False ADS-B traffic targets around Everett WA

[PT20J]

For the last month or so I have been randomly getting false targets showing at or near my position and altitude (GTX 345 transponder). Crown Aviation at KPAE has also received numerous reports of this primarily in the area between Everett WA and the San Juan Islands. There are numerous reasons why this can happen, but one is that apparently if the ground software dissociates your aircraft position from its track then it can report your own aircraft as a target. Because this has persisted off and on, I have reported it to the FAA ADSB team.

If anyone else has had this problem and cares to report it, here is the link to the online form (If you have tried and failed to use this form in the past, it was broken but has now been fixed):

https://www.faa.gov/air_traffic/technology/adsb/adsb_reports/

Skip

[Utah20Gflyer]

I routinely get a traffic target with .25 miles and same altitude as me.  Maybe every couple flights.  It lasts around 10 seconds and then disappears.  I have a Uavionix wing beacon for ADSB out and use a Sentry for In.  Its pretty annoying because when the audible traffic alert comes up as quarter mile and same altitude it gets the blood pressure up until I realize it's just myself.  I haven't learned to ignore it yet which is probably a good thing.  

I've heard other pilots locally say the same thing happens to them.  

[PT20J]

  On 11/26/2022 at 12:56 AM, Utah20Gflyer said:

I routinely get a traffic target with .25 miles and same altitude as me.  Maybe every couple flights.  It lasts around 10 seconds and then disappears.  I have a Uavionix wing beacon for ADSB out and use a Sentry for In.  Its pretty annoying because when the audible traffic alert comes up as quarter mile and same altitude it gets the blood pressure up until I realize it's just myself.  I haven't learned to ignore it yet which is probably a good thing.  

I've heard other pilots locally say the same thing happens to them.  

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Cannot hurt to report it. One thing I learned is that the FAA doesn't monitor ADS-B performance very closely. The GBTs (ground-based transceivers) have some gross error detection, but as long as it's not affecting air traffic control, they don't really know of a problem unless someone complains. And I don't think they get many problem reports. When I used the form to report a different problem a while back it was broken and the FAA didn't even know it was broken until I told them about it. When my report did go through, I got a quick response from an engineer that was very helpful.

Skip

[Bolter]

  On 11/26/2022 at 12:12 AM, PT20J said:

For the last month or so I have been randomly getting false targets showing at or near my position and altitude (GTX 345 transponder). Crown Aviation at KPAE has also received numerous reports of this primarily in the area between Everett WA and the San Juan Islands. There are numerous reasons why this can happen, but one is that apparently if the ground software dissociates your aircraft position from its track then it can report your own aircraft as a target. Because this has persisted off and on, I have reported it to the FAA ADSB team.

If anyone else has had this problem and cares to report it, here is the link to the online form (If you have tried and failed to use this form in the past, it was broken but has now been fixed):

https://www.faa.gov/air_traffic/technology/adsb/adsb_reports/

Skip

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I have seen this, but only on 2 flights, since 30-Oct, and both north of PAE (my base), and east of Whidbey.  Cannot recall exactly the locations.  I occasionally see myself on ADS-B in other places, but the persistence of it was different these occasions, so it caught my attention.  That it happened twice in sequential flights, even more interesting.  And now you mention it, so I feel better about my avionics.  I will report this as well.  

-dan

[amillet]

Happened to me near 0S9 last weekend 

 

[A64Pilot]

  On 11/26/2022 at 12:56 AM, Utah20Gflyer said:

I routinely get a traffic target with .25 miles and same altitude as me.  Maybe every couple flights.  It lasts around 10 seconds and then disappears.  I have a Uavionix wing beacon for ADSB out and use a Sentry for In.  Its pretty annoying because when the audible traffic alert comes up as quarter mile and same altitude it gets the blood pressure up until I realize it's just myself.  I haven't learned to ignore it yet which is probably a good thing.  

I've heard other pilots locally say the same thing happens to them.  

