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"Confirm you are not a flight of two."

DanH

Legacy Member
Mentor
I think we discussed this some time ago, but don't recall if there was an explanation.

Sometimes I contact an approach for a VFR arrival, they give me a squawk, and then come back with a request to confirm I do not have a formation partner. The scope is displaying two targets close together, when I am in fact alone.

System is a common combination, a Garmin 327 and a GDL82 ADS-B, running in anonymous mode.

Anyone know the reason?
 
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Antenna echo?

Anything strange about your antenna placement? Reflection off those nice gear legs?
 
On occasion I will see a "double image" of one target on my Skyview Classic screen. Sometimes the image is of my aircraft and sometimes the image is of another aircraft. It is my understanding that such images are an artifice of the ADSB system and are associated with issues within the FAA's ground network.
 
ADS-B dual images....etc.....

On occasion I will see a "double image" of one target on my Skyview Classic screen. It is my understanding that such images are an artifice of the ADSB system and are associated with issues within the FAA's ground network.

I will get these occasionally was well. It surprises me (although nothing should) that ATC would ask as I would think they see them often as well, likely NOT usually a flight of two. It may be something to do with the ground towers. It may be that the ADS-B broadcast is on two frequencies and they are both being picked up. An interesting quirk. One of, I think, many. My favorite is a target that suddenly shows up as a near-by problem, the Fish Finder turns red as in NEAR and it suddenly disappears! Um....what? Or the targets that are close but invisible. Another favorite...... It is a new system and things need to get ironed out......:rolleyes:
 
Certainly can see the possibility that the ground system isn’t properly reconciling the minor difference between your ATCRBS reply, and UAT output position.
 
To Dan's original question -

Target tracking software on the FAA's TRACON and Center computers runs a "fusion" algorithm to combine individual returns from primary radar, Mode 3/S ATCRBS transponder and ADS-B to (hopefully) produce one target for the controller. If there's common data between the Mode 3/S and ADS-B returns, like ICAO address or Mode 3 code, correlation is easy. Since you're running in UAT anonymous mode, there's no discrete Mode 3 code or ICAO address to correlate between the returns. Your ADS-B position is far, far more accurate than primary radar or ATCRBS, so in the absence of Mode 3/ICAO correlation there may be some times when the fusion software isn't sufficiently sure that your ATCRBS and ADS-B are from the same aircraft, hence the target split.

Carl, there's definitely latency in the system. ATC approach radars rotate at ~12 rpm so there's the possibility of up to a 5ish second latency in Mode 3/C report changes.

HTH

Dave
 
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I can verify what Dave is saying by experience. I now leave my ADS-B abled and have had a problem since. There is also a Garmin update that you can do to the GDL82 that is supposed to prevent "ghosting", but I have not done it.
 
Haha, I did that a few weeks ago. Albeit not in a 747, but in a work plane nonetheless. I'll call it "delusions of more fun".
 
I can verify what Dave is saying by experience. I now leave my ADS-B abled and have had a problem since. There is also a Garmin update that you can do to the GDL82 that is supposed to prevent "ghosting", but I have not done it.

Should have read "NOT HAD A PROBLEM SINCE"
 
They probably ask you to confirm because there are a lot more people flying formation in RV's than in Cessnas...
 
To Dan's original question -

Target tracking software on the FAA's approach radar computers runs a "fusion" algorithm to combine individual returns from primary radar, Mode C/S transponder and ADS-B transponder to produce (hopefully) one target for the controller. If there's common data between the Mode C/S and ADS-B returns, like ICAO address or Mode C code, correlation is easy. Since you're running in UAT anonymous mode there's no discrete Mode C code or ICAO address to correlate between the returns. Your ADS-B position is far, far more accurate than primary radar or Mode C so in the absence of Mode C/ICAO correlation, there may be some times when the fusion software isn't sufficiently sure that your ATCRBS and ADS-B are from the same aircraft, hence the target split.

Thanks Dave, that makes sense. Sounds like I need to conduct an experiment next time it happens, by going back to 1200, turning the anonymous mode off, then getting a new squawk from the controller. If it then comes up with one aircraft instead of two, mystery solved.

After a bit more thought, perhaps I just need to wait 10 minutes and then ask the controller if the phantom return went away on its own. Why? When I switch from 1200 to an assigned squawk, anonymous mode should shut down automatically; the system should begin to transmit a real ICAO address. The ATC system should then be able to combine returns, given a little time.
 
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System is a common combination, a Garmin 327 and a GDL82 ADS-B, running in anonymous mode.

Let me know when you're ready to step up to a 1090ES solution with in/out all in one box (345) and be done with these UAT quirks. :D
 
Let me know when you're ready to step up to a 1090ES solution with in/out all in one box (345) and be done with these UAT quirks. :D

The quirk appears to relate to using anonymous mode, unavailable with 1090ES. Not advantageous.
 
