Ghost In The Machine

Byssokol

Ghost In The Machine

At least once a week somebody sends me an email about an ADS-B traffic target that suddenly appeared on their EFB app, often just a hundred feet above or below, and less than half a mile behind. The reporter is never able to spot the target, which appears to doggedly follow them for a few minutes before disappearing. No, this is not a UFO or a new stealth drone. It’s just an annoying (and occasionally panic-inducing) artifact of the way ADS-B works.

No, you are not being followed by one of these.

No, you are not being followed by one of these.

If you’re being chased by ghosts, don’t worry – you’re not alone. It’s not an issue that’s specific to FlightBox or to whatever EFB app you’re using. Every ADS-B In system occasionally displays a ghost, even for those of us equipped with ADS-B Out. Here’s what appears to be happening:

You’re receiving ADS-B traffic data from the FAA ground towers. Those towers are receiving data from secondary surveillance radar. The radar data is collected at various center and terminal radar sites and sent to the towers for processing and broadcast. The software in the towers use the “hockey puck” algorithm to determine what traffic targets get broadcast.

Apparently, the delay between the time the radar site picks up a target and the time it arrives in your cockpit can vary quite a bit. Most of the time your EFB is able to determine which of the various targets it’s receiving is your aircraft (often referred to as “ownship”) which it filters out. But if the latency (time lag in delivery) is too great, the software can’t be absolutely sure if a given target is really “you” so, so it displays it – frequently with a traffic conflict warning.

Theoretically, this shouldn’t happen with aircraft that are equipped with ADS-B Out. Each Out-equipped aircraft broadcasts a unique identifier (ICAO code) which should allow both the towers and your local software to filter it out, preventing ghosts. But even the towers sometimes can’t correlate between your radar target and your position as received directly from your ADS-B Out transmissions. They fall victim to the same latency-induced issues that plague your EFB. Rather than take a chance on miss-reporting a conflict, they rebroadcast the radar data as an anonymous target which suddenly appears on your screen as a ghost.

The ghost effect appears to be worse when you’re in an area covered by multiple radars and multiple ADS-B towers. In talking with ADS-B experts, it does not sound like there’s any obvious fix for this. Lower latency links between the radar sites and the ADS-B towers can help, but even that’s not a guaranteed fix. As we get closer to 2020 and more aircraft are equipped with ADS-B Out, it should get better. Until then, don’t panic every time a mysterious traffic target appears on your screen. But don’t ignore it, either – that way lies midair ugliness.

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15 Comments so far

Jon TrachtaPosted on  6:52 pm - Jul 7, 2016

Thanks Steve, I had this occur on my first cross country flight with flight box displaying on a mini-ipad with WingX software. I did some s turns the first time thinking maybe it was Customs as my home base is close to the border. However, it occurred again on my return flight in almost the same area about 50 dme north of Tucson Intl. Thanks for the info. Was wondering if i needed a fix.

Wade EaklePosted on  4:27 pm - Jul 12, 2016

Steve, I’ve updated my FlightBox per the instructions on the Update Tutorial, but still get “Stratus” in the main heading, even after getting the “success” pop-up. Otherwise, all indications are that version v0.8r2 (db130aab76) installed OK.

Anything else I should do?

    ssokolPosted on  7:14 pm - Jul 12, 2016

    Try doing a “hard refresh” on the page (Ctrl + F5 on Windows, Cmd + Shift + R on Mac) to force it to reload the base page. If the update “took” then you should see “FlightBox – Powered by Stratux” at the top of the screen.

AndrewPosted on  11:00 pm - Jul 14, 2016

v1.0r1 of stratux out any time frame for one of your read only versions? by the way box still working well.

ssokolPosted on  6:03 pm - Jul 15, 2016

Chris and the core team have kicked around the idea of declaring a 1.0 but I’m not sure if the current release (v0.9rX) will be it. We may hold out until the AHRS algorithm is done and included.

Tyler TrainaPosted on  6:48 pm - Jul 21, 2016

I had this issue become 10x worse when using the built in (external) GPS in FlightBox. When I used my DualGPS over Bluetooth and FlightBox over WiFi, this issue was basically non-existent. Any idea why this would make a difference?

BrianPosted on  10:45 pm - Aug 3, 2016

Can this work with Garmin Pilot?

Leslie PartridgePosted on  8:08 am - Aug 9, 2016

When the ghost traffic appeared on my display I too did some fancy air work to see who was below me and on my tail. (Up to three targets.) At the time it seemed to me that it was a glitch in the system but then I am using an independent unit that is sensing the transponder returns of the airplane that I am flying. I attributed the ghosts to the latency between the two units and the ground and am glad to see this blog on the ghost topic. What is the solution?

Maybe I missed something in the setup of the flight box so that it links to my aircraft transponder (?). Is it possible to link to the aircraft transponder / altitude encoder so that I /we are putting out one traffic identity instead of two? Thanks for the support.

    ssokolPosted on  2:14 pm - Aug 10, 2016

    Hi Leslie,

    Unfortunately there is not real “cure” for this. If you have an ADS-B Out system installed in your aircraft you will want to add the ICAO code to the “Hex Code” field on the FlightBox settings page. That will help to filter out ownship messages which are relayed over ADS-R. It won’t do anything to help with the false targets that result from latency between ATC radar and the ADS-B uplink.

    Thanks,

    Steve

RandolPosted on  5:45 pm - Aug 10, 2016

Steve,
I’ve done everything possible (as far as I know) and cannot get GPS information from Flightbox to show on my Samsung Tab 7 or Samsung Tab 3 using AVARE and the AVARE External I/O app. Yes, I have tried both the Flightbox internal GPS and the alternate external one. I’ve also tried several Browsers.The Flightbox status page shows number of GPS seen and number of GPS tracked, but never any in solution and no fix. In looking at the GPS in menu, there is a position and track and ground speed, so some GPS is there, but on Avare map it always shows No GPS Signal therefore; no navigation. I have uninstalled and installed the Avare apps several times. Any suggestions would be greatly appreciated. I like everything about Avare and intend to use it and will try anything to get Flightbox to work.

RandolPosted on  7:39 pm - Aug 10, 2016

Steve,
Disregard my prior post/question. In doing everything again (for the umpteenth time), Flightbox suddenly worked with AVare. Now I see and use GPS information on the Avare map. I wish I knew what I did differently, but I think I did everything the same as before.
I still have to try it all out when I can get to a tower location.
Randol

ssokolPosted on  9:49 pm - Aug 10, 2016

Glad it started working for you. I like the free aspect of Avare, but the external devices interface is kludgy and makes it hard to track down issues. Please let me know how it works out in the air.

Thanks,

Steve

Tony YaconoPosted on  7:12 pm - Sep 3, 2016

Steve I can not get the display on my pad as I can on my I Phone or Wing X All I can get is a little box in the corner showing little dot and a figure What is wrong? how do I correct this? I am using True Flight
Thank You Tony

    ssokolPosted on  1:04 pm - Sep 6, 2016

    Hi Tony,

    I’m afraid I don’t know – I don’t have a TrueFlight device. You’ll need to contact the True Flight team for support.

    Sorry,

    Steve

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