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I get that often, right after takeoff from my home field, as I climb it stops

[skykrawler]

The TIS broadcast by the GBT potentially includes traffic from UAT, 1090, and secondary radar.   The secondary radar targets will not have a mode-s ID - these are typically displayed with a different symbol on a CDTI.   It's possible the ground station does not receive your airborne transmission or fails to associate it with the secondary radar target.  The target is added to the TIS-B without an identifier.    The airborne receiver typically notices a target with it's own identifier and does not add it to the display bus.  Receiver/display equipment is a factor.  I'm on the east coast and have had this happen - but it is rare.   Love my traffic display in the panel.

This is one way this can happen.

https://support.foreflight.com/hc/en-us/articles/205482188-What-do-ADS-B-ADS-R-and-TIS-B-refer-to-when-displayed-with-a-traffic-target-

[FlyingDude]

Happened to me a few times in metro Detroit area. I’ll report it. Thanks for the post and raising awareness.

[A64Pilot]

In my case I think it’s ground radar picking me up, but for whatever reason it doesn’t associate the target with me, perhaps latency in the system? So system gets my current location and due to latency where I was a short time ago, but soon associates the two targets, ADSB and Radar together as one.

That’s a pure guess and I am not any kind of ADSB guru, but it seems logical.

On boats it’s not called ADSB but AIS and on boats you could get similar issues when in port where there were ground based receivers receiving your AIS and retransmitting it.

Rare but you could be at anchor down below and the collision alarm sounds, scares the stew out of you

[PT20J]

False targets are inevitable and will happen occasionally during normal operation. In my 2.5 years flying with the GTX 345, they have been extremely rare. However, the issue I reported has been happening off an on, and when it happens, it happens with great frequency. On a recent flight to the San Juan islands, it happened about once every 30 seconds during climb out and cruise for the first half of the flight. 

TIS-B client broadcasts are derived from fused data from multiple secondary radars and alignment issues can create false targets. Here are a couple of excerpts from the SBS Description Document that describe this.

 

There are several possible causes, but the one involving ground based processing (which is likely if multiple aircraft are reporting the same issue in a geographic area) is:

[A64Pilot]

Back to the AIS thing, this is called “Ownship” I’m not sure how but the AIS usually combines ownship transmission from ground station to its own GPS location and knows that’s you and does not report another ship real close by. Each AIS transmission identifies you by code, so maybe that’s how.

What’s different in AIS and ADSB is AIS has no raw RADAR,  the ground stations only know your location because your AIS broadcasts it, it then retransmits your location, I guess just in case other boats don’t have line of sight with you or don’t receive for some reason. I believe due to latency in the system if your moving you get a signal of you are in a slightly different location than you are now, so it sets of an alarm.

I believe ADSB could be doing something similar, ground stations either receiving your ADSB and retransmitting it, but you have moved, so you get the aircraft at 5 O’clock at 1/4 mile.

I believe my false alerts have always been a little behind me, sometimes off to the side, but never ahead, which I think lends credence to my theory. 

Are you guys false alerts always off to the side or behind?

My ADSB in is a Garmin GDL-39

[Igor_U]

Skip

Thanks for posting the link. I had this issue of the false target in last couple of flights. Most notable was yesterday over 0S9 while maneuvering during my BFR.

"Traffic" popped on my ipad and my CFI friend's phone and got our attention. 

[jaylw314]

FWIW, in my ADS-B In device (Stratux), there's a configuration setting to enter your own Mode S code (hex).  This filters out your ownship track which ADS-B will happily rebroadcast out to you.

I don't know if that's a config item for other receivers, though.

[PT20J]

To understand the problem, it helps to review how ADS-B traffic services work.

ADS-B OUT equipped aircraft transmit their ID and state vector (horizontal and vertical position, and horizontal and vertical velocities) to ATC. ADS-B IN equipped aircraft will receive these transmissions from nearby aircraft and display them as targets. These will be most of the targets you see. There is also an ADS-R service that retransmits 1090 target info on the UAT frequency and vice versa so that single-transceiver equipped aircraft can see nearby aircraft transmitting on the other frequency.