ATC approach radars rotate at 12 rpm so there's the possibility of up to a 5 second latency in Mode C report changes.

Dave
And DOD radars, used by FAA center controllers, rotate at 10 rpms so an even greater possibility of latency error.

The quirk appears to relate to using anonymous mode, unavailable with 1090ES. Not advantageous.
Guess you just have to "pick your poison" and decide which is best for you.

:cool:
 
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I've had "Traffic" announced in my Dynon Skyview system when I have changed transponder codes. For example, passing over LAX in the special flight rules corridor, you are supposed to go from squawking 1200 to 1201. Usually, shortly after the code is swapped, I'll get a ghost aircraft appear where there is just me. I've also noticed this if you make a sudden course change. I think the latest SW update for Dynon has fixed some of these issues.
 
My -8 has an Appareo Stratus transponder with a Stratus receiver. On the last couple of flights, ForeFlight has called traffic on my 6 o’clock, same altitude, but only momentarily. I had not made any changes to the transponder code, etc. Any thoughts?
 
Flying today, and MGM approach wasn't busy, so experiment time.

Yes, it appears to be a ATC system latency issue, combined with an inability to tie an anonymous return to a subsequent or previous coded return.

Squawking 1200 in anonymous, then punching in an assigned squawk, results in two returns on the controller's screen, and a collision alarm, which makes his heart jump for a moment. The anonymous VFR return disappears after 10 seconds or so, leaving only the assigned squawk with N number, and a puzzled controller.

Changing back to VFR/anonymous does the same thing in reverse...two returns, an alarm, then a disappearing assigned squawk after 20 seconds or so.

Today's operator was surprised to learn ADS-B could be anonymous. He didn't seem real happy to hear it. Suddenly getting two returns with a collision warning is a PITA for him, so I can understand his feelings. Assuming VFR cruise (no flight following), it appears the cure is to disable anonymous mode in 1200 before contacting the approach facility and accepting an assigned squawk. That will be the next test.

Chops to Dave!
 
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Squawking 1200 in anonymous, then punching in an assigned squawk, results in two returns on the controller's screen, and a collision alarm, which makes his heart jump for a moment. The anonymous VFR return disappears after 10 seconds or so, leaving only the assigned squawk with N number, and a puzzled controller.
Very cool Dan, I was hoping you'd experiment and I'm glad to see your results. Your UAT transponder transmits about once a second and if it takes MGM's radar and tracking software a couple of sweeps to process your Mode 3 transponder reply - which looks like the case here - then a track split is no surprise.

Side note - if anyone is interested in hearing from ATC controllers what they see from ADS-B on their displays, check out episodes 108 through 111 of the Opposing Bases podcast at https://www.opposingbases.com/ which is produced by two controllers who are also active pilots. It's one of my favorite aviation podcasts.

HTH

Dave
 
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My -8 has an Appareo Stratus transponder with a Stratus receiver. On the last couple of flights, ForeFlight has called traffic on my 6 o’clock, same altitude, but only momentarily. I had not made any changes to the transponder code, etc. Any thoughts?
I'm not super familiar with how Stratus boxes identify ownship ADS-B replies, but my guess is they don't. It's likely up to ForeFlight software to decide that a really close ADS-B target is in fact your airplane and occasionally the software gets it wrong. It's tuned to identify your ownship ADS-B reply but also has to account for the possibility that it's *not* you and there's really another airplane out there. That sorting process isn't perfect, especially if ForeFlight doesn't know anything about your transponder like ICAO address, etc. Maybe it does, but I'm not familiar with how the ID process works.

My $0.02, HTH.

Dave
 
I'm not super familiar with how Stratus boxes identify ownship ADS-B replies, but my guess is they don't.
On the StratuX (not StratuS) boxes, you can enter your hex code in the settings so it can filter that as an ownship. The data never gets to Foreflight, FltPlanGo, etc. as it's filtered right in the Stratux system.
 
I'm not super familiar with how Stratus boxes identify ownship ADS-B replies, but my guess is they don't. It's likely up to ForeFlight software to decide that a really close ADS-B target is in fact your airplane and occasionally the software gets it wrong. It's tuned to identify your ownship ADS-B reply but also has to account for the possibility that it's *not* you and there's really another airplane out there. That sorting process isn't perfect, especially if ForeFlight doesn't know anything about your transponder like ICAO address, etc. Maybe it does, but I'm not familiar with how the ID process works.
Some good information on ownship ghosts in ForeFlight -

https://support.foreflight.com/hc/e...-mirroring-me-while-using-my-Stratus-receiver

HTH

Dave
 
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