My Cessna 150F has never had ADS-B out. It has had the same N number since 1967. But yet, FlightRadar24 shows a flight a few weeks ago, perfectly tracing the coast north of San Diego, 2,000 miles from my home airport in Kentucky.

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How is it possible for ADS-B tracks to show up under the wrong N number?

It is not one blip of data with a bad ID…it’s hundreds of data points over 50 minutes with no discernible glitches at all.

I searched the web and this StackExchange for several variations of “ads-b wrong plane” with no results coming close to matching. Plenty of discussion of poor position accuracy and other anomalies…but none about wrong N numbers that I found.

UPDATE: I found the name for this, which yields better search results: "Call sign mismatch" or its abbreviation, CSMM.

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    $\begingroup$ At a guess I'd say that the ADS-B unit in the aircraft you saw hasn't been configured properly (perhaps it's a new installation). It shouldn't happen, but people are fallible. There's a good article on I Fly America that discusses it. $\endgroup$ Aug 30 '21 at 3:43
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    $\begingroup$ I assume you didn't leave your keys in the ignition? $\endgroup$
    – Richard
    Aug 30 '21 at 15:31

FR24 may not actually receive any N-number from the aircraft. It's not unlikely that it only receives the Mode-S transponder code and cross-references that with a database to your N-number.

So the real question is, how is it possible that another plane broadcasts the same Mode-S code? There's really only two options here:

  • a receiver was set up incorrectly. FR24 is based on a network of volunteers, so perhaps one had a faulty setup. Or there was a bug at FR24.
  • the transponder was configured incorrectly. Somebody has to put in the code in the transponder. Perhaps they made a mistake. Perhaps the transponder had a fault. I think this is the more likely option.

Seeing as no further flights are logged on FR24 since then with your N-number, either way the problem seems to be rectified.

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    $\begingroup$ I would not exclude the possibility of a bug in any of the software involved in the FR24 network. $\endgroup$
    – bogl
    Aug 30 '21 at 8:39
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    $\begingroup$ An incorrect receiver setup is unlikely to be the cause. I can't think of any probable setup mistake that could cause this, since the 24 bit address is so fundamental to the way ADS-B works. I would rule it out. The transponder being configured incorrectly, or having a hardware issue (in case of hard-wired 24 bits), is a common cause of 24 bit confusion. $\endgroup$
    – DeltaLima
    Aug 30 '21 at 9:02
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    $\begingroup$ @RalphJ Mode-S is not the same as ADS-B $\endgroup$
    – Sanchises
    Aug 30 '21 at 13:18
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    $\begingroup$ I used to develop air traffic software. I remember seeing a case where 2 aircraft at the same airport were broadcasting the same Mode-S at the same time. The explanation I was given was it was probably degradation of the memory in the transponder, likely a single bit flipped from 0 to 1 or vice versa. $\endgroup$
    – rtaft
    Aug 30 '21 at 14:02
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    $\begingroup$ @RalphJ The FAA has an alhorithm that assigns 24 bit addresses based on the registration. If you know the algorithm you can relate any US registration to its 24 bit address and vice versa. I am pretty sure that FR24 uses this algorithm to convert US 24 bit addresses that are not in their database to the registration nr. $\endgroup$
    – DeltaLima
    Aug 30 '21 at 14:18

Flight tracking sites like FR24 use both ADSB data and official radar tracks.

The first possibility is that someone misconfigured their ADSB to use your hex code/tail number. This seems unlikely since setup is normally done by an avionics technician, but it’s not impossible. It becomes more likely if the plane is used for operations like Angel Flight, where the pilot has to frequently change their own ADSB setup due to using mission callsigns, which is unfortunately not a user-friendly operation with most systems.

The other possibility is that the pilot called ATC for flight following, and the controller heard or typed the wrong tail number when creating their beacon code. Controllers are human too and do make mistakes occasionally.

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    $\begingroup$ I've noticed this often locally where I'll see an ADS-B track with the correct tail number for an airplane I recognize and a duplicate one (presumably from official radar tracks) that has a smaller "typo" version of the registration. $\endgroup$
    – Jeff B
    Sep 2 '21 at 4:30

ADS-B codes in the US are calculated based on the N-number. If you ever get a Mode S transponder for your airplane, your ADS-B transponder code will be "AC534B". That same calculation can be run in reverse. If someone sets the code "AC534B" in their transponder, they'll show up on FR24 and other flight-tracking sites as "N8933S".

ADS-B transmissions can also include an optional "callsign" field up to eight characters long. Flight-tracking sites tend to display this in preference to the tail number because knowing that SWA 1691 is on final approach to Los Angeles is more useful than knowing that N7879A is on final approach. If someone set their callsign to "N8933S", it will be displayed instead of the actual tail number.

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    $\begingroup$ Radarbox is willing to say that the track did belong to a transponder with the code AC534B, not a callsign, and that they received the data from FAA SWIM, not a private ADS-B receiver. $\endgroup$
    – hobbs
    Aug 31 '21 at 4:37

It seems many ADS-B transponders lets the pilot reconfigure their callsign in the unit: https://www.aopa.org/news-and-media/all-news/2017/october/pilot/adsb-changing-flight-id

This is news to me. I was not aware this is pilot-configurable.

This is typically done for a special kind of flight, such as Medivac, Angel Flights, or scheduled commercial flights. This allows the aircraft ID in the ADS-B data stream to match the ID filed on a flight plan. In fact, it seems that it is required that they match.


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