Not all aircraft are ADS-B equipped. To display these targets, TIS-B service is used. ATC surveillance systems (primarily secondary radar) send target information to the ADS-B control stations (there are three in the US) where this data is fused with ADS-B data and correlated to defined tracks.  If a non ADS-B target track aligns with an ADS-B target track then the two are assumed to be the same target and that target is not transmitted over TIS-B. But, if the tracks become uncorrelated, then the target is transmitted and you get a TIS-B target from your ownship at or near your position. This can be caused by a problem with the fusion server and the FAA has no way of knowing unless it receives reports.

Skip

[Pinecone]

I have gotten that false target a couple of times near my home field of 0W3.

Next time, I will make note and report it.  It DOES get your heart rate up.

[PT20J]

UPDATE:

The FAA finally received enough reports to track down the problem to the Seattle ASR-9 radar azimuth being slightly out of alignment. This should be fixed shortly. Thanks to everyone who reported an issue. They can't fix it if they don't know about it 

Skip

[Bolter]

  On 12/15/2022 at 6:51 PM, PT20J said:

UPDATE:

The FAA finally received enough reports to track down the problem to the Seattle ASR-9 radar azimuth being slightly out of alignment. This should be fixed shortly. Thanks to everyone who reported an issue. They can't fix it if they don't know about it 

Skip

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I did file (based on your suggestion), and got a personal email reply from the FAA as part of the investigation.  Glad that they found something.  -dan

[PT20J]

Also, for everyone who wondered: I did confirm with Garmin that all Garmin ADS-B In equipment filters out ownship ADS-B targets to eliminate the possibility of picking up your own ADS-B out transmission should it be reflected back by the ground.

[EricJ]

  On 12/15/2022 at 6:51 PM, PT20J said:

UPDATE:

The FAA finally received enough reports to track down the problem to the Seattle ASR-9 radar azimuth being slightly out of alignment. This should be fixed shortly. Thanks to everyone who reported an issue. They can't fix it if they don't know about it 

Skip

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That's a pretty cool outcome!   This feels like a good nerd victory.

 

[BobbyH]

  On 12/15/2022 at 6:51 PM, PT20J said:

The FAA finally received enough reports to track down the problem to the Seattle ASR-9 radar azimuth being slightly out of alignment.

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Thanks Skip, that's good to know.  Just reading this thread and remembering a number of times, both North and South of Seattle getting the ghosted targets from my own airplane.  Although frustrating and concerning a few times, wondering where that plane came from, it wasn't there a minute ago, my anxiety disappears when the target disappears too, knowing it's just "the other me".  Appreciate your chasing this down.  

[hammdo]

Had that happen today — gets your attention… wonder if the DFW area had a ‘ghost’ problem.. where’s ghostbusters when you need them ;o)

-Don

[PT20J]

  On 12/16/2022 at 2:51 AM, hammdo said:

Had that happen today — gets your attention… wonder if the DFW area had a ‘ghost’ problem.. where’s ghostbusters when you need them ;o)

-Don

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Maybe. Here's the link to report it: https://www.faa.gov/air_traffic/technology/adsb/adsb_reports/

Most of the targets you see are other airplane's ADS-B out transmissions picked up by your avionics. However, in some airspace there may be legal non-ADS-B equipped airplanes that ATC picks up on radar. To make sure you can see them, their position and altitude is broadcast using TIS-B. The problem is that your airplane is also picked up on radar and if it get's broadcast as a TIS-B target it will show up as a ghost at your approximate position and altitude. The way the ground system filters this out is to look at your ADS-B altitude and track and compare it with the radar target altitude and track. If they match, then the radar target is assumed to be also an ADS-B target and is not transmitted as a TIS-B target. If the radar slips out of alignment (they all drift and have to be realigned periodically) then the radar target can be offset enough from the ADS-B target and it gets transmitted as  TIS-B target creating a ghost. Eventually this problem will get corrected during periodic maintenance, but you can expedite it by filling out a report. 

Skip

[Ragsf15e]

Also, “we” (as in GA) are mostly the only ones using tis b.  It’s not something the airlines use.  It’s for us, and it’s pretty useful in some out of the way places where more planes aren’t equipped with adsb out.  Believe it or not, they’re out there!