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Thread: Garmin GDL 82 ADS-B Problem I can't explain

  1. #1

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    Garmin GDL 82 ADS-B Problem I can't explain

    So here are the facts:

    !. Don't see my aircraft on flight aware
    2. I could follow the aircraft prior to correcting my N number at the request of the FAA, must match the registration and not the N number.
    3. So far no other aircraft can see me who as a 39 or 52,or any other 1090, or UAT in
    4. Performed 2 separate Public ADS-B Performance reports in a Bravo airspace with a squawk code and I received two passes Passes in the form of PDF.s for the FAA so as far as they are concerned its working perfectly
    5. Performed multiple flights inside the 30 mile vale on separate different days without getting a squawk code and when request a report I get the following email message:


    Your Performance Report request for Tail Number N427SC on 5/29/2021 failed to generate a result.

    If you believe this is an error, please respond to 9-AWA-AFS-300-ADSB-AvionicsCheck@faa.gov and include the following information:
    1. Aircraft registration number and flight identification code,
    2. Flight date and time,
    3. Make/model of ADS-B transmitter and GPS, and
    4. Any ADS-B avionics operating abnormalities observed or reported during the associated flight.

    I have made a request for per the email and waiting.

    6. I have run the Garmin install tool diagnostic test and ground test at least 5 times, all test perfect, over and over

    Does anybody know what is or isn't happening?

    The totally odd thing is it worked perfectly for a year and the only change was to my N number which was performed using the Garmin Install tool for the GDL and a laptop . I have NX427SC per FAA regulations on my aircraft, however the FAA will not allow registration of NX so I had to register the N number as N427SC, X is for experimental and is permitted instead of 2 inch " Experimental" lettering somewhere inside the cockpit.

  2. #2
    aviationinfo's Avatar
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    I have the GDL-82 and it has always worked perfectly.

    Do you have the Anonymous feature installed in your setup? If the switch is enabled in your software and the physical switch selected, it should keep your N# from showing up on the public websites like FlightAware and FlightRadar24 yet not from the FAA, by design. Could your switch have failed, or something else in the software setup feature set incorrectly by mistake during or before/after running the diagnostic so many times? Just a thought.
    Last edited by aviationinfo; 05-31-2021 at 11:49 PM.
    Aviationinfo

  3. #3
    fancypants's Avatar
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    Do you have a Garmin ADS-B in device (GDL 52 or 39) and a phone you can run Garmin Pilot on? There's an ADS-B compliance tool you can use within Garmin Pilot when paired with a Garmin ADS-B in device. It's useful in cases like this. It will show you basically everything that your ADS-B out unit is broadcasting. You can accomplish the same thing with a Stratux device, but it's more complicated.

    When your GDL-82 is in anonymous mode, you are not transmitting your assigned ICAO address or your N-number. All anyone sees, including the FAA, is a randomized temporary ICAO address. If you try to pull a PAPR report for an anonymous mode flight you won't get a result. This all fits with aviationinfo's suspicion that you've got anonymous mode switched on.

    If that's not it, your GDL 82 might be in a weird state somehow. Your ICAO address should be A51733 to match your N number. I'd reset the configuration to factory and start over.

  4. #4

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    Thank you for your comments. My 82 has worked perfect as well , right up until the change. The ICAO address is correct and remember it passes all ground checks when hooked to a laptop, all . It does have a anonymous switch which also passes all ground checks when hooked to a lap top including turning in on and off. I responded to the FAA request per there instructions and they have no record. But when I get a squawk ID in Bravo the from Tampa Approach control I become visible and I have in my possession 2 Passed Reports so again according to the FAA my aircraft is compliant and approved. Non of this makes any sense.
    Starting over and re configuring the 82 is the only thing I haven't performed. I'm Hoping Garmin can shed some light on this problem.

  5. #5
    hotrod180's Avatar
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    I agree, it sounds like some sort of anonymous issue.
    I have gotten a good PAPR when my Tailbeacon was in anonymous mode,
    but on other occasions I get a "failed to generate a report".
    I'd say leave it out of / disable anonymous mode, and try it all again.

