality isn’t as good due to the lack of historical use in many FIRs.


For Mode S the ident is carried in a Comm-B response to a BDS2,0 interrogation from a SSR (you do need those interrogations before you see responses, but it’s a very common interrogation):

FWIW DF=18 messages generated by mlat-client don’t include an ident. If you’re feeding both the raw data & mlat output into Planeplotter, then it’s probably just picking up on the Comm-B responses. Or you may be seeing real TIS-B messages rather than mlat-client-generated synthetic messages.

As mduell says, the quality of the idents does vary…


Interesting. If I point Planepoltter to port 30105 on my Pi, which should be MLAT-only messages from the mlat-client, some of the DF18 messages as seen by Planeplotter DO contain an ident! This can’t be coming from anywhere but Flightaware, as the receiver and planeplotter sharing are excluded (I have native Planeplotter sharing disabled).




Strange, there is no code that generates them:

(callsign is unused, and isn’t set in the FA case anyway)


The flight id must have been from residual planeplotter data from the combined feeds after I switched ports.

After running 30 minutes or so connected to port 30105, I see no flight id in the MLAT data.

Thanks for the response!




It appears the flight ID is not sent in a Mode-S all-call DF11 response, only the ICAO code.

A short-form DF 5 Surveillance Identity Response does seem to include the flight ID, but not sure.
EDIT: I checked and only the Squawk (encoded in the 13-bit ID field as 4 octal digits) and ICAO address/parity in the AP field are sent in DF5 - No alphanumeric pilot-entered flight ID.

The long-form DF21 Comm B Identity Response definitely has the flight id, as you mentioned.

Around Chicago, the rate of DF21 messages is very, very low, at least as received by my station, which has an excellent antenna and receiver system. DF5s are quite common though.

I seem to get about 20% of the Piaware MLAT flights populated with flight number from the local receiver in Planeplotter, but it varies wildly.

I doubt I ever pick up TIS-B broadcasts directly, but I do occasionally receive them when propagation to a ground station is favorable.




Yeah, this is all correct.

DF11 carries address only.
DF4 carries altitude (and address xored into CRC)
DF5 carries squawk (and address xored into CRC)
DF20 carries altitude + Comm-B reply (and address xored into CRC)
DF21 carries squawk + Comm-B reply (and address xored into CRC)

One type of Comm-B reply will carry the ident.

DF21 (and DF20) are triggered by a SSR interrogation which asks for a particular transponder register (e.g. the BDS2,0 register carries the ident). What sort of replies you hear is very dependent on what nearby SSRs are asking for - it varies regionally, e.g. in Europe SSRs tend to interrogate for a lot of extra data (e.g. the EHS registers that carry airspeed/heading and autopilot data) but it’s less common to see those interrogated in the US.


So, DF20s could theoretically also contain flight number as well, as both DF20 and DF21 have identical Comm-B fields (if the appropriate Comm-B message type is used)? I suspect that’s what is happening as I see about a dozen flight numbers pop up on MLAT targets in 30 seconds, but the DF21 message counter doesn’t budge.

To give a feel, after 112,597,062 total messages only 2,915 are DF21 and 110,678 are DF20, That’s in 2 days, 2 hours and 30 minutes of receiver operation. Those numbers include combined receiver and Piaware MLAT messages.




An additional observation. Even though DF20 messages are called “Comm B Altitude Replies” and DF21s “Comm B Identity Replies”, either can convey pilot-entered flight ID information in the Comm B data block of the message. Both message types convey the ICAO hex address. DF20s convey altitude as well in a dedicated field. DF21s convey the traditional Mode-A/C squawk in a dedicated field, hence the message name including the term “Identity”.

I can correlate the appearance of flight identifier data for Flighaware MLAT tracks in planeplotter to the arrival of DF20 messages in modesmixer2s web interface statistics page. The arrival rate is slow enough to make the visual correlation.

As mentioned, other types of data besides flight identity can be encoded in the Comm B data block of either message. In the Chicago area most DF20s seem to contain the flight ID. DF21s are exceptionally rare.