    I think the NX vs N tail number is a moot point--
    they might allow NX on the airplane, but what does your airworthiness and/or registration say?
    Cessna Skywagon-- accept no substitute!

  6. #6

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    Registration does not include the “X”, that is only permitted to be displayed per 14CFR45.22. I’m surprised that the GDL-82 let you input an NX as the registration. That should have caused a red flag with the ICAO code. If it were me, I think I would go back to square 1 and change the N# and ICAO codes to something totally different,” he put


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  7. #7

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    square 1 is next. The 82 took the NX number fine and worked till the changed. Registered the plane as N427SC, adsb sent out NX , this is why the FAA contacted me , to correct the problem.

  8. #8

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    Quote Originally Posted by fancypants View Post

    When your GDL-82 is in anonymous mode, you are not transmitting your assigned ICAO address or your N-number. All anyone sees, including the FAA, is a randomized temporary ICAO address.
    Can you please quote some reference to support that. It is my understanding that anonymous mode still transmits the assigned ICAO hex code. Using an ICAO hex code different from that assigned in the registration requires a PIA application:

    "4. FAA Private ICAO Address (PIA) ProgramPIA is another program offered by the FAA that provides the highest level of privacy currently available. Essentially, the PIA program allows aircraft owners to get a special, non-published ICAO address, which is linked to their aircraft. Think of it as a “Virtual Tail number” for your aircraft that you program into your ADS-B unit. It does not replace your aircraft’s tail number; it’s an unpublished address that the FAA links to the N-Number painted on your aircraft (and associated ICAO number) in its private PIA database. You can still use your public ICAO/N-Number anytime you want, as long as you re-configure your ADS-B unit to send out the public number. In fact, you’ll need to do that anytime that you leave U.S. airspace because private ICAO addresses may only be used in the U.S."
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  9. #9
    fancypants's Avatar
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    The GDL82 configuration tool accepts any alphanumeric tail number and generates the permanent ICAO address from that. If I remember correctly, it doesn't do any validation aside from ensuring the ICAO address is not 000000 or FFFFFF.

    If you can get the Garmin ADS-B compliance tool running in the Pilot app, I'd be curious to know what the GDL82 is outputting right now in its broken state.

  10. #10
    fancypants's Avatar
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    Quote Originally Posted by frequent_flyer View Post
    Can you please quote some reference to support that. It is my understanding that anonymous mode still transmits the assigned ICAO hex code. Using an ICAO hex code different from that assigned in the registration requires a PIA application:

    "4. FAA Private ICAO Address (PIA) ProgramPIA is another program offered by the FAA that provides the highest level of privacy currently available. Essentially, the PIA program allows aircraft owners to get a special, non-published ICAO address, which is linked to their aircraft. Think of it as a “Virtual Tail number” for your aircraft that you program into your ADS-B unit. It does not replace your aircraft’s tail number; it’s an unpublished address that the FAA links to the N-Number painted on your aircraft (and associated ICAO number) in its private PIA database. You can still use your public ICAO/N-Number anytime you want, as long as you re-configure your ADS-B unit to send out the public number. In fact, you’ll need to do that anytime that you leave U.S. airspace because private ICAO addresses may only be used in the U.S."
    I don't have a reference handy, but the specification describes the content of ADS-B out packets in great detail, including the algorithm used to generate the anonymized ICAO codes. I've verified the operation of the GDL82's anonymous mode using raw packet dumps obtained from a modified Stratux setup. With a GDL82 running the most recent firmware, anonymous mode does not output the permanent ICAO address at any time. I investigated this over a year ago, so it might take me a bit to dig up my notes.

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    Quote Originally Posted by fancypants View Post
    I don't have a reference handy, but the specification describes the content of ADS-B out packets in great detail, including the algorithm used to generate the anonymized ICAO codes. I've verified the operation of the GDL82's anonymous mode using raw packet dumps obtained from a modified Stratux setup. With a GDL82 running the most recent firmware, anonymous mode does not output the permanent ICAO address at any time. I investigated this over a year ago, so it might take me a bit to dig up my notes.
    Thanks. I did a bit of digging and found this is covered in AC 20-165B para 3.7.2.3. Also found that the PIA program is not applicable to UAT only 1090.

    It appears that anonymous mode does give some protection from public surveillance as long as it's supported by the equipment manufacturer.
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  12. #12
    aviationinfo's Avatar
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    Quote Originally Posted by frequent_flyer View Post
    Thanks. I did a bit of digging and found this is covered in AC 20-165B para 3.7.2.3. Also found that the PIA program is not applicable to UAT only 1090.

    It appears that anonymous mode does give some protection from public surveillance as long as it's supported by the equipment manufacturer.
    Firstly, Fortysix12 looks like you’re on the way to solving the issue.

    Secondly— sorry for the thread creep, but I was under the impression that the Anonymous mode does not affect the FAA’s ability to see the aircraft info. Is that correct? I leave mine in anonymous most of the time and have never heard anything.
    Aviationinfo

  13. #13

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    Here is one of my pass reports. Perfect except as noted.
    Attached Files Attached Files

  14. #14
    fancypants's Avatar
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    Quote Originally Posted by aviationinfo View Post
    Firstly, Fortysix12 looks like you’re on the way to solving the issue.

    Secondly— sorry for the thread creep, but I was under the impression that the Anonymous mode does not affect the FAA’s ability to see the aircraft info. Is that correct? I leave mine in anonymous most of the time and have never heard anything.
    I've seen people say that FAA can see who you are in anonymous mode, but my own experimentation and interpretation of the documentation seems to disagree with that statement.

    Assuming you're:
    * using a uAvionix or GDL82 UAT 978 ADS-B unit
    * running current firmware
    * operating in anonymous mode
    then you are not broadcasting your permanent ICAO address. You are broadcasting a temporary ICAO address derived from 1) your permanent ICAO address and 2) a concatenation of some of the least significant bits of your latitude and longitude at startup for randomization. Could it be more random? Yes, definitely. But no one (not even FAA) can see data that isn't there.

    A few caveats, though:
    1) The first packet transmit from a GDL82 in anonymous mode sometimes contains all of the information one would need to reverse the algorithm applied to generate the temporary ICAO address and reconstruct the permanent ICAO address. So if one were to capture the very first transmission at startup, one might be able to identify the unit's N number. I haven't tested this on a uAvionix unit, but I assume it's the same situation. This is a bummer. Those first packets usually go out in between hangar rows, not in the line of sight of FAA towers or FlightAware receivers, so the risk of exposure is lessened somewhat.
    2) A more complex analysis over time might yield some guesses as to the unit's true identity. Just an idea, not going to expound on it here.
    3) There are other methods one could conceivably use, like RF fingerprinting, to identify a particular transmitter. Outside the scope of this discussion.

    I'm not aware of anyone (neither FAA nor commercial entity) actually applying the techniques above to unmask an anonymous ADS-B user. I think the situation would have to be pretty extraordinary for that to happen. Doesn't mean it hasn't happened or won't happen in the future. I think more transparency in enforcement actions relying on ADS-B data would be a very good thing.

    This is my understanding of the current situation with anonymous mode. I'd be happy to be proven wrong and learn something new.

  15. #15

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    I'm going to reconfigure and test, if that is a failure, then I'm going to do an exchange with Garmin at a price of course, even though its only 2 months out of warranty. Cost is $475. But let me iterate a couple of points.

    1. I am able to generate the report only when I"m under a positive radar control with a Squawk code.
    2. I am invisible to everyone if I"m not with a code and unable to generate a report from the FAA
    3. This is all performed with the anonymous mode off .

    If you interested further , look up my N number on Flightaware. N427SC, I flew to Melrose under positive radar control with Jax,
    the second one minute report southwest of Ocala is without a squawk code and I can't explain it.
    Keep you posted and thank you for all the comments and suggestions. I will get to the bottom of it, just running out of time before I"m off to Idaho, on the 14th, actually Montana first stop.
    Cheers.

  16. #16
    spinner2's Avatar
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    Quote Originally Posted by Fortysix12 View Post
    I'm going to reconfigure and test, if that is a failure, then I'm going to do an exchange with Garmin at a price of course, even though its only 2 months out of warranty. Cost is $475. But let me iterate a couple of points.

    1. I am able to generate the report only when I"m under a positive radar control with a Squawk code.
    2. I am invisible to everyone if I"m not with a code and unable to generate a report from the FAA
    3. This is all performed with the anonymous mode off .
    .
    I have a GDL82 in my Cub also with a Becker transponder. No issues in the 2 years or so I’ve had it.

    Readng your first item above made wonder if the problem could be with your transponder? Could it not be actually transmitting correctly when 1200 is dialed into it? Anonymous mode only works when the transponder is emitting 1200.
    "Fast is fine, but accuracy is everything." Wyatt Earp

  17. #17

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    Quote Originally Posted by spinner2 View Post
    I have a GDL82 in my Cub also with a Becker transponder. No issues in the 2 years or so I’ve had it.

    Readng your first item above made wonder if the problem could be with your transponder? Could it not be actually transmitting correctly when 1200 is dialed into it? Anonymous mode only works when the transponder is emitting 1200.

    Interesting, and that is what Garmin is suggesting to. There could be a relationship. The totally odd thing that I can't shake is that Inside a vale, invisible to everyone with out a squawk code, totally visible to ATC with a squawk code but invisible to aircraft.

    That's an easy test. put in my own squawk for just a moment and see what happens. That is a good idea.

  18. #18
    JimParker256's Avatar
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    Quote Originally Posted by Fortysix12 View Post
    Here is one of my pass reports. Perfect except as noted.
    The footnote on your PAPR clearly states "the above FP ID Mismatch field can be disregarded when no flight plan was filed for the associated flight." So, unless you were on a flight plan at the time, that is indeed a "passing" PAPR.

    On the other hand, if you were on a flight plan, there is still an issue.
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  19. #19
    hotrod180's Avatar
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    Quote Originally Posted by fancypants View Post
    I've seen people say that FAA can see who you are in anonymous mode, but my own experimentation and interpretation of the documentation seems to disagree with that statement.
    Assuming you're:
    * using a uAvionix or GDL82 UAT 978 ADS-B unit......
    I've gotten PAPR's when I was operating in anonymous mode that didn't generate any results.
    But other anonymous mode flights did generate a report.
    I don't recall if maybe some were in "rule airspace" and others weren't.
    ??
    Cessna Skywagon-- accept no substitute!

  20. #20
    fancypants's Avatar
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    Quote Originally Posted by hotrod180 View Post
    I've gotten PAPR's when I was operating in anonymous mode that didn't generate any results.
    But other anonymous mode flights did generate a report.
    I don't recall if maybe some were in "rule airspace" and others weren't.
    ??
    Kind of impossible to say why that would happen without a complete log of every packet your skybeacon/tailbeacon broadcast during the flight. I suspect the unit was in non-anonymous mode for at least a portion of those flights that you were able to generate a report for. Wild ass guesses as to how that could have happened: lost GPS fix, power cycle due to voltage dip during start, gremlins? Of course there's still the possibility that FAA somehow knows who you are in anonymous mode, but I can't find any hard evidence to support this. Wish I could explain it - either way.

    I've tried to pull PAPR reports for some of my anonymous mode flights and they never find anything associated with my permanent ICAO address. Would be interesting to see if a report can be generated off of the temporary ICAO address.

  21. #21
    hotrod180's Avatar
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    I don't worry too much of I'm truly anonymous to the feds when in anonymous mode.
    I had my tailbeacon set for anon when I first installed it,
    but switched anon mode off on a trip so my wingmen could keep track of me easier.
    Just never bothered to switch it back.
    FWIW I've tried to track myself on FlightAware,
    it comes up that I've "opted out of tracking".
    I don't remember ever doing so, but it sounds like something I'd do so I'm sure I did.
    Cessna Skywagon-- accept no substitute